US Climate Variability and Predictability Program

The weakened Atlantic Meridional Overturning Circulation changes marine heatwaves

Marine heatwaves (MHWs) feature prolonged extreme warm events in global oceans that have dramatic impacts on marine ecosystems and fisheries. Thereby, finding the cause of these extreme events is of central importance. Atmospheric blocking highs, local air–sea interactions, and interannual and decadal variability have been suggested to drive the MHWs that have occurred in the Pacific and Atlantic Oceans.

The impact of AMOC slowdown on marine heatwaves from CCSM4 simulations.

Figure: The impact of AMOC slowdown on MHW (a) duration and (b) annual mean frequency and (c) intensity during 2061-2100 estimated from CCSM4 simulations. Dotted areas indicate that the AMOC impact is not statistically significant at the 95% level.

In a recent paper published in Geophysical Research Letters, Ren and Liu proposed a new physical driver — the Atlantic Meridional Overturning Circulation (AMOC) and its related slowdown — can modulate global MHWs through centuries. They applied a novel method to the climate model CCSM4 to isolate and quantify the AMOC effect. They found that during the past four decades, the AMOC induced changes in MHWs are mainly significant in the so-called North Atlantic warming hole (NAWH) region to the south of Greenland where the AMOC plays an essential role in producing a relative cooler mean-state SST. Towards the end of the 21st century, the AMOC impact will become evident in an even broader area covering much of the North Atlantic, North Pacific, and Arctic Oceans. This weakened ocean circulation brings about a bipolar-seesaw like change in MHW frequency, making MHWs more frequent in the Southern Hemisphere but less frequent in the Northern Hemisphere. Particularly, the NAWH region would reach a near-permanent MHW state without an AMOC slowdown. MHWs there would have ultra-long durations, diminished annual frequencies, and enhanced magnitudes.

The results from this study highlight the importance of the AMOC as a key ocean circulation system and to the long-term variations of climate extremes such as MHWs. Moreover, the AMOC impact is of a global scale and becomes increasingly significant. This means that MHWs off the California coast will be affected remotely by the weakened AMOC in the current century.

Written by
Wei Liu, University of California, Riverside

Xianglin Ren and Wei Liu

University of California, Riverside