346
V. Barale and M. Gade
limited by sunlight, but is always limited by nutrient availability—a condition relaxed only in the colder season, when (wind-driven) convection processes can enrich
the euphotic zone with nutrients from deeper layers. However, some of the details
appearing in the ws pattern—in particular the late summer (September) maximum
in the northern sub-basin, and both early fall (October) and spring (April) relative
minima in the southern sub-basin—can not be easily associated with this general
scheme. Instead, they seem to conincide rather well with the monsoon reversal periods, when the wind over the southern sub-basin, first aligned with that over the
northern basin (late summer), starts blowing from the opposite, southerly, direction
(early fall), and then again when the wind field over both sub-basins aligns once
more from the same, northerly, direction (spring).
The spatial heterogeneity of this seasonal sequence above is illustrated in some
detail by the chl and ws climatological (1999–2009) monthly mean images, derived
from the SeaWiFS and QuikSCAT historical data sets, and shown in Figs. 17.5
and 17.6 respectively. These composites (also arranged from July to June) present a
highly structured pigment field and wind field, exhibiting a large degree of variability
(both in space and time) for all sub-basins already identified in the annual mean
images.
In July, August and September, low chl values characterize most of the Red
Sea, in particular in the interior of the northern sub-basin, while the southern basin
maintains higher values only in near-coastal areas. Note that, in summer, the climatic
conditions of the southern sub-basin are such that some areas remain permanently
covered by clouds (mostly in July), for the entire SeaWiFS period of coverage, so
as to prevent the computation of a mean value. Furthermore, note that the highest
values recorded in August for the southern sub-basin appear to be located in the
Bab-el-Mandeb and in the Gulf of Aden, along the southeastern coastline, possibly
having more to do with local oceanographic conditions, than with those typical of the
Red Sea proper. Sofinaos and Johns (2007) suggest that in summer the basic structure
of the three-layer flow, in the southernmost part of the basin, involves banking toward
the coasts. In particular, the Red Sea Outflow Water should be banked toward the
western coast, and the Gulf of Aden Intermediate Water, and surface waters, toward
the eastern coast. Indeed, such an inflow from the Gulf ofAden, composed of nutrientrich waters upwelled by the summer monsoon wind pattern over the Arabian Sea,
would provide the apt process for generating the enhanced patterns observed in the
southeastern coastal zone in August.
The corresponding ws images show that, in this period, northerly winds prevail
over the entire Red Sea, with mean speed increasing over the northern sub-basin and
decreasing over the southern sub-basin. The summer chl minimum is followed by
increasing values in October, November and December, starting from the south and
progressing northward.
Note also the persistent traces of intense eddy-like circulation features, around
20
◦ N, in the October transition period. This corresponds to a period of wind reversal
over the southern sub-basin, from northerly to southerly winds, as well as increasing
mean wind speed. After reaching their maximum extent in January and February,
V. Barale and M. Gade
limited by sunlight, but is always limited by nutrient availability—a condition relaxed only in the colder season, when (wind-driven) convection processes can enrich
the euphotic zone with nutrients from deeper layers. However, some of the details
appearing in the ws pattern—in particular the late summer (September) maximum
in the northern sub-basin, and both early fall (October) and spring (April) relative
minima in the southern sub-basin—can not be easily associated with this general
scheme. Instead, they seem to conincide rather well with the monsoon reversal periods, when the wind over the southern sub-basin, first aligned with that over the
northern basin (late summer), starts blowing from the opposite, southerly, direction
(early fall), and then again when the wind field over both sub-basins aligns once
more from the same, northerly, direction (spring).
The spatial heterogeneity of this seasonal sequence above is illustrated in some
detail by the chl and ws climatological (1999–2009) monthly mean images, derived
from the SeaWiFS and QuikSCAT historical data sets, and shown in Figs. 17.5
and 17.6 respectively. These composites (also arranged from July to June) present a
highly structured pigment field and wind field, exhibiting a large degree of variability
(both in space and time) for all sub-basins already identified in the annual mean
images.
In July, August and September, low chl values characterize most of the Red
Sea, in particular in the interior of the northern sub-basin, while the southern basin
maintains higher values only in near-coastal areas. Note that, in summer, the climatic
conditions of the southern sub-basin are such that some areas remain permanently
covered by clouds (mostly in July), for the entire SeaWiFS period of coverage, so
as to prevent the computation of a mean value. Furthermore, note that the highest
values recorded in August for the southern sub-basin appear to be located in the
Bab-el-Mandeb and in the Gulf of Aden, along the southeastern coastline, possibly
having more to do with local oceanographic conditions, than with those typical of the
Red Sea proper. Sofinaos and Johns (2007) suggest that in summer the basic structure
of the three-layer flow, in the southernmost part of the basin, involves banking toward
the coasts. In particular, the Red Sea Outflow Water should be banked toward the
western coast, and the Gulf of Aden Intermediate Water, and surface waters, toward
the eastern coast. Indeed, such an inflow from the Gulf ofAden, composed of nutrientrich waters upwelled by the summer monsoon wind pattern over the Arabian Sea,
would provide the apt process for generating the enhanced patterns observed in the
southeastern coastal zone in August.
The corresponding ws images show that, in this period, northerly winds prevail
over the entire Red Sea, with mean speed increasing over the northern sub-basin and
decreasing over the southern sub-basin. The summer chl minimum is followed by
increasing values in October, November and December, starting from the south and
progressing northward.
Note also the persistent traces of intense eddy-like circulation features, around
20
◦ N, in the October transition period. This corresponds to a period of wind reversal
over the southern sub-basin, from northerly to southerly winds, as well as increasing
mean wind speed. After reaching their maximum extent in January and February,