Satellite-observed SST and chlorophyll reveal contrasting dynamical-biological effects of mesoscale eddies in the North Atlantic

Abstract

Abstract The influence of mesoscale eddies on chlorophyll (Chl) has received significant attention due to Chl being a proxy for phytoplankton, which plays a crucial role in marine ecosystems. Solely relying on the analysis of satellite-observed Chl poses challenges in determining the phytoplankton response to mesoscale eddies. To address this, our study takes a collaborative approach, utilizing satellite-derived sea surface temperature anomalies (SSTA) and chlorophyll anomalies (CHLA) to comprehensively investigate the dynamical-biological processes associated with eddies in the subtropical and mid-latitude North Atlantic. In the subtropics, the patterns in CHLA and SSTA predominantly exhibit a dipole nature, with the dipole component providing more than 70% of the explained variance (EV). This suggests that eddy stirring is the dominant mechanism driving the observed anomaly patterns. Conversely, in the mid-latitudes, the monopole components (TM ) explain more than 60% of the EV, implying a more influential role for eddy trapping and vertical modulations. The signs of the TM of eddy SSTA persist throughout their lifetime, being consistent with the lowering (raising) of isopycnals within AEs (CEs). However, the subtropical CHLA response is higher in AEs than CEs, indicating additional factors, such as eddy-induced Ekman pumping and/or mixing to a deeper level may be important. This finding is also corroborated by subsurface observations from Argo floats. At mid-latitudes, there is a clear inverse correspondence between the CHLA and mixed layer depth. In contrast, no significant correlation is observed in the subtropics, except during winter when a positive relationship emerges. These patterns suggest that phytoplankton exhibit highly diverse responses to the physical dynamics associated with eddies. Our work offers a method to estimate eddy dynamical-biological impacts on phytoplankton using satellite products, compensating for the limitations of in-situ observations. It also reveals potential contributions to marine primary production, global carbon cycles, and the development of biogeochemical models.