The Indian ocean is bounded by the Eurasian continental landmass on the north – responsible for the unique monsoonal seasonality in the north Indian Ocean which results in a complete reversal in the winds and surface currents every six months. Peninsular India divides the north Indian ocean into two zones – the Arabian Sea and the Bay of Bengal. Asymmetric distributions of monsoonal energy and the water fluxes make these two zone hydrographically quite different.
Winds are generally stronger in the Arabian Sea with more evaporation as compared to precipitation and small river runoff leading to a negative water balance. The Bay of Bengal on the other hand has excess precipitation over evaporation and receives the bulk of the river runoff from the adjoining landmass. Accordingly, the upper water column in the Bay of Bengal is very strongly stratified and coastal upwelling, if and when it occurs, is quite weak. The Arabian Sea, on the other hand, houses one of the five major upwelling zones of the world.
In the northern hemisphere when winds blow parallel to the coast from the north, they tend to drive the ocean currents to the right of the wind direction; effectively pushing the surface waters offshore. As this water is pushed away, colder water from the below rises to replace it. This process which brings up the cold, nutrient rich waters to the surface is called upwelling (Fig. 1)
Upwelling in the eastern Arabian sea
As with the other parts of the north Indian ocean, circulation along the west coast of India also varies with the seasons, reversing in April and October (Fig 2). In winter when the winds are relatively weak, the surface current – the West India Coastal Current (WICC) – flows poleward (towards the north) and is associated with downwelling and well-oxygenated conditions over the continental shelf. During the summer season, however, the region bears typical characteristics of an eastern boundary environment, an equatorward (towards the south) flowing WICC, a poleward flowing undercurrent – the West India Undercurrent (WIUC), and coastal upwelling.
The upwelling begins sometime in April along the southwest coast and progressively moves northward such that there is a time lag between its onset (and demise) from south to north. The upwelled water is derived from the WIUC that originates in the south (off Sri Lanka) and is, therefore, fresher and slightly more oxygenated. However, as the water ascends over the Indian continental shelf, it loses more oxygen. The extent of oxygen depletion increases northward because, first, its concentration in the upwelling water at its source (i.e., at the shelf break) decreases and, second, the shelf width increases northward.
The Indian upwelling zone along the west coast is a rare eastern-boundary (of the Arabian Sea) upwelling environment receiving large freshwater inputs as a result of intense rainfall in the coastal zone and in the adjoining Western Ghats. This leads to the formation of a warmer, fresher water layer, a few metres in thickness, which floats over the cold, saline upwelled water. The establishment of very strong stratification very close to the surface is aided by slow upwelling.
In the southern region (off Kerala, south of Cochin), the cold upwelled water does reach the surface and get transported beyond the relatively narrow shelf. Elsewhere, upwelling is manifested merely by shoaling of the thermocline (Fig. 3) over the inner and mid shelf regions but to depths shallow enough to allow utilisation of nutrients by phytoplankton. Consequently, primary productivity (PP), mainly fuelled by upwelled nutrients with its rate being at maximum a few metres below the surface, is quite high. The resultant elevated oxygen demand in the poorly ventilated and poorly oxygenated upwelled water ensures almost complete removal of dissolved oxygen.
During the peak of the upwelling season (in September), almost the entire Indian shelf (and probably also some part of the Pakistani shelf) is bathed with the upwelled water having low oxygen concentrations. Occupying an area of at least 1,80,000 sq km the coastal hypoxic (oxygen depleted) zone of the eastern Arabian Sea is by far the largest of all coastal hypoxic systems, formed both naturally as well as due to cultural eutrophication (process by which a body of water becomes enriched with nutrients that stimulate the growth of algae). This is in sharp contrast to conditions prevailing in the western Arabian Sea where upwelling is far more vigorous, and the upwelled waters spend relatively short time over the shelf such that suboxia (extreme hypoxia) over the shelf occurs seldom, if ever.
