1. Regional Interests in Ocean, Climate and Coastal Observing System

S.R. Shetye               -  National Institute of Oceanography, Dona Paula, Goa 403 004, India
K. Radhakrishnan     -  Indian National Centre for Ocean Information Services, Jubilee Hills, Hyderabad 500 003, India

Of the three major oceans – Pacific, Atlantic, and Indian – the Indian is the only one that does not touch both the North and the South Polar regions. This is a consequence of the presence of the Asian continent to the north. It restricts the ocean to the south of about 25 N. The restriction has important implications to the climate of the region and to the working of the ocean. The most important implication to the climate is the large seasonal changes that occur in the patterns of wind and of precipitation over ocean and the surrounding lands (Figure 1). In this overview we examine some of the important features of the response of the Indian Ocean to forcing by the wind and to other boundary effects. A priority of an observing system for the ocean should be to monitor these features

Figures 2 and 3 taken from Schott and McCreary (2001) summarize the large-scale circulation during January-February and July-August, respectively. South of about 10 S the direction of the currents depicted in the figures remains approximately unchanged from season to season. Here the annual mean transport can be understood reasonably well using the Sverdrup relation. Added to the Sverdrup flow are: the Indo-Pacific Throughflow that enhances the transport of the South Equatorial Current; the Agulhas Current, the Mozambique Current and the East African Coastal Current that form the western boundary current system along the East African coast; and, the Leeuwin Current System along the western Australian coast that has a strong surface flow and an undercurrent of similar magnitude.

In contrast to the circulation in the south, the region north of about 10 S exhibits a circulation that is strongly seasonal, as indicated by the differences in Figures 2 and 3 in this region. This is a consequence of the monsoons, the Southwest Monsoon (June-September) winds and precipitation being much stronger than those in the Northeast Monsoon (November-February). During the transition between the two Monsoons, May and October, the equatorial Indian Ocean exhibits a unique feature: eastward Wyrtki Jets whose transports have been estimated to be between 12-20 Sv (106 m3 s-1). The highly seasonal circulation north of 10 S, including the Wyrtki Jets, can be understood as superposition of tropical and coastal locally- and remotely-forced low-frequency (annual, semi-annual, etc.) waves. The waves can lead to strong boundary currents, the most prominent example being the Somali Current during the Southwest Monsoon.

Two mediterranean seas - the Red Sea and the Persian Gulf – join the northern part of the Indian Ocean and have remarkable influence on its salinity, and consequently on density structure. The magnitude of this influence can be appreciated from the Temperature-Salinity diagram of the Indian Ocean water masses given in Figure 4. The Red Sea Water and the Persian Gulf Water that is injected into the Arabian Sea at sub-surface levels leaves this semi-enclosed basin more saline than any other part of the Indian Ocean. In contrast, the high precipitation over northeastern part of the ocean (Figure 1) makes the surface waters of the Bay of Bengal and the Anadaman Sea the least saline.

The northern part of the Indian Ocean basin receives net heat influx across the air-sea interface. An important question for the global climate in general and climate of the region in particular is, how does the ocean circulation remove this heat? Following a synthesis of available data and ideas, Godfrey et al. (1995), proposed that the removal of heat takes place with the help of two overturning cells, one shallow (few hundred meters deep) and the other deep (in excess of 2000 m). In the shallow cell northward transport across the equator occurs in the western boundary and the southward transport occurs in the Ekman layer of the open ocean. In the deeper overturning cell northward flow is in the deep western boundary currents. Nature of the southward flow in this cell is not known. In view of the profound implications of these cells to the climate of the region, providing data for studying the cells, particularly the shallow one, ought to be a priority of a climate observing system.

It is now well established that SST anomalies can provide advance warning of impending episodes of anomalies in climate elements. Godfrey et al. (1995) have pointed out that SST anomalies in the Indian ocean that get generated by the combined effect of circulation, air-sea fluxes, and mixed-layer dynamics, have been associated with a number of climate elements relevant to the region. These are: El Nino and Southern Oscillation, tropospheric Quasi-Biennial Oscillation, intraseasonal oscillations in the north Indian Ocean, east African rainfall anomalies, south Australian rain (often described as "Nicholls Dipole"), and the long-term trend of SST. To these six, we should add the recently discovered (Saji et al., 1999; Webster et al., 1999) Indian Ocean Dipole (IOD). It has been suggested that the IOD forms a coupled ocean-atmosphere system, and hence monitoring the SST anomalies associated with it offers the promise of prediction of related seasonal climatic anomalies.

An issue that an observing system of this region must address is monitoring for evolution of storms. These are most prominent in two areas: a zonal band centered approximately around 10 S; and, the Bay of Bengal and eastern Arabian Sea. Both areas show distinct seasonal preference for cyclogenesis. In the south the preferred period is December-March with a maximum in January. In the north the primary maximum is in November with a secondary maximum in May. The Bay of Bengal also breeds depressions that migrate west-northwestward bringing rainfall to the Indian subcontinent. These generally occur during active spells of the Southwest Monsoon when the Inter Tropical Convergence Zone hangs over the region. An observing system must not only keep track of storms, but also monitor changes in sea-level due to storm surges in coastal areas. Storm surges in the Bay of Bengal have been amongst the deadliest anywhere, and have a history of major losses of life and property. An added interest now in sea level monitoring comes from two considerations: climate change is expected to lead to a long-term sea-level rise; and, frequency of storms – and hence storm surges – is expected to increase due to global warming. Both have serious implications to low-lying coastal areas, including coral islands.

