DATE2016-05-31 14:55:03
AUTHORSD. Rizou (1), H. Preka (1), H. Flocas (1), M. Hatzaki (2)
  1. National And Kapodistrian University Of Athens, Faculty Of Physics, Dept. Of Environmental Physics And Meteorology Athens (Greece)
  2. National And Kapodistrian University Of Athens, Faculty Of Geology & Geoenvironment, Dept. Of Geography & Climatology Athens (Greece)
ABSTRACTIt is well known that a teleconnection pattern is established between the Indian summer monsoon (ISM) and the eastern Mediterranean climate (Ziv et al., 2004; Tyrlis et al.,2012 Rizou et al., 2015). The excitation of Rossby waves is one of the mechanisms responsible for this type of teleconnection. Rodwell and Hoskins (1996, 2001) showed with the aid of idealized simulations that the strong diabatic heating associated with the rainfall in the ISM region, induces a Gill-type Rossby wave response to the west of the monsoon region, which interacting with the southern flank of the mid-latitude westerlies, produces a region of adiabatic descent over North Africa and the eastern Mediterranean (EM). The objective of this study is to examine the propagation and characteristics of the Rossby wave trains responding to the convection forcing and extending to the eastern Mediterranean during the years of strong and weak activity of ISM on inter-annual and intra-annual basis. Daily NCEP/NCAR reanalysis datasets on a 2.5ºx2.5º grid are employed for the period 1948-2013. The selection of strong and weak years is based on All India Rainfall Index (AIRI). Summer refers to the peak ISM season, namely June, July, August, September (JJAS). The forcing of the Rossby waves is examined with the aid of the Rossby wave source (RWS), which is divided in two terms: the generation of vorticity by divergence (S1) and the advection of absolute vorticity by divergent flow (S2). Monthly composite means and anomalies of RWS, S1 and S2 for strong and weak years are calculated, along with velocity potential, absolute vorticity, zonal wind and divergence at 200 hPa. Furthermore, correlation maps between Rossby waves and AIRI were constructed to identify the relationship pattern of Rossby waves with the monsoon rainfall. It was found that an intense Rossby wave structure forms during the strong ISM years, which becomes more determined in July and August. This train seems to derive from subtropics, where deep convection occurs during Indian summer monsoon and mainly during July and August, when the diabatic source moves northwards over the Bay of Bengal, south of the westerly subtropical jet. The train seems to propagate westwards towards the Middle East and the Mediterranean during specific periods of the strong years.