Patterns of the Oceanic Redistribution of Heat and Carbon during Global Warming Hiatus Periods in a Large Earth System Model Ensemble

Abstract:

The redistribution of the ocean’s heat content generated by internal variability likely plays an important role for the observed slowdown in the rate of global warming since 1998. So far, the importance of the Equatorial Pacific was highlighted, with periods of frequent and/or intense La Niña phases having been identified as periods of intensified ocean heat uptake, keeping the rate of global mean warming low. Here we challenge this view by examining changes in the oceanic heat and carbon content during periods with particularly low (and high) trends in global mean surface temperature (GMST) in a 30-member ensemble simulation with the coupled carbon-climate Earth System Model GFDL ESM2M under a historical/RCP8.5 scenario over 1950-2100.

Between 2005 and 2024, we identified 41 (out of 330 possible) hiati, i.e., running decades with a linear decadal GMST trend of zero or lower. While decadal hiati are relatively frequent in our ensemble, the probability of a hiatus lasting 15 years or longer is only 2%. During an average hiatus event, only 37% of the global vertical heat redistribution in the ocean above 110m occurs in the Equatorial Pacific between 20°N and 20°S caused by increased upwelling associated with stronger trade winds. While the Atlantic north of 30°S contributes another 30% to the global anomaly in ocean heat content above 110m, the heat anomaly in the upper 110m of the Indian Ocean and the Southern Ocean is small. Many individual hiati are even less dominated by the Tropical Pacific than the average hiatus: In 4 out of 41 Hiatus decades, the SST anomaly in the ENSO3.4 region is zero or positive. This finding challenges the popular view of the Equatorial Pacific being the single most important region and indicates that the surface warming pattern may be very different during future Hiatus decades. The changes in ocean carbon uptake and content and similarities to ocean heat content changes during Hiatus periods will also be discussed.