Indian Ocean Coastal Biome
305
During this season, the cyclonic circulation of the Arabian Sea causes downwelling of
isopleths near the coast. Coastal currents respond to this local wind forcing but there
is some question about the actual forcing of the southward flow that occurs during the
Northeast Monsoon: thermohaline driving, river runoff, alongshore wind stress, and a
continuation of the equatorward EICC have all been invoked.
The Southwest Monsoon is initiated in the south in May and June, then spreads
northward and continues until October or even November. Accordingly, mean flow is
reversed during this period and the longshore components of the coastal current suggest
that it is mainly barotropic and wind-driven (Antony and Shenoi, 1991). It is also clear
that what causes the reversal of flow at the end of the year is the reversal of the monsoon
winds. The flow proper to each monsoon forms, of course, the outer edge of the eastern
limb of the Arabian Sea gyre; during the Southwest Monsoon this is accompanied by
upsloping of the thermocline toward the coast and intrusion of subpycnocline, hypoxic
water onto the shelf which may persist during the entire Southwest Monsoon (Wooster
et al., 1967).
Local processes, such as wind-forced coastal upwelling, are complicated in this way
by the distantly forced effects of the spin-up of the whole Arabian Sea gyre that occurs
before the effects of local wind stress are evident. We should expect, therefore, that a
simple model of coastal upwelling forced by Ekman divergence (e.g., Mathew, 1982)
should not entirely fit observations and the relative significance of geostrophic and winddriven upwelling on this coast should be examined holistically. Geostrophic upsloping of
density contours starts in April, several months before local wind stress could force local
upwelling, and occurs because offshore isopleths begin to slope upward toward the shore
as the Southwest Monsoon spins up an anticyclonic gyre (Longhurst and Wooster, 1990;
McCreary et al., 1993). Neritic processes then erode the shoaled pycnocline. Later in the
season, continuing equatorward wind stress transports the now-shallower surface layer
offshore, leading to Ekman upwelling along the coastline. At such low latitudes as these,
the required wind stress is relatively small compared with the situation at midlatitudes:
off Cochin at 10
N, the same upwelling strength as that off California at 40
N is forced by
a much weaker coastwise wind stress. Thus, upwelling is a consequence of both remotely
forced baroclinic adjustment (Wyrtki, 1973a) and an equatorward component of wind
stress along the coast during the Southwest Monsoon.
The Laccadive Islands provide a western barrier to circulation between the southern
part of this area and the open Arabian Sea. Colborn (1975) characterizes this province as
having a shallow permanent thermocline, which breaks the surface during the Southwest
Monsoon—having a mixed-layer depth varying from 25 m (Southwest Monsoon) to 75 m
(Northeast Monsoon).
Regional Response of the Pelagic Ecosystem
This is yet another coast with a diatom bloom that responds to local seasonal upwelling,
although satellite images show that the entire coastline is characterized by a strong and
permanent nearshore band of relatively high chlorophyll concentration, even during
periods when upwelling is not anticipated. This signal is generally attenuated toward the
south and is widest at the Gulf of Cambay (20
S), where it occupies the entire gulf. It
is likely that at least part of this signal is caused by DCOM, known to interfere with the
interpretation of satellite images of the Amazon plume (q.v.). Nevertheless, the seasonal
upwelling that is associated with the Southwest Monsoon does unequivocally force a
bloom that is strongest on the southern segment of the shelf.
During the preupwelling period, when the pycnocline starts to slope upward toward
the coast, equatorward flow along the coast has a low offshore component, and nutrient
levels are low. The mixed layer is thin and subsurface water with low oxygen content has
305
During this season, the cyclonic circulation of the Arabian Sea causes downwelling of
isopleths near the coast. Coastal currents respond to this local wind forcing but there
is some question about the actual forcing of the southward flow that occurs during the
Northeast Monsoon: thermohaline driving, river runoff, alongshore wind stress, and a
continuation of the equatorward EICC have all been invoked.
The Southwest Monsoon is initiated in the south in May and June, then spreads
northward and continues until October or even November. Accordingly, mean flow is
reversed during this period and the longshore components of the coastal current suggest
that it is mainly barotropic and wind-driven (Antony and Shenoi, 1991). It is also clear
that what causes the reversal of flow at the end of the year is the reversal of the monsoon
winds. The flow proper to each monsoon forms, of course, the outer edge of the eastern
limb of the Arabian Sea gyre; during the Southwest Monsoon this is accompanied by
upsloping of the thermocline toward the coast and intrusion of subpycnocline, hypoxic
water onto the shelf which may persist during the entire Southwest Monsoon (Wooster
et al., 1967).
Local processes, such as wind-forced coastal upwelling, are complicated in this way
by the distantly forced effects of the spin-up of the whole Arabian Sea gyre that occurs
before the effects of local wind stress are evident. We should expect, therefore, that a
simple model of coastal upwelling forced by Ekman divergence (e.g., Mathew, 1982)
should not entirely fit observations and the relative significance of geostrophic and winddriven upwelling on this coast should be examined holistically. Geostrophic upsloping of
density contours starts in April, several months before local wind stress could force local
upwelling, and occurs because offshore isopleths begin to slope upward toward the shore
as the Southwest Monsoon spins up an anticyclonic gyre (Longhurst and Wooster, 1990;
McCreary et al., 1993). Neritic processes then erode the shoaled pycnocline. Later in the
season, continuing equatorward wind stress transports the now-shallower surface layer
offshore, leading to Ekman upwelling along the coastline. At such low latitudes as these,
the required wind stress is relatively small compared with the situation at midlatitudes:
off Cochin at 10
N, the same upwelling strength as that off California at 40
N is forced by
a much weaker coastwise wind stress. Thus, upwelling is a consequence of both remotely
forced baroclinic adjustment (Wyrtki, 1973a) and an equatorward component of wind
stress along the coast during the Southwest Monsoon.
The Laccadive Islands provide a western barrier to circulation between the southern
part of this area and the open Arabian Sea. Colborn (1975) characterizes this province as
having a shallow permanent thermocline, which breaks the surface during the Southwest
Monsoon—having a mixed-layer depth varying from 25 m (Southwest Monsoon) to 75 m
(Northeast Monsoon).
Regional Response of the Pelagic Ecosystem
This is yet another coast with a diatom bloom that responds to local seasonal upwelling,
although satellite images show that the entire coastline is characterized by a strong and
permanent nearshore band of relatively high chlorophyll concentration, even during
periods when upwelling is not anticipated. This signal is generally attenuated toward the
south and is widest at the Gulf of Cambay (20
S), where it occupies the entire gulf. It
is likely that at least part of this signal is caused by DCOM, known to interfere with the
interpretation of satellite images of the Amazon plume (q.v.). Nevertheless, the seasonal
upwelling that is associated with the Southwest Monsoon does unequivocally force a
bloom that is strongest on the southern segment of the shelf.
During the preupwelling period, when the pycnocline starts to slope upward toward
the coast, equatorward flow along the coast has a low offshore component, and nutrient
levels are low. The mixed layer is thin and subsurface water with low oxygen content has