International Journal of Marine Science2016, Vol.6, No.50, 1-9
7
The existence of LNLC areas in Arabian Sea and Bay of Bengal is further supported by climatic SST image and a
spatial anomaly index of SST (Figure 5a, 5b). High SST values >28
C persists throughout the year in this region
with very little inter annual variability.
Figure 5 a) Climatic SST map derived from NOAA-AVHR (1998-2007) b) Spatial anomaly index map
In the Arabian Sea LNLC area, SST values range from 30
-32
C whereas in Bay of Bengal, SST values are
slightly lowered ranging from 27
-29
C. This may be attributed to the increased duration of cloud cover during an
annual cycle. Spatial anomaly index of SST shows that LNLC area of Arabian Sea is warmer (0.5-0.7
0
) compared
to the climatic basin –wide mean indicating a permanently stratified ocean with very little vertical mixing of upper
mixed layer and deeper waters. The LNLC region in Bay of Bengal is cooler (-0.1-0.4
0
) than the mean climatic
basin scale SST. However, this region is more oligotrophic (as seen from the chlorophyll images) compared to
Arabian Sea LNLC. Influence of less saline waters flowing to the Bay during the post monsoon period may
stratify the upper mixed layer leading to poorer mixing and oligotrophic conditions.
Primary production (Figure 6) in gC m
-2
y
-1
computed using climatic mean chlorophyll a and other variables in an
analytical model depicts very low productivity values ranging from 0.2 – 0.3 gC m
-2
y
-1
with a mean value of 0.22
gC m
-2
y
-1
(SD
0.054) in the Arabian Sea LNLC area. Bay of Bengal LNCL area also showed primary production,
values ranging from 0.2-0.24 gC m
-2
y
-1
with a mean value of 0.22 gC m
-2
y
-1
(SD
3.9).
The low standard deviation, indicating almost uniform low production for all the years resulted in a CV of only
1.8 % compared to 5.4 % (CV) in the Arabian Sea LNCL. Euphotic depth estimated from the model is around 78
m for both the areas. This shows that LNCL area is embedded in a typical oligotrophic region of the ocean basin.
(Figure 7)