An outstanding issue from the large-scale biogeochemistry of the Indian Ocean is ventilation of the basin. Being restricted to tropics in the north, the sub-tropical convergence zone typically found in other basins does not exist here. The net result is that the basin to the north of the equator must be largely ventilated by waters that subduct in the south Indian Ocean and then make their way to the north. This, together with high productivity of the Arabian Sea, leads to the formation of an intense oxygen minimum layer, a special feature of the hydrography of the ocean.

The large-scale processes described above have impacts on the shallow shelf and estuarine zone of the ocean. In addition, the coastal zone comes under the influence of factors - tides, effect of local winds, influence of river runoff, etc. - that differ from location to location. The variety of coastal physical processes encountered in the North Indian Ocean can be appreciated from three recent reviews: Schumann (1998) on southeast Africa and Madagascar; Shetye and Gouveia (1998) on coastal north Indian Ocean; and, Church and Craig (1998) on the Australian coast. An observing system will need to take into account the variations in demands on the system from location to location. Moreover, these demands include keeping track of not just the physical processes, but also ecological and biogeochemical processes.

The southern and central parts of the west coast of India offer an example of such demands. The coast supports a large fishery industry. Hence there is considerable interest in understanding its ecology and biogeochemistry. The sub-surface waters here are oxygen deficient, a result of the ventilation characteristics of the North Indian basin as well as other local factors such as the high productivity of the Arabian Sea in general and the coastal region in particular. The region experiences upwelling during the Southwest Monsoon, and the oxygen deficient waters are brought close to the surface. Sometimes the entire shelf is covered by waters with O2<0.5 ml l-1. Naqvi et al. (2002) have pointed out that, though it is recognized that the cause of this low oxygen waters is natural, there is now concern that anthropogenic effects, such as fertilizer inputs from land, may be aggravating the situation. The fertilizer inputs could enhance the naturally high primary productivity rate bringing about a shift in the structure of the ecosystem. Monitoring the state of biogeochemistry and ecology of the region would be a worthwhile goal of a coastal observing system for the region. The observing system will also need to generate time-series data to resolve the strong seasonal cycle that this region exhibits.

Design of an observing system for an area is critically dependent on understanding of the area. While it is fair to say that a first order understanding of the large-scale oceanography of the Indian Ocean is available, there exist long stretches in the coastal ocean (for example, eastern Bay of Bengal and Northern Arabian Sea), where basic information on oceanography is not available. An observing system will therefore need to operate initially in an "exploration mode" for a number of areas of the ocean. It might be efficient to explore these areas using numerical models before field observations begin.

In summary, the regional interests that concern the climate module of the Indian Ocean observing system are reasonably well defined, thanks to the efforts of many researchers during the last decade (see, for example, Godfrey et al., 1995). The methods for monitoring under this module are also well established with XBT surveys, current meter moorings, satellite-based remote sensing, and drifters/floats forming the most important arms of the system. In contrast, the interests in the coastal module are much larger and more complex and generally differ from location to location. As a result, a comprehensive monitoring system is harder to implement. An added complication is that there are stretches of the coastal areas whose basic oceanographic features are yet to be defined.

 

References

Figure 1. Winds and monthly precipitation during January (top) and July (bottom)

Figure 2: A schematic representation of currents observed during January-February. The currents identified are: South Equatorial Current (SEC); South Equatorial Counter Current (SECC); Northeast and Southeast Madagascar Current (NEMC and SEMC); East Africa Coastal Current (EACC); Somali Current (SC), West India Coastal Current; Lakshadweep High (LH); East India Coastal Current (EICC); Northeast Monsoon Current (NMC); South Java Current (JC); and, Leeuwin Current (LC). Also shown are transports in Sv (106 m3 s-1) across sections shown as red lines. The Indo-Pacific Throughflow, which enters from the east, influences both the SEC and the LC. The figure is taken from Schott and McCreary (2001).

 

Figure 3: A schematic representation of currents (green) observed during July-August. The currents identified are: South Equatorial Current (SEC); South Equatorial Counter Current (SECC); Northeast and Southeast Madagascar Current (NEMC and SEMC); East Africa Coastal Current (EACC); Somali Current (SC), Southern Gyre (SG), Great Whirl (GW), and associated upwelling wedges (in blue); Socotra Eddy (SE); Ras al Hadd Jet (RHJ) and upwelling wedge off Oman; West India Coastal Current; Lakshadweep Low (LL); East India Coastal Current (EICC); Southwest Monsoon Current (SMC); Sri Lanka Dome (SD); and, Leeuwin Current (LC). Also shown are transports in Sv (106 m3 s-1) across sections shown as red lines. The Indo-Pacific Throughflow, which enters from the east, influences both the SEC and the LC. The figure is taken from Schott and McCreary (2001).

Figure 4: Temperature-Salinity diagram of Indian Ocean water masses taken from Schott and McCreary (2001). The acronyms used are: Bay of Bengal (BB), northern Arabian Sea (AS), equatorial region of western basin (EQ), South Equatorial Current (SEC), western exit of Indonesian Throughflow (ITF), Leeuwin Current (LC), Somali Current (SC), Circumpolar Deep Water (CDW), Indian Deep Water (IDW), Antarctic Intermediate Water (AAIW), Indian Central Water (ICW), Red Sea Water (RSW), Persian Gulf Water (PGW), and Arabian Sea Water (ASW)