Abstract
European heat waves result from large-scale stationary waves and have major impacts on the economy and mortality. However, the dynamical processes leading to and maintaining heat waves are still not well understood. Here we use a nonlinear stationary wave model (NSWM) to examine the role played by anomalous stationary waves and how they are forced during heat waves. For our study, we use the Japanese Reanalysis (JRA-55) data for the period 1958 through 2017. We show that the NSWM can successfully reproduce the main features of the observed anomalous stationary waves in the upper troposphere. Our results indicate that the dynamics of heat waves are nonlinear, and transient momentum fluxes are the primary drivers of the observed anomalous stationary waves. The contribution from orographic forcing is moderate and mainly through nonlinear interactions with diabatic heating. Further decomposition of the transients indicates that the high-frequency transient vorticity fluxes make dominant contributions. Furthermore, our results reveal that the response to heating located in the tropical Indian Ocean and the west Pacific region is primarily responsible for maintaining the observed anomalous stationary waves linked to European heat waves. This is confirmed by exploring the relationship between heat waves and the Indian Ocean Dipole strength. The heating in the mid-latitude and tropical Atlantic region plays a secondary role. Our results suggest that European heat waves are potentially predictable by considering the nonlinear effects involved in anomalous stationary waves and the heating sources in the nearby and remote tropical region.
Highlights
Mid-latitude heat waves are expected to become more frequent and intense due to anthropogenic global warming (Perkins 2015; Kornhuber et al 2019)
To explore the links between European heat waves and large-scale teleconnection patterns and corresponding sea surface temperature anomalies, the daily North Atlantic Oscillation (NAO) index, and the monthly index representing the strength of the Indian Ocean Dipole (IOD) and the NINO3.4 index which describes El Niño and La Niña events are obtained from the National Center for Environmental Prediction/Climate Prediction Center https://www.cpc.ncep.noaa.gov/
While previous studies indicated that the North Atlantic mid-latitude sea surface temperature forcing is important for European heat waves (Cassou et al 2005; Ghosh et al 2017, 2019), here we demonstrate that IOD and tropical heating in west Pacific are responsible for longlived European heat waves (38 cases at least with a threshold of 5 days)
Summary
Mid-latitude heat waves are expected to become more frequent and intense due to anthropogenic global warming (Perkins 2015; Kornhuber et al 2019). Over Europe, persistent blocking anti-cyclones are part of slow-moving planetary waves or quasi-stationary Rossby waves (Kornhuber et al 2017) These quasi-stationary waves are the primary drivers of the mid-latitude weather variations on weekly or monthly timescales, and their activities further impact on the position of the jet streams and act as guides for transient perturbations such as storms and blocking events (Sellevold et al 2016; Held et al 2002). There is no consensus regarding the locations where heating can force persistent blocking systems over Europe Building on these findings, our study aims to investigate the dynamical mechanism of maintaining anomalous stationary waves and identify the key region of tropical heating related to European heat waves.
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