Hydrological Cycle Changes Explain Weak Snowball Earth Storm Track Despite Increased Surface Baroclinicity

Tiffany A Shaw,R J Graham

doi:10.1029/2020gl089866

Tiffany A Shaw, R J Graham

Open Access

https://doi.org/10.1029/2020gl089866

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Abstract

AbstractSimulations show that storm tracks were weaker during past cold, icy climates relative to the modern climate despite increased surface baroclinicity. Previous work explained the weak North Atlantic storm track during the Last Glacial Maximum using dry zonally asymmetric mechanisms associated with orographic forcing. Here we show that zonally symmetric mechanisms associated with the hydrological cycle explain the weak Snowball Earth storm track. The weak storm track is consistent with the decreased meridional gradient of evaporation and atmospheric shortwave absorption and can be predicted following global mean cooling and the Clausius‐Clapeyron relation. The weak storm track is also consistent with decreased latent heat release aloft in the tropics, which decreases upper tropospheric baroclinicity and mean available potential energy. Overall, both hydrological cycle mechanisms are reflected in the significant correlation between storm track intensity and the meridional surface moist static energy gradient across a range of simulated climates between modern and Snowball Earth.

Highlights

Storm tracks are regions where midlatitude cyclones occur most frequently and several modern theories connect their intensity to surface baroclinicity
Relative to the modern climate, the Snowball Earth simulations have more ice, larger Northern Hemisphere (NH) surface baroclinicity, weaker meridional surface moist static energy (MSE) gradient, weaker stationary eddy MSE flux (Figure 1d), and weaker storm track intensity
We examined whether zonally symmetric mechanisms associated with the hydrological cycle can account for the weaker Snowball Earth storm track despite increased surface baroclinicity

Summary

Introduction

Storm tracks are regions where midlatitude cyclones occur most frequently and several modern theories connect their intensity to (near-) surface baroclinicity (meridional temperature gradient, Chang et al, 2002; Held, 2018; Shaw et al, 2016). The surface baroclinicity-intensity connection is supported by the seasonality of storm track intensity in modern reanalysis data (see Figure S5 in O'Gorman, 2010) and idealized simulations of annual mean storm track intensity across a range of climates without ice (see Figure 4 in O'Gorman & Schneider, 2008) (see Figure 1 in Caballero & Langen, 2015) It is assumed in energy balance models (EBMs) based on surface temperature (Mbengue & Schneider, 2018; North, 1975) and surface moist static energy (Frierson et al, 2007). All of the mechanisms are based on dry zonally asymmetric dynamics; for example, they appeal to changes in stationary

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