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:

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:

Plain Language Summary: Midlatitude storms transport water vapor poleward and upward. When ascending, the air cools, causing the vapor to condense, releasing latent heat. The latent heating boosts the ascent in which it occurs and amplifies the storms originally responsible for the heating. This circular chain of events couples latent heating and storms in a nonlinear relationship we call the latent heating feedback. We simulate an atmosphere where latent heating is static and not a consequence of warm, moist air ascending. Comparing this to an atmosphere with realistic latent heating, we show that realistic latent heating leads to more intense storms traveling further poleward, especially west of North America and Europe. Simultaneously, the longitudinally averaged jet streams and storms respond by retracting toward the equator, leaving reduced westerlies and a double jet tendency over North America and Europe. Previous works tend to focus on the effect of latent heating on the average atmospheric state. Our work shows that this effect is only part of the story and that the latent heating effect on storms directly causes regional differences that climate models struggle with.

Abstract:

Plain Language Summary: The Pacific Northwest (PNW) experienced a record‐breaking heatwave during the summer of 2021, resulting in significant adverse effects on both human society and ecosystems. A heavy rainfall band was observed stretching from south China to south of Japan 1 week prior to the heatwave, fueling the debate over whether the monsoon activity contributed to this event. Our study found that while the monsoon activity typically has a cooling effect on the PNW's climate, in this particular year, it had a warming effect and thus contributed to this specific heatwave. This unusual warming effect was driven by a stronger and more northward‐shifted Pacific jet stream, which altered the extratropical response to the monsoon, resulting in an anticyclonic pattern over the PNW instead of the typical cyclonic response seen under average climatic conditions. Therefore, it is important to distinguish between the general question of whether monsoon can influence such events on average, and the specific question of whether it did in any specific case. We argue that when discussing the influence of large‐scale climate drivers on extremes, it is crucial to clearly state whether the focus is on the general potential for influence or on the specific role in a particular event.

Abstract:

Evidence has emerged of a discrepancy in tropical Pacific sea surface temperature (SST) trends over the satellite era, where most coupled climate models struggle to simulate the observed La Niña-like SST trends. Here we highlight wider implications of the tropical Pacific SST trend discrepancy for global circulation trends during boreal winter, using two complementary methods to constrain coupled model SST trends: conditioning near-term climate prediction (hindcast) simulations, and pacemaking coupled climate simulations. The robust circulation trend response to constraining the tropical Pacific SST trend resembles the interannual La Niña response. Constraining tropical Pacific SST robustly reduces tropical tropospheric warming, improving agreement with reanalyses, and moderately shifts the zonal-mean jets poleward. It also improves surface air temperature and precipitation trends in ENSO-sensitive regions, such as the Americas, South Asia, and southern Africa. Our results underline the importance of tropical Pacific SST for achieving confidence in multidecadal model projections.