An Introduction to the Earth, Oceans, Climate and Oceanography

S. R. Shetye

The universe formed sometime between 13 and 14 billion years ago (BYA), and the Earth about 4.55 BYA. The early Earth was fully molten. The surface cooled slowly, forming the solid crust within 150 million years. From 4 to 3.8 BYA, Earth underwent a period of heavy asteroidal bombardment. Steam escaped from the crust while more gases were released by volcanoes. Additional water was imported by arrival of comets. Clouds formed as the planet cooled. Rain gave rise to the oceans about 3.8 BYA, may be earlier.

Life on earth probably originated about 3.8 BYA in the oceans when chemical composition of the ocean and the atmosphere was very different from what it is today. About 3 BYA, something similar to modern photosynthesis developed and oxygen was produced. Over time, it transformed Earth's atmosphere to its current state. Some of the oxygen reacted to form ozone, which collected in a layer near the upper part of the atmosphere. By blocking the ultraviolet radiation, it allowed cells to colonize the surface of the ocean and ultimately the land. Fish, the earliest vertebrates, evolved in the oceans around 530 million years ago (MYA). During the last 500 million years at least three major extinction events occurred that affected life on Earth severely: 488 MYA; 250 MYA, when 95% of life on Earth died; and, 65 MYA when a meteorite likely struck, and most large animals, including the non-avian dinosaurs, became extinct.

As biology in the oceans and on land was evolving, geometry of the oceans was changing. The supercontinent Columbia is believed to have existed from around 1.8 to 1.5 BYA. 300 MYA the most recent supercontinent, Pangaea, formed. It broke up into Laurasia and Gondwana about 180 MYA. India, which formed a large block of Gondwana broke about 125 MYA. It collided with Asia about 45 MYA, forcing the crust to buckle and forming the Himalayas. It is estimated that by about 11 MYA the India started experiencing the monsoons.

Many scientists believe that a very severe ice age began around 770 MYA. Surface of all the oceans completely froze (Snowball Earth). Eventually, after 200 million years, enough carbon dioxide escaped through volcanic outgassing, and the resulting greenhouse effect raised global temperatures. Since then Earth's climate has gone through distinct periods of warming and cooling, most of them due to changes in solar radiation received by the Earth. Climate change is accompanied by changes in sea level: when the Earth warmed (cooled), the sea level went up (down).

Exploration of the oceans played a major role in piecing together the information on much of the changes that have taken place on the Earth since its formation. Early exploration of the oceans was limited to its surfaces. The 19th century saw maturing of ocean exploration. A major milestone was the Challenger expedition (1872-76) organized by Britain. The modern phase of oceanography of the waters around India began with the International Indian Ocean Expedition in 1960s. The National Institute of Oceanography, Goa, was founded in 1966.

Looking into the Earth

P. Dewangan and R. Mukhopadhyay
Geological Oceanography

Five billion years ago the Earth was formed in a massive conglomeration and bombardment of meteorites and comets. The immense amount of heat energy released by the high-velocity bombardment melted the entire planet, and it is still cooling off today. Denser materials like iron (Fe) from the meteorites sank into the core of the Earth, while lighter silicates (Si), other oxygen (O) compounds, and water from comets rose near the surface. As a result of this accretion, the Earth has a layered structure like an onion. Geologists collect information about Earth's remote interior from several different sources. Some rocks found at the earth's surface, known as kimberlite and ophiolite, originate deep in the crust and mantle. Some meteorites are also believed to be representative of the rocks of the Earth's mantle and core. These rocks provide geologists with some idea of the composition of the interior. Another source of information, while more indirect, is perhaps more important. That source is earthquake, or seismic waves. When an earthquake occurs anywhere on Earth, seismic waves travel outward from the earthquake's center. The speed, motion, and direction of seismic waves changes dramatically at different levels within Earth, known as seismic transition zones. Therefore, scientists can make various assumptions about the earth's character above and below these transition zones through careful analysis of seismic data. The gross structure of the Earth is illustrated in Figure 1.

The outermost layer of Earth is the crust, or the thin "shell" of rock that covers the globe. There are two types of crust: the continental crust, which consists mostly of light-colored rock of granitic composition that underlies the earth's continents; and the oceanic crust, which is a dark-colored rock of basaltic composition that underlies the oceans. One of the most important differences between continental and oceanic crust is their difference in density. The lighter-colored continental crust is also lighter in weight, with an average density of 2.6 g/cm3 (grams per cubic centimeter), compared to the darker and heavier basaltic oceanic crust, which has an average density of 3.0 g/cm3. It is this difference in density that causes the continents to have an average elevation of about 600 m above sea level, while the average elevation (depth) of the ocean bottom is 3,000 m below sea level. The heavier oceanic crust sits lower on the earth's surface, creating the topographic depressions for the ocean basins, while the lighter continental crust rests higher on the earth's surface, causing the elevated and exposed continental land masses. Another difference between the oceanic crust and continental crust is the difference in thickness. The heavier oceanic crust forms a relatively thin layer of 5-10 km, while the continental crust averages about 35 km thick but can reach up to 70 km in certain sections, particularly those found under newly elevated and exposed mountain ranges such as the Himalayas.

The base of the crust (both the oceanic and continental varieties) is determined by a distinct seismic transition zone called the Mohorovi?ic discontinuity. The Mohorovicic discontinuity, commonly referred to as "the Moho" is named after the Croatian seismologist Andrija Mohorovi?ic. The Moho is the transition or boundary zone between the bottom of the earth's crust and the underlying unit, which is the uppermost section of the mantle called the lithospheric mantle. Like the crust, the lithospheric mantle is solid, but it is considerably more dense. Because the thickness of the earth's crust varies, the depth to the Moho also varies from 5-10 km under the oceans to 35-70 km under the continents. The Moho is defined by the level within Earth where P wave velocity increases abruptly from an average speed of about 6.9 km/s to about 8.1 km/s.

