Cloud feedbacks in extratropical cyclones: insight from long-term satellite data and high-resolution global simulations

Abstract. A negative extratropical shortwave cloud feedbackdriven 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 feedbackpredicted by GCMs. This feature has been proposed to be due to transitions from ice to liquid with warming. To gain insight into the shortwavecloud feedback we examine extratropical cyclonevariability and the response of extratropical cyclonesto transient warming in GCM simulations. Multi-Sensor Advanced Climatology Liquid Water Path(MAC-LWP) microwave observations of cycloneproperties 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- cyclonevariability 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 cycloneLWP in GCMs in response to a transient 4 K warming can be predicted based on the relationship between the WCB moisture flux and cycloneLWP 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 cycloneLWP over the Southern Ocean in observations and GCMs is consistent with changes in the moisture flux. We propose two cloud fee dbacks 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 cyclonesunder 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 cycloneLWP 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 cyclonecomposites is examined to understand what cyclonicregimes the mixed-phase cloud fee dback is relevant to. At a fixed WCB moisture flux cycloneLWP 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-topphase partitioning observed by the Atmospheric Infrared Sounder(AIRS) indicates that phase transitions may be driving increases in LWP in the poleward half of cyclones.