Gulf Stream cross-frontal exchange: possible mechanisms to explain interannual variations in phytoplankton chlorophyll in the Slope Sea during the SeaWiFS years

Abstract

During the first 4 years of the SeaWiFS mission (September 1997 through August 2001), the spring blooms in the Slope Sea increased in magnitude. During the same time, the mean path of the Gulf Stream shifted northward. The northward trend of the Gulf Stream is evidenced in satellite sea-surface temperature imagery as well as in situ temperature, salinity, and current vector information collected by the merchant vessel Oleander on its weekly trip between New Jersey and Bermuda. Surface temperature and salinity increased in the Slope Sea over the 4 years. It surprised us to find a collective increase in phytoplankton chlorophyll, temperature, and salinity, contrary to the commonly observed inverse relationship between temperature and chlorophyll in surface waters of this region. While the Gulf Stream surface waters are depleted of nutrients and low in biomass content, waters at depth are rich in nutrients. Although the Gulf Stream usually serves as a barrier between Sargasso waters to the south and Slope waters to the north, cross-stream exchange occurs when there is upward flow along isopycnals toward the surface waters of the Slope Sea. Under certain conditions, warm-core rings and associated streamers shed by the Gulf Stream may also bring nutrient-rich water up into the euphotic zone. The outflow of surface water from the Labrador Sea appears to influence both the size and horizontal transport of the Slope Sea. When the Slope waters are warm and saline there is less Labrador water present, resulting in less dilution of the Gulf Stream waters leaking into the Slope Sea and less horizontal advection within its cyclonic gyre. While the intensity of the spring blooms during this period has dramatic interannual variability, we found the total surface chlorophyll concentration integrated over the Slope Sea remains nearly unchanged. This suggests that the Labrador is not the major supplier of nutrients, but rather that the Slope Sea receives a steady nutrient supply from the sub-surface Gulf Stream waters.