A NUMERICAL INVESTIGATION OF DYNAMICS, THERMODYNAMICS AND MIXED-LAYERPROCESSES IN THE INDIAN-OCEAN

A 2 1/2-layer, thermodynamic numerical model is used to study the dyna mics, thermodynamics and mixed-layer physics of Indian Ocean circulati on. A surface mixed layer of temperature T(m) is imbedded in the upper layer of the model, and entrainment and detrainment in the mixed laye r are determined by wind stirring and surface cooling. There is also d etrainment w(d) through the base of the upper layer that models subduc tion. Monthly climatological data, including air temperature T(a) and specific humidity q(a), are used to force the model, and model sea sur face temperature (SST), T(m), is used to determine the sensible and la tent heat fluxes. With a few notable exceptions, our main-run solution compares well with observed current and SST data; this is particularl y true for T(m), which typically differs from observed SST by less tha n 0.5-1.0-degrees-C. Our analyses focus on three topics: the relative importance of remote versus local forcing, the thermodynamic processes that determine the model SST field, and the development of meridional circulation cells. There are a number of examples of remotely forced circulations in our main run. During the spring a northeastward counte rcurrent flows against the prevailing winds along the Somali coast nor th of 4-degrees-N, and from October through February a southwestward S omali Undercurrent is present from the tip of Somalia to 3-degrees-N; both of these flows result in part from forcing during the previous So uthwest Monsoon. From March through May there is another southwestward Somali Undercurrent south of 7-degrees-N, generated primarily by the propagation of a Rossby wave from the west coast of India. The current s along the west coast of India are either strongly influenced or domi nated by remote forcing from the Bay of Bengal throughout the year. A northeastward flow is well established along the east coast of India i n March, long before the onset of the Southwest Monsoon; it is remotel y forced either by upwelling-favorable, alongshore winds elsewhere wit hin the Bay of Bengal or by negative wind curl in the western Bay. Fin ally, the Agulhas Current is strengthened considerably in a solution t hat includes throughflow from the Pacific Ocean. To investigate the re lative importance of thermodynamic processes, we carried out a series of test calculations with various terms dropped from the T(m)-equation . There is little effect on T(m) when the sensible heat flux is set to zero, or when the solar radiation field is replaced by a spatially sm oothed version. When temperature advection is deleted, T(m) is most st rongly affected near western boundaries since isotherms are no longer shifted there by the swift currents; the annual-mean, surface-heat-flu x field QBAR is also changed, with QBAR becoming more positive (negati ve) to compensate for the absence of warm (cold) currents. Without ent rainment cooling, T(m) never cools during the summer in the intense up welling regions in the northern ocean, and the annual-mean heat gain t hrough the ocean surface (the area integral of QBAR over the basin) re verses to become a net heat loss. In individual tests without entrainm ent cooling, with T(a)=T(m), and with q(a) set to 80% of its saturated value q(s), model SST warms near the northern and southern boundaries during their respective winters by about 1-degrees-C, indicating that several processes contribute to wintertime cooling. The T(m) field de grades considerably in a single test run with both T=T and q(a)=0.8q(s ), so that one or the other of these external forcing fields is requir ed to be able to simulate SST accurately. The annual-mean circulation has two meridional circulation cells. In the Tropical Cell, water subd ucts in the southern ocean, flows equatorward in the lower layer of th e western-boundary current, and is entrained back into the upper layer in the open-ocean upwelling regions in the southern ocean. In the Cro ss-Equatorial Cell, the subducted water crosses the equator near the w estern boundary, where it is entrained in the regions of intense coast al upwelling in the northern ocean. The strength of the cells is direc tly related to the assumed magnitude of the subduction rate w(d), but their structure is not sensitive to the particular parameterization of w(d) used.