ABSTRACT
S E A L E V E L V A R I A B I L I T Y A N D S U R F A C E D Y N A M I C S IN H I E I N T E R I O R
OF THE PHILIPPINE ARCHIPELAGO
AdviserDr. C e s a r L. Villanoy
Lei Ian I A. SoIei3
University o f t h e Philippines, 2012
T h e d o m i n a n t modes o f variability within the interior o f t h e Philippine archipelago were
e x a m i n e d from t i m e - f r e q u e n c y analysis of sea level and currents measured using an arrav
of shallow pressure g a u g e s , high frequency radars and m o o r e d acoustic Doppler current
profiler that were deployed during the intensive observation period of the Philippine
Straits D y n a m i c s Rxpcrimcnt (PhilEx) in 2007-2009. T h e seasonal signal associated with
the reversing East Asian-Western Pacific monsoon s y s t e m s w a s the m o s t dominant m o d e
of variability and the d y n a m i c response of sea level and currents to this a t m o s p h e r e driven oscillation is clearly demonstrated by the large a m p l i t u d e fluctuation of sea level
and by the c h a n g e in the direction and intensity of f l o w at seasonal scales. In Panay
Strait, sea level variability associated with the seasonal m o n s o o n s led to depressed sea
level near the west coast of Panay d u r i n g the northeast m o n s o o n ( N E M ) and a u g m e n t e d
sea level d u r i n g the southwest m o n s o o n ( S W M ) . The m a g n i t u d e of the response o f sea
level to the monsoon w i n d s was found to be largely d e f i n e d by m o d i f i c a t i o n s to local
wind r e g i m e brought a b o u t by island topography and orientation of the c o a s t During the
N E M , a strong westward wind Jet w a s observed at the leeward side of the mountain gap
at the northern part of Panay Island w h i c h enhanced o f f s h o r e s u r f a c e transport along the
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trajectory of the wind j e t . This explains the larger negative sea level a n o m a l i e s observed
at the northern part of t h e strait. M e a n w h i l e , higher sea level a n o m a l i e s during the S W M
at the northern part of the strait is attributed to the presence of a headland which causes a
pile up o f w a t e r hence the higher a n o m a l i e s observed. The westward wind jet was also
found to influence s u r f a c e currents in Panay Strait. The positive shear of the wind j e t led
to the formation of a c y c l o n i c eddy between f > anay fsland and C u y o Island (in the vicinity
of the strait) during the N E M . T h e cyclonic eddy led to southward f l o w offshore and the
associated return f l o w is believed to e n h a n c e northv/ard flow near-coast. The northward
surface f l o w is a p r e d o m i n a n t feature in Panay Strait. Even during the southwest
monsoon ( S W M ) w h e n a reversal is already seen in the s u b s u r f a c e flow, flow near
surface w a s still n o r t h w a r d . For this reason, the c y c l o n i c eddy is taken to only modulate
the northward flow near-coast and the predominantly northward How is believed to be
maintained by several processes that may occur at different times.
Apart f r o m the seasonal signal associated with the reversing m o n s o o n s , intraseasonal
oscillations (ISO) w e r e also found to spread over a wide range of frequencies. T w o ISO
with periods of ~8 and - 2 5 da ys were found to have significantly larger contributions to
the sea level spectrum. T h e s e ISO occurred year-round but were found to be stronger
during the N E M . In the ease of the - 2 5 day oscillation, an unusually high peak was
observed in February 2 0 0 8 which was related to a persistently northerly surge that has
been previously associated with c h a n g e s in the mesoscale eddy activity in the adjacent
Mindoro Strait. T h e northerly surge resulted from an e x t r e m e cold a n o m a l y (EGA) event
affecting the Southeast Asian region during that period which, in turn, is believed to be a
result of the c o - o c c u r r e n c e of the d o m i n a n t ISO and the 2 0 0 7 - 2 0 0 8 La Nina. From these
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results, the necessity to define strait dynamics, as in the case of Panay. based on
m o d i f i c a t i o n s to tlow across different time scales of variability is highlighted.
!t has been previously suggested that remote forcing can also d e f i n e transport across the
M i n d o r o - P a n a y Strait complex and in an examination of Kelvin w a v e propagation as a
potential m e c h a n i s m for remote forcing of the P a c i f i c O c e a n on the interior of the
archipelago and on the South China Sea (SCS), sea level a n o m a l i e s have been found to
travel f r o m the east coast o f t h e Philippines through the Surigao Strait and Sibutu Passage
and out into the S C S through the M i n d o r o Strait. C o m p a r i s o n o f t h e t i m i n g of these
Kelvin w a v e s with theoretical phase speeds for Kelvin w a v e s in the region suggest that
the Kelvin waves likely travel along the ea.st coast o f t h e Philippines, down south, and
into the Sibutu Passage where it then propagates a l o n g the coast of the islands at the
eastern side o f t h e Sulu Sea, up to M i n d o r o Strait and out to the S C S . W h e n c o m p a r e d
with the total sea level in Pandan (along Panay Strait), it w a s estimated that Kelvin w a v e s
may a c c o u n t for a s much as 5 0 % o f t h e variability observed a l o n g the strait. A s such,
while variability is believed to be largely atmosphere-driven (i.e. by m o n s o o n s and
a t m o s p h e r i c ISO), the remote influence of the surrounding bodies of water (i.e. the
remote f o r c i n g of the Pacific O c e a n ) cannot be disregarded.
K e y w o r d s : Variability, monsoons, intraseasonal oscillations, sea level, currents, Kelvin
w a v e propagation
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