Temporal and spatial variability of chlorophyll-a in the western subarctic Pacific determined from satellite and ship observations from 1997 to 1999

Abstract

Abstract The objectives of this study are to describe and understand the processes controlling the temporal and spatial variability of chlorophyll-a (chl-a) in the subarctic northwestern Pacific Ocean from 1997 to 1999. Remotely sensed data from multi sensors, including ocean color (OCTS and SeaWiFS), sea-surface temperature (SST, AVHRR), and wind (SSM/I) datasets were utilized for the purpose of this study. Ocean-color imagery clearly showed seasonal and interannual variability in the spatial abundance and distribution of chl-a in the study area. Chl-a concentrations were generally low (0.53±0.24 mg m−3) for most part of the year, except for a few peaks (1.0–2.0 mg m−3) observed in the spring and fall bloom seasons (May, June, September, and October). Chl-a concentrations (>10 mg m−3) were consistently high along the Kuril Islands and in the coastal waters around the Kamchatka Peninsula, and in 1998 they were clearly higher than in 1999. The Western Subarctic Gyre (WSG) was characterized by positive sea-surface temperature anomalies (SSTA) during the summer to fall of 1998. These anomalies appear to be a high-latitude response to 1997/1998 ENSO event. High concentrations of chl-a appeared in WSG in September–November only in 1998. We suggest that the high chl-a around the WSG from summer to fall in 1998 was facilitated by (1) negative SSTA in winter, in spite of the negative wind anomaly, which provided larger amount of nutrients to the sea surface, (2) positive wind anomaly from April to June, which might cause light limitation of the phytoplankton growth, due to deepening of the surface-mixed layer in summer, and (3) positive SSTA, even with the slightly positive wind anomaly, which was accompanied with the water-column stabilization and hence recovered light limitation of the phytoplankton. Warmer SST also might have enhanced the phytoplankton growth. During summer to fall in 1999, the phytoplankton biomass between 42°N and 43°N along 165°E was greater than in other years. The enhanced chl-a concentration coincided with a distinct frontal temperature gradient located between 40°N and 45°N in September 1998 and 1999. The peak chl-a concentration in September 1999 (>2.0 mg m−3) within the temperature front was much larger than in 1998 (about 0.7 mg m−3). The position of this front in September 1999 (41°N) was south of its position in 1998 (43°N). Convergence of chl-a is linked to the temperature gradient and southward expansion of well-mixed subarctic water that is cold, low in salinity, and nutrient-rich. Our results showed significant interannual biological variability, and illustrate how remote sensing with multi sensors can aid in monitoring relationships between local ecosystems and global climate change.