A multi-year La Niña event sustained an exceptionally persistent chlorophyll-a enhancement in the Patagonian Shelf

Between mid-2010 and early 2012, the Southern Patagonian Continental Shelf (SPCS) experienced an exceptionally intense and persistent phytoplankton bloom, during which chlorophyll-a (Chl-a) concentrations doubled the climatological mean during the austral autumn and winter. As a result, annual Chl-a levels exceeded the long-term average by more than 80%. Here, we investigate the climatic and oceanographic drivers underlying the magnitude and duration of this anomalous event. Using 24 years of ocean colour observations combined with atmospheric reanalysis data and hydrodynamic model outputs, we identify large-scale atmosphere-ocean coupling associated with the concurrent multi-year La Niña (LN) and positive Southern Annular Mode (SAM) phases as the dominant driver of the extraordinary bloom. At the regional scale, this sustained climate event enhanced wind-driven exchange between western and eastern Patagonia, promoting the intrusion of low-salinity (<34.5), potentially nutrient-rich Pacific water through the Magellan Strait into the Atlantic Ocean (>0.12 Sv). Additional processes, including episodic wind weakening over the SPCS, a reduced autumn-winter mixed layer depth (MLD), and associated modifications of the northward-directed shelf and offshore transport, likely further enhanced phytoplankton production, distinguishing this event from other comparable climatic episodes. These findings highlight the critical role of interactions between major climate variability modes in shaping ocean productivity in the SPCS.