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Hadas Saaroni 1 , Baruch Ziv 2 , Tzvi Harpaz 1 , Eran Beja 1 and Pinhas Alpert 3

CIRCULATIONS AND MECHANISMS GOVERNING THE SUMMER TEMPERATURE REGIME IN THE EASTERN MEDITERRANEAN. 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

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Hadas Saaroni 1 , Baruch Ziv 2 , Tzvi Harpaz 1 , Eran Beja 1 and Pinhas Alpert 3

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  1. CIRCULATIONS AND MECHANISMS GOVERNING THE SUMMER TEMPERATURE REGIME IN THE EASTERN MEDITERRANEAN Hadas Saaroni1, Baruch Ziv2, Tzvi Harpaz1, Eran Beja1 and Pinhas Alpert3 1Dep. of Geography, Tel Aviv University, Israel 2The Open University of Israel 3Dep. of Geophysics, Tel Aviv University, Israel 2nd ESF MedCLIVAR workshop, October 8-10, 2007

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

  3. MATERIALS Study period: Mid-summer (Jul - Aug) 1948-2006 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

  4. GOVERNING SYNOPTIC PATTERN AND DYNAMIC FACTORS

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

  6. Result: minimum moisture over the N. hemisphere Long-term mean Specific Humidity (gK/g) averaged over 500-300 hPa levels for Jul-Aug

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

  8. 12 16 20 24 Lower level cool advection from the Mediterranean 850 hPa temperature & wind vectors, Jul-Aug

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

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

  11. P (hPa/day) T (K/day) The lower-level advection dominates the inter-diurnal temp. variations The pressure gradient Cyprus-Egypt reflects the advection effectiveness Correlation between p&t Jul-Aug 1989: -0.48

  12. GOVERNING CIRCULATIONS AND TELE-CONNECTIONS

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

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

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

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

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

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

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

  21. R=-0.63 (1989) Some reservations concerning the Asian Monsoon – EM tele-connection The inter-diurnal correlation is not evident! Correlation between the vertical air velocity, at 600 hPa – India & at 150 hPa – EM (1 day lag) Jul-Aug 1948-2004

  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

  23. ANALYSIS OF EXTREME EVENTS

  24. Occurrence of ‘hot’ and ‘cool’ events (1948-2002) The ‘hot’ tail - heat waves - dominates Hot and cool events according to their duration Hot/cool day definition: Temp. exceeding 1 STD

  25. The ‘hot’ tail increased during the last decades 1948-1975 1976-2002 Changes from 1948 to 2002

  26. Characterizing 3 groups of days: (based on 850-hPa Temp. for JA, 1975-2006) Upper 5% percentile – ‘hot days’ Lower 5% percentile– ‘cool days’ Median 5% percentile– ‘normal days’

  27. 850 hPa Temp. anomaly ‘cool’ days -4.8 850 hPa Temp. anomaly ‘hot’ days +5.4 850 hPa Temp. anomaly ‘normal’ days In the normal days the entire MB is ‘normal’ The temperature anomalies have synoptic- scale, ~1,500 Km

  28. 850 hPa Temp. ‘normal’ days 850 hPa Temp. ‘cool’ days 22.6 17.9 850 hPa Temp. ‘hot’ days Similar pattern, except for a difference in the temperature gradient Larger gradient implies more effective cool advection 28.1

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

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

  31. 500 hPa Temp. anomaly ‘normal’ days 500 hPa Temp. anomaly ‘cool’ days +0.2 -0.8 500 hPa Temp. anomaly ‘hot’ days