Abrupt Increase in Marine Heatwave Accumulated Days Over the Gulf Stream Extension Around 2013 Linked to Subtropical Eastern Pacific Warming

Abstract

Marine heatwaves (MHWs) exert profound impacts on marine ecosystems and socio‐economic systems. Although individual extreme events have been widely investigated, the mechanisms underlying decadal‐scale abrupt shifts in MHW regimes under global warming remain poorly understood. Here, we identify a pronounced abrupt change in MHW accumulated days over the Gulf Stream (GS) extension around 2013, marked by a stepwise increase of 8.75 days substantially exceeding the long‐term linear trend. Using a random forest framework, we show that a rapid buildup of subsurface ocean heat content (OHC), rather than anomalous surface heat fluxes, is the dominant contributor to this abrupt change. Mechanistic analyses further indicate that this subsurface heat accumulation is dynamically associated with a northward displacement of the Gulf Stream, driven by anomalous southerly winds. By combining observations with CESM2 pacemaker experiments and targeted AGCM sensitivity simulations, we trace the origin of these atmospheric anomalies to an abrupt warming in the subtropical eastern Pacific. This remote forcing excites a transcontinental, quasi‐stationary Rossby wave train, which sustains a deep, equivalent‐barotropic anticyclonic circulation over the GS Extension. Our results identify a previously underappreciated teleconnection linking Pacific climate variability to Atlantic subsurface heat redistribution, underscoring the capacity of remote atmospheric forcing to induce abrupt MHWs abrupt changes in western boundary current regions.