Role of the equatorial Indian Ocean processes on the climate variability (EIO-CLIVAR)
Objectives:
To understand the dynamics and the long-term variability of ocean currents in the equatorial Indian Ocean To study the upper ocean variability in the thermohaline structures, currents (VM-ADCP), nutrients, chlorophyll and Primary Production in the equatorial Indian Ocean To study the deep-sea circulation in the equatorial Indian Ocean To study the dynamics and thermodynamics of the equatorial Indian Ocean by enhancing the time-series current measurements through additional moorings towards predicting the intra-seasonal and interannual variability. To study the biogeochemistry of the equatorial Indian Ocean by enhancing the multi-disciplinary observations through repeat hydrographic sections and additional sensors (physical and biochemical) and sediment traps on the moorings towards understanding the regional CO2 flux. To address the impact of convection over the equatorial Indian Ocean on the monsoon activity over the Indian sub-continent. To study the role of basin scale oceanic processes on the moisture transport into the Indian sub-continent to understand the hydrological cycle over Indian subcontinent.
General description:
Among the world Oceans, the tropical ocean basins, encompassing the equatorial regions, have special place in modulating the regional as well as global climate. The research on the equatorial regions has been recognized since the last 3 decades and the Equatorial Pacific and Atlantic were largely studied with respect to equatorial dynamics and air-sea interaction through observations and models. One best example is the El Nino and El Nino-Southern Oscillation (ENSO) in the equatorial Pacific and its linkage to the global climate and its possible influence on the Indian Monsoon Rainfall. Amongst the three equatorial regions, the research on the equatorial Indian Ocean has gained momentum with the set up of the Indian Ocean Panel (IOP) as a part of the CLIVAR and GOOS programs. The Indian Ocean as such is typical for its northern extent being limited to the tropical belt and experiences semi-annual wind forcing. The unique features of the equatorial Indian Ocean are the occurrence of equatorial Jets at semi-annual intervals during the monsoon transitions (April-May and October – November) and the absence of equatorial upwelling on the annual scale, unlike the equatorial Pacific. Most of the equatorial Indian Ocean comes under the influence of the Indo-Pacific warm pool. Cross-equatorial flow both in the atmosphere and in the upper ocean in the western equatorial Indian Ocean is the most dominant feature during the southwest monsoon. The eastern equatorial Indian Ocean, being influenced by the Indo-Pacific warm pool, acts as the source region for atmospheric convection that triggers westerly wind bursts over the western equatorial Pacific thereby affecting the upper oceanic conditions there leading to the generation of El Nino over the equatorial Pacific. The equatorial dynamics holds on the linear wave theory and explains that the dynamics are dominated by the propagation of low frequency waves – both Kelvin and Rossby waves. At interannual time scales, an interesting feature highlighting the variability in the Sea Surface Temperature (SST) with cold temperature anomalies off Sumatra coast in the eastern equatorial Indian Ocean and warm temperature anomalies in the western equatorial Indian Ocean is reported recently. This feature is recognized as the internal mode of the Indian Ocean and is widely known as the Indian Ocean Dipole/Zonal Mode (IODZM). The influence of the IODZM on the upper ocean variability and the subcontinent monsoon rainfall are yet to be understood.
While many theoretical and numerical modeling studies are in progress, long-term ocean observations including that of currents are lacking in the equatorial Indian Ocean. During late 1970s, there were time series measurements for a shorter period in the western and central equatorial Indian Ocean. During the World Ocean Circulation Experiment (WOCE), the German group deployed current meter moorings in the region between the Equator and south of Sri Lanka coast along 80E.
In order to enhance the time series observations and to study the intraseasonal and interannual variability of currents in the eastern equatorial Indian Ocean, a long-term time-series current measurements program has been established at the National Institute of Oceanography (NIO) with the funding support from the former Department of Ocean Development (DOD) and presently the Ministry of Earth Sciences (MoES) through the Indian National Center for Ocean Information System (INCOIS), Hyderabad.
One component of the Supra Institutional Project (SIP1306) looks into the long-term measurements of currents in the equatorial Indian Ocean through current meter moorings along the equator under the MoES funded Ocean Observing System (OOS) program, so far, a 4 year time-series record was obtained at 93°E, 3 year time series record at 83°E and 2 year time series record at 76°/77°E.
A second component of the Supra Institutional Project (SIP1306) looks into the physical forcing and the biogeochemical response of the equatorial Indian Ocean through repeated water-column measurements along two cross-equatorial sections (77oE & 83oE) under the funding by Ministry of Earth Sciences (MoES) through the Indian National Center for Ocean Information System (INCOIS), Hyderabad.
Externally funded projects linked under SIP1306
GAP 1970 GAP 2030
Mooring
Cruise
Collaborations
Scientific components and achievements:
The long-term measurements of currents at the equator, 93°E mooring location in the eastern equatorial Indian Ocean revealed the presence of high frequency intraseasonal variability of period 10-20 day period (biweekly) in the meridional currents [Murty et al., 2002 and Figure 2 (lower panel)].
The zonal velocity is dominantly the low frequency semi-annual variation with superimposed intraseasonal variability (~40-60 day period) (Figure 2 top panel). The observed currents are compared with the simulations from an Ocean General Circulation Model (OGCM) [Center for Oceanic and Atmospheric Sciences, Indian Institute of Science, Bangalore). A good comparison is revealed at 10-20 day biweekly period both from observations and model simulations [Sengupta et al., 2004) (Figure 3).
The international collaboration for the Indian Ocean observations for basin scale studies is shown in Figure 4, while Figure 5 highlights the proposed mooring array [RAMA] as part of IndOOS. The realization of RAMA moored array with the efforts from India, US, Japan and China is shown in Figure 6.
