Abstract
Proxy-based hydro-climate reconstructions in the Levant suggest enhanced water availability during the last glacial maximum (LGM) compared to present-day conditions. To date, the governing hypothesis is that additional water availability may be directly linked to increased Cyprus Low frequency and intensity in the region. However, this paradigm has not been tested in a modelling framework. With this aim, we analysed results from a weather type classification algorithm and regional climate simulations. The weather type classification is applied to ERA5 reanalysis data for present-day (1979–2018) and two Paleoclimate Modelling Inter-comparison Project PMIP3/PMIP4 pre-industrial and LGM model runs. Dynamical downscaling of the two models with the regional Weather Research and Forecast model shows that the present hydro-climate can largely be reproduced. Our simulations suggest that both evaporation and precipitation were lower in the LGM compared to pre-industrial conditions, and that their relative changes can thus most likely explain the additional water availability during that time. Indeed, evaporation in the eastern Mediterranean is reduced to a higher degree (∼−33%) as compared to precipitation (∼−20%) during the LGM. Particularly, lower evaporation during LGM summer may have sustained the year-round wetter conditions in the Levant. In addition, we find significant changes in Cyprus Low characteristics for the LGM. The simulated daily precipitation associated with Cyprus Lows is significantly lower than pre-industrial values (reduction of 26%–29%), whereas the wind intensity is stronger (increase of 7%–8%). Finally, a significant increase in Cyprus Low frequency during LGM winter is likely (+22%). Indeed, our findings are in line with a plethora of proxy-based reconstructions, and provide a reinterpretation of the driving mechanism of water availability, i.e. strong changes in evaporation rather than precipitation. This study places projected hydro-climatic drying of the Levant in a long timescale perspective. As such, it improves our understanding of the physical processes influencing the hydrological cycle in this vulnerable region, situated on the border between sub-tropical and mid-latitude climate zones.
Highlights
The past hydro-climate conditions in the Levant attract continued interest
Evaluation of weather research and forecast model (WRF) pre-industrial (PI) simulations As we focus on the hydro-climate, the capabilities of the WRF-PI runs to simulate recent hydroclimate conditions in comparison with ERA5 are first examined
The simulations show a steep gradient of E and P from west to east and north to south, which is indicated in ERA5 data (figures 2(a) and (d))
Summary
The past hydro-climate conditions in the Levant attract continued interest. The reasons for this are two-fold: (a) the Levant is situated in a unique transition zone between mid-latitude temperate climate in the north and sub-tropical arid climate in the south.As such, even small large-scale climatic fluctuations may have a detrimental influence on the region’s hydro-climate (Kushnir et al 2017). (b) The region was one of the major pathways out of Africa during relatively wet periods. Most of the annual precipitation is restricted to a small number of days (Saaroni et al 2010) These rainy events are often associated with a cyclone at surface level and a trough or cutoff low at upper levels, transporting cold air masses from the north over the warm Mediterranean Sea (Alpert and Reisin 1986, Shay-El and Alpert 1991). The surface level trough axis may be tilted towards the Mediterranean Sea, when convoyed by an upper-level ExtraTropical trough covering the Levant. Such conditions favour the development of short-lived heavy precipitation events at south-eastern locations (Krichak et al 1997, 2012). The typical mean sea level pressure distribution for a randomly selected Cyprus Low and Red Sea Trough is shown in figures 1(a) and (b)
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