Upwelling and eddy features in the Eastern Sulu Sea from satellite data and
field measurements
Irene D. Alabia1 and Cesar L. Villanoy1
1Marine
Science Institute, Physical Oceanography Laboratory, University of the
Philippines-Diliman Quezon City 1101 Philippines. E-mail:
irenealabia@gmail.com
Surface chlorophyll and Sea Surface Temperature (SST) patterns in the Eastern
Sulu Sea were examined using MODIS data and field measurements. Upwelling
and gyral structures were identified in the Eastern region of the basin during the
Northeast monsoon period. Another strong coastal upwelling in Northern
Zamboanga region was also seen from the images. These regions exhibit high
chlorophyll concentrations and relatively lower temperatures. In-situ surface
measurements of chlorophyll and SST sampled in the area during northeast
monsoon period (December 2007) revealed similar patterns. The formation of
these structures is forced by the reversing monsoon system. Wind jets are
formed as Northeasterly wind funnels through the narrow inter-island channels.
This formation results to the generation of upwelling and eddy structures.
Upwelling areas were characterized as areas with highly positive Ekman
pumping velocities. Wind velocity patterns during the same period showed the
intensification of wind vectors in the inter-island gaps.
The satellite-derived datasets for chlorophyll and SST were also subjected to
EOF analysis to further examine spatial and temporal trends. From the results of
the analysis, the first 3 modes of each of the datasets represented the major
components of the total variances. The first spatial mode obtained from
chlorophyll data explains 79% of the total variance and represents the contrast
between high-chlorophyll shallow areas and low-chlorophyll open ocean. The
second (5%) and third (2%) spatial modes constitutes the seasonal distribution
trends during northeast monsoon and southwest monsoon periods, respectively.
The areas along the island gaps and northern Zamboanga coast showed
elevated chlorophyll patterns. The results of the analysis for SST revealed strong
seasonal signal during the northeast monsoon (December-March) period. The
first spatial mode accounts for 33% of the total variance and shows the low
temperature pattern in the inter-island channels. The southwest monsoon (JuneAugust) and transition (April-May) signals were represented by the second (14%)
and third (8%) spatial modes.
Keywords: Upwelling, MODIS, EOF analysis, cyclonic eddy, Sulu Sea