Slowdown of the thermohaline circulation causes enhanced maritime climate influence and snow cover over Europe

Abstract

The ocean thermohaline circulation (THC) in the Atlantic is generally accepted to contribute to the comparatively mild climate of western and northern Europe. Global climate models (Manabe and Stouffer, 1995; Vellinga and Wood, 2002) and palaeo‐observations (McManus et al., 2004) associate periods of weak or absent Atlantic THC with considerably lower temperatures in and around the northern North Atlantic. However, it is uncertain whether such change would spread longitudinally around the globe or would be limited to a narrow strip near the coast (Vellinga and Wood, 2002; Mikolajewicz et al., 1997). Moreover, the relatively low spatial resolution of global climate models has prohibited more detailed statements about possible or probable THC‐induced climate change in Europe. Here, we first use a global climate model to perform a THC‐slowdown sensitivity experiment, with a weakening of the MOC by about 50%, and the associated control experiment. The two global simulations are then used to force a regional climate model for Europe; the regional control and sensitivity simulations are analyzed here. We find stronger maritime influence over Europe than in the case with no MOC weakening, which surprisingly results in cooling (because sea surface temperatures drop), contrary to today's conditions, where maritime influence leads to milder conditions in Europe. Lower temperatures cause reduced precipitation, increased snow cover and higher albedo leading to positive feedback.