Underlying the crust is the mantle. The uppermost section of the mantle, which is a rigid layer, is called the lithospheric mantle. This section extends to an average depth of about 70 km, although it fluctuates between 50-100 km. The density of this layer is greater than that of the crust, and averages 3.3 g/cm3. But like the crust, this section is solid and brittle, and relatively cool compared to the material below. This rigid uppermost section of the mantle (the lithospheric mantle), combined with the overlying solid crust, is called the lithosphere, which is derived from the Greek word lithos, meaning rock. At the base of the lithosphere, a depth of about 70 km, there is another distinct seismic transition called the Gutenberg low velocity zone. At this level, the velocity of S waves decreases dramatically, and all seismic waves appear to be absorbed more strongly than elsewhere within the earth. Scientists interpret this to mean that the layer below the lithosphere is a softer zone of partially melted material (with between 1-10% molten material). This "soft" zone is called the asthenosphere, from the Greek word asthenes meaning weak. The asthenosphere extends to a depth of about 250 km. Below that depth, seismic wave velocity increases, suggesting an underlying denser, but solid phase. The rest of the mantle, from the base of the asthenosphere at 250 km to the core at 2,900 km, is called the mesosphere (or middle sphere). There are mineralogical and compositional changes suggested by sharp velocity increases within the mesosphere. Notably, there is a thin zone at about the 400 km depth attributed to a possible mineralogical change (presumably from an abundance of the mineral olivine to the mineral spinel), and there is another sharp velocity increase at about the 660 km level, attributed to a possible increase in the ratio of iron to magnesium in mantle rocks. The region between 400 and 660 km is known as the transition zone, and it forms the lower part of the upper mantle. The lower mantle (660-2,900 km) contains 72.9% of the mantle-crust mass and is probably composed mainly of silicon, magnesium, and oxygen. The velocity increases linearly in the lower mantle.

At a depth of 2,900 km there is another abrupt change in the seismic wave patterns, known as the Gutenberg discontinuity, or more often referred to as the core-mantle boundary (CMB). At this level, P waves decrease while S waves disappear completely. Since S waves cannot be transmitted through liquids, it is believed that the CMB denotes a phase change from the solid mantle above, to a liquid outer core below. This phase change is believed to be accompanied by an abrupt temperature increase of 1,300° F (704° C). This hot, liquid outer core material is denser than the cooler, solid mantle, probably due to a greater percentage of iron. It is believed that the outer core consists of a liquid of 80-92% iron, alloyed with a lighter element. The actual boundary between the mantle and the outer core is a narrow, uneven zone that contains undulations that may be 5-8 km high. These undulations are affected by heat-driven convection activity within the overlying mantle, which may be the driving force for plate tectonics. The interaction between the solid mantle and the liquid outer core is very important to Earth dynamics for another reason. It is the eddies and currents in the core's iron-rich fluids that are ultimately responsible for the earth's magnetic field. Although the core-mantle boundary is currently situated at a depth of about 2,900 km, this depth has not been constant through geologic time. As the heat of the earth's interior is constantly but slowly dissipated, the molten core within the earth gradually solidifies and shrinks, causing the core-mantle boundary to slowly move deeper and deeper within the earth's core.

There is one final, even deeper transition evident from seismic wave data. Within Earth's core, at the 5,100 km level, P waves speed up and are reflected from yet another seismic transition zone. This indicates that the material in the inner core from 5,100-6,370 km is solid. The phase change from liquid to solid is probably due to the immense pressures present at this depth. In addition to this phase change in the inner core from liquid to solid, seismic wave velocities, as well as the earth's total weight, suggest that the inner core has a different composition than the outer core. This could be accounted for by a relatively pure iron-nickel composition for the inner core. Although no direct terrestrial evidence for a solid iron-nickel inner core exists, comparative evidence from meteorites supports this theory.

Plate Tectonics

A. K. Chaubey
Geological Oceanography

The theory of continental drift, which paved the way for discovery of plate tectonics, was put forward by Alfred Lother Wegener - a meteorologist from Germany - in 1912. The theory states that continents are not fixed, but have been slowly wandering during the course of Earth's geological history. Although Wegener's continental drift theory was later disproved, it was one of the first times that the idea of crustal movement had been introduced to the scientific community; and it has laid the groundwork for the development of modern plate tectonics.

In the early 1960s, the emergence of the theory of plate tectonics started a revolution in the earth sciences after more and more evidences were found to support the idea that the plates were moving continously over geologic time. Paleomagnetic observations and seafloor spreading records have provided the rock-solid evidences for establishing the theory of plate tectonics. Scientists have continuously verified and refined this theory, and now have a much better understanding of how our planet has been shaped by plate-tectonic processes. We now know that, directly or indirectly, plate tectonics influences nearly all geologic processes, past and present. Indeed, plate tectonics theory has proven to be as important to the earth sciences as the discovery of the structure of the atom was to physical sciences and the theory of evolution was to the life sciences.

Tectonics is the study of the forces within the Earth that give rise to continents, ocean basins, mountain ranges, earthquake belts and other large-scale features of the earth's surface. The theory of plate tectonics states that the outer rigid layer (about 70-100 km thick) of the earth called lithosphere, is divided into number of zones. These zones are called lithospheric plates upon which continents and ocean floor lie. The plates are in continuous motion at a speed of few centimeters per year over the asthenosphere, which is highly viscous, easily deformable layer between upper and lower mantle. The relative motion between the plates produce new crust at mid-oceanic ridges, consume crust at subduction zones and conserve the crust along the transform faults. Apart from normal process of construction and destruction at plate boundaries, plates do undergo break-ups and unifications. The lithospheric plates were reconfigured several times by continental rifting, ridge jumps and ridge propagation from the origin of the Earth to the present. All of the big geologic phenomena, earthquakes, volcanoes, mountain building, occur at plate boundaries.
In this lecture, we discuss some fundamentals of plate tectonics and attempt to understand new insights into its power and its limitations.

Seawater and its constituents

P. V. Narvekar & M. Dileep Kumar
Chemical Oceanography

Seawater is a natural soup of chemical substabces. Over 90 elements listed in periodic table are known to occur in seawater. Substances can occur in dissolved and particulate forms where the particles range from colloidal to aggregates. Total dissolved salt content of seawater is expressed as salinity, which on an average is 35 PSU. Sodium amd Chloride ions are the most abundant. Physicochemical properties of seawater depend on salinity, temperature and pressure changes in the oceans. Among these properties density assumes the greatest significance as it determines the global circulation, particularly in the deep oceans.

Dissolved ions in seawater can occur in many forms or species. The forms and their abundances of an element is determined by its atomic properties and the consequent behaviour. Elements may occur in forms of free, pairs of ions or complexed ions. An attraction between cation and anions in seawater results in ion pair or complexation. Similarly cations, particularly, metal ions react with organic ligands and form organometalic substances. Speciation of elements is important as it determines the behaviour or fate of that substance in the oceans. Dissolved and particulate substances in seawater are practically differentiated by separating them through filtration (e.g. through 0.45 µm filters).

Two master variables, pH and pE, determine the elemental speciation in natural waters. The pE is a redox indicator, which is susceptible to oxygen availability. The pH is the acid-base indicator determined by the carbon dioxide chemistry, particularly HCO₂- - CO₃²- equilibrium, in seawater. Besides carbonate ions, borate and silicates occur in seawater. Therefore, amount of protons required to neutralize these anions is termed 'alkalinity'.

