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

Variability in the position and strength of the subtropical jet (STJ) and polar front jet (PFJ) streams has important implications for global and regional climate. Previous studies have related the position and strength of the STJ to tropical thermodynamic processes, whereas the position and strength of the PFJ are more associated with mid-latitude eddies. These conclusions have largely resulted from studies using idealized models. In this study, ERA-Interim reanalysis and CMIP6 global climate models are used to examine month-to-month and interannual variability of the wintertime Northern Hemisphere (NH) STJ and PFJ. This study particularly focuses on the regional characteristics of the jet variability, extending previous studies on zonal-mean jet streams. Consistent with idealized modeling studies, a close relationship is found between tropical outgoing longwave radiation (OLR) and the STJ, and between mid-latitude surface temperature gradients and the PFJ. Variations of both jets are also linked to well-known teleconnection patterns. Variations in tropical convection over the Pacific Ocean are associated with variations of the NH STJ at most longitudes, with different phases of the El Niño-Southern Oscillation (ENSO) associated with the shift and strengthening of the STJ in different regions. CMIP6 models generally capture these relationships, but the models’ tropical convection is often displaced westward when compared to observations, reflecting a climatological bias in OLR in the western tropical Pacific Ocean in many models. The displaced tropical convection in models excites different paths of Rossby wave propagation, resulting in different ENSO teleconnections on the STJ over North America and Europe.

Clouds and their associated radiative effects are a large source of uncertainty in global climate models. One region with particularly large model biases in cloud radiative effects (CRE) is the Southern Ocean. Previous research has shown that there are many dynamic “cloud controlling factors” that influence shortwave CRE, and that three important cloud controlling factors over the Southern Ocean are mid-tropospheric vertical velocity, surface thermal advection, and Estimated Inversion Strength (EIS), which have been shown to influence shortwave CRE on monthly timescales. Model errors may thus arise from biases in representing cloud controlling factors (atmospheric dynamics), in representing how clouds respond to those cloud controlling factors (cloud parameterizations), or some combination thereof. This study extends previous work by examining cloud controlling factors over the Southern Ocean on daily timescales in both observations and global climate models. This allows the cloud controlling factors to be examined in the context of transient weather systems, such as extratropical cyclones, and in the context of high pressure quiescent scenes. Composites of EIS and mid-tropospheric vertical velocity are constructed around extratropical cyclones to examine how the different dynamic cloud controlling factors influence shortwave CRE around the cyclone and how similar the model cyclones are to observations. On average, models tend to produce a realistic cyclone, when compared to observations, in terms of the dynamic cloud controlling factors. The difference between observations and models instead lies in how the models’ shortwave CRE responds to the cyclone dynamics. In particular, the models’ cloud radiative effects are too sensitive to perturbations in mid-tropospheric vertical velocity and, as a result, they tend to produce clouds that are too bright in the cold frontal region of the cyclone and too dim in the center of high pressure systems.