Surface Fluxes Modulate the Seasonality of Zonal-Mean Storm Tracks

Abstract

The observed zonal-mean extratropical storm tracks exhibit distinct hemispheric seasonality. Previously, the moist static energy (MSE) framework was used diagnostically to show that shortwave absorption (insolation) dominates seasonality but surface heat fluxes damp seasonality in the Southern Hemisphere (SH) and amplify it in the Northern Hemisphere (NH). Here we establish the causal role of surface fluxes (ocean energy storage) by varying the mixed layer depth d in zonally symmetric 1) slab-ocean aquaplanet simulations with zero ocean energy transport and 2) energy balance model (EBM) simulations. Using a scaling analysis we define a critical mixed layer depth dc and hypothesize 1) large mixed layer depths (d > dc) produce surface heat fluxes that are out of phase with shortwave absorption resulting in small storm track seasonality and 2) small mixed layer depths (d < dc) produce surface heat fluxes that are in phase with shortwave absorption resulting in large storm track seasonality. The aquaplanet simulations confirm the large mixed layer depth hypothesis and yield a useful idealization of the SH storm track. However, the small mixed layer depth hypothesis fails to account for the large contribution of the Ferrel cell and atmospheric storage. The small mixed layer limit does not yield a useful idealization of the NH storm track because the seasonality of the Ferrel cell contribution is opposite to the stationary eddy contribution in the NH. Varying the mixed layer depth in an EBM qualitatively supports the aquaplanet results.

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

“…The second configuration is a slab ocean aquaplanet general circulation model with modern obliquity, greenhouse gases, zero ocean energy transport, and thermodynamic sea ice (a motionless single slab, Giorgetta et al, 2012), hereafter referred to as AQUA. The modern AQUA simulation has a 50‐m mixed layer depth (Barpanda & Shaw, 2020; Donohoe et al, 2014). Here we vary the mixed layer depth as a simple way of simulating a range of climates between modern and Snowball Earth.…”

“…The storm track interacts with the terms on the right hand side of (5); thus, it is difficult to infer causality when the MSE framework is used diagnostically. However, Shaw et al (2018) and Barpanda and Shaw (2020) showed that external parameters in the MSE framework (e.g., insolation and mixed layer depth) can form the basis of predictions, scaling estimates, and thereby causal understanding.…”

Opposite Responses of the Dry and Moist Eddy Heat Transport Into the Arctic in the PAMIP Experiments

“…Poleward heat transport (PHT) works toward energy balance by moving heat from warmer to colder regions (e.g., Held, 2001; Trenberth & Stepaniak, 2003), primarily via atmospheric but also via oceanic pathways (Czaja & Marshall, 2006; Held, 2001). Atmospheric warming from anthropogenic climate change is also spatially varying, for example, in tropical upper‐ and Arctic lower‐tropospheric warming (e.g., Deser et al., 2016; Vallis et al., 2015), which entails changes to PHT that also occur via the atmospheric pathway (e.g., Alexeev & Jackson, 2013; Barpanda & Shaw, 2020; Hwang & Frierson, 2010; Hwang et al., 2011; Shaw et al., 2018). Changes in PHT in observations have also been linked to reduced reflected shortwave radiation due to sea ice loss to reduced high‐latitude PHT (Hartmann & Ceppi, 2014).…”

The Role of Water Vapor in the Response of the Extratropical Circulation of Earth‐Like Planets to Obliquity Changes

“…The MSE budget has the appeal of interpreting the tropical and extratropical atmospheric circulations (including baroclinic eddies) as part of the global energy budget and relating them to sources and sinks of energy through the top and the bottom of the atmospheric column, as well as atmospheric and surface energy storage. By applying this framework to the understanding of the seasonality of zonal-mean storm tracks in aquaplanet simulations at Earth's obliquity, Barpanda and Shaw (2020) for instance showed how a large heat capacity of the lower boundary buffers insolation through surface fluxes, resulting in small seasonality of the net energy input and, with it, of the storm tracks, as for instance seen in Earth's Southern Hemisphere. For small mixed layer depths, they found a large seasonality of the net energy input, which leads to a large storm track seasonality, but also an increasing role for atmospheric energy storage.…”