Persistent warm and dry extremes over the eastern Mediterranean during winter: The role of North Atlantic blocking and central Mediterranean cyclones

Abstract

Persistent dry winter events over the eastern Mediterranean (EM) disrupt the rainy (winter) season precipitation patterns and dramatically reduce water availability in the region. Here we objectively identify persistent dry, warm winter events over Israel, and apply a Lagrangian approach to three case-studies, aiming to understand the relation between the synoptic setting, precursor Rossby waves, and how the dry, warm conditions emerge. Self-organizing map classification of atmospheric profile data over Israel shows that the most persistent (at least 5 days) dry and warm winter events are induced by a stagnant upper tropospheric ridge over the EM, a pronounced trough or cut-off low over the western/central Mediterranean and blocking over the North Atlantic. The leading mechanisms of the warm and dry conditions are: adiabatic heating during slantwise subsidence, heating by sensible heat fluxes from the surface, and advection of warm and dry continental air. The relative contributions of the mechanisms and geographical locations of the back trajectories vary greatly both within and among the events. In addition, the Atlantic blocking and EM ridge are supported by upstream diabatic heating in warm conveyor belts of North Atlantic cyclones and Mediterranean cyclones, respectively. A quantitative classification of the back trajectories above the Atlantic and the Mediterranean has shown transitions from adiabatic to diabatic contributions and vice versa along these paths. The sequential relation between Atlantic ridge (or block), trough over Europe and ridge over EM and/or west Russia places local persistent warm and dry extremes into a large-scale context and thus provides new opportunities for understanding their predictability at weather and climate scales.

On the persistence of warm and cold spells in the Northern Hemisphere extratropics: regionalisation, synoptic-scale dynamics, and temperature budget

“…JJA cold spells are associated with enhanced RRWP frequency in Western Europe only (Figure 12-b), but we find a significant link of DJF warm spells to RRWPs in Western North America, the North Atlantic and the Eastern Mediterranean (Figure 12-c). In the Eastern Mediterranean, these RRWPs may develop downstream of the strong Greenland blocking that occurs during warm spells (Berkovic and Raveh-Rubin, 2022). In Western North America and the North Atlantic (region 15 on Figure 3-c, one of the few non spatially-coherent ones), RRWPs may be part of a persistent wave train associated with the so-called "North American winter temperature dipole" (Singh et al, 2016).…”

Weather persistence on sub-seasonal to seasonal timescales: a methodological review

“…The residence time approach can characterise episodic, state, and also global stationarity, and is particularly useful for predictability and risk assessment (De Luca et al, 2019;Francis et al, 2020;Berkovic and Raveh-Rubin, 2022). From the time perspective, large values of 𝑅(𝑡) indicate the most stationary periods.…”

“…From the time perspective, large values of 𝑅(𝑡) indicate the most stationary periods. A minimum threshold is often defined to separate stationary from non-stationary periods: for instance, 2 days for weather regimes (De Luca et al, 2019;Francis et al, 2020) and extreme precipitation (Du et al, 2022), 5-6 days for warm spells (Berkovic and Raveh-Rubin, 2022;Rousi et al, 2022b), 5-25 days for geopotential anomalies (Dole and Gordon, 1983), or 2 seasons for droughts (Ford and Labosier, 2014). In the phase space, the stationarity of any state x 0 can be described from the distribution of its residence times 𝑅 x 0 ∼ P {𝑅(𝑡) s.t.…”

Weather persistence on sub-seasonal to seasonal timescales: a methodological review

“…( 14)-( 15) take an Eulerian perspective; their parallel in the Lagrangian perspective is the concept of "survival time", i.e., the duration of a specific weather system or pattern along its trajectory (Liu et al, 2018;von Lindheim et al, 2021). The residence time approach can characterize episodic, state and global quasi-stationarity and is particularly useful for predictability and risk assessment (De Luca et al, 2019;Francis et al, 2020;Berkovic and Raveh-Rubin, 2022). From the time perspective, large values of R(t) indicate the most quasi-stationary periods.…”