Dr Matt Patterson

Abstract:

Europe has seen a rapid increase in the frequency and intensity of hot extremes in recent decades. In this study it is shown, using ERA5 reanalysis data 1960–2021, that the hottest summer days in North-West Europe are warming approximately twice as fast as mean summer days. Moreover, this pattern stands out as relatively unusual across the Northern Hemisphere. It is also shown that comprehensive climate models fail to capture this difference in trends. A hypothesis is suggested to explain the differential rate of warming between the mean and hottest days, namely that the hottest days are often linked to warm advection from Iberia and North Africa, areas that are warming faster than North-West Europe. This hypothesis can account for about 25% of the difference between ERA5 and a climate model ensemble and hence further research is needed to understand the drivers of the differing trends in mean and extreme temperature.

Abstract:

Since the 1980s, external forcings from increasing greenhouse gases and declining aerosols have had a large effect on European summer temperatures. These forcings may therefore provide an important source of forecast skill, even for timescales as short as a season ahead. However, the relative importance of external forcings for seasonal forecasts has thus far received little attention, particularly on a regional scale. In this study, we investigate forcing-induced skill by comparing the near-surface temperature skill of a multi-model ensemble of seasonal predictions from the Copernicus Climate Change Service archive to that of an uninitialised ensemble of Coupled Model Intercomparison Project phase 6 projections for European summers (June–July–August) spanning the years 1993–2016. As expected, predictive skill over southern Europe is larger for initialised seasonal predictions compared to uninitialised climate projections. However, for northern Europe, we find that predictive skill is generally small in current seasonal models and surprisingly even smaller compared to uninitialised climate projections. These results imply that further research is necessary to understand the role of external forcing on seasonal temperature variations over Europe.

Abstract:

The East Asian summer jet (EASJ) is an important component of the East Asian summer monsoon system and its variability is correlated with precipitation and surface temperature variations over this region. Whilst many studies have considered the interannual variability of the EASJ, less is known about variations on a decadal time-scale. This study investigates the relationship between decadal EASJ variability and sea surface temperatures (SSTs) and thus the potential predictability that SSTs may provide. Given the relatively short observational record, we make use of the long pre-industrial control simulations in the Coupled Model Intercomparison Project phase 6 (CMIP6) in addition to a large ensemble of atmosphere-only experiments, forced with random SST patterns. We then create an SST-based reconstruction of the dominant modes of EASJ variability in the CMIP6 models, finding a median EASJ–reconstruction correlation for the dominant mode of 0.43. Much of the skill in the reconstruction arises from variations in Pacific SSTs, however the tropical Atlantic also makes a significant contribution. These findings suggest the potential for multi-year predictions of the EASJ, provided that skilful SST forecasts are available.

Abstract:

Stationary wave changes play a significant role in the regional climate change response in Southern Hemisphere (SH) winter. In particular, almost all CMIP5 models feature a substantial strengthening of the westerlies to the south of Australia and enhancement of the subtropical jet over the eastern Pacific in winter. In this study we investigate the mechanisms behind these changes, finding that the stationary wave response can largely be explained via reductions in the magnitude of the upper level Rossby wave source over the tropical / subtropical East Pacific. The Rossby wave source changes in this region are robust across the model ensemble and are strongly correlated with changes to low latitude circulation patterns, in particular, the projected southward migration of the Hadley cell and weakening of the Walker circulation. To confirm our mechanism of future changes, we employ a series of barotropic model experiments in which the barotropic model is given a background state identical to a particular CMIP5 model and an anomalous Rossby wave source is imposed. This simple approach is able to capture the primary features of the ensemble mean change, including the cyclonic anomaly south of Australia, and is also able to capture many of the inter-model differences. These findings will help to advance our understanding of the mechanisms underpinning SH extratropical circulation changes under climate change.