A document by Tiffany A Shaw. Click on the document to view its contents.

Stratospheric Sudden Warmings (SSWs) predominantly occur in the Northern Hemisphere with only 1 major event recorded in the Southern Hemisphere in the satellite era. Investigating factors that contribute to this asymmetry can help to reveal the cause of SSWs and lead to improved forecasts. Here we use climate model simulations to investigate the impact of boundary conditions (topography and ocean circulation) on the asymmetry. Flattening topography eliminates Northern Hemisphere SSWs, while removing the ocean meridional overturning circulation reduces their frequency by half. The SSW response to boundary conditions is controlled by decrease in hemispheric asymmetry of eddy heat flux. The reduction is driven by a decrease in amplitude of both eddy meridional wind and eddy temperature, as well as an increase in the difference between their phases. The results suggest boundary conditions play an important role in shaping SSWs, especially topographic forcing, but that the boundary condition interactions are nonlinear.

The modern “wet” tropics are dominated by the Intertropical Convergence Zone, however “dry” tropics likely occurred in Earth’s history. It is unclear how the tropics change between wet and dry climates because recent progress has focused on modern and warmer climates. We show the tropical hydrological cycle undergoes a wet-to-dry regime transition when surface wetness is decreased in a general circulation model. The dry regime occurs when precipitation is suppressed by negative evaporation. The regime transition is dominated by near-surface relative humidity, in contrast to our traditional understanding which assumes changes in relative humidity are small. We show near-surface relative humidity changes are controlled by re-evaporation of stratiform precipitation. The moistening effect of re-evaporation is non-local: re-evaporation happens near the lifting condensation level and moisture diffuses downward to the near-surface. Our results provide a first step toward understanding tropical hydrological cycle changes between wet and dry climates.

Energy balance regimes (e.g., Radiative Convective Equilibrium or RCE) qualitatively characterize the low, mid, and high latitudes of Earth’s modern climate. Currently we do not have a complete quantitative understanding of the spatio-temporal structure of energy balance regimes. Here we use the vertically-integrated moist static energy budget to define a nondimensional number that quantifies when and where RCE is approximately satisfied in Earth’s modern climate. We find RCE exists yearround in the tropics and also in the Northern midlatitudes during summertime. Decomposing the seasonal RCE regime transition in the midlatitudes over land and ocean shows that RCE occurs predominantly over land. We use idealized models (energy balance and aquaplanet models) to test the hypothesis that the RCE regime occurs during midlatitude summer for land-like (small heat capacity) surface conditions. Consistent with the hypothesis, an aquaplanet model configured with a shallow mixed layer depth transitions to RCE in the midlatitudes during summertime whereas it does not for a deep mixed layer depth.

Climate models project that tropical warming is amplified aloft in response to increased CO$_2$. Amplification aloft is expected following moist adiabatic adjustment and the Clausius-Clapeyron relation. Here, we show that moist adiabatic adjustment overpredicts the multi-model mean temperature response at 300 hPa by 12.9–25.3\% across the model hierarchy. We show that overprediction is influenced by at least three mechanisms: large-scale circulation, direct effect of CO$_2$, and convective entrainment. Accounting for the large-scale circulation and the direct effect of CO$_2$ reduces overprediction by 5.7\% and 3.8\% respectively, but does not eliminate it. To test the influence of entrainment, we vary the Tokioka parameter in aquaplanet simulations with and without a large-scale circulation. When varying the climatological entrainment rate in the aquaplanet, overprediction varies from 6.7–17.9\%. The sensitivity of overprediction to climatological entrainment rate in the aquaplanet configured in radiative-convective equilibrium agrees well with the predictions of zero-buoyancy bulk-plume models.

Simulations show storm tracks were weaker during past cold, icy climates relative to the modern climate despite increased surface baroclinicity. Previous work explained the weak storm track using dry zonally-asymmetric mechanisms associated with orographic forcing. Here we show zonally-symmetric changes in the hydrological cycle explain the weak Snowball Earth storm track. According to the moist static energy framework, the weak storm track is connected to decreased latent heat flux (evaporation) and meridional surface moist static energy gradient. The weak storm track can be predicted using the Clausius-Clapeyron relation and a surface ice albedo. 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, and is significantly correlated with the meridional surface moist static energy gradient. Our insights may apply to other climates such as the Last Glacial Maximum.