17 Basic Ecosystem Dynamics in the Red Sea as Seen by Sundry . . .
353
monsoon-driven atmospheric forcing on the chl variability. Data from ten consecutive
full-year cycles, from July 1999 to June 2009, were used to compute chl ABV and ws ABV
from monthly composite images. A correlation between these parameters has been
attempted, to explore the basic ecosystem dynamics of the Red Sea.
The SeaWiFS imagery details the diverse characteristics of the northern (oligotrophic) sub-basin and the southern (mesotrophic) sub-basin, between which a
central, transitional area presents more variable environmental conditions. Interestingly, hardly any subdivision between a strictly pelagic region and a coastal zone
appears anywhere in the Red Sea, with the exception of that occurring between the
basin interior and the broad shelf areas, dotted by numerous islands, of the south.
The observed seasonal pattern is essentially bimodal, with a fall-winter period
of extended blooming, which progresses from south to north, followed by a springsummer period of much reduced blooming, at least in the northern sub-basin. Overall,
this annual cycle seems to be governed by the climatic characteristics of the basin,
the monsoon-driven wind regime in particular, and by the ensuing thermohaline
circulation.
Given that fertilization in the Red Sea occurs mainly via the inflow from the Gulf of
Aden and the southern part of the basin, chl values should indeed start to increase with
the inflow of surface waters from the Gulf of Aden throughout phase one (October
to December) of the fall-winter period. Thus, relative chl maxima appear first in the
southern sub-basin, between October and November, and then progress northward
in the northern sub-basin, between November and December. From December to
January, Ekman transport due to the NE monsoon would contribute to spread surface
waters toward the northern region, thus fostering the chl absolute maximum observed
in the north during phase two (January to March) of the fall-winter period. The flux of
nutrients of southern origin would then stop, or be much reduced, during the spring
inter-monsoon, from March to May. Hence the decreasing chl values toward the end
of the fall-winter period, and the continuing decrease in phase one (April to June)
of the spring-summer period.
Following the May absolute minimum, starting in June and through phase two
(July to September) of the spring-summer period, the intrusion of intermediate waters
from the Gulf of Aden, driven by the SW monsoon, would favour the mixing of deep
nutrients into the upper layer of the southern sub-basin. This could fertilize the
southern Red Sea and result in the chl relative maxima recurring in summer months,
between July and September. At the end of this phase, following the September
minimum, the conventional anti-estuarine regime of the Red Sea would take over
again, with the Red Sea overflow coupled to the inflow from the Gulf of Aden leading
once again to the fall-winter situation detailed above.
Acknowledgments Analyses and visualizations used in this paper were produced with the online
data services featured by the Goddard Earth Sciences (GES) and Data Information Services Center
(DISC) Interactive Online Visualization ANd aNalysis Infrastructure (GIOVANNI), of the National
Aeronautic and Space Administration (NASA); by the Global Marine Information System (GMIS),
of the Joint Research Centre (JRC), European Commission (EC); and by Remote Sensing Systems
(RSS), sponsored by the NASA Ocean Vector Winds Science Team. We acknowledge the Mission
353
monsoon-driven atmospheric forcing on the chl variability. Data from ten consecutive
full-year cycles, from July 1999 to June 2009, were used to compute chl ABV and ws ABV
from monthly composite images. A correlation between these parameters has been
attempted, to explore the basic ecosystem dynamics of the Red Sea.
The SeaWiFS imagery details the diverse characteristics of the northern (oligotrophic) sub-basin and the southern (mesotrophic) sub-basin, between which a
central, transitional area presents more variable environmental conditions. Interestingly, hardly any subdivision between a strictly pelagic region and a coastal zone
appears anywhere in the Red Sea, with the exception of that occurring between the
basin interior and the broad shelf areas, dotted by numerous islands, of the south.
The observed seasonal pattern is essentially bimodal, with a fall-winter period
of extended blooming, which progresses from south to north, followed by a springsummer period of much reduced blooming, at least in the northern sub-basin. Overall,
this annual cycle seems to be governed by the climatic characteristics of the basin,
the monsoon-driven wind regime in particular, and by the ensuing thermohaline
circulation.
Given that fertilization in the Red Sea occurs mainly via the inflow from the Gulf of
Aden and the southern part of the basin, chl values should indeed start to increase with
the inflow of surface waters from the Gulf of Aden throughout phase one (October
to December) of the fall-winter period. Thus, relative chl maxima appear first in the
southern sub-basin, between October and November, and then progress northward
in the northern sub-basin, between November and December. From December to
January, Ekman transport due to the NE monsoon would contribute to spread surface
waters toward the northern region, thus fostering the chl absolute maximum observed
in the north during phase two (January to March) of the fall-winter period. The flux of
nutrients of southern origin would then stop, or be much reduced, during the spring
inter-monsoon, from March to May. Hence the decreasing chl values toward the end
of the fall-winter period, and the continuing decrease in phase one (April to June)
of the spring-summer period.
Following the May absolute minimum, starting in June and through phase two
(July to September) of the spring-summer period, the intrusion of intermediate waters
from the Gulf of Aden, driven by the SW monsoon, would favour the mixing of deep
nutrients into the upper layer of the southern sub-basin. This could fertilize the
southern Red Sea and result in the chl relative maxima recurring in summer months,
between July and September. At the end of this phase, following the September
minimum, the conventional anti-estuarine regime of the Red Sea would take over
again, with the Red Sea overflow coupled to the inflow from the Gulf of Aden leading
once again to the fall-winter situation detailed above.
Acknowledgments Analyses and visualizations used in this paper were produced with the online
data services featured by the Goddard Earth Sciences (GES) and Data Information Services Center
(DISC) Interactive Online Visualization ANd aNalysis Infrastructure (GIOVANNI), of the National
Aeronautic and Space Administration (NASA); by the Global Marine Information System (GMIS),
of the Joint Research Centre (JRC), European Commission (EC); and by Remote Sensing Systems
(RSS), sponsored by the NASA Ocean Vector Winds Science Team. We acknowledge the Mission