Subtropical gyre variability as seen from satellites

摘要

A satellite multi-sensor approach is used to analyse the biological response of open ocean regions of the subtropical gyres to changes in physical forcing. Thirteen years (1998-2010) of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll-α (chl-α) data, combined with concurrent satellite records of sea-surface temperature (SST) and sea level height, were analysed to investigate the seasonal and inter-annual variability of chl-α concentration within these immense so-called ocean deserts. The seasonal variability of chl-αwarming/cooling of surface waters. Summer warming promotes shallower mixed layers and lower chl-α due to a reduction of vertical mixing and consequently a decrease in nutrient supply. The opposite happens during the winter cooling period. Therefore, long-term trends in SST have the potential to cause an impact on the inter-annual variability of chl-α. Our analyses show that, during the 13 whole years of SeaWiFS data record, the North Pacific, Indian Ocean and North Atlantic gyres experienced a decrease in chl-α of 9%, 12% and 11%, respectively, with corresponding SST increases of 0.27℃, 0.42℃ and 0.32℃, respectively. The South Pacific and South Atlantic gyres also showed warming trends but with weak positive trends in chl-α that are not statistically significant. We hypothesize that the warming of surface waters in these two gyres is counterbalanced by other interacting physical- and biological-driving mechanisms, as indicated in previous studies.