Late Summer chlorophyll blooms in the oligotrophic North Pacific Subtropical Gyre

In the late summer of 1997, 1999, and 2000 satellite observations of surface chlorophyll from SeaWiFS recorded large blooms in the oligotrophic North Pacific Subtropical Gyre (NPSG) near 30°N and between 130–160°W. The largest blooms covered more than 350,000 km2 and lasted as long as 4 months. They are distinct from the surface seasonal cycle of chlorophyll in both timing and amplitude. The blooms are not associated with either SSH or SST anomalies indicative of changes in subsurface structure, nor do they appear to be forced by nutrient fertilization from dust deposition or rainfall. These blooms are compared with summer blooms that have been previously observed in this region by in situ studies, and some potential causes for them are discussed.