Connection between Eastern Mediterranean seasonal mean 500 HPA height and sea-level pressure patterns and the spatial rainfall distribution over Israel

This study introduces it new conceptual model to explain the recently observed changes in winter precipitation over Israel. The model is based on our earlier published work (where a connection was reported between the occurrence of major rain days (MRDs) in different parts of the country and three prototypes, A, B and C, of the 500 hPa trough axis orientation prevailing on MRDs) and on additional results obtained by an extension of that work in the present paper. The first part of the present study is devoted to the extension of our early work. Composite techniques have been used on National Center for Atmospheric Research (NCAR)-National Meteorological Center grid-point data for the rain seasons 1981-82 to 1985-86 to identify the sea-level pressure (SLP) distribution associated with each of the three 500 hPa prototypes. Prototype A (trough axis oriented from northwest to southeast, earlier shown to be associated with MRDs in northern Israel) was found in the present work to be associated with a surface low in the vicinity of Antalya, southern Turkey. Prototype B (trough axis oriented from north to south, earlier shown to be associated with MRDs in central Israel) was found in the present work to be associated with a surface low over southeastern Turkey. Prototype C (trough axis oriented from northeast to southwest, earlier shown to be associated with MRDs in southern Israel) was found to be associated with elevated surface pressure over northwestern Turkey and a trough over eastern Turkey. In the second part of the study we used our results to construct a conceptual model of the mechanism responsible for the relative increases in seasonal (winter) rainfall over the southern part of the country and the decrease over the north. Using National Centers for Environmental Prediction-NCAR reanalysis data for the period 1982-2000, we demonstrated that the direct atmospheric agent responsible for this change in the spatial rainfall distribution is an increased frequency of occurrence of 500 hPa troughs oriented from northeast to southwest (Prototype C) accompanied by prominent positive SLP anomalies centred over Turkey. Our analysis further shows that these atmospheric systems are consistent with the persistence of a positive phase of the North Atlantic Oscillation on the one hand and with latest IPCC predictions of precipitation patterns over the eastern Mediterranean basin on the other hand.