Abstract
Abstract. A negative extratropical shortwave cloud feedback driven by changes in cloud optical depth is a feature of global climate models (GCMs). A robust positive trend in observed liquid water path (LWP) over the last two decades across the warming Southern Ocean supports the negative shortwave cloud feedback predicted by GCMs. This feature has been proposed to be due to transitions from ice to liquid with warming. To gain insight into the shortwave cloud feedback we examine extratropical cyclone variability and the response of extratropical cyclones to transient warming in GCM simulations. Multi-Sensor Advanced Climatology Liquid Water Path (MAC-LWP) microwave observations of cyclone properties from the period 1992–2015 are contrasted with GCM simulations, with horizontal resolutions ranging from 7 km to hundreds of kilometers. We find that inter-cyclone variability in LWP in both observations and models is strongly driven by the moisture flux along the cyclone's warm conveyor belt (WCB). Stronger WCB moisture flux enhances the LWP within cyclones. This relationship is replicated in GCMs, although its strength varies substantially across models. It is found that more than 80 % of the enhancement in Southern Hemisphere (SH) extratropical cyclone LWP in GCMs in response to a transient 4 K warming can be predicted based on the relationship between the WCB moisture flux and cyclone LWP in the historical climate and their change in moisture flux between the historical and warmed climates. Further, it is found that that the robust trend in cyclone LWP over the Southern Ocean in observations and GCMs is consistent with changes in the moisture flux. We propose two cloud feedbacks acting within extratropical cyclones: a negative feedback driven by Clausius–Clapeyron increasing water vapor path (WVP), which enhances the amount of water vapor available to be fluxed into the cyclone, and a feedback moderated by changes in the life cycle and vorticity of cyclones under warming, which changes the rate at which existing moisture is imported into the cyclone. Both terms contribute to increasing LWP within the cyclone. While changes in moisture flux predict cyclone LWP trends in the current climate and the majority of changes in LWP in transient warming simulations, a portion of the LWP increase in response to climate change that is unexplained by increasing moisture fluxes may be due to phase transitions. The variability in LWP within cyclone composites is examined to understand what cyclonic regimes the mixed-phase cloud feedback is relevant to. At a fixed WCB moisture flux cyclone LWP increases with increasing sea surface temperature (SST) in the half of the composite poleward of the low and decreases in the half equatorward of the low in both GCMs and observations. Cloud-top phase partitioning observed by the Atmospheric Infrared Sounder (AIRS) indicates that phase transitions may be driving increases in LWP in the poleward half of cyclones.
Highlights
Constraining the change in cloud reflectivity in response to warming is key to offering a more accurate prediction of 21st century climate change. Caldwell et al (2016) showed that uncertainty in shortwave cloud feedback represented the largest contribution to uncertainty in climate sensitivity in the Coupled Model Intercomparison Project Phase 5 (CMIP5) generation of models
With the caveat in mind that we must infer the partitioning of liquid between rain and cloud in the observations, we evaluate the ability of a wide array of global climate models (GCMs) to simulate the response of cyclone-mean liquid water path (LWP) (LWPCM) to the warm conveyor belt (WCB) moisture flux in the Southern Ocean (30–80◦ S)
We have examined the behavior of extratropical cyclones in models and long-term microwave observations for the period 1992–2015
Summary
Constraining the change in cloud reflectivity in response to warming is key to offering a more accurate prediction of 21st century climate change. Caldwell et al (2016) showed that uncertainty in shortwave cloud feedback represented the largest contribution to uncertainty in climate sensitivity in the Coupled Model Intercomparison Project Phase 5 (CMIP5) generation of models. We will show that the mixed-phase cloud feedback does not explain all of the observed variability or trends in extratropical LWP within cyclones in both observations and GCMs by showing that changes in moisture flux into the cyclones predict the majority of the change in LWP in response to warming. This is done by sorting our observations and simulations into cyclonic regimes across the extratropics. We hope that the relationships between synoptic state and cyclone cloud LWP in this work provide a clear benchmark that models may be evaluated against and will reduce uncertainty related to the extratropical shortwave cloud feedback in models
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