Jp. Mccreary et al., A NUMERICAL INVESTIGATION OF DYNAMICS, THERMODYNAMICS AND MIXED-LAYERPROCESSES IN THE INDIAN-OCEAN, Progress in oceanography, 31(3), 1993, pp. 181-244
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.