Abstract

Abstract. This study focuses on a single Mediterranean hurricane (hereafter medicane), to investigate its response to global warming during the middle of the 21st century and assesses the effects of a warmer ocean and a warmer atmosphere on its development. Our investigation uses the state-of-the-art regional climate model WRF to produce the six-member, multi-physics ensembles. Results show that our model setup simulates a realistic cyclone track and the transition from an initial disturbance to a tropical-like cyclone with a deep warm core. However, the simulated transition occurs earlier than for the observed medicane. The response of the medicane to future climate change is investigated with a pseudo global warming (PGW) approach. This is the first application of the PGW framework to medicanes. The PGW approach adds a climate change delta (defined as difference between future and present climate) to WRF's boundary conditions which is obtained for all prognostic variables using the mean change in an ensemble of CMIP5 simulations. A PGW simulation where the climate change delta is added to all prognostic variables (PGWALL) shows that most of the medicane characteristics moderately intensify, e.g. surface wind speed, uptake of water vapour, and precipitation. However, the minimum sea level pressure (SLP) is almost identical to that under present climate conditions. Two additional PGW simulations were undertaken; One simulation adds the projected change in sea surface and skin temperature only (PGWSST) while the second simulation adds the PGW changes to only atmospheric variables (PGWATMS); i.e. we use present-day sea surface temperatures. These simulations show opposing responses of the medicane. In PGWSST, the medicane is more intense than PGWALL as indicated by lower SLP values, the stronger surface wind, and the more intense evaporation and precipitation. In contrast, the medicane in PGWATMS still transitions into a tropical-like cyclone with a deep warm core, but the PGWATMS medicane weakens considerably (SLP, surface wind, and rainfall decrease). This difference can be explained by an increase in water vapour driven by the warmer ocean surface (favourable for cumulus convection). The warmer and drier atmosphere in PGWATMS tends to inhibit condensation (unfavourable for cumulus convection). The warmer ocean and warmer atmosphere have counteracting effects which leads to only a modest enhancement of the medicane by global warming. The novel approach in this study provides new insights into the different roles of warming of the ocean and atmosphere in medicane development.

Highlights

  • It is well known that severe cyclonic storms occur in the Mediterranean Sea, in particular from September to March (e.g. Cavicchia et al, 2013)

  • The response of the cyclone tracks to climate change seems different between PGWALL/PGWSST and PGW changes to only atmospheric variables (PGWATMS) and we examine the response of other cyclone features in the pseudo global warming (PGW) experiments

  • In this study we investigated the impacts of future climate change on a tropical-like cyclone formed in the Mediterranean Sea in a PGW framework with the Weather Research and Forecasting (WRF) regional climate model

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Summary

Introduction

It is well known that severe cyclonic storms occur in the Mediterranean Sea, in particular from September to March (e.g. Cavicchia et al, 2013). Jin, 1996; Caniaux et al, 2011) Another difference between medicanes and tropical cyclones is that the formation of medicanes is generally triggered by an intrusion of trough-like systems or cut-off lows over the Mediterranean (Fita et al, 2006; Chaboureau et al, 2012; Fita and Flaounas, 2018; Bouin and Lebeaupin Brossier, 2020). While the mechanism of medicanes differs to a large extent from that of tropical cyclones, understanding the response of the medicane features to anthropogenic global climate change is important for mitigating future risks associated with natural hazards. We use a PGW framework to investigate the impacts of global warming on the development and intensity of medicane Rolf (Miglietta et al, 2013; Ricchi et al, 2017; Dafis et al, 2018).

WRF Simulation of Rolf under present climate
WRF simulation of Rolf under warmer climate
Simulation of medicane Rolf under present climate
Concluding remarks
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