Tim Woollings

Abstract:

Extratropical cyclones are expected to be more diabatically driven in a warmer world, in line with the 6-7% increase in precipitable water per degree of global-mean surface temperature increase. This leads to a preferential strengthening of the most intense cyclones in a warmer climate as a result of increased latent heating (LH), accompanied by a decrease in the strength of weaker cyclones. In this study, using data from new CESM model experiments, and employing a storm-centric potential vorticity (PV) budget, we estimate the contribution of LH to storm intensification across height and storm lifecycle. We use an objective algorithm to track the cyclones, and a suitable storm-compositing method to compute the spatial and temporal patterns of PV generated from diabatic and adiabatic processes. To isolate the intensification of storms due to PV generation from other processes like storm propagation, we develop a novel storm-averaging methodology.  Using this methodology, we investigate how the magnitude and pattern of PV produced from LH are modified when the sea surface temperature is uniformly increased by 4K. Focusing on the strongest cyclones in the southern hemisphere, we show that the increase in low-level PV generated in cyclones in the warmer model run can be almost entirely attributed to changes in the strength and pattern of LH. By also comparing winter and summer cyclones in our model runs, we obtain a consistent pattern of how the LH contribution to cyclone intensification changes from a cooler to a warmer environment. Finally, we show that our methodology also works well for cyclones in reanalysis data (MERRA2). Given the socio-economic impacts of severe storms, this study provides valuable insights into the processes that govern cyclone intensification, and how they are expected to change in a warmer world. We also quantify the increase in cyclone strength with warming, which can support policymakers in anticipating and mitigating the effects of these events.

Abstract:

All Mediterranean-type climate regions have experienced recent wintertime precipitation declines, contributing to severe droughts in many cases. Understanding whether these declines are driven primarily by changes in large-scale circulation, atmospheric moisture, or submonthly weather systems is critical for interpreting past trends and anticipating future hydroclimate risk. We use constructed circulation analogues together with a Reynolds-decomposition moisture budget to diagnose the respective roles of dynamic circulation change, thermodynamic humidity change, and submonthly eddy activity in driving these wintertime precipitation trends.We apply both approaches to observations and reanalyses, multiple large climate model ensembles, and a preindustrial control simulation to understand how these processes regulate moisture convergence and precipitation variability across Mediterranean-type climate regions. Circulation analogue results indicate that observed wintertime precipitation declines are predominantly dynamically driven. However, the thermodynamic drying inferred from the analogue method is stronger than that simulated by large ensembles in all Mediterranean-type regions. Moisture budget diagnostics additionally highlight a substantial contribution from submonthly eddy trends in some locations.By directly comparing the two frameworks, we highlight that estimates of dynamic and thermodynamic trends can depend strongly on the diagnostic method used. In particular, dynamically driven moisture anomalies and changes in submonthly variability can contaminate thermodynamic estimates derived from both approaches. Using the large ensembles, we show that thermodynamic trends inferred from the two methods can even differ in sign. These results underscore the importance of combining multiple diagnostic methods to more robustly quantify the influence of large-scale circulation and humidity changes on regional precipitation decline.

Abstract:

While a range of processes have been linked to uncertainty in tropical precipitation minus evaporation (P–E) and circulation changes, growing evidence links cloud-radiative changes to inter-model spread. Radiation-locking studies further demonstrate strong sensitivities of circulation and P–E to cloud-radiative changes in aquaplanet models; however, the physical mechanisms linking CO2-driven cloud-radiative changes to tropical circulation and P–E responses remain poorly understood. Here, we use the radiation-locking technique to elucidate these mechanisms in a climate model configured with realistic continents, sea ice, and a seasonal cycle, with the ocean represented by a slab ocean model with prescribed climatological q-fluxes. We introduce a novel analytical framework in which the P–E response is analysed as a function of climatological P–E, enabling direct comparison with thermodynamic scaling arguments.Despite inducing weak surface warming, CO2-driven cloud-radiative changes substantially modify the tropical hydrological response, driving a robust wet-gets-drier, dry-gets-wetter P–E pattern that opposes the canonical wet-gets-wetter, dry-gets-drier signal associated with climate warming. Moisture and moist static energy budget analyses show that this response is driven by a weakening of the tropical overturning circulation associated with enhanced upper-tropospheric cloud-radiative heating. Sea surface temperature pattern changes induce additional P–E responses, including a poleward shift of precipitation maxima over the Indian and western Pacific Oceans. Our results demonstrate that circulation changes strongly shape tropical P–E responses to cloud-radiative changes, and that the balance between dynamic and thermodynamic responses may be a key control on inter-model spread. We further highlight the coupling between cloud-radiative heating and latent heat release as critical for the resulting circulation response.

Abstract:

Midlatitude storms transport warm and moist air poleward and upward, releasing latent heat. Latent heating is thus organized by thecirculation but then modifies temperature gradients and winds, constituting a nonlinear feedback. We define the latent heating feedbackas the effects that arise from latent heating being coupled with the circulation. Because of its nonlinearity, the climatic effects of thisfeedback are difficult to isolate and remain poorly understood.By decoupling latent heating from the circulation in an atmospheric general circulation model, we show that the latent heating feedbackenhances storm track eddy diffusivity, modifying eddy heat fluxes beyond changes in mean baroclinicity. Simultaneously, tracked stormsoccur at lower latitudes, intensify more, and propagate further poleward, while the subtropical jet strengthens as coupled latent heatingpreserves lower latitude baroclinicity. The feedback response supports the idea that diabatic effects cause the “too zonal, tooequatorward” storm track biases in climate models.Finally, we extend the analysis to climate change experiments where we isolate the contribution from the latent heating feedback onstorm intensity and eddy kinetic energy as the world warms. The feedback is most important in summer where it accounts for most of thechanges in eddy kinetic energy. In winter, the feedback is constrained. Isolating the latent heatingfeedback helps to quantify how storminess changes as the atmosphere warms, which climate models currently struggle with.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>In contrast to global warming trends, much of Eurasia experienced a winter cooling trend over 1990–2014. Some studies have proposed a causal link between this regional cooling, particularly strong over Siberia, to coincident reductions in Arctic sea-ice extent. However, free-running historical climate models overwhelmingly simulate a forced Eurasian warming signal, leading other studies to suggest that internal variability explains the observed cooling. Here, we use retrospective seasonal climate predictions to highlight a robust dynamical link between Siberian cooling and upstream north-east Atlantic atmospheric circulation changes. Examining the interannual predictability of these circulation patterns, we find spuriously weak but skilful model signals. When these weak dynamical signals are corrected, stronger low-frequency variability in downstream Siberian temperature also emerges, with half of the observed 1990–2014 cooling simulated. Our results suggest that Eurasian decadal climate variability is at least partly driven by a predictable atmospheric circulation response to slowly evolving boundary conditions.</jats:p>