Assaf Hochman | The Hebrew University of Jerusalem (original) (raw)
Papers by Assaf Hochman
Bulletin of the American Meteorological Society
Scientific Reports
Humanity is close to characterizing the atmospheres of rocky exoplanets due to the advent of JWST... more Humanity is close to characterizing the atmospheres of rocky exoplanets due to the advent of JWST. These astronomical observations motivate us to understand exoplanetary atmospheres to constrain habitability. We study the influence greenhouse gas supplement has on the atmosphere of TRAPPIST-1e, an Earth-like exoplanet, and Earth itself by analyzing ExoCAM and CMIP6 model simulations. We find an analogous relationship between CO2 supplement and amplified warming at non-irradiated regions (night side and polar)—such spatial heterogeneity results in significant global circulation changes. A dynamical systems framework provides additional insight into the vertical dynamics of the atmospheres. Indeed, we demonstrate that adding CO2 increases temporal stability near the surface and decreases stability at low pressures. Although Earth and TRAPPIST-1e take entirely different climate states, they share the relative response between climate dynamics and greenhouse gas supplements.
Science of The Total Environment
International Journal of Climatology
The Astrophysical Journal
The atmospheres of rocky exoplanets are close to being characterized by astronomical observations... more The atmospheres of rocky exoplanets are close to being characterized by astronomical observations, in part due to the commissioning of the JWST. These observations compel us to understand exoplanetary atmospheres, in the voyage to find habitable planets. With this aim, we investigate the effect that CO2 partial pressure (pCO2) has on exoplanets’ climate variability, by analyzing results from ExoCAM model simulations of the tidally locked TRAPPIST-1e exoplanet, an Earth-like aqua-planet, and Earth itself. First, we relate the differences between the planets to their elementary parameters. Then, we compare the sensitivity of the Earth analog and TRAPPIST-1e’s surface temperature and precipitation to pCO2. Our simulations suggest that the climatology and extremes of TRAPPIST-1e’s temperature are ∼1.5 times more sensitive to pCO2 relative to Earth. The precipitation sensitivity strongly depends on the specific region analyzed. Indeed, the precipitation near mid-latitude and equatorial s...
Earth System Dynamics
Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on ... more Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, "Sharav" Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and subtropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).
Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on ... more Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, "Sharav" Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and subtropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).
Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on ... more Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, "Sharav" Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and subtropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).
Environmental Research Letters
Proxy-based hydro-climatic reconstructions over the Levant suggest enhanced water availability du... more Proxy-based hydro-climatic reconstructions over 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 over the region. However, this paradigm has not been tested in a modelling framework. With this aim, we analyzed 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 PMIP3/PMIP4 pre-industrial and LGM model runs. Dynamical downscaling of the two models with the regional WRF 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 wa...
International Journal of Climatology
Global climate models provide only partial information on local-scale phenomenon, such as precipi... more Global climate models provide only partial information on local-scale phenomenon, such as precipitation, primarily due to their coarse resolution. In this study, statistical downscaling algorithms, based on both weather regimes and past analogues, are operated for 18 Israeli rain gauges with an altitude ranging between −200 and~1,000 m above sea level (ASL). To project seasonal precipitation over Israel and its hydrologic basins, the algorithms are applied to six Coupled Model
Science Advances
The atmosphere is a chaotic system displaying recurrent large-scale configurations. Recent develo... more The atmosphere is a chaotic system displaying recurrent large-scale configurations. Recent developments in dynamical systems theory allow us to describe these configurations in terms of the local dimension—a proxy for the active number of degrees of freedom—and persistence in phase space, which can be interpreted as persistence in time. These properties provide information on the intrinsic predictability of an atmospheric state. Here, this technique is applied to atmospheric configurations in the eastern Mediterranean, grouped into synoptic classifications (SCs). It is shown that local dimension and persistence, derived from reanalysis and CMIP5 models’ daily sea-level pressure fields, can serve as an extremely informative qualitative method for evaluating the predictability of the different SCs. These metrics, combined with the SC transitional probability approach, may be a valuable complement to operational weather forecasts and effective tools for climate model evaluation. This n...
