WCD-21. Dynamics of a Persistent Gulf Stream Heatwave

Abstract
The sea surface temperature (SST) is a key indicator of climate change, shaped by the coupled circulations of the atmosphere and ocean. Here I report a recent stepwise increase in SSTs within the Gulf Stream region, and uncover a mechanism for its persistence through a full decade. In 2012, the Gulf Stream about 130 km off the U.S. Mid-Atlantic coast warmed by nearly 3°C, in excellent agreement with high-resolution global climate model simulations subject to a doubling of atmospheric carbon dioxide. Amplified warming along the poleward edges of the subtropical ocean gyres is expected to accompany their poleward shifts as a dynamical response to widening Hadley cells. Concurrent sea level and wind observations reveal a positive feedback mechanism whereby the induced changes in ocean currents advect warm SSTs, inducing a surface wind acceleration that maintains the initially amplified sea level gradient. Accounting for the negative feedback associated with evaporative heat loss implies a time scale of about four months for the observed warming. In light of these results, abrupt emergence and persistence of marine heatwaves should be an expected attribute of the future ocean, particularly for the Gulf Stream and other gyre limit regions.