Sensitivity of ocean heat uptake to changing atmospheric forcing in the context of climate variability

Abstract

The global warming hiatus, a period of slowdown increase in air surface temperature despite continued emissions of greenhouse gases, is a complex phenomenon that challenges our understanding of climate processes and feedback mechanisms. Potential explanations include deep ocean heat uptake and the influence of internal climate variability. However, rather than focusing on individual processes within specific basins, there is a need for a complete picture of the ocean mechanisms responsible for the hiatus. This research used the adjoint model of the MIT General Circulation Model (MITgcm) to analyze how atmospheric forcing patterns drive changes in Vertical Heat Flux (VHF), focusing on the 10-year averaged global VHF into the volume from 300m depth to the bottom. Implementing the adjoint model to studying the hiatus is an innovative approach of our study. By examining the adjoint sensitivities, we identified optimal atmospheric forcing patterns for increasing VHF, revealing that intensified trade winds in the tropical Pacific and Atlantic, stronger westerlies in subtropical regions and the Southern Ocean, and enhanced meridional winds along major coastlines all contribute to VHF. Specific regional patterns of temperature-related forcings also play a role, with surface warming in higher latitudes and cooling in the tropics being effective for VHF increase. Projecting historical forcing anomalies onto these sensitivity patterns allowed us to quantify the influence of different forcings on VHF during the period 1979-2008. Anomalies are computed with respect to the climatology of the period 1948-1968, characterized by small changes in the ocean heat content. The zonal wind, especially over the Southern Ocean, emerged as the dominant contributor, underscoring the role of adiabatic processes in deep ocean heat uptake. The Pacific Ocean also appeared as a significant region where atmospheric forcings have a major impact on VHF changes, followed by the Atlantic, particularly in areas like the Greenland and Labrador Seas, although with a relatively minor contribution. The Interdecadal Pacific Oscillation (IPO) and the Southern Annular Mode (SAM) were found to be the primary drivers of changes in VHF among the climate modes analyzed, followed by the Atlantic Multidecadal Oscillation (AMO). While the IPO and AMO explain the entire contribution to VHF changes due to the variability of atmospheric forcings in the Pacific and Atlantic Oceans, respectively, the SAM accounts for only one-quarter of the zonal wind variability in the Southern Ocean, which is the main driver of VHF changes. These climate modes drive distinct physical processes that could have influenced VHF. Strengthened easterlies in the tropical Pacific during the hiatus, including the fraction related to the IPO and La Niña, are thought to have caused the thermocline to shoal in the east, bringing cooler, deeper waters to the surface and lowering sea surface temperatures. In the central-western Pacific, these stronger easterlies lead to the accumulation of warm surface waters, which may have, in turn, deepened the thermocline. This deepening of the thermocline could have facilitated the downward transport of heat into the subsurface, potentially contributing to enhanced deep ocean heat uptake and the hiatus. Similarly, the enhanced westerlies in the Southern Ocean, including the fraction related to the SAM, likely increased northward Ekman transport and Ekman pumping, drawing deeper, denser, cooler waters to the surface. This process could have strengthened the Deacon Cell and tilted the isopycnals, allowing warm surface waters to move downward along the slope and sequester heat in deeper ocean layers. Increased downward VHF in the Atlantic Ocean, primarily in the Labrador Sea and Greenland Sea, was driven by warming in the surface layers. This surface warming was likely mixed into the deep ocean through the deep mixed layer left by previous strong convection periods, which may have enhanced deep ocean heat uptake and contributed to the global warming hiatus. Although the contribution from Pacific climate modes is dominant, the Southern Ocean and the Atlantic also play substantial roles, underscoring the global nature of the processes involved in the hiatus. Notably, much of the Southern Ocean’s influence on heat uptake cannot be directly tied to the specific climate mode analyzed. Yet, the zonal wind anomalies in this region remain crucial in driving the global warming hiatus.