Theoretical and Applied Climatology
The Eastern Mediterranean resides on the border between the temperate and semi-arid and arid clim... more The Eastern Mediterranean resides on the border between the temperate and semi-arid and arid climate zones, and is thus influenced by both mid-latitude and sub-tropical weather systems. Precipitation and extreme weather in this region are mainly associated with either Cyprus Lows or the “wet” Red Sea Troughs. Current regional climate projections indicate that the region may become warmer and drier in future decades. Here, we analyze the influence of enhanced greenhouse gas forcing on the climatological properties of the ‘wet’ and ‘dry’ Red Sea Trough (WRST & DRST, respectively). With this aim, a regional synoptic classification and a downscaling algorithm based on past analogs are applied to eighteen rain stations over the main ground water basins in Israel. The algorithms are applied to the NCEP/NCAR reanalysis data for 1986–2005 and to eight CMIP5 model simulations for the historical (1986–2005) and end of the century (2081–2100) climate conditions according to the RCP8.5 scenario...
Proxy-based hydro-climatic reconstructions over the Levant suggest enhanced water availability du... more Proxy-based hydro-climatic reconstructions over 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 over the region. However, this paradigm has not been tested in a modelling framework. With this aim, we analyzed 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 PMIP3/PMIP4 pre-industrial and LGM model runs. Dynamical downscaling of the two models with the regional WRF 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 subtropical and mid-latitude climatic zones.
Point by point response to Reviewer 1: Reviewer 1: In this paper, the authors employ an approach ... more Point by point response to Reviewer 1: Reviewer 1: In this paper, the authors employ an approach from dynamical systems theory to quantify the (intrinsic) predictability of atmospheric states based on reanalysis data during cold and hot extremes over the Eastern Mediterranean. This is complemented with GEFS reforecasts, which are used to infer forecast uncertainty, or practical predictability. While the distinction and investigation of practical and intrinsic predictability is not new (e.g. Melhauser & Zhang, 2012), I am not aware of any comparable publications in the context of heatwaves. In addition, a simple Lagrangian model is used to reveal the origin of near-surface air masses during hot and cold extreme events. The resulting paper is nicely structured, not too lengthy and certainly an interesting read. I only have two minor comments and a few additional comments, questions and suggestions, as the manuscript is well written and understandable. Response: Thank you for the posit...
The Continuous Wavelet Transform (CWT) is a frequently used tool to study periodicity in climate ... more The Continuous Wavelet Transform (CWT) is a frequently used tool to study periodicity in climate and other time series. Periodicity plays a significant role in climate reconstruction and prediction. In numerous studies, the use of CWT revealed Dominant Periodicity (DP) in climatic time series. Several studies suggested that these "natural oscillations" would even reverse global warming. It is shown here that the results of wavelet analysis for detecting DPs can be miss-interpreted in the presence of local singularities that are manifested in lower frequencies. This may lead to false DPs detection. In CWT analysis of synthetic and real-data climatic time series, with local singularities, CWT indicates on a low frequency DP even if there is no true periodicity in the time series. It is argued that this is an inherent general property of CWT. Hence, applying CWT to climatic time series should be re-evaluated and more careful analysis of the entire wavelet power spectrum is required, focusing on high frequencies as well. Thus, a cone-like shape in the wavelet power spectrum most likely indicates the presence of a local singularity in the time series rather than a DP, even if the local singularity has an observational or a physical basis. It is shown that analyzing the derivatives of the time series may be helpful in interpreting the wavelet power spectrum. Nevertheless, this is only a partial remedy that does not completely neutralize the effects caused by the presence of local singularities.
Climate Dynamics
Here, we analyze future projections of cyclone activity in the Mediterranean region at the end of... more Here, we analyze future projections of cyclone activity in the Mediterranean region at the end of the twenty-first century based on an ensemble of state-of-the-art fully-coupled Regional Climate System Models (RCSMs) from the Med-CORDEX initiative under the Representative Concentration Pathway (RCP) 8.5. Despite some noticeable biases, all the RCSMs capture spatial patterns and cyclone activity key characteristics in the region and thus all of them can be considered as plausible representations of the future evolution of Mediterranean cyclones. In general, the RCSMs show at the end of the twenty-first century a decrease in the number and an overall weakening of cyclones moving across the Mediterranean. Five out of seven RCSMs simulate also a decrease of the mean size of the systems. Moreover, in agreement with what already observed in CMIP5 projections for the area, the models suggest an increase in the Central part of the Mediterranean region and a decrease in the South-eastern par...
