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Hadas Saaroni 1, Baruch Ziv 2, Tzvi Harpaz 1, Eran Beja 1 and Pinhas Alpert 3 1 Dep. of Geography, Tel Aviv University, Israel 2 The Open University of.

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Presentation on theme: "Hadas Saaroni 1, Baruch Ziv 2, Tzvi Harpaz 1, Eran Beja 1 and Pinhas Alpert 3 1 Dep. of Geography, Tel Aviv University, Israel 2 The Open University of."— Presentation transcript:

1 Hadas Saaroni 1, Baruch Ziv 2, Tzvi Harpaz 1, Eran Beja 1 and Pinhas Alpert 3 1 Dep. of Geography, Tel Aviv University, Israel 2 The Open University of Israel 3 Dep. of Geophysics, Tel Aviv University, Israel 2nd ESF MedCLIVAR workshop, October 8-10, 2007 CIRCULATIONS AND MECHANISMS GOVERNING THE SUMMER TEMPERATURE REGIME IN THE EASTERN MEDITERRANEAN

2 OUTLINE Governing synoptic pattern & dynamic factors Circulations & tele-connections Analysis of extreme events

3 MATERIALS Study period: Mid-summer (Jul - Aug) Main Data Source: NCEP-NCAR CDAS-1 archive (Kalnay et al., 1996; Kistler et al., 2001) Air-trajectories: NOAA HYSPLIT4 Model, 1997 Data processing and display: MatLab and GrADS softwares


5 Long-term mean 500-hPa Omega for Jul-Aug Upper-level factor: permanent subsidence NCAR-NCEP CDAS-1 archive

6 Result: minimum moisture over the N. hemisphere Long-term mean Specific Humidity (gK/g) averaged over hPa levels for Jul-Aug

7 Long-term mean sea level pressure (hPa), Jul-Aug The Persian Trough with the NW Etesian winds H L H

8 850 hPa temperature & wind vectors, Jul-Aug Lower level cool advection from the Mediterranean

9 The main dynamic factors: Upper-level subsidence warming Lower–level cool advection cooling

10 Annual 850 hPa temperature in 32.5ºN, 35ºE a. Time series of 1989 b. Total STD a. b. The balance may explain the high persistency in temp.

11 Correlation between p& t Jul-Aug 1989: The pressure gradient Cyprus-Egypt reflects the advection effectiveness The lower-level advection dominates the inter-diurnal temp. variations T ( K/day) P (hPa/day)


13 According to Rodwell & Hoskins (1996): The subsidence over the East Mediterranean owes its existence to the Asian Monsoon No subtropical descent during summer, i.e., no Hadley circulation exists We examine: The impact of the Asian Monsoon on the inter-diurnal variations The existence of the Hadley Cell Signature

14 Long-term mean Vertical- zonal Cross-Section for Jul-Aug Closed circulation connects the EM to the Asian Monsoon, and another circulation – to the west W E

15 Long-term mean Vertical- meridional Cross-Section for Jul-Aug A signature of the Hadley Cell do exists S N

16 168h back-trajectories for a typical summer day The EM is connected to Europe (low-level), the African Monsoon (mid-levels) and Asian Monsoon (higher-levels)

17 Isentropic cross-section of wind field (440K): Jul-Aug A distinct circulation connecting the EM with the Asian Monsoon is well seen

18 Inter-diurnal variation of vertical velocity in the EM (150 hPa, right axis) and Mid-Asia (600 hPa): Jul-Aug 1989 The EM subsidence is highly correlated (r = -0.63) with ascendance over Mid Asia, with 1 day lag EM Mid-Asia

19 vertical advection Horizontal advection The inter-diurnal variations in horizontal & vertical advections are negatively correlated (- 0.37) Contribution of horizontal & vertical advections to the 850-hPa daily temperature in the EM for Jul-Aug 1989

20 Adiabatic warming over The EM increases Advective cooling over EM increases TEMPERATURE IS BALANCED Subsidence in EM increases Etesian winds strengthen Proposed mechanism balancing the temperature variations (Ziv et al. 2004) Pressure over Mid-Asia drops Updraft over Mid-Asia increases Asian Monsoon strengthens

21 Correlation between the vertical air velocity, at 600 hPa – India & at 150 hPa – EM (1 day lag) Jul-Aug The inter-diurnal correlation is not evident! Some reservations concerning the Asian Monsoon – EM tele-connection R=-0.63 (1989)

22 In order to explain the summers with no correlation we intend to: Search for correlations with other locations within the Asian Monsoon Look for competing tele-connections (e.g., to the west, Hadley circulation) Concentrate on long periods with near-normal temperatures


24 The hot tail - heat waves - dominates Hot and cool events according to their duration Hot/cool day definition: Temp. exceeding 1 STD Occurrence of hot and cool events ( )

25 Changes from 1948 to 2002 The hot tail increased during the last decades

26 Characterizing 3 groups of days: (based on 850-hPa Temp. for JA, ) Upper 5% percentile – hot days Lower 5% percentile – cool days Median 5% percentile – normal days

27 The temperature anomalies have synoptic- scale, ~1,500 Km 850 hPa Temp. anomaly normal days 850 hPa Temp. anomaly hot days 850 hPa Temp. anomaly cool days In the normal days the entire MB is normal

