Abstract
The sea surface temperature (SST) in the subpolar North Atlantic decreased during the past century, a remarkable feature known as the “warming hole (WH).” It is commonly claimed that the WH results from the slowdown of the Atlantic meridional overturning circulation. However, using an ensemble of the Community Earth System Model coupled to a slab ocean model simulation, we show the atmosphere alone can account for ∼50% of the observed cooling trend in the subpolar North Atlantic and ∼90% of the cooling relative to the temperature change of the global ocean. We find this cooling is caused by increased local westerlies in response to external forcing that enhance heat loss from the ocean through turbulent heat fluxes. This cooling is partly compensated by the damping in SST and the warming due to the rise of greenhouse gases. We further suggest that wind‐driven ocean processes can enhance the cooling in the real world, but that nonetheless the ultimate driver resides in the atmosphere.
Plain Language Summary
The global sea surface temperature (SST) shows a positive trend of ∼0.55 K century−1 during the last century (1920–2005), due primarily to the rise in atmospheric CO2 values. In contrast, the SST in the subpolar North Atlantic has cooled at a rate of ∼0.4 K century−1, known as the “warming hole.” Many previous studies using climate models link the cooling SST trend to a slowdown of the Atlantic meridional overturning circulation (AMOC), a planetary‐scale oceanic circulation transporting heat northward to the high‐latitude North Atlantic. However, direct observations linking the cooling SST trend with the slowdown of AMOC remains absent, due to the lack of long‐term AMOC observations. Using a climate model without variable ocean currents, we find the atmosphere alone can produce ∼50% of the observed cooling trend in the past century. The cooling trend results from the enhanced surface westerly winds that remove heat from ocean surface by enhancing air‐sea temperature differences and surface wind strength. This cooling trend is partially compensated by the warming due to the rise of greenhouse gases and the damping effect in SST. We further show that wind‐driven ocean processes can enhance the cooling. Our mechanism is consistent with observed changes in the North Atlantic Ocean and atmosphere.
Key Points
A warming hole appears in a slab ocean model, weaker in absolute magnitude than the observed, but close after removing the global average
The warming hole is driven by intensified surface westerlies in response to external forcings that lead to cooling via turbulent heat flux
A novel methodology is developed to identify compensating effects of damping by sea surface temperature and the rise of greenhouse gases