Main groups of substances in seawater are major, minor and nutrient elements. Dissolved salt content in seawater is mainly constituted by 11 ions that are present at more than parts per million (ppm or mg/l) levels. Since the relative concentrations of these ions in seawater are unaffected by mixing or biological processes these are termed conservative elements. Therefore, ratios of these major ions remain nearly constant any where in the ocean. Particularly useful is ion to chloride ratio that facilitates understanding the behaviour of substances. Minor elements occur at < ppm concentrations. As changes in their concentrations brought about by mixing and biological processes are significant this group of trace substances are said to be non-conservative elements.

Biological processes release many organic substances into seawater. These range from methane to the most complex humic substances. Different organic compounds behave differently under various physicochemical conditions in oceans. Conversion from dissolved to particulate organic forms will have strong influence not only on carbon cycle but also on many other dissolved constituents, including trace metals, in the ocean.

Control of seawater composition

M. Dileep Kumar
Chemical Oceanography

Global cycles of elements involve transient residence during their passage through various geological reservoirs. The major geological reservoirs, in the present context, include crust (geosphere), oceans (hydrosphere) and air (atmosphere). Seawater has originally formed at the early stages of the formation of our planet. Reactions between acidic gases, released during the violent volcanism, and basalt rock in the presence of water led to the formation of seawater. Thereafter, the oceans continued to receive material inputs from rivers, atmosphere, volcanoes and sediments.

Comparison of the compositions of rain, river and seawaters reveals material transport pathways and the significance of weathering on the continents. Rain and seawater are rich in Na+ and Cl- but river water in Ca²⁺ and HCO₃-. Conservative behaviour of some elements enables their enrichment in seawater whereas many are in trace (at parts per billion, ppb, or less) quantities. The seawater salinity is nearly constant at 35 PSU in the global ocean. Changes in trace element composition occur from place to place. Seawater composition can be considered to be at a steady state, in general.

For the elements in a reservoir to be at steady state the total inputs should equal the total outputs. Removal of elements is expected to occur through precipitation of least soluble compounds. However, at the concentrations of trace substances in seawater, and at a pH of ~8 and a pE of ~12, precipitation of many elements either by acid-base or redox equilibria is not possible. Alternative mechanisms will have to be at work. Incorporation of substances during the formation and subsequent turnover of organic matter (i.e. biological cycle) in the ocean plays a significant role in material transportation within the water column but in shorter time scales. Under these conditions adsorption is the most important process. Physical adsorption can also be supplemented by chemical complexation between ligands on particle surfaces and dissolved ions. Substances thus get attached to particles in suspension that later aggregate into particles and settle in marine sediments. Despite the slow settling rates of particles ( a few mm or less in 1000 years in the deep ocean) the adsorption-complexation-scavenging mechanisms are responsible for removal of many trace elements in the ocean.

The reactivity and behaviour of elements are reflected in their life times in the oceans. A measure of this is 'residence time'. Major element residence times are of the order of millions of years but those of trace substances are about a few hundreds-to-thousands of years. Knowledge of oceanic residence times is important in understanding global mass balances and surficial cycles.

Geochemical cycles
Origin of seawater
Sources of elements
Rain, river and seawaters
Conservative and non-conservative elements
Removal mechanisms
Precipitation-dissolution
Adsorption/desorption
Complexation-scavenging
Biogeochemical cycles
Residence times

Ocean Biogeochemistry

M. Dileep Kumar
Chemical Oceanography

Geochemical processes at the Earth surface are continuously being modified after the emergence of life. The biogeochemistry is the study of understanding the geochemical and biological processes controlling cycles of substances. The biogeochemistry assumes global significance in not only regulating abundances of natural greenhouse gases in the atmosphere but also in the evolution of present oxygen levels. Oceans occupy ~70% of the Earth surface with a potential to modulate the atmospheric composition. For instance, oceans contain about 60 times more carbon dioxide than that in atmosphere, wherein a minor change in air-sea exchange of this gas can affect its abundance in the atmosphere. Biogeochemistry is generally identified with the cycles and processes of elements C, N, S, P and O as these build the organic material, the turnover of which also influences cycles of many other elements.

Ocean biogeochemistry involves inter-conversions of materials between organic and inorganic forms in seawater. These ocean biogeochemical processes are continuously under the influence of external forcings induced by wind, river discharges, volcanic depositions etc. Ocean biogeochemistry mainly consists of physical and biological pumps. Transports of dissolved and particulate substances in the oceans in space (vertical and horizontal) and time form the physical pump. Synthesis of organic matter in the sunlit surface layers, its subsequent decomposition in surface or deep waters and sinking of undecomposed biological materials from surface to deep ocean constitute the biological pump. The rate of biological materials sinking is known as export flux.

Transport and transformations of organic matter influences not only the abundances of CO₂ species of pCO₂, HCO₂- and CO₂^2- but also of other elements, nitrogen in particular. Oxygen is used in the decomposition of organic matter. In oxygen deficient environments, bacteria turn to nitrate as the alternative oxidant where it is reduced to elemental nitrogen. An important byproduct of nitrification and denitrification processes in natural waters is nitrous oxide, which has much greater greenhouse warming potential than CO₂. Exchanges of materials across air-sea, land-ocean and sediment-water interfaces, which is largely regulated by biogeochemical processes in the water column, are important in global cycles of materials. Coastal pollution may trigger eutrophication that can significantly alter the nature and extent of biogeochemical processes.
In the last one-and-a-half century the atmospheric CO₂ levels have rised significantly; attributed to the warming of atmosphere. One of the means to remove the human induced CO₂ in air is ocean fertilization. Iron fertilization has been found to enhance the primary production in areas of high nitrate and low chlorophyll (HNLC). Subsequent sequestration of organic matter into deep-sea sediments facilitates long-term delay in returning CO₂ back into air.

