Unraveling Eastern Mediterranean Daily Maximum / Minimum Temperature's Shifts Linked to Shifting Synoptic Systems' Regimes

The dependence of long-term changes in maximum and minimum temperatures on the synoptic systems’ regimes over the Eastern Mediterranean (EM) region has been studied for the period of 1948/1-2023/6. The method compares monthly time series of TMAX and TMIN averaged over daily data from the Israeli weather stations with monthly percentages of each EM synoptic class.

Step 1. Synoptic systems’ time series. By extending the semi-objective synoptic classification (SC) algorithm, previously developed for daily 12Z and applied in numerous studies over the past two decades, a sub-daily SC was conducted using NCEP/NCAR reanalysis data (at 00Z, 06Z, 12Z and 18Z). The original (at 12Z) definitions of 19 EM synoptic classes comprising 5 major synoptic groups (Cyprus Lows, Red Sea Troughs, Persian Troughs, Sharav Lows and Highs) have been retained. With the combined efforts of regional experts, a separate training set was prepared for each standard time based on sea level pressure charts. The resulting sub-daily SC revealed sharp differences between the daytime and nighttime percentages of synoptic systems, as well as profound differences in their long-term changes.

Step 2. The TMAX, TMIN time series. The daily 2-m data of the maximum and minimum temperatures (TMAX, TMIN) were taken from the database of the Israel Meteorological Service. The data coverage allowed us to combine records for 103 locations. We then filled in the missing daily data at each location based on its most correlated neighbor in each calendar month. Finally, we averaged the completed data into one regional monthly TMAX and one TMIN time series.

Step 3. TMAX, TMIN vs. synoptic systems. TMAX is most often observed at local time of 14:00 in winter and 15:00 in summer (daylight saving time), which corresponds in the EM region to universal time 12Z. Therefore, TMAX was tied to synoptic classes at 12Z. TMIN is most often observed at local time of 5:00 a.m. in winter and 6:00 a.m. in summer (daylight saving time), which corresponds in the EM region to universal time 03Z. This is midway between standard 00Z and 06Z of the NCEP/NCAR reanalysis. To tie TMIN to synoptic classes at 03Z, we estimated percentages for each synoptic class at 03Z as its averaged percentages at 00Z and 06Z.

Main Results. We considered correlations between TMAX, TMIN and synoptic classes for different calendar periods. Among the results, high correlations, both positive and negative, were found between TMAX, TMIN and governing synoptic systems. Strong associations between individual synoptic classes and TMAX, TMIN were revealed by: (i) refining the SC on the 6-hourly timescale; (ii) separate SC definitions for deep and shallow low-pressure synoptic systems; (iii) separate SC definitions for different locations of low-pressure centers (in cyclones) or axes (in troughs) on complex EM topography, consisting of sea and land surfaces and prolonged coastline; (iv) exploring synoptic connections for each calendar month, each standard EM season (winter DJFM, summer JJAS, and spring and autumn in between) and specific groups of months (e.g., DJF, JFM). The results make it possible to provide synoptic explanations for long-term changes in TMAX, TMIN over the Eastern Mediterranean region in 1948-2023, as well as to open up prospects for a long-term forecast of regional TMAX, TMIN based on much more reliable climate forecasts for changes in synoptic systems’ regimes.