Oceanography The Official Magazine of
The Oceanography Society
Volume 28 Issue 04

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Volume 28, No. 4
Pages 46 - 53


Shifts in Chlorophyll a off Eastern Luzon, Philippines, Associated with the North Equatorial Current Bifurcation Latitude

By Olivia C. Cabrera , Cesar L. Villanoy, Irene D. Alabia , and Arnold L. Gordon 
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Article Abstract

The circulation pattern off the east coast of Luzon, Philippines, as observed during two oceanographic cruises conducted in May/June 2011 and April/May 2012, features a northward-flowing current (the nascent Kuroshio) bracketed by an anticyclonic eddy to its northeast and a cyclonic eddy to its southwest. A previous study showed that a more northerly location of the North Equatorial Current (NEC) bifurcation during the 2011 cruise resulted in entrainment of more subtropical water from the Kuroshio into upper thermocline waters, while a more southerly bifurcation location during the 2012 cruise injected more tropical NEC water into upper thermocline waters offshore of eastern Luzon. Here, we show that the change in source of upper thermocline waters in this region can also be recognized in the distribution pattern of chlorophyll a. Horizontally averaged chlorophyll a profiles showed higher concentrations in the upper 125 m of the water column during the 2011 cruise when the bifurcation latitude was 13°N–14°N than during the 2012 cruise when the bifurcation latitude was observed at 10°N–11°N. Chlorophyll a data overlain on regional currents show that the 2011 nascent Kuroshio had a higher chlorophyll a content, supplied by the Kuroshio recirculation from the north, while the increased NEC content of the 2012 nascent Kuroshio was accompanied by reduced chlorophyll a within the upper thermocline, supporting the notion that horizontal advection plays an important role in chlorophyll variability off eastern Luzon. Using sea surface height anomalies as a proxy for upper ocean heat content, we show that warmer waters from the NEC supply the nascent Kuroshio when the location of the NEC bifurcation is more southerly. Empirical orthogonal function analysis of Moderate Resolution Imaging Spectroradiometer chlorophyll a data from 2003 to 2013 shows that the primary mode of chlorophyll a variability offshore of eastern Luzon was positively correlated with the NEC bifurcation latitude, further establishing that warmer, oligotrophic waters from the NEC supply the nascent Kuroshio when NEC bifurcation is southerly.


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