Séminaire

In this work we made use of historical winds records taken aboard ships to reconstruct a series of the prevalent summer northerly winds (Etesian winds) over the eastern Mediterranean since 1880. Previous studies have shown a significant link between the frequency and strength of these winds and the strength of the Indian Summer Monsoon (ISM) but this relationship had only been studied in detail for the second half of the 20th century due to the lack of wind measures in the eastern Mediterranean for previous periods. Here we present a new climatic index, the so-called “Etesian Wind Index” (EWI), defined as the percentage of days with prevalent northerly wind (wind blowing from 305º to 335º) in a fixed region [20ºE-30ºE, 32ºN-37ºN]. By using this source of historical wind observations, we have been able to compute EWI index since 1880 and analyze the long term variability of the Etesians and its relation with the ISM at an unprecedented temporal coverage.
A running correlation analysis  revealed a strong and significant positive correlation between the EWI and the strength of the ISM for the period 1960-1980, more markedly in July  and August. This result is in accordance with other recent studies. However, we have found that the correlation fades out in the first half of the 20th century (1900-1950) and in the period 1980-2012, even showing  significant negative values around the subperiod 1920-1950.
Similar index to the EWI was computed using the ERA-20C reanalysis. Despite the fact that both indices show some discrepancies before 1950, the correlation analysis with the ISM revealed similar results, pointing out a strong loss of EWI-ISM correlation in the first half of the 20thcentury and from 1980 onwards and a marked positive correlated period between 1960 and 1980, specially in August.
Furthermore, during the period of strong positive correlation between the Etesians and the ISM , the Etesians variability is controlled by two pressure centers, one corresponding to the heat Asian Monsoon low and the other to the low level high pressure center located over central Europe and Western Mediterranean , whereas in a period of non-significant correlation, the central-west Mediterranean  high pressure center plays the main role in the Etesians variability. In addition, composite difference maps ,between extreme ISM years, point out a change in the monsoon dynamics affecting the eastern Mediterranean regarding periods of increase and decrease correlation. The ability of CMIP5 models in reproducing this varying relationship between the Etesians and the ISM, as well as future trends in this teleconnection, is currently being investigated.

benjamin.sultan@locean.upmc.fr