A ribbon of dark water: phytoplankton blooms in the meanders of the Pacific North Equatorial Countercurrent

In the far southwestern corner of the North Pacific Ocean, the Mindanao Current retroflects and flows northeast into the nascent North Equatorial Countercurrent (NECC). Ocean-color images show that phytoplankton blooms occur both in the nearly closed cyclonic loop of the retroflection, and in the meandering NECC, which were particularly strong during the El Niño event of 1997–98. In the winter of 1997–98, the bloom persisted for about 4 months, and, at its peak, extended for several thousand kilometers along the current stream. The NECC meanders are observed in SeaWiFS and OCTS images at various times throughout the nearly 5 years of data examined, but regionally averaged chlorophyll concentrations during the 1997–98 El Niño were higher than at any time since. The current meanders are sometimes visible in SeaWiFS images during winter, spring and summer, but not in autumn, consistent with the seasonal cycle of formation and decay of the Mindanao Dome (MD). Although the MD has a cyclonic circulation pattern, vertical nutrient flux appears to be maximal in the NECC, which forms its southern boundary, rather than in the MD itself. The appearance of phytoplankton blooms along the stream of the NECC is most plausibly attributed to upwelling associated with current meandering, although El Niño-induced shoaling of the thermocline and seasonal Ekman pumping also may contribute. In addition, transport of nutrient-rich water from the South Pacific by the New Guinea Coastal Undercurrent (NGCUC), which is maximal in boreal winter, creates a gradient of nutrient concentration across the NECC stream with much greater concentrations to the south at a given density (J. Geophys. Res. 103 (1998) 12959). In 1997-98 a confluence of all of these factors occurred. Shoaling of the thermocline and an intensified NECC due to the El Niño event coincided with the seasonal formation of the MD and increased westward transport of nutrient-rich water by the NGCUC to produce the highest chlorophyll concentrations observed in 5 years of ocean-color data. Similar though weaker blooms were observed in the spring of 2003 in association with the very weak 2002–03 El Niño.