Tim Woollings

Abstract:

Atlantic multidecadal variability (AMV) is a well-known mode of climate variability with well-understood impacts on several aspects of Northern Hemisphere climate. However, its impact on heatwaves, a type of heat extremes that is increasingly affecting human societies, remains less well understood. The influence of AMV on extratropical summer heatwaves in the Northern Hemisphere is analyzed with a suite of coupled climate model experiments from the Decadal Climate Prediction Project. Our analysis suggests that AMV exerts substantial influence on the heatwave frequency (HWF) and heatwave number (HWN) of heatwaves over subtropics and midlatitudes in the Northern Hemisphere. Compared to the widespread seasonal mean warming response, these heatwave hotspots are less expansive geographically. The warm AMV phase (AMV+) as opposed to the cold phase (AMV−) drives a global stationary wave anomaly that links hotspots of HWF and HWN increases over North America, North Africa, central/western Asia, and parts of East Asia. Such dynamic impacts of AMV on heatwaves are more significant than the thermodynamic impacts of a warmer ocean surface. Hence, mean surface warming alone due to the warming effects of AMV+ versus AMV− does not necessarily equate to more frequent heatwaves. Furthermore, precipitation and surface heat fluxe responses further amplify the HWF increases. By further comparing the tropical and extratropical portions of AMV imposed in model simulations, we emphasize that linear and nonlinear interactions of these features strongly shape the impacts of AMV. We further discuss the mechanisms for and causes of model-observation discrepancies and inter-model uncertainties in the influence of AMV on atmospheric circulation and summer heatwaves, in terms of atmospheric circulation response in North Atlantic-Europe and jet waveguide effects. This highlights some challenges in pinpointing the influence of AMV on heatwaves, and improved understanding of it is necessary for more accurate predictions and projections of heatwaves.

Abstract:

Abstract Greenland atmospheric blocking, a persistent anticyclonic pattern, strongly influences local and regional weather and climate. It is known to significantly exacerbate Greenland ice sheet melt and mass loss in summer as well as influence atmospheric conditions over the North Atlantic. Greenland blocking has been observed to increase in intensity since the summer of 2000s, but this trend has partly reversed after 2012. This decadal variability is highly correlated with the negative phase of the North Atlantic Oscillation (NAO), the dominant pattern of climate variability in the North Atlantic. However, summer NAO shows different temporal variation in June in comparison with later summer months, i.e., July and August. In this study, we analyse the individual summer months in turn to evaluate differences between their respective spatial patterns of Greenland blocking events. We use different approaches including a Self-Organising Map (SOM) to evaluate individual blocking days, and an event-based analysis to assess the development of blocking events over the course of 7 days. The results show that spatial patterns of Greenland blocking are similar between July and August but are distinctly different in June. In particular, Greenland blocking in June is strongly related to cyclonic wave breaking over the eastern Atlantic. Our analysis using wave activity flux of the zonally varying mean flow reveals a distinct pattern of wave energy and pseudo-momentum associated with cyclonic wave breaking prior to Greenland blocking high anomalies in June, in contrast to the other two summer months. This might partially explain the difference in the spatial patterns and evolution of blocking in June compared with July and August.

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.