High sensitivity of tropical precipitation to local sea-surface temperature - Nature.com

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Precipitation and atmospheric circulation are the coupled processes through which tropical ocean surface temperatures drive global weather and climate^1–5. Local ocean surface warming tends to increase precipitation, but this local control is hard to disentangle from remote effects of conditions elsewhere. Such remote effects occur, for example, from El Niño Southern Oscillation (ENSO) events in the equatorial Pacific, which alter precipitation across the tropics. Atmospheric circulations associated with tropical precipitation are predominantly deep, extending up to the tropopause. Shallow atmospheric circulations^6–8 impacting the lower troposphere also occur, but the importance of their interaction with precipitation is unclear. Uncertainty in precipitation observations^9,10 and limited observations of shallow circulations¹¹ further obstruct understanding of the ocean’s influence on weather and climate. Despite decades of research, persistent biases remain in many numerical model simulations^12–18, including excessively wide tropical rainbands^14,18, the ‘double-intertropical convergence zone (ITCZ) problem’^12,16,17 and too-weak responses to ENSO¹⁵. These demonstrate stubborn gaps in our understanding, reducing confidence in forecasts and projections. Here we show that the real world has a high sensitivity of seasonal tropical precipitation to local sea-surface temperature. Our best observational estimate is 80% precipitation change per g/kg change in the saturation specific humidity (itself a function of the ocean surface temperature). This observed sensitivity is higher than in 43 of the 47 climate models studied, and is associated with strong shallow circulations. Models with more realistic sensitivity have smaller biases across a wide range of metrics. Our results apply to both temporal and spatial variation, over regions where climatological precipitation is around 1 millimetre per day or greater. Novel analysis of multiple independent observations, physical constraints and model data, underpin these findings. The spread in model behaviour is further linked to differences in shallow convection, providing a focus for accelerated research, to improve seasonal forecasts through multidecadal climate projections.

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October 26, 2020 at 11:23PM
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High sensitivity of tropical precipitation to local sea-surface temperature - Nature.com

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