Bulletin of the American Meteorological Society
Scientific Reports
Humanity is close to characterizing the atmospheres of rocky exoplanets due to the advent of JWST... more Humanity is close to characterizing the atmospheres of rocky exoplanets due to the advent of JWST. These astronomical observations motivate us to understand exoplanetary atmospheres to constrain habitability. We study the influence greenhouse gas supplement has on the atmosphere of TRAPPIST-1e, an Earth-like exoplanet, and Earth itself by analyzing ExoCAM and CMIP6 model simulations. We find an analogous relationship between CO2 supplement and amplified warming at non-irradiated regions (night side and polar)—such spatial heterogeneity results in significant global circulation changes. A dynamical systems framework provides additional insight into the vertical dynamics of the atmospheres. Indeed, we demonstrate that adding CO2 increases temporal stability near the surface and decreases stability at low pressures. Although Earth and TRAPPIST-1e take entirely different climate states, they share the relative response between climate dynamics and greenhouse gas supplements.
Science of The Total Environment
International Journal of Climatology
The Astrophysical Journal
The atmospheres of rocky exoplanets are close to being characterized by astronomical observations... more The atmospheres of rocky exoplanets are close to being characterized by astronomical observations, in part due to the commissioning of the JWST. These observations compel us to understand exoplanetary atmospheres, in the voyage to find habitable planets. With this aim, we investigate the effect that CO2 partial pressure (pCO2) has on exoplanets’ climate variability, by analyzing results from ExoCAM model simulations of the tidally locked TRAPPIST-1e exoplanet, an Earth-like aqua-planet, and Earth itself. First, we relate the differences between the planets to their elementary parameters. Then, we compare the sensitivity of the Earth analog and TRAPPIST-1e’s surface temperature and precipitation to pCO2. Our simulations suggest that the climatology and extremes of TRAPPIST-1e’s temperature are ∼1.5 times more sensitive to pCO2 relative to Earth. The precipitation sensitivity strongly depends on the specific region analyzed. Indeed, the precipitation near mid-latitude and equatorial s...
Earth System Dynamics
Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on ... more Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, "Sharav" Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and subtropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).
Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on ... more Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, "Sharav" Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and subtropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).
Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on ... more Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, "Sharav" Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and subtropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).
Environmental Research Letters
Proxy-based hydro-climatic reconstructions over the Levant suggest enhanced water availability du... more Proxy-based hydro-climatic reconstructions over 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 over the region. However, this paradigm has not been tested in a modelling framework. With this aim, we analyzed 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 PMIP3/PMIP4 pre-industrial and LGM model runs. Dynamical downscaling of the two models with the regional WRF 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 wa...
International Journal of Climatology
Global climate models provide only partial information on local-scale phenomenon, such as precipi... more Global climate models provide only partial information on local-scale phenomenon, such as precipitation, primarily due to their coarse resolution. In this study, statistical downscaling algorithms, based on both weather regimes and past analogues, are operated for 18 Israeli rain gauges with an altitude ranging between −200 and~1,000 m above sea level (ASL). To project seasonal precipitation over Israel and its hydrologic basins, the algorithms are applied to six Coupled Model
Science Advances
The atmosphere is a chaotic system displaying recurrent large-scale configurations. Recent develo... more The atmosphere is a chaotic system displaying recurrent large-scale configurations. Recent developments in dynamical systems theory allow us to describe these configurations in terms of the local dimension—a proxy for the active number of degrees of freedom—and persistence in phase space, which can be interpreted as persistence in time. These properties provide information on the intrinsic predictability of an atmospheric state. Here, this technique is applied to atmospheric configurations in the eastern Mediterranean, grouped into synoptic classifications (SCs). It is shown that local dimension and persistence, derived from reanalysis and CMIP5 models’ daily sea-level pressure fields, can serve as an extremely informative qualitative method for evaluating the predictability of the different SCs. These metrics, combined with the SC transitional probability approach, may be a valuable complement to operational weather forecasts and effective tools for climate model evaluation. This n...