Global Thermohaline Circulation
V. S. N. Murty
Physical Oceanography

Deep-sea measurements way back in the 17th century fascinated the researchers to identify the existence of a large water column of cold waters below about 1 km depth. The theories put forwarded since then explained the filling up of the deep-sea basins with the cold circumpolar waters and their spread into the tropical regions as well. The circulation of the waters in the oceans results mainly due to tidal forces, wind stress, and density differences. The density of sea water is controlled by its temperature (thermo) and its salinity (haline), and the circulation driven by density differences is thus called the Thermohaline Circulation. The cold polar waters are very dense in nature and sink to greater depths. The sinking of cold dense water at high latitudes (eg., the Greenland Sea, the Norwegian Sea and the Labrodar Sea) is due to temperature and salinity differences, and this sinking and spreading of cold waters is known as the thermohaline circulation or the Meridional Overturning Circulation. The cold, dense water gradually warms and returns to the surface, throughout the world's oceans. The surface and subsurface currents, the sinking regions, and the return of water to the surface form a closed loop, the thermohaline circulation or global thermohaline conveyor belt. This is driven primarily by the formation and sinking of deep water in the Norwegian Sea, and is thought to be responsible for the large flow of upper ocean water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelogo. The critical part of the thermohaline circulation is the sinking in the North Atlantic Ocean because the Atlantic is much more saline (and hence, denser), compared to the north Pacific Ocean. It is more saline because it is warmer and thermohaline circulation from the tropical and South Atlantic brings in warm water. Therefore, the thermohaline circulation appears to be self-sustaining. And if some event occurs to break this self-sustaining chain of processes, then there is the potential for the circulation to break down rapidly (i.e., over several decades) and to remain in a reduced-circulation state for several centuries. Some fairly simple models of the world's oceans do simulate a rapid break down of the thermohaline circulation, when the density of the water in the North Atlantic Ocean is lowered by adding fresh water (rain) and/or by warming. Increased rainfall and warming over the North Atlantic are both expected as a result of increased greenhouse gas concentrations, and so it can be argued that global warming may cause a rapid collapse of the thermohaline circulation. Due to the interactions between many components of the climate system, it is not a simple matter to estimate how different our climate would be without the current thermohaline circulation. Certainly the biggest impact would be on the temperature over the North Atlantic and Europe. Climate modeling studies are underway to estimate the impact of break down of the thermohaline circulation. The preliminary climate model studies indicated that cooling of ~8°C around the coast of Greenland, but with more moderate cooling (<2°C) over most of Europe.

Scientists believe that the impact of the global thermohaline circulation in the Indian Ocean is the flow of warm and less saline Pacific Ocean waters into the southern equatorial Indian Ocean through the Indonesian Archipelogo. This inflow is more known as the Indonesian Throughflow, which varies from 2 to 10 Sv (1 Sv = 106 m3/s), and exhibits both seasonal and interannual variability. In the Indian Ocean, the mean heat flux per unit area north of 20°S is about 40 Wm-2 and it is estimated that this heat energy has to be exported southwards across the Indian Ocean equator at an annual rate of about 0.5 X 1015 W. This is equivalent to an average upwelling velocity of 6 x 10-5 cm/s from 2000 m over the whole Arabian Sea and Bay of Bengal. This rate of upwelling is large compared to the rate of global upwelling velocity of 4 x 10-5 cm/s.

One might speculate that the main reason for a relatively large heat flux over the Indian Ocean - and hence for a large deep upwelling velocity - is that the Indian Ocean is confined to low latitudes. The insolation is strongest in the tropics, so the average heat flux for the Indian Ocean would naturally get weighted towards a higher value. The heat flux received at the ocean surface gets disturbed by two agencies (1) the meridional overturning cells; and (2) the horizontal wind driven gyres in the upper oceans. In the latter case, the subtropical gyres carry heated water from the tropics into mid-latitudes, where the higher temperatures cause heat loss from the ocean, and thereby, a negative, zonally averaged surface heat flux results in. The equatorial gyres, on the other hand, confine the heated water to the tropics, reinforcing the heat input there and leaving only meridional overturning as sole agent to distribute the heat. It is estimated that the south Indian Ocean exports heat to the rest of world's oceans (towards south) at a rate of 0.69 x 1015 W at 18°S and 0.25 x 1015 W at 32°S.

Global wind-forced circulation

D. Shankar and S. R. Shetye
Physical Oceanography

The oceans form a rather unconventional tank. Their horizontal dimension stretches to thousands of kilometres, but the vertical dimension is generally less than 4 km. This low ratio of vertical-to-horizontal dimension (called aspect ratio), that the earth is a sphere that rotates on its axis, and the stable stratification - lighter water overlying heavier water - lead to the dynamics of the ocean being very different from that seen in fluid flows in laboratories or in engineering. The low aspect ratio leads to motion being mostly horizontal and to the ocean being in hydrostatic equilibrium. The rotation leads to the Coriolis force (a pseudo-force like the centrifugal force) playing a crucial role.

The winds blowing on the surface of the ocean exert a stress on the ocean surface and transfer momentum to the ocean. The resulting motion is most noticeable within the upper 1 km, but the strongest motions are in the uppermost few hundred metres. An implication of the Coriolis force is that currents are not in the direction of the wind. The direct effect of wind forcing is confined to a surface boundary layer called the Ekman layer, in which the net motion of water is to the right of the wind (in the northern hemisphere). Below the Ekman layer, direct wind forcing is not important, and the primary force balance is between the pressure-gradient force and the Coriolis force. This balance is called geostrophy. In a flow in geostrophic balance, the current flows not from high to low pressure, as we are used to seeing in daily life, but along isobars (like the winds in cyclones). In the northern hemisphere, the current flows with the higher pressure on its right.

The large-scale circulation in the world oceans is dominated by the anticyclonic sub-tropical gyres, in which a strong poleward (towards the pole) current exists near the western boundary (east coast of a continent) and a gentler equatorward drift is seen over the rest of the basin. In the Atlantic and Pacific oceans, the strong western boundary currents are called the Gulf Stream and Kuro Shio respectively. These gyres are forced by the large-scale pattern of winds. The winds are easterly (from the east) over the equatorward half of the gyre and westerly over the poleward half. This leads to an anticyclonic wind pattern, which forces the equatorward drift over most of the basin. The poleward western boundary current is needed to balance the equatorward mass transport of this flow.

This description and theory, however, assume a steady state. This is a good approximation for the regime of the "steady" easterly trade winds, but not for the north Indian Ocean, over which the winds reverse with season. This leads to a strong seasonal variation in the circulation in this basin.

Wind-forced circulation in the north Indian Ocean

D. Shankar and S. R. Shetye
Physical Oceanography

Seasonal variations in winds are common all over the world oceans. The equatorial low-pressure regime and the sub-tropical highs undergo seasonal changes in magnitude and location. The change over the north Indian Ocean is, however, unique in its intensity. The winds actually reverse direction with season. They blow generally from the northeast, from the high over the Indian subcontinent to the low over the equatorial Indian Ocean, during November-March (northeast or winter monsoon), like the trade winds over the other ocean basins. During May-September (southwest or summer monsoon), however, the southeasterly trade winds cross the equator and the winds over the north Indian Ocean blow generally from the southwest; the low-pressure regime shifts northward to lie over the hot Indian subcontinent. The summer monsoon winds are stronger than the winter monsoon winds. In response to these changing winds, the circulation in the top 200 m of the north Indian Ocean shows a distinct seasonal cycle.
In the Arabian Sea, the current off Somalia and Oman is the equivalent of the western boundary currents elsewhere in the world oceans. The Somali Current flows equatorward during the winter monsoon and poleward (crossing the equator) during the summer monsoon. Its dynamics, however, is quite different from that of the Gulf Stream or Kuro Shio. During the summer monsoon, when the Somali Current flows poleward, water upwells off Somalia, lowering the sea surface temperature. These cold waters are spread eastward by the Ekman drift.

