Climate dynamics

Abstract:

The North Atlantic sector has been identified as a region where the 11‐year solar cycle has small but potentially non‐negligible impacts on winter climate, but a debate persists about the robustness of such impacts. This work explores the signatures of the 11‐year solar cycle over the North Atlantic in the ERA5 and 20th Century Reanalysis datasets. The results confirm previous studies with a robust positive boreal winter response in mean‐sea‐level pressure (mslp) in the region of the Azores at lags of three years after solar maximum. The spatial evolution of the response is examined in detail by first decomposing the mslp time series into the dominant modes of North Atlantic winter mslp variability, including the North Atlantic Oscillation (NAO), the East Atlantic (EA) and the Scandinavian patterns, before performing a multilinear regression analysis. We find that the maximum 11‐year solar response in the December–January–February (DJF) average does not project directly onto the NAO. However, when the early/late‐winter responses are examined separately, a statistically significant NAO response is seen in late winter (January–February) at lag 0–1 years and a statistically significant NAO response is also seen at lag +3 years in early winter (November–December). These results are consistent with predicted responses from previously proposed top‐down influences from the stratosphere in late winter followed by the re‐emergence of a signal from underlying sea surface temperatures in early winter. However, the NAO response is not the primary contributor to the total DJF response at lag +3 years. A previously unidentified solar‐cycle response in the EA pattern is found in late winter at lag +3 years with larger amplitude than the NAO response. The evolution of the DJF mslp response over the Azores region can thus be understood as a summation of the NAO and EA patterns at lag +3 years.

Abstract:

When decomposing the atmospheric flow into a basic state and perturbations, the perturbations are generally interpreted as the contribution from chaotic non-linear weather. We explore the link between day-to-day weather and the climatological zonal mean perspective on zonal momentum in more detail by systematically linking eddy momentum fluxes to weather events. Specifically, we first decompose the full momentum flux divergence into contributions from mean flow and perturbations both in the time and zonal direction as well as their combinations, and then systematically relate synoptic jets, cyclones, and Rossby wave breaking events to the instantaneous momentum fluxes. We thus construct a step-by-step link between the time-zonal mean perspective on momentum flux convergence and the synoptic perspective.With this approach, we show that both the time and zonal averaging are a residual of a large compensation of momentum flux convergence and divergence. In both dimensions, the mean must be regarded as a residual that is at least an order of magnitude smaller than the original signal. Further, a large fraction of eddy momentum flux convergence and divergence occurs in association with weather features, with synoptic jets alone accounting for 60-80% of the convergence from the subtropics throughout the mid-latitudes. Rossby wave breaking, on the other hand, only features less than 30% of the momentum flux convergence in the midlatitudes.Finally, the attribution of the full-field momentum flux convergence is nearly indistinguishable from the attribution of eddy-momentum flux convergence, irrespective of whether the eddies are defined as perturbations in time, zonal direction, or the combination of both. The effect of stationary waves to the momentum fluxes is thus implicitly included in the selected transient weather events.

Abstract:

Sea surface temperature (SST) trends over the satellite era show a pronounced cooling over the tropical south-eastern Pacific and enhanced warming over the West Pacific warm pool. By contrast, climate models tend to warm across all longitudes in the tropical Pacific. What does this discrepancy mean for climate model trends outside the tropical Pacific? Does capturing the observed pattern of tropical Pacific SST warming help to resolve other trend discrepancies in models? We use two complementary methods to constrain boreal winter SST trends in coupled models: pacemaker experiments, and conditioned near-term climate predictions (hindcasts). We find that the global response to constraining tropical Pacific SST trends resembles the interannual La Niña response. The Pacific SST trend explains 33-39% of the poleward zonal-mean jet shift seen in the models, and is associated with robustly reduced tropical tropospheric warming trends consistent with reanalyses. It also improves surface air temperature and precipitation trends in ENSO-sensitive regions, such as the South Asia, southern Africa, and the Americas. Our results highlight the importance of resolving discrepancies in the tropical Pacific for building confidence in climate model trends globally.

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.