عنوان مقاله [English]
The teleconnection pattern (EA /WR) plays an important role in the fall and winter weather of Europe and Southwest Asia (SWA). The purpose of the present research is to find out how critical (strong) phases of this teleconnection influences atmospheric circulation and consequently affects weather of EurAsia by investigation of eddy energy fluxes and different conversion terms in the tendency of eddy kinetic energy (EKE) equation. First, by applying the monthly indices of EA/WR which are taken from the Climate Prediction Center (CPC), we derived 37 critical positive months (CPM) and 38 critical negative months (CNM). Then by using NCEP / NCAR reanalysis data of the years 1950-2014 for the 4-month of November to February (NDJF), we computed the ensemble mean (averaging over CPM and CNM separately) and anomalies of different meteorological quantities with respect to long mean (65-year period). The energetic terms which have been investigated include baroclinic conversion (BCC), barotropic conversion (BTC), conversion of total energy flux (CTF), ageostrophic flux (CAF) and baroclinic generation (BCG). The ensemble mean and vertical average of the energetic terms in a domain of 90W to 90E and 20N to 80N were computed.
The first part of the paper is devoted to dynamic analysis of the ensemble mean and anomalies of the meteorological quantities in the critical phases of EA/WR. In the second part, an energetic approach is employed to study the effects of the wave train anomalies on the North Atlantic and Mediterranean storm tracks along with its effects on the SWA. As the subtropical cyclonic activities in the latter region is so much dependent on the strength and position of subtropical jet stream, the wind speed at 250 hPa level as well as BTC of energy between EKE and the mean kinetic energy have a key role in this topic.
The results obtained indicate that EKE of the Eastern Mediterranean is not significantly different in the two phases, but in the negative (positive) phase of EA/WR there is a marked increase (decrease) of EKE in the south of Europe and the west of Mediterranean regions. Also, in the north and east of Europe and in the west of Russia in the negative (positive) phase the EKE decreases (increases). As a result, the south-east (north-west) pathway of the North Atlantic storm track to the south (north) of Europe is strengthened in the negative (positive) phase, which can spread to the southwest of Russia (east of Mediterranean). So the connection of the Mediterranean storm track to the Atlantic storm track is stronger in CNM compared to CPM.
In the both phases, in the middle and lower troposphere a see-saw anomaly pattern was observed between the northwest and the southeast of the Mediterranean Sea. Anomalous atmospheric circulations of SWA and the Eastern Mediterranean Sea are similar to that in the center of West Russia. In the positive (negative) phase of the EA/WR, the cyclonic (anticyclonic) circulation in the Middle East increases (decreases) the BCG and BCC as well as heat and humidity fluxes and intensifies (weakens) the subtropical jet. These features would result in the possibility of strengthening (weakening) of cyclonic activity in the Eastern Mediterranean.
In addition to the above characteristics, in the CPM, the formation of strong total energy flux divergence in the east of Mediterranean could act as a source of energy for downstream propagating waves and therefore enhance the activity of Eastern Mediterranean storm track towards Iran. In Iran, despite of the existence of intense jet in the southern part, BTC anomaly is negative, while in the north of Red Sea (the southern flank of the jet) it is positive which may be a result of less cyclone passage towards the south of the jet and more dissipation in comparison to the long mean in the northern flank of the jet.
In the CNP, although based on the analysis of energitc terms the connection of the Eastern Mediterranean with the storm track of the North Atlantic is well established, but the formation of anticyclonic circulations in the SWA results in weakening of the activity, the passage of the cyclones and thereby the possibility of "downstream development" in the SWA. In this phase, the divergence of the ageostrophic flux (negative CAF) and increase in BCG (generation of the eddy available potential energy) can make the Mediterranean center as a source of eddy energy for the northeast of Africa and the Eastern Mediterranean. The dynamic anomalous circulations of the EA/WR pattern and EKE diagnostics confirm that in the positive phase, in contrast to the negative phase, the stretch of the Eastern Mediterranean storm track through the Middle-East is more active.