Kennelly, Maureen A., and James A. Yoder

Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, Tel: 401-874-6679, Fax: 401-874-6728, Email: m.kennelly@gso.uri.edu.

Seasonal and ENSO variability in global ocean phytoplankton chlorophyll

The 4-year, calibrated SeaWiFS data set provides a means to determine seasonal and other sources of phytoplankton variability on global scales, which are important components of the total variability associated with ocean biological and biogeochemical processes. We used empirical orthogonal function (EOF) analysis on a 4-year time series of global SeaWiFS chlorophyll a imagery (1ºx1º spatial resolution and 8-day temporal resolution) to quantify the major seasonal (as well as the 1998 ENSO) signals in phytoplankton biomass between 50ºS and 50ºN, and then a second analysis to quantify summer patterns at higher latitudes. Among the important effects we resolved are a 6-month phase shift in maximum chlorophyll a concentrations between subtropical (winter peaks) and subpolar (spring-summer peaks) waters, greater seasonal range at high latitudes in the Atlantic compared to the Pacific, an interesting phasing between spring and fall biomass peaks at high latitudes in both hemispheres, and the effects of the 1998 ENSO cycle in the tropics. Our results show that dominant seasonal and ENSO effects are captured in the first 6 of a possible 184 modes, which explain 68% of the total temporal variability associated with the global mean chlorophyll a pattern. Finally, the dominant global patterns are consistent with the results of ocean models of seasonal dynamics based on seasonal changes to the heating and cooling cycles of the upper ocean.