Another distinct feature of the Arabian Sea circulation is seen off southwest India, where the sea level and associated circulation exhibit a distinct annual cycle. A sea-level high, called the Lakshadweep High, forms in the southeastern Arabian Sea during December and spreads westward. A low, the Lakshadweep Low, forms during the summer monsoon; it also spreads westward. Associated with this annual cycle is a reversal in the West India Coastal Current (WICC). The WICC flows poleward during the winter monsoon and equatorward during the summer monsoon. Off southwest India, however, the existence of the Lakshadweep High and Low results in the WICC in this part of the coast often flowing opposite to the WICC farther north.

During the winter monsoon, a seasonal sub-tropical gyre forms in the Bay of Bengal; its western boundary current flows poleward along the east coast of India. The current along the Indian east coast is called the East India Coastal Current (EICC). A weaker poleward flow exists during the summer monsoon, when the strong southwesterlies blow over the bay. So, the EICC is strongest (March-April) not when the winds are strongest (July-August), but when they are weak. When the summer monsoon winds collapse in September, the EICC reverses to flow equatorward. This equatorward EICC brings down south the low-salinity waters from the northern Bay of Bengal. These low-salinity waters are carried into the southeastern Arabian Sea by the westward Winter Monsoon Current (WMC) and the poleward WICC.

Among the most striking features of the wind-forced circulation in the north Indian Ocean are the seasonally reversing monsoon currents. They flow across the basin, transferring heat and salt between the Arabian Sea and the Bay of Bengal. The WMC flows westward from the bay to the Arabian Sea during winter. During summer, the Summer Monsoon Current (SMC) flows eastward from the Arabian Sea to the bay.

The distinct seasonal cycle in the wind forcing implies that classical, steady-state theories built to explain the observed circulation elsewhere in the world oceans (like, for example, the sub-tropical gyres and the western boundary currents in the Atlantic) fail in the north Indian Ocean. A viable theory has to account for the time-varying forcing and response. Such a theoretical framework has been assembled over the last decade-and-a-half to explain the observed seasonal cycle of circulation in the basin. It shows that the Arabian Sea, the Bay of Bengal, and the equatorial Indian Ocean function as a single dynamical entity, making the current at any location in the basin a response not just to local winds, but also to winds blowing elsewhere in the basin.

Oceans and climate

S.S.C. Shenoi and D. Shankar
Physical Oceanography

Solar heating is distributed unequally over the Earth's surface. Oceanic motion makes an important contribution to the transport of heat and reduces the equator-pole temperature gradient over the Earth. On an average, the ocean transports as much heat to the higher latitudes as does the atmosphere. The larger heat capacity of water compared to air makes it possible to do this with currents that are much weaker than the winds. Oceanic circulation therefore has a close relation to climate and affects it on a range of space and time scales. On long time scales, the oceans participate in determining climate through the global conveyor belt, which is affected by changes intrinsic to the atmosphere and the ocean and by the changes in solar heating due to variations in the Earth's orbit around the Sun.

A more easily observable example of the ocean's role in climate is El Niño, a phenomenon that occurs in the equatorial Pacific Ocean. Under normal conditions, the easterly (from the east) trade winds maintain a reservoir of warm water in the western Pacific off Indonesia and Papua New Guinea. This warm water supports strong atmospheric convection. As a result, rainfall in this region is among the highest in the world. During the summer monsoon this band of high rainfall extends into the Indian Ocean and over the Indian subcontinent. The rising air that is responsible for this rainfall moves across the Pacific basin and sinks over the cool waters off Peru on the eastern side. When an El Niño occurs, the waters off Peru warm, and these warm waters spread westward, increasing the sea surface temperature across the eastern and central Pacific. This suppresses convection over Indonesia and the western Pacific. The effect of El Niños is not restricted to the equatorial Pacific. The large expanse of the basin, which covers almost half the globe, ensures that El Niño has a global impact on climate. During El Niño, with the atmospheric convection over the western Pacific being suppressed and the band of high rainfall shifting eastward, there is a tendency for rainfall over India also to decrease.

Dramatic advances in satellite technology have led to the recent discovery of an El-Niño-like oscillation in the equatorial Indian Ocean. It has been called the Indian Ocean Dipole Mode. Under normal conditions, the band of warm waters in the western Pacific extends across the north Indian Ocean. The eastern equatorial Indian Ocean is usually warmer than its western counterpart. When the positive phase of the dipole occurs, as it did in 1997, sea surface temperature decreases in the east and increases in the west. Recent research suggests that the dipole has a significant influence on the rainfall over India.
The Arabian Sea and the Bay of Bengal also exercise a profound influence on climate. Though both are located in the same latitude band and receive the same amount of solar radiation from the Sun, the Bay of Bengal is much warmer than the Arabian Sea and many more storms brew over the bay. The depressions that form over the northern Bay of Bengal move northwestward across the Indo-Gangetic plains, bringing rain to most of northern India. Over the Arabian Sea, rainfall is much less on an average. The ocean plays a major role in keeping the Arabian Sea relatively dry. Recent research shows that there are two causes. First, the winds over the Arabian Sea are stronger because of the presence of the mountains of East Africa. These strong winds force a much more vigorous oceanic circulation and the heat received at the surface is transported southward and into the deeper ocean. The winds over the Bay of Bengal, in contrast, are more sluggish and the bay is unable to remove the heat received at the surface. Second, the bay receives more rainfall; it also receives more freshwater from the large rivers, especially the Ganga and the Brahmaputra, that empty into it. This freshens the surface of the bay and stabilizes the water column, making it more difficult for the winds to mix the warm, stable surface layer with the cooler waters below. In the Arabian Sea, there is no such stabilizing effect. As a consequence, the mixing with the cooler waters below is more vigorous. Since a sea surface temperature of about 28°C is necessary for convection to take place in the atmosphere, this condition is satisfied in the Bay of Bengal, but not in much of the Arabian Sea. Thus, in spite of their geographical similarities, the two arms of the north Indian Ocean are strikingly different when it comes to climate.

Waves, Tides and Shallow water processes

A.S. Unnikrishnan
Physical Oceanography

The time scales and spatial scales associated with the physical processes in the nearshore regions are different from those in the open ocean. In the open ocean, variability of physical phenomena occurs in time scales ranging from intraseasonal, seasonal and interannual. The spatial scales associated with these phenomena are of the order of hundreds to thousands of kilometers. However, in the nearshore regions, variability in time occurs ranging from a few seconds to a few days. The currents are formed due to breaking of waves, that come from the open ocean and driven by tides, besides local wind-driven effects etc. The spatial scales associated with these phenomena are relatively small due to sharp changes in bottom topography and shoreline configuration. Currents, sea surface elevations, salinity distribution etc. change over short time scales and small spatial scales. Studying these processes has applications in navigation, in determining fate of pollutants, sediment transport etc.

