shelves, is surrounded by arid lands to the west and north: Somalia, the southeast face
of the Arabian Peninsula, and Pakistan. The eastern side borders western India. The
north end has two evaporation basins attached to it by narrow straits, the Red Sea and
the Persian Gulf. Like the Mediterranean, these are “negative estuaries” or
evaporation basins, with surface water flowing in at the mouths and very saline water
flowing out over sills and then sinking into middle depths. The surface over a modest
area is diluted by the Indus River outflow at the very north end and by a suite of
smaller rivers draining western India. The Bay of Bengal also has narrow shelves and
a deep center basin, but has neither desert lands surrounding nor evaporation basins
drawing from and draining into it. Two great rivers, the Ganges and the Brahmaputra,
freshen the surface out to the open Indian Ocean south of Sri Lanka. Much of the
coastal zone is mangal, i.e. saline sedimentary areas supporting mangrove forests.
Arabian Sea
(^) Monsoons are the typifying feature of Arabian Sea seasonality, and in fact of the
seasonal cycling of the entire Indian Ocean. In winter, moderate winds blow from
northeast to southwest and accelerate surface waters in that direction, with the
Coriolis effect shifting the flow toward the Arabian coast and sustaining downwelling.
Thus, nutrient-depleted waters are carried into the gulf and support only low
productivity. In the northern spring, the winds come around to the southwest, blowing
both along the Somali–Arabian coast and across the open gulf toward the Himalayas.
High evaporation into the initially arid desert air moistens it, and then cooling by
uplift over the Indian Peninsula condenses the water, producing a prolonged rainy
season on the eastern side. The spring-summer southwest monsoon is very powerful
with winds of 40 knots sustained for months that produce waves >10 m and drive the
Somali Current along the coast of the Horn of Africa at the world’s greatest surface
velocity, to 7 knots. It also produces strong coastal upwelling offshore of Somalia,
Yemen, and Oman, with prolonged intervals of abundant stocks of large
phytoplankton. In addition, the portion of this wind blowing straight across the gulf
from the tip of Somalia toward the Himalayas, called the Finlater Jet, has strong wind-
stress curl on both sides. To the right there is Ekman pumping, moving nutrients up to
support a deep-water bloom 500 km wide and months in duration. To the left, there is
Ekman pumping which moves water down, partially separating this “Finlater bloom”
from the coastal bloom. The bloom across the northern Arabian Sea can sustain
chlorophyll levels in excess of 20 mg m−3 for months and primary-production rates in
excess of 1 g C m−2 d−1. The combination of a very long seasonal phytoplankton
bloom and the countervailing directions of deep and surface flows (toward and away
from the continent) creates a level of minimum motion, through which passes
copious, oxygen-consuming organic matter exported from the surface. The layer from
∼150 to 1000 m is persistently anoxic.