Upwelling in western Arabian sea
The most intense upwelling in the Arabian Sea occurs along its western boundary. This process although restricted to the southwest monsoon (SWM, from June to September), affects a very large area because of its vigorous circulation during this season. Nutrient-rich subsurface waters brought in by the three main coastal upwelling zones, off Somalia, Yemen, and Oman, spread far and wide (up to 1000 km from the coast), triggering extensive phytoplankton blooms. When the phytoplankton die and begin to sink to the ocean floor, they are broken down by bacteria. During this process the bacteria use up oxygen and if the concentration of the dead plants is large enough, the bacteria could consume all the oxygen in a region.
In the western Arabian Sea, the high productivity which results from phytoplankton blooms contributes to sustenance of an oxygen minimum zone (OMZ) that is the thickest (~1 km) found anywhere in the open ocean. This in conjunction with the large coastline-to-ocean area ratio results in the north Indian ocean basins containing a disproportionately large (two-third) area of global continental margin in contact with oxygen-depleted waters. The upper boundary of the OMZ in the open ocean generally lies between 100 and 150 m depths. However, uplift of the thermocline in coastal areas sometimes brings the oxygen-depleted waters over to the shelf. This has long been known to occur along the west coast of India during the SWM. Interest in this system has been rekindled in recent years due to ongoing changes in distribution of oxygen all over the world oceans. Also, a relationship appears to exist between the intensity of coastal upwelling/oxygen deficiency and the west coast fisheries – there seems to be a remarkable revival of fisheries since 2003 apparently due to a relaxation of the intensity of oxygen deficiency.
Oxygen deficiency trends over Arabian Sea
The Candolim Time Series (CaTS) – a fixed site off the coast of Goa, regularly sampling and collecting data since 1977, shows substantial interannual changes both in the duration (onset/demise) and intensity of the oxygen deficiency. The record is not long enough to unambiguously distinguish the effects of human activities from natural variability, but it does raise interesting possibilities of links with various physical and biological phenomena. One important aspect of the CaTS data, however is the lack of a secular trend (i.e., steadily intensifying anoxia as inferred from early observations). Nonetheless, conditions prevailing today are arguably more severe than those inferred from historical data that goes back to the 1950s.
The most extensive data (on salinity, temperature, and oxygen) was generated under UNDP/FAO-sponsored Integrated Fisheries Project (IFP) along a number of cross-shelf sections in various seasons from 1971 to 1975. An examination of data subsequently collected by the National Institute of Oceanography (NIO) also shows that the subsurface environment was denitrifying but not sulfate reducing at least until the 1980s. When the oxygen is depleted in a region the bacteria turn to nitrates and denitrification occurs as nitrates are consumed rapidly. After this, the bacteria eventually turn to reducing sulphate – leading to anoxia.
What caused the shift from the suboxic to anoxic conditions in the Arabian Sea is still not clear. The two possible factors are altered hydrography/circulation and nutrient loading from land. While the relative roles of hydrography/circulation and eutrophication cannot be inferred from the available sedimentary data, the inferred higher productivity, regardless of the cause, is consistent with an intensified oxygen deficiency. This increase in productivity has been attributed to human activities. The huge increase in fertiliser consumption in South Asia, as well as release of reactive nitrogen have rendered coastal waters in the region highly eutrophic. As a result of high turbidity of estuarine waters during the SWM, nutrients are not fully utilised by the phytoplankton, thus allowing their export to open waters, supplementing supply through upwelling and contributing to the high PP over the inner and mid-shelf regions.
Thus, it has been hypothesised that elevated nutrient loading, mainly through atmospheric deposition, is the most likely cause of the intensification of seasonal oxygen deficiency since the early 1970s. This shift is superimposed on natural variability on interannual to decadal timescales as exemplified by the very intense anoxic conditions recorded during 1998–2002 arising from hydrographic changes (upwelling and stratification). However, how these two factors combine to control variability of oxygen deficiency is not clear. Some trade-off between these opposing processes must be occurring, but how this trade-off varies with the strength of the SWM is not known. Also, what is the relative role of local processes such as precipitation and local winds, versus remote forcing of circulation in determining the hydrographic structure along the Indian west coast is yet to be understood.
Future research must address these issues in view of recent reports of expansion of oxygen-deficient zones in the coastal ocean in response to anthropogenic nutrient loading and/or changes in circulation as well as the projected intensification of coastal upwelling as a result of global warming.