28 Similar pattern, except for a difference in the temperature gradient Larger gradient implies more effective cool advection 850 hPa Temp. hot days 850 hPa Temp. cool days 850 hPa Temp. normal days

29 Back-trajectories for the groups of the hot, cool and normal days Lower- mid-levels cool advection is weakest in hot days Horizontal projection View from south No differences in upper-levels

30 No substantial differences in the upper- level temperatures over the EM 500 hPa Temp. hot days 500 hPa Temp. cool days500 hPa Temp. normal days

31 In both hot and cool days – negative anomalies is found in the EM, BUT their locations are different 500 hPa Temp. anomaly hot days 500 hPa Temp. anomaly cool days500 hPa Temp. anomaly normal days

32 The temp. difference is concentrated in the lower 3 Km Temperature profiles for the hot, cool and normal days

33 Hot days: retreat of the Persian Trough deflects the Etesian winds & shortens its path over the sea 925 hPa GPH normal days925 hPa GPH cool days 925 hPa GPH hot days The Persian Trough persists in all of them

34 The difference in cool advection explains the difference in temperature 925 hPa GPH anomaly normal days925 hPa GPH anomaly cool days 925 hPa GPH anomaly hot days + - Enhanced westerly component - + Reduced westerly component

35 700 hPa GPH normal days Cool days: Enhanced trough over the EM 700 hPa GPH cool days700 hPa GPH hot days Hot days: The Subtropical High extends over the EM This suggests that mid-level dynamics controls lower- level temperature

36 The dominant factor is the lower-level cool advection Profiles of dT (day -1 ) imparted by horizontal advection (dashed) & vertical motion (full) for the hot, cool & normal days Contribution of horizontal advectionContribution of vertical motion

37 Surprisingly, the weakest subsidence is in the hot days! Omega profiles for the hot, cool andnormal days This finding deserves further investigation

38 DYNAMIC CLASSIFICATION OF EXTREME EVENTS (Preliminary results) Extreme events reflect breaking of the seasonal prevailing regime, presumably due to an influence of external circulations The events are classified according to the main factor for temperature change

39 COOL EVENTS All of them had common characteristics, somewhat similar to the winter Cyprus Low

40 The cool tongue is to the northwest Wind&Temp. 850 hPa, 9/7/95 Temp. anomaly Typical cool event increased Etesian winds combined with cold surge in the Aegean Sea -8

41 GPH 500 hPa, 9/7/ hPa GPH anomaly The upper-level trough seems to be the cause for that -8

42 HOT EVENTS 1. Subtropical - The subtropical high intensifies and expands 2. Tropical - Northward shift and breaking of the subtropical high enables tropic penetration 3. Baroclinic - A dynamic ridge as a part of Rossby wave H H H L H L

43 Subtropical events: 500 hPa GPH 500 hPa GPH anomaly Intensification and northward expansion of the Subtropical high

44 Warming over Greece and the EM eliminates the northwesterly cool advection from the sea Wind & Temp. 850 hPa - 24/7/ hPa Temp. anomaly +10 Example for a subtropical event

45 Tropical events: 500 hPa GPH 500 hPa GPH anomaly Breaking of the subtropical high enables tropic penetrations by the upper level southerly winds

46 Upper level cyclone in Egypt, producing southerly winds over the Levant 500 hPa GPH 500 hPa GPH anomaly Example for a tropical event 500 hPa GPH 12 Aug 85

47 The Etesian winds veered to easterly, implying continental hot advection The warm anomaly is over the Levant Wind & Temp. 850 hPa - 12/8/ hPa Temp. anomaly +6

48 Tropical events were identified according the 500 hPa relative humidity (>30%) Tropical events Non-Tropical

49 Upper level humidity (500 hPa) Subtropical: 24 Jul 07 Tropical: 13 Aug 85 Baroclinic: 27 Jul 02

50 A dynamic ridge ahead of a pronounced trough over the central Med. induces intense subsidence 500 hPa GPH 500 hPa GPH anomaly Baroclinic events:

51 The non-tropical events are divided to subtropical and baroclinic according to the STD of GPH along 37.5°N (between 10°E- 35°E) baroclinic subtropical

52 Both, the upper level ridge and the lower level temp. anomaly reached the EM from west 850 hPa temp. anomaly Example for a baroclinic event 500 hPa GPH 30 Jul 02

53 CONCLUDING REMARKS (1) Two competing factors dominate the EM: Upper-level subsidence and lower-level cool advection These factors are negatively correlated part of the time, then stabilize the temperature The EM is tele-connected to the Asian Monsoon, South Europe and the eastern African Monsoon (Hadley Circulation) Otherwise, the lower-level advection dominates the inter-diurnal temperature variations

54 CONCLUDING REMARKS (2) Cool events result from cold surges over Greece together with intensified Etesian winds. They are somewhat similar to the winter Cyprus Lows Three scenarios were identified for the development of hot events, according to the main factor that breaks the regional temperature balance: 1.Subtropical events: elimination of the cool air source by subsidence 2.Tropical events: break of the Etesian winds 3.Baroclinic events: increased subsidence Extreme events result from upper-level synoptic factors, but the thermal processes are confined to the lower-levels

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