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Natuna eddy (Akhir
2012
; Chu et al.
1998
) is also observed north of Natuna Island
centred at 5N 108E. The general surface circulation in October (Fig.
6
a) is
relatively weak compared to the first monsoon transition period (April). During
this period, major part of the southward surface current is flowing from the
Vietnam Coast towards Borneo Island and is also embedded with eddies and
meanders. North eastward flow prevails along the coast of Borneo. The anti-
cyclonic eddy observed in the north and southeast of Natuna Island which is
formed during SW monsoon (August) is still visible. In December, the surface
circulation has been strengthened (Fig.
6
b). The southward flowing current from
Vietnam coastal region turns direction to northwest along the Borneo coastal area.
All eddies which observed during October totally disappeared during this season.
5 Conclusion
The present study derives the surface circulation off northwest Borneo Island using
the satellite altimetry data. The SLA estimated from altimeter is consistent with
the tide gauge measurements at Kota Kinabalu and Bintulu. The geostrophic
current pattern derived by combining the SLA and 1992-2002 Mean Dynamic
Ocean Topography well agree with the drifting buoy flow pattern. The general
pattern of surface circulation in the central region of SSCS is cyclonic in April,
October and December. Along the northwest coast of Borneo Island, the surface
current is toward north eastward during April. While in August, the surface cir-
culation is rather complex and the surface flow is northward near the Borneo
Island. In October, the surface circulation is relatively weak in the region. By
December, the circulation has again strengthened and strong southward surface
current from the Vietnam Coast is clearly visible. The current is north eastward
along the coast of Borneo Island during December.
Number of eddies and meandering patterns are found in seasonal surface cir-
culation of the study region. A cyclonic eddy at around 61N, 113E is clearly
identified. This eddy is weak in August and not visible during October and
December. An anti-cyclonic eddy is found south of Natuna Island in April but
diffused by August. Furthermore, a cyclonic and anti-cyclonic eddy found off
the Sarawak and northeast of Natuna Island respectively during April and which
are present in August and October but dispersed by December. A mesoscale anti-
cyclonic eddy is found in the north of Natuna Island during August which sustains
up to October.
Acknowledgments The authors would like to thank the TUDelft NOAA and Altimetrics LLC
for providing altimetry data. Special thanks are due to MOHE, UTM and VOTE NO. (01G01)
for funding this project. The 1992-2002 mean ocean dynamic topography data has been obtained
from Nikolai Maximenko (IPRC) and Peter Niiler (SIO).
