Influence of contrasting sea surface temperature warming patterns on atmospheric circulation and cloud feedbacks

Abstract

Climate sensitivity, defined as the global-mean surface temperature change due to a doubling of atmospheric CO2, is a key metric for quantifying the Earth system response to increasing greenhouse gases. Estimates of climate sensitivity vary widely, making it difficult for societies to prepare for the impacts of climate change. Uncertainty in climate sensitivity is driven primarily by uncertainty in how clouds will respond to warming. But how clouds respond to climate change depends strongly on the geographic pattern of warming: the so-called ‘pattern effect’. This recently-discovered phenomenon is crucial to narrowing uncertainty in climate projections, yet fundamental understanding of the processes underpinning the pattern effect is underdeveloped. In particular, the potential role of changes in atmospheric circulation as a crucial link between warming patterns and cloud feedbacks remains unclear.  Here we use a series of idealised GCM simulations and a moist static energy (MSE) framework to investigate the coupling between tropical sea surface temperature (SST) warming, circulation changes and cloud feedbacks. In the simulations the SST of different ‘patches’ of the tropical ocean are perturbed, resulting in strongly non-linear cloud responses. We demonstrate that the circulation response is also non-linear and closely coupled to the cloud response. Specifically, SST warming in the west Pacific leads to a reduction in ascent fraction – the proportion of the atmosphere that is ascending at 500 hPa – over the tropical ocean, associated with an increased top-of-atmosphere shortwave cloud radiative effect.  In contrast, SST warming in the east Pacific has little effect on ascent fraction. We develop a framework for estimating ascent fraction as a function of near-surface MSE, inclusive of an entraining-plume model to account for dry-air mixing into moist ascending air. We demonstrate how this framework can provide insight into both the circulation changes associated with patterned SST warming and the resulting cloud feedbacks.