Hydrological Cycle Changes Explain Weak Snowball Earth Storm Track Despite Increased Surface Baroclinicity. Issue 20 (16th October 2020)

• Record Type: Journal Article
• Authors: Shaw, Tiffany A.; Graham, R. J.
• Is Part Of: Geophysical research letters. Volume 47:Issue 20(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-10-16
• Subjects: climate dynamics -- storm tracks; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL089866
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Simulations 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. Plain Language Summary: Storm tracks (low‐ and high‐pressure weather systems) dominate Earth's climate in the middle latitudes. Several modern theories connect storm track intensity to the equator‐to‐pole near‐surface temperature gradient. However, it has been known for some time that this connection fails when applied to simulations of past cold, icy climates such as the Last Glacial Maximum and Snowball Earth. Previous work explained the weak North Atlantic storm track during the Last Glacial Maximum using dry longitudinally dependent dynamical mechanisms associated with orographic forcing. Here we show that the weak Snowball Earth storm track can be explained by hydrological cycle changes that are independent of longitude. In particular, the weak storm track is consistent with the decreased equator‐to‐pole gradient of evaporation, absorption of sunlight, and surface moist static energy, which follow global mean cooling and the Clausius‐Clapeyron relation. The decreased equator‐to‐pole surface moist static energy gradient is correlated with decreased latent heat release aloft in the tropics, which weakens the potential energy that is available to be converted into kinetic energy. Key Points: Snowball Earth storm track is weaker than modern following decreased gradient of latent heat flux, shortwave absorption and latent heat release aloft The weaker Snowball Earth storm track can be predicted following global‐mean cooling and the Clausius‐Clapeyron relation The significant correlation between storm track intensity and surface moist static energy gradient reflects the importance of the hydrological cycle