Interannual-to-Decadal Variability in the Bifurcation of the North Equatorial Current off the Philippines

Abstract

Satellite altimeter sea surface height (SSH) data from the past 17 yr are used to investigate the interannual-to-decadal changes in the bifurcation of the North Equatorial Current (NEC) along the Philippine coast. The NEC bifurcation latitude migrated quasi decadally between 10° and 15°N with northerly bifurcations observed in late 1992, 1997–98, and 2003–04 and southerly bifurcations in 1999–2000 and 2008–09. The observed NEC bifurcation latitude can be approximated well by the SSH anomalies in the 12°–14°N and 127°–130°E box east of the mean NEC bifurcation point. Using a 1 ½-layer reduced-gravity model forced by the ECMWF reanalysis wind stress data, the authors find that the SSH anomalies in this box can be simulated favorably to serve as a proxy for the observed NEC bifurcation. With the availability of the long-term reanalysis wind stress data, this helps to lengthen the NEC bifurcation time series back to 1962. Although quasi-decadal variability was prominent in the last two decades, the NEC bifurcation was dominated by changes with a 3–5-yr period during the 1980s and had low variance prior to the 1970s. These interdecadal modulations in the characteristics of the NEC bifurcation reflect similar interdecadal modulations in the wind forcing field over the western tropical North Pacific Ocean. Although the NEC bifurcation on interannual and longer time scales is generally related to the Niño-3.4 index with a positive (negative) index corresponding to a northerly (southerly) bifurcation, the exact location of bifurcation is determined by wind forcing in the 12°–14°N band that contains variability not fully representable by the Niño-3.4 index.

Interannual and Decadal Variability in Tropical Pacific Sea Level

“…Previous work has linked western Pacific sea level changes on decadal time scales to the North Pacific Gyre and the bifurcation latitude of the North equatorial current [30,31]. Wind forcing in the 12 • -14 • N band has a more significant influence on this bifurcation latitude than the Niño 3.4 index [32]. Therefore, our choice of region for tropical zonal wind stress would not capture this relationship.…”

Air–sea CO<sub>2</sub> flux in the Pacific Ocean for the period 1990–2009

“…Long-term time-series records of oceanic CO 2 appear to show a decrease in the positive trends in pCO 2 sw and DIC in the eastern to southern rim of the subtropical cell and in its tropical branch after the strong warm event of ENSO in 1997-1998(Dore et al, 2003Keeling et al, 2004;Ishii et al, 2009;Midorikawa et al, 2012). A change in the subtropical cell (Qiu and Chen, 2010) is a likely driver, but the mechanism driving a decrease in the rate of increasing pCO 2 sw and its possible linkage to PDO is not fully understood. Regarding a potential change in the seasonal cycle in pCO 2 sw, Rodgers et al (2008) and Gorgues et al (2010) argued that over decadal timescales in the North Pacific there is a divergence between the trend in winter and summer that occurs in the absence of trends in the circulation state of the ocean, and consequently a decadal trend arises towards an increased seasonal cycle.…”

Discovery of a spawning area of the common Japanese conger Conger myriaster along the Kyushu-Palau Ridge in the western North Pacific

“…The collections of C. myriaster larvae were limited to the northern part of our survey area (17-21°N), which belongs to the marginal area adjacent to the northernmost part of the NEC (south of approximately 15°N [40]) and covers the STCC region (north of 19-20°N [40][41][42][43]). In contrast, the larvae were absent from the southern region (south of 16°N), which belongs to the mainstream of the NEC as a stable westward current [40].…”