The two most important phenomena that cause variability in the coastal ocean are due to surface waves and tides. Wind waves are the most common form of a class of waves called surface gravity waves. They are called gravity waves because the Earth's gravity pulls the water particles back to equilibrium once the wind disturbs the ocean surface and perturbs it from equilibrium by kinetic energy into water. Wind waves form in water that is deeper than half their wavelength; hence they are called deep-water waves. The momentum gained by the water decays with depth, and there is no motion due to these waves in deeper water. The pattern associated with these waves moves at speeds of 1 to 100 m/s, their wave lengths range from 1-1000 m and their periods range from a few seconds to minutes. The waves generated in the open ocean, when they reach coastal waters undergo transformations mainly due to the shallow water effects. The waves undergo refraction depending on the bathymetry of the region and break very near the shore giving rise to littoral currents, which control sediment movements.

Tides are called shallow water waves, because they occur in water that is shallower than their wave length. The speed of shallow water waves is given by ?gh, where g is the acceleration due to gravity and h is the ocean depth. Tides have wavelengths ranging from 100-10000 km and periods of 12.5 (semi-diurnal) and 24 hours (diurnal). Along the Indian coast, the tides are mostly mixed type. Along the Indian coasts, the tidal ranges are low in the southern side, which gradually increase towards the north. Correspondingly, tidal currents are weaker along the southern part of the Indian coast than in the northern side. The magnitude of tidal currents increases northward, and it reaches very high values in regions such as Gulf of Kutch and Gulf of Khambhat, where speeds exceeding 2 m/s have been measured.

Coastal eco-system

A. C. Anil
Marine Corrosion and Material Research

Coastal environment embraces wide variety of habitats. Among them rocky shores and other man made structures provide unique habitat for many organisms. The factors that regulate the population of any given organism are regulated by events across trophic levels. In the coastal eco system this is further influenced by the anthropogenic pressures.

"Barnacle" a shelled organism, is common world over either in the expanses of inter tidal region or on the surfaces of the man made structures. Barnacles are sessile from their juvenile stage. However, their life cycle includes planktonic larval stages. The typical pattern of larval development in barnacles include stages (nauplii) dependent on food organisms such as phytoplankton and a terminal non feeding substratum exploring stage (cypris). Such a diverse mode of life in a single organism provides a unique case for elucidating the complexities of sustaining life in the coastal environment and is addressed in the lecture.

Beaches and Shoreline

S. Prasanna Kumar
Physical Oceanography

Beach is an extremely fragile buffer zone between the coast and the ocean, which usually has a gentle slope and is a site of intense air-sea-land interactions. The seaward extent of the beach is the low water line and the landward extent normally is the sand dune or a cliff. It consists of unconsolidated sediments such as sand, pebbles and sometimes boulders. Beaches are very important in our day-to-day life not only from the point of leisure and tourism, but also lively hood of fishing community as well as waste disposal. Hence it is important to understand the characteristics and variability of this dynamic zone for the better management.

The physical forces that influence the beach are the tides, waves and winds. The response of the beach to each of this forcing will depend on the type of material it consists of, the near shore bathymetry, the source of beach material etc. In this talk, we will examine each one of the above in detail. The discussion will cover the tide producing force, characteristics of spring and neap tide and its effect on beach sediment movement, the generation of gravity waves, its propagation and transformation (shallow water wave equation), wave breaking and energy dissipation in the near shore region, littoral currents and rip currents, and near shore sediment transport. The seasonal variability of beaches along the Indian coast will be examined with some examples. Finally, instrumentation for the monitoring of beach variability and the role of remote sensing in study of beach dynamics will be discussed.
We will also briefly discuss, the potential dangers such as drowning associated with beach recreational activities and how to avoid such situation.

Marine Pollution Studies - An overview

Anupam Sarkar
Chemical Oceanography

The marine environment is being contaminated at an alarming rate by various types of pollutants such as organic (pesticides residues, polychlorinated biphenyls, polychlorinated dibenzo-dioxins, polychlorinated dibenzo-furnas, polycyclic aromatic hydrocarbons, tributyl tin, etc) as well as inorganic (toxic heavy metals, lead, mercury, cadmium, arsenic, copper, cobalt, nickel etc) compounds. The main sources of such contamination are the discharge of municipal wastes, industrial effluents, and extensive shipping activities all over the world. Of these, persistent organic pollutants are of great significance because of their high persistence, lipophilicity, and toxic potentials. The impact of these pollutants can be enormous. They can cause serious damage to the physiological systems of the marine organisms leading to death. Moreover, human health can be greatly affected due to ingestion of such contaminants through the food chain.

It is therefore, a prime need of the hour to evaluate the levels of contamination of the marine environment and to understand the mechanisms of their interaction with the physiological system of the marine organisms. In order to assess the impact of various types of pollutants we need to address the following questions?

1. What are the sources of pollution ?
2. How can we estimate the levels of contamination ?
3. What are the methods of identification and quantitation of organic contaminants ?
4. What are the impacts of pollutants on marine organisms ?
5. How biomarkers are useful for estimation of the impact of pollutants ?
6. What are the role biomarkers in marine pollution monitoring ?
7. Is there any solution to such problem of marine pollution ?

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Estuarine Processes - I (Physical)

A. S. Unnikrishnan
Physical Oceanography

The classical definition of an estuary as given by Pritchard is " A semi enclosed body of water having a free connection with the open sea in which sea water is measurably diluted with fresh water drained from land ''. The importance of estuaries lie in the fact that they are the spawning and nursing grounds for many fishes. The banks of estuarine channels form a favoured location for human settlements, which use the estuaries for fishing and commerce, but nowadays also for waste disposal. Many estuaries are locations of some of the major sea ports.

Estuaries are classified into salt wedge type, partially stratified type and well mixed type. Salt wedge estuaries have a typical circulation pattern of surface downstream freshwater flow and the bottom upstream saltwater flow. In well mixed estuaries, the tidal ranges are high and their action dominates that of river flow. In well mixed estuaries, the tidal action is very strong that the flow is nearly vertically homogeneous.

The tidal propagation in a well mixed estuary can be modelled using a two- dimensional model. In these estuaries, currents are mainly driven by tides and changes in tidal propagation, as the tides propagate into the estuary determine estuarine circulation pattern. A depth-averaged model, which makes use of vertically integrated equations of momentum and equation of continuity, can be solved numerically to simulate the tidal elevations and currents inside the estuary. These models have wide applications in pollutant dispersal studies, navigation, sediment transport etc.