Theoretical and Applied Climatology
The Eastern Mediterranean resides on the border between the temperate and semi-arid and arid clim... more The Eastern Mediterranean resides on the border between the temperate and semi-arid and arid climate zones, and is thus influenced by both mid-latitude and sub-tropical weather systems. Precipitation and extreme weather in this region are mainly associated with either Cyprus Lows or the “wet” Red Sea Troughs. Current regional climate projections indicate that the region may become warmer and drier in future decades. Here, we analyze the influence of enhanced greenhouse gas forcing on the climatological properties of the ‘wet’ and ‘dry’ Red Sea Trough (WRST & DRST, respectively). With this aim, a regional synoptic classification and a downscaling algorithm based on past analogs are applied to eighteen rain stations over the main ground water basins in Israel. The algorithms are applied to the NCEP/NCAR reanalysis data for 1986–2005 and to eight CMIP5 model simulations for the historical (1986–2005) and end of the century (2081–2100) climate conditions according to the RCP8.5 scenario...
Proxy-based hydro-climatic reconstructions over the Levant suggest enhanced water availability du... more Proxy-based hydro-climatic reconstructions over 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 over the region. However, this paradigm has not been tested in a modelling framework. With this aim, we analyzed 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 PMIP3/PMIP4 pre-industrial and LGM model runs. Dynamical downscaling of the two models with the regional WRF 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 subtropical and mid-latitude climatic zones.
Point by point response to Reviewer 1: Reviewer 1: In this paper, the authors employ an approach ... more Point by point response to Reviewer 1: Reviewer 1: In this paper, the authors employ an approach from dynamical systems theory to quantify the (intrinsic) predictability of atmospheric states based on reanalysis data during cold and hot extremes over the Eastern Mediterranean. This is complemented with GEFS reforecasts, which are used to infer forecast uncertainty, or practical predictability. While the distinction and investigation of practical and intrinsic predictability is not new (e.g. Melhauser & Zhang, 2012), I am not aware of any comparable publications in the context of heatwaves. In addition, a simple Lagrangian model is used to reveal the origin of near-surface air masses during hot and cold extreme events. The resulting paper is nicely structured, not too lengthy and certainly an interesting read. I only have two minor comments and a few additional comments, questions and suggestions, as the manuscript is well written and understandable. Response: Thank you for the posit...
The Continuous Wavelet Transform (CWT) is a frequently used tool to study periodicity in climate ... more The Continuous Wavelet Transform (CWT) is a frequently used tool to study periodicity in climate and other time series. Periodicity plays a significant role in climate reconstruction and prediction. In numerous studies, the use of CWT revealed Dominant Periodicity (DP) in climatic time series. Several studies suggested that these "natural oscillations" would even reverse global warming. It is shown here that the results of wavelet analysis for detecting DPs can be miss-interpreted in the presence of local singularities that are manifested in lower frequencies. This may lead to false DPs detection. In CWT analysis of synthetic and real-data climatic time series, with local singularities, CWT indicates on a low frequency DP even if there is no true periodicity in the time series. It is argued that this is an inherent general property of CWT. Hence, applying CWT to climatic time series should be re-evaluated and more careful analysis of the entire wavelet power spectrum is required, focusing on high frequencies as well. Thus, a cone-like shape in the wavelet power spectrum most likely indicates the presence of a local singularity in the time series rather than a DP, even if the local singularity has an observational or a physical basis. It is shown that analyzing the derivatives of the time series may be helpful in interpreting the wavelet power spectrum. Nevertheless, this is only a partial remedy that does not completely neutralize the effects caused by the presence of local singularities.
Climate Dynamics
Here, we analyze future projections of cyclone activity in the Mediterranean region at the end of... more Here, we analyze future projections of cyclone activity in the Mediterranean region at the end of the twenty-first century based on an ensemble of state-of-the-art fully-coupled Regional Climate System Models (RCSMs) from the Med-CORDEX initiative under the Representative Concentration Pathway (RCP) 8.5. Despite some noticeable biases, all the RCSMs capture spatial patterns and cyclone activity key characteristics in the region and thus all of them can be considered as plausible representations of the future evolution of Mediterranean cyclones. In general, the RCSMs show at the end of the twenty-first century a decrease in the number and an overall weakening of cyclones moving across the Mediterranean. Five out of seven RCSMs simulate also a decrease of the mean size of the systems. Moreover, in agreement with what already observed in CMIP5 projections for the area, the models suggest an increase in the Central part of the Mediterranean region and a decrease in the South-eastern par...