INDIAN OCEAN ONE-TIME HYDROGRAPHY (WORLD OCEAN CIRCULATION EXPERIMENT)

The intermediate and abyssal circulation in the Central Indian Basin has been studied only sketchily, and the sources, sinks and budgets of deep water will be explored with the new stations from this and the other WHP cruises which cross it. The deep circulation is overwhelmingly constrained by the many mid-ocean ridges which criss-cross the Indian Ocean. In this section we examine the possible source of deep water for the Central Indian basin from the West Australian basin across the saddle in the Ninetyeast Ridge near 29 S. Since the overall upwelling rate from the abyss to the thermocline north of 30S appears to be much greater than in the Pacific Ocean, the obvious question is where most of the enhanced upwelling occurs and whether it is primarily linked to the monsoon and equator or to the complex deep topography.

The 1995 repetition of the eastern part of I5E is meant to determine if there were changes in the properties of waters flowing into and out of the West Australia Basin. The biggest differences might be expected in the upper ocean water masses, including the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW); a shift in deep properties, if found, would be more remarkable.

The cruise track is shown in figure 1. The latter portion was a nominal repeat of the 1987 section (Toole and Warren, 1993). The goals of the sampling were to obtain a section through the center of the Central Indian Basin, and to repeat the crossing of the northward flow of deep water just to the west of Australia. Particular attention was paid to a potential source of deep water for the Central Indian Basin, through a sill in the NinetyEast Ridge, located at about 28S. It was also possible to deviate from the 32S section, and sample in the deep water south of Broken Ridge instead of along the top of the ridge. Between Broken Ridge and Australia we chose to move the section slightly north of the original position of I5E in order to resolve whether the deep flow splits around Dirck Hartog Ridge.

Preliminary results reported in Talley and Baringer (1995) show the very fresh water (<33.3 off Sri Lanka and <34.2 at 5S) originating in the Bay of Bengal is evident at the surface, extending southward from Sri Lanka to a front at 10S (Figure 2). Below the fresh surface water lies an intense halocline: 1 psu change in 30 meters. The saline water (>35.3) just below the halocline is of Arabian Sea origin. The temperature, salinity and density fields all indicate the existence of a well-defined equatorial undercurrent centered at about 80 meters on this March section, in accord with climatology and results of current meter arrays near this location (Knox and Anderson, 1985; Schott et al., 1994). The surface countercurrent has velocities of about 70 cm/sec based on ADCP measurements. The equatorial currents are clearly confined between fronts at 4N and 4S, and the low salinity surface water has higher values than to the north and south of this equatorial channel; the halocline is also more intense and both suggest equatorial upwelling.

One of the most striking features on the section is the front at 10S which separates the tropical waters from those of Indonesian throughflow origin (Figure 3). This jet lies between 10 and 14S and appears to extend almost exactly zonally westward from Java, carrying a surface-to-intermediate depth layer of relatively fresh water, although not as fresh as the Bay of Bengal water at the surface. Oxygen and nutrients in the jet are similar to those of the tropical Indian Ocean

One of the primary purposes of the WHP Indian Ocean expedition is to determine the fate of the deep water which enters from the south. The Central Indian Basin is among the most removed from the deep water source in the south since its bottommost water is fed from the east across the Ninety-East Ridge, especially across the sill near 11S (Warren, 1981). Our section down the middle of the basin confirms the importance of the 11S sill compared with the others. By far the largest puddle of water at 80E with temperature less than 1C (Figure 4) is centered at 9S with the largest geostrophic shear signature there, indicating a source which is most likely to be the 11S sill. A secondary bottom puddle of cold water lies centered at 18S; this is probably a southward extension of the 11S bottom water (Warren, 1982).

The deep water lying above the bottom water does not follow such a circuitous route from the south, but rather enters the Central Indian Basin directly across a saddle at about 35S. This section shows some evidence for the input of this deep water, which is marked by high salinity and high oxygen, centered at 2500-3500 meters (Figure 5).

Our cruise timing permitted sampling the 28S sill with three small sections. We found clear evidence of the westward flow of bottom water into the Central Indian Basin, where it turns to the south. The properties of this layer clearly indicate an origin east of the Ninety-East Ridge and north of Broken Plateau. The layer is colder than 1.2C, fresher than 34.725psu and denser than 45.89 sigma 4. The net transport appears to be about 2 to 2.5 Sv.

The eastward leg of our cruise crisscrossed the 1987 section at about 32S (Toole and Warren, 1993). the 1995 section shows a net northward flow +5.2 Sv below 2000 dbar consisting of +6.6 northward flow in the boundary current and -1.4 Sv southward flow in the interior (Figure 6). Toole and Warren (1991) and Warren (1981) both obtained a net northward flow of 6 Sv into the West Australian Basin consisting of 7 Sv in the boundary current and -1 Sv southward return flow (see Figure 3 for the comparison to Toole and Warren's (1991) transport). The general location and magnitude of the deep flows are remarkably similar.

Chereskin, T.K., W.D. Wilson, H.L. Bryden, A. Ffield, and J. Morrison, 1997. Observations of the Ekman balance at 8°30'N int he Arabin Sea during the 1995 southwest monsoon, Geophysical Research Letters, 24 (21), 2541-2544.

Ffield, A., 1997. GRL special section: WOCE Indian Ocean expedition, Geophysical Research Letters, 24 (21), 2539-2540.

Ffield, A., J. Toole, and D. Wilson, 1997. Seasonal Circulation in the South Indian Ocean, Geophysical Research Letters, 24 (21), 2773-2776.

Gordon, A.L., S. Ma, D.B. Olson, P. Hacker, A. Ffield, L.D. Talley, D. Wilson, M.O. Baringer, 1997. Advection and Diffusion of Indonesian Throughflow within the Indian Ocean South Equatorial Current, Geophysical Research Letters, 24, 2573.

McCarthy, M.C., L.D. Talley, and M.O. Baringer, 1997. Deep Upwelling and Diffusivity in the Southern Central Indian Basin, Geophysical Research Letters, 24 (21), 2801-2804.

Talley, L. D. and M. O. Baringer, 1995. Preliminary results from WHP Sections I8N/I5E in the Central Indian Ocean. WOCE Newsletter, 21, 35-38.

Talley, L. D. and M. O. Baringer, 1997. Preliminary results from WHP Sections I8N/I5E in the Central Indian Ocean. Geophysical Research Letters, 24 (21), 2789-2792.

Toole, J. M. and B. A. Warren, 1993. A hydrographic section across the subtropical South Indian Ocean. Deep-Sea Research, 40 (10), 1973-2019.

Warren, B.A., 1981. Deep circulation of the World Ocean. In Evolution of Physical Oceanography, MIT Press, 6-41. Warren, B.A., 1982. The deep water of the Central Indian Basin. J. Mar. Res., 42 (Suppl)., 823-860.