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V. Barale and M. Gade
relative minimum of October, a maximum is reached already in November, in the
southern sub-basin, and then in December, elsewhere. A second phase follows, from
January to March, with chl values decreasing everywhere. Starting from the absolute
maximum of January, a relative minimum for the period is reached again in March. In
spring-summer, again a first phase, from April to June, shows the chl values continue
their decline, reaching a minimum around May and perhaps starting to increase again
in June, in both the northern and southern sub-basins. In a second phase, from July
to September, the increase seems to continue, with a relative maximum in August,
for the southern sub-basin.
The ws values (still Fig. 17.9, left panels) seem to follow the same general pattern
of recurrent maxima in fall-winter followed by minima in spring-summer, at least at
the basin scale. The winds of the northern and southern sub-basins, however, tend
to behave in rather different ways. During spring-summer, when they are aligned
in the SW monsoon (i.e. northerly everywhere), ws is rather constant at first, only
to reach a relative maximum in September, in the north, and a relative minimum in
October, in the south. In this period, northerly winds are reinforced in the northern
sub-basin, but will eventually die off in the southern sub-basin, before reversing
their direction. During fall-winter, when the NE monsoon induces winds of opposite
direction (i.e. southerly) in the southern sub-basin, ws increases from October on,
and then decreases to a minimum in April. While the general fall-winter vs springsummer bimodal scheme can be readily associated with the wind forcing required to
have increased vertical mixing, and a consequent enhanced fertility of surface waters,
in fall-winter, the diverse sub-basin pattern followed by ws in spring-summer does
not seem to have any significant impact on the chl field.
The linear regression of the monthly mean chl ABV and ws ABV (Fig. 17.9, right
panels) indicates that the correlation is positive, but very small (R
2 = 0.43), at the
basin scale. Conversely, the correlation is extremely small (R
2 = 0.19) or virtually non
existent (R
2 = 0.06) in the northern and southern sub-basins, respectively. Introducing
a time lag between chl ABV and ws ABV (i.e. taking into account that there might be a
delay of one or more months in the chl response to changes in ws), the correlation
does not improve, indeed it decreases further. Once again, the general trend of
fall-winter maxima followed by spring-summer minima suggests that the general
blooming pattern of the Red Sea is indeed related to the classical “tropical sea”
scheme, driven by wind forcing in wintertime. But, at the same time, it appears that
specific, recurrent blooming episodes, in the southern Red Sea in particular, are not
related directly to wind forcing (rather to other factors such as the periodic exchange
of water with the Arabian Sea, via the Gulf of Aden and Bab-el-Mandeb).
17.5 Conclusions
Space and time heterogeneity of algal blooming in the Red Sea was studied using
a SeaWiFS-derived (and in part MODIS-derived) time series of chl data. Further, a
QuikSCAT-derived time series of ws data was also considered, to assess the impact of
V. Barale and M. Gade
relative minimum of October, a maximum is reached already in November, in the
southern sub-basin, and then in December, elsewhere. A second phase follows, from
January to March, with chl values decreasing everywhere. Starting from the absolute
maximum of January, a relative minimum for the period is reached again in March. In
spring-summer, again a first phase, from April to June, shows the chl values continue
their decline, reaching a minimum around May and perhaps starting to increase again
in June, in both the northern and southern sub-basins. In a second phase, from July
to September, the increase seems to continue, with a relative maximum in August,
for the southern sub-basin.
The ws values (still Fig. 17.9, left panels) seem to follow the same general pattern
of recurrent maxima in fall-winter followed by minima in spring-summer, at least at
the basin scale. The winds of the northern and southern sub-basins, however, tend
to behave in rather different ways. During spring-summer, when they are aligned
in the SW monsoon (i.e. northerly everywhere), ws is rather constant at first, only
to reach a relative maximum in September, in the north, and a relative minimum in
October, in the south. In this period, northerly winds are reinforced in the northern
sub-basin, but will eventually die off in the southern sub-basin, before reversing
their direction. During fall-winter, when the NE monsoon induces winds of opposite
direction (i.e. southerly) in the southern sub-basin, ws increases from October on,
and then decreases to a minimum in April. While the general fall-winter vs springsummer bimodal scheme can be readily associated with the wind forcing required to
have increased vertical mixing, and a consequent enhanced fertility of surface waters,
in fall-winter, the diverse sub-basin pattern followed by ws in spring-summer does
not seem to have any significant impact on the chl field.
The linear regression of the monthly mean chl ABV and ws ABV (Fig. 17.9, right
panels) indicates that the correlation is positive, but very small (R
2 = 0.43), at the
basin scale. Conversely, the correlation is extremely small (R
2 = 0.19) or virtually non
existent (R
2 = 0.06) in the northern and southern sub-basins, respectively. Introducing
a time lag between chl ABV and ws ABV (i.e. taking into account that there might be a
delay of one or more months in the chl response to changes in ws), the correlation
does not improve, indeed it decreases further. Once again, the general trend of
fall-winter maxima followed by spring-summer minima suggests that the general
blooming pattern of the Red Sea is indeed related to the classical “tropical sea”
scheme, driven by wind forcing in wintertime. But, at the same time, it appears that
specific, recurrent blooming episodes, in the southern Red Sea in particular, are not
related directly to wind forcing (rather to other factors such as the periodic exchange
of water with the Arabian Sea, via the Gulf of Aden and Bab-el-Mandeb).
17.5 Conclusions
Space and time heterogeneity of algal blooming in the Red Sea was studied using
a SeaWiFS-derived (and in part MODIS-derived) time series of chl data. Further, a
QuikSCAT-derived time series of ws data was also considered, to assess the impact of