Estuarine processes - II (Biological)

X. N. Verlecar
Biological Oceanography

Estuaries, in the major cities, have always fascinated man, by providing a close glimpse of natural habitat, despite their attempt to pollute or reclaim this environment. The biologists have become interested in estuaries as areas in which, the responses of plants and animals, to severe environmental gradients could be studied. The estuarine water is of mixed origin, with fresh water supplied by the river and land runoff and sea water by the state of tide. The water of mixed origin does not occupy the constant position. The zone of mixing can remain static if the fresh water discharge is fairly constant throughout the year and the tidal incursion negligible. These two important points are not applicable to Indian conditions where the freshwater regime becomes highly variable with the onset of monsoon and the tidal range becomes dependent with the geographic location of the estuary and the phases of moon. Gradients of salinity for example and the problems of living in turbid water or a muddy substrate, prevents most of the animal species from the adjacent sea or rivers from entering the estuaries. In spite of these problems, the life in the estuaries remains abundant because of constant supply of rich food, to sustain the most productive animal biomass in this natural ecosystem. The mixtures of salt and fresh water present in the estuary, challenges the physiology of the animals, for which few of these are able to adapt.

In India, estuaries, like any other country have been the focal point of lot of human activities. Major of these activities include development of ports and harbours, exploitation of fish throughout the year and particularly during the monsoon season when fishing in the sea, because of turbulent conditions, gets considerably reduced or becomes suspended. These activities have increased human settlement around the estuaries with the consequent stress on their natural habitats such as mangroves, coral reefs, fisheries, etc. Mumbai city alone, which has developed long bays and creeks of Colaba, Thane, Mahim, etc with population density of 25,000/km² , generates about 2.2 x 106 m³/day of domestic wastes. Much of these wastes enter the bays, creeks and the harbour areas.
Estuarine productivity

Estuaries are rich in nutrients and phytoplankton productivity is high when turbidity is low. In turbid estuaries such as the Kochi Backwaters, even though there is sufficient solar radiation, restriction of light penetration in water column seriously limits productivity. Water temperature does not show any direct effect on productivity but changes in salinity become important in favouring the phytoplankton productivity. The peaks of production occur in monsoon when the salinity in the backwaters is low. Annual cycle of productivity in the Kochi Backwaters shows only brief pulses of blooms. While nutrients are high in monsoon, it may be that nutrients alone are not conducive for substantial increase in phytoplankton production. Similar feature is also observed in the Mandovi-Zuari estuaries in Goa, where high turbidity restricts the phytoplankton productivity in monsoon, although the availability of nutrients from land drainage is high.

Copepods form the most important component of zooplankton in almost all of Indian estuaries. The occurrence of other organisms is largely dependent on the site of observation in the estuary showing an assortment of marine, brackish water and freshwater forms. For example, in the Hooghly Estuary in the upper regions, which largely remain freshwater dominated, Pseudodiaptomus sp. and Oithona sp. were common, while Cyclops and Mesocyclops were recorded in the freshwater zone. Other crustaceans such as ostracods, mysids, cumaecans, amphipods, tanaidacean and decapods were not very abundant, but crustacean larvae formed a significant part of the zooplankton biomass.

Detritus
Detritus largely consists of both living and inert materials in suspension, which
continuously settle to the bottom. Following an increase in turbidity, the euphotic zone becomes very narrow resulting into a large fall-out of plant and animal materials including faecal pellets. This material, which has been commonly fermed as organic detritus, gets deposited on the sediment as a superficial layer. It has been defined as all types of biogenic material in various stages of microbial decomposition. It is derived either by autochthonous sources (from within the environment) such as phytoplankton, submerged vegetation, mudflat algae, filamentons algae, etc., or by allochthonous sources (from outside the environment) which include mangrove leaves, shore material washed during the high tide, wind blown matter, organic material coming from outside, etc. It also contains fine silt and sand particles around which organic matter adheres and forms aggregates. Continous collections of detritus were made for one year from the Cochin Backwaters and its visual description, chemical composition, etc., have been described earlier (Qasim & Sankaranarayanan, 1972). Caloric value of the detritus from backwaters ranged from 200 to 500 cal/g dry wt. From the Zuari Estuary the caloric value of detritus was found to be much higher. It ranged between 173 to 6057 callg dry wt (average 1463). This is because the detritus from Zuari contains substantial quantities of decaying mangrove leaves in addition to contribution from autochthonous sources, the latter seems to be in very little quantities.

Benthos
There are few studies carried out on the bottom communities of the estuaries of the east coast of India. But those along the west coast of India, have been studied well enough to be described. Polychaetes, gastropods, bivalves, nemertines, gastrotrichs, sipunculids, isopods, amphipods, etc., largely formed the macrobenthic fauna. While meiofauna include nematodes, foraminiferans, ostracods, turbellarians, lamellibranchs, etc.Changing hydrographical conditions associated with tidal incursion, flushing of the estuary and seasonal changes induced by the monsoon, determine the abundance and composition of the benthic fauna.

Food chain cycle, derived any one estuary is applicable to practically all estuaries of east and west coasts of India where monsoon cycle is responsible for the dilution of estuaries leading to enrichment with nutrients followed by pulses of phytoplankton blooms and catastrophic effect on zooplankton population. The return of zooplankton to the estuary following an increase in salinity and the appearance of carnivores for a short period is a typical cycle shown by practically all the estuaries which have been studied. Benthic population, on the other hand, remains largely stable except for minor changes seen among the sensitive species. Thus, the multiple pathways leading to branching of the food web satisfy the food requirement of all types of estuarine animals. This is an adaptive response to the environment. Food web cycle, pollution and associated factors controlling the production and decomposition processes will be discussed in details.

Circulation along the Indian coast

S.S.C. Shenoi and A.S. Unnikrishnan
Physical Oceanography

The currents along the coast of India are due to two causes: forcing by winds and the tides. The cross-shore current on the continental shelf is dominated by the tidal component, but it is the winds that contribute much more to the along-shore component. Farther offshore, the contribution of tides is negligible and the current is forced by the wind.

Like wind-forced currents elsewhere in the north Indian Ocean, the wind-forced coastal current off India reverses with season. Off the Indian east coast, the East India Coastal Current (EICC) flows poleward during January-September and equatorward during the rest of the year. It is stronger during March-April, when the winds are weak, than during July-August, when the strong southwest monsoon winds blow along the coast. Off the Indian west coast, the situation is a little more complicated. The West India Coastal Current (WICC) flows poleward during January-April and equatorward during the rest of the year. Off southwest India (Kerala), however, the WICC often flows in a direction opposite to that along the rest of the west coast. This is due to the existence of the Lakshadweep High and Low, a high and low in sea level that form in the region during winter and summer respectively.

