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Comparison of Seasonal Cycles of Phytoplankton Chlorophyll, Aerosols, Winds and Sea-Surface Temperature off Somalia

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Relation http://eprints.cmfri.org.in/12413/
doi: 10.3389/fmars.2017.00386
 
Title Comparison of Seasonal Cycles of
Phytoplankton Chlorophyll, Aerosols,
Winds and Sea-Surface Temperature
off Somalia
 
Creator Shafeeque, Muhammed
Sathyendranath, Shubha
George, Grinson
Balchand, A N
Platt, Trevor
 
Subject Oceanography
Seawater study
 
Description In climate research, an important task is to characterize the relationships between
Essential Climate Variables (ECVs). Here, satellite-derived data sets have been used to
examine the seasonal cycle of phytoplankton (chlorophyll concentration) in the waters
off Somalia, and its relationship to aerosols, winds and Sea Surface Temperature
(SST). Chlorophyll-a (Chl-a) concentration, Aerosol Optical Thickness (AOT), Ångström
Exponent (AE), Dust Optical Thickness (DOT), SST and sea-surface wind data for a
16-year period were assembled from various sources. The data were used to explore
whether there is evidence to show that dust aerosols enhance Chl-a concentration in
the study area. The Cross Correlation Function (CCF) showed highest positive correlation
(r2 = 0.3) in the western Arabian Sea when AOT led Chl-a by 1–2 time steps (here, 1
time step is 8 days). A 2 × 2◦ box off Somalia was selected for further investigations.
The correlations of alongshore wind speed, Ekman Mass Transport (EMT) and SST
with Chl-a were higher than that of AOT, for a lag of 8 days. When all four variables
were considered together in a multiple linear regression, the increase in r2 associated
with the AOT is only about 0.02, a consequence of covariance among AOT, SST,
EMT and alongshore wind speed. The AOT data show presence of dust aerosols
most frequently during the summer monsoon season (June–September). When the
analyses were repeated for the dust aerosol events, the correlations were generally
lower, but still significant. Again, the inclusion of DOT in the multiple linear regression
increased the correlation coefficient by only 2%, indicating minor enhancement in
Chl-a concentration. Interestingly, during summer monsoon season, there is a higher
probability of finding more instances of positive changes in Chl-a after one time step,
regardless of whether there is dust aerosol or not. On the other hand, during the
winter monsoon season (November–December) and rest of the year, the probability
of Chl-a enhancement is higher when dust aerosol is present than when it is absent.
The phase relationship in the 8-day climatologies of Chl-a and AOT (derived from
NASA’s SeaWiFS and MODIS-A ocean colour processing chain) showed that AOTled Chl-a for most of the summer monsoon season, except when Chl-a was very high,
during which time, Chl-a led AOT. The phase shift in the Chl-a and AOT climatological
relationship at the Chl-a peak was not observed when AOT from Aerosol Climate Change
Initiative (Aerosol-CCI) was used.
 
Date 2017
 
Type Article
PeerReviewed
 
Format text
 
Language en
 
Identifier http://eprints.cmfri.org.in/12413/1/Grinson%20George_2017_Frontiers%20in%20Marine%20Science_4_Comparison%20of%20Seasonal%20Cycles.pdf
Shafeeque, Muhammed and Sathyendranath, Shubha and George, Grinson and Balchand, A N and Platt, Trevor (2017) Comparison of Seasonal Cycles of Phytoplankton Chlorophyll, Aerosols, Winds and Sea-Surface Temperature off Somalia. Frontiers in Marine Science, 4. pp. 1-15.