Abstract
AbstractEquilibrium climate sensitivity (ECS) is a theoretical concept which describes the change in global mean surface temperature that results from a sustained doubling of atmospheric CO2. Current ECS estimates range from ∼1.8 to 5.6 K, reflecting uncertainties in climate feedbacks. The sensitivity of the lower (1,000–700 hPa) and upper (500–200 hPa) troposphere to changes in spatial patterns of tropical sea surface temperature (SST) have been proposed by recent model studies as key feedbacks controlling climate sensitivity. We examine empirical evidence for these proposed mechanisms using 14 years of satellite data. We examine the response of temperature and humidity profiles, clouds, and top‐of‐the‐atmosphere radiation to relative warming in tropical ocean regions when there is either strong convection or subsidence. We find warmer SSTs in regions of strong subsidence are coincident with a decrease in lower tropospheric stability (−0.9 ± 0.4 KK−1) and low cloud cover (∼−6% K−1). This leads to a warming associated with the weakening in the shortwave cooling effect of clouds (4.2 ± 1.9 Wm−2K−1), broadly consistent with model calculations. In contrast, warmer SSTs in regions of strong convection are coincident with an increase in upper tropospheric humidity (3.2 ± 1.5% K−1). In this scenario, the dominant effect is the enhancement of the warming longwave cloud radiative effect (3.8 ± 3.0 Wm−2K−1) from an increase in high cloud cover (∼7% K−1), though changes in the net (longwave and shortwave) effect are not statistically significant (p < 0.003). Our observational evidence supports the existence of mechanisms linking contrasting atmospheric responses to patterns in SST, mechanisms which have been linked to climate sensitivity.
Highlights
Clouds are a major source of uncertainty in physical climate models (Boucher et al, 2013; Soden & Held, 2006; Vial et al, 2013)
We find warmer sea surface temperature (SST) in regions of strong subsidence are coincident with a decrease in lower tropospheric stability (−0.9 ± 0.4 KK−1) and low cloud cover (∼−6% K−1)
SST warming in the past decades has been concentrated in the tropical marine ascent regions and has been linked to dampened warming through an associated increase of low cloud in subsidence regions (Zhou et al, 2016)
Summary
Clouds are a major source of uncertainty in physical climate models (Boucher et al, 2013; Soden & Held, 2006; Vial et al, 2013). An important step in the mechanism is the connection between the lower and upper troposphere and the extent to which relative warming at the surface is able to propagate up and influence the free troposphere In this hypothesis, if the SST warming is concentrated in subsidence regions, much of the warming is effectively “trapped” under the temperature inversion which caps the boundary layer at around 700 hPa (Andrews & Webb, 2018; Zhou et al, 2016) leading to warmer, moister air in the lower troposphere, while humidity above the inversion remains low (Figure 1a). By utilizing both the short- and longwave part of the spectrum, including separating the longwave window and nonwindow effects, we interpret TOA radiation flux changes with respect to these atmospheric and cloud changes
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