The above description is largely based on hydrography1. Direct measurements using current meters suggests that while this picture of the coastal currents is true on the average, there are considerable fluctuations from day to day. Such measurements, however, have been made for relatively short durations (like, say, a month) and are not yet sufficient to tell us how the EICC and WICC vary at frequencies higher than the seasonal.

The cross-shore component of the coastal current is dominated by the tides. These tidal currents are barotropic in nature, i. e, they are vertically homogeneous. In many regions near the coast, especially on shallow shelves, these barotropic currents are stronger than the baroclinic components, which are driven by large-scale winds.

Tidally driven currents exhibit variability on time scales of hours and therefore are important for determining the pollutant dispersal, navigation, sediment transport, etc. For instance, along the west coast of India, the intensity of tidal currents increases gradually northward from the southern side. Along this coast, the width of the continental shelf is very narrow in the southern side and broadens gradually till central west coast (off Mumbai). Barotropic tides coming from the open ocean, after entering into the shelf region, undergo amplification while approaching towards the shore and generate stronger currents. In the Gulf of Kutch and Gulf of Khambhat, where the tidal ranges are very high, currents having magnitudes of about 2 m/s have been observed. The other part of the north Indian Ocean with a broad shelf is the northern Bay of Bengal, where the shelf width is about 200 km. Hence, tidal currents are extremely important in the Bombay-High region off Mumbai and in the northern bay.

1Direct measurement of currents is difficult in the oceans. In the absence of direct measurements, two methods have been used extensively to map the large-scale, seasonal circulation in the world oceans. The first method exploits geostrophy to construct dynamic topographies of the sea surface relative to some level. Oceanographic surveys are used to collect vertical profiles of temperature and salinity (hydrography) in a region. Since the velocity of currents near the surface is much greater than that in the deep ocean, it is possible to use the geostrophic balance to compute the average current, at a given depth relative to some other depth, between two locations. When this is done for the surface in a region like the north Indian Ocean, we obtain the dynamic topography of the sea surface, which gives an idea of the transport with respect to some depth. Oceanographers commonly use 1000 m as a reference depth, also called the level of no motion. The second method makes use of ships. The deviation of a ship from its planned trajectory (due to winds and currents) is used to compute surface currents; these are called ship drifts. Each of these two methods has its own drawbacks. Dynamic topographies constructed from hydrographic data give the geostrophic transport relative to an assumed level of no motion, typically 1000 m, and are therefore unreliable in the vicinity of a coast; also, they cannot account for the non-geostrophic transport. Ship drifts, on the other hand, are influenced by the wind and are usually too noisy to yield a clear, coherent picture of the basin-scale circulation.

Coastal Hazards: A broader perspective to address beyond Tsunami

Govind Ranade
Geological Oceanography

The killer waves generated by the recent Tsunami resulted in heavy losses in terms of property and human life for various countries including India. This has called for the attention of the planners and the scientific community to evolve a comprehensive action plan to address our preparedness towards various coastal hazards. Tsunami has only been a sporadic event for the Indian coast. There are coastal hazards both natural and manmade, which have lead to disasters and destruction that have not been noticed with the similar seriousness. This could perhaps be because they occurred as regional disturbances. India has a long coastline (~7500 km) and a large Exclusive Economic Zone (EEZ) (~2 mi. sq. km.) that includes two major groups of islands, all of which are susceptible to different coastal hazards. Peninsular India comprises of nine populous states, with a significant component of their economy in some way related to the sea. This includes fishing, shipping, offshore oil industry, ports and harbors, tourism and allied industries. The long-term effects of such calamities, which can significantly affect and as well alter the natural environment, need to be addressed. Natural Coastal hazards affecting the coastline include coastal erosion, marine inundation from storms, high waves, tsunamis, sea-level rise, inundation from coastal stream flooding, coastal landslides, volcanic & seismic activity, algal blooms, effects on the productivity around the coastal zones and the created hazards like sewerage discharge, toxic industrial effluent discharge, coastal constructions without consideration of geological set up of the coastline. Although, unforeseen such events disrupt not only the normal lives and livelihood of the people, but also the economics of the country especially when the population growth along the coasts is increasing considerably.

The impacts of the coastal hazards are becoming alarmingly costly and devastating. The frequency of these hazards is on an increasing trend. The prime factors for this increase are attributed to the changing global climate and their cyclic tendencies and the increased population in the coastal areas. Also, with the better understanding of the causative factors for the disaster, they are being noticed as new source of challenge to be dealt with.

It is therefore needed to have a holistic view of all the types of coastal hazards in totality. This would help in developing an effective system to provide a solution in understanding of coastal hazards and evolve strategies for our preparedness concerning hazard mitigation. There is a need of concerted effort of the scientific community, planners and the social sector to arrive at developing such a system. This would first involve a detailed study of the Indian coastline, both near shore and offshore in generating a volume of scientific data covering all the physical, geological and biological processes that makes the complex coastal system along the Indian coastline. This data can then be used for developing model studies under different simulated hazard conditions to give us a deep insight into the causative effects of the damage that would prevail on occurrence of such a coastal hazard. Managing a calamity of such nature along the Indian coastline, is therefore a challenge for the scientific community, general masses and the planners of this country.

The after hazards mitigation programme therefore needs to be planned as per the nature of calamity and the hazard type, based on the outcome of the study of scientific analysis of all hazard types. The data provided by the scientific community therefore will have a great bearing for the planners, NGOs and the government agencies in developing suitable mitigation packages as well as providing preventive methods in some cases.

Life in the Oceans

Chandralata Raghukumar
Biological Oceanography

Oceans are home to some of the most diverse life forms. These vary from several meters to less than a micron in size, drab to stunning looking, sedate to constant swimmers, those which eat from anything to everything to those which are choosey about their meals. As sunlight is the most important element for life on earth, the oceans are divided into 5 zones based on penetration of the sunlight. They are the epipelagic or sunlit zone, the mesopelagic or twilight zone, the bathypelagic or midnight zone and the abyssal or pitch black bottom zone. The temperature in these zones range from 30°C on the surface to 1-2°C at the bottom in the deep sea. The hydrostatic pressure increases by 1 bar at every 10 m depth. Therefore, the life in this constant saline fluid varies tremendously.

The marine habitats include nutrient-poor oligotrophic open oceanic waters, comparatively nutrient-rich coastal waters, coral reef atolls (the oases in oceanic deserts) the hydrothermal vents with metal-rich fluids with temperatures of 200-350°C, cold-seeps, the estuaries, the mangrove swamps, the intertidal beaches and rocky shores. The life forms have adapted various strategies (form and function) to live and survive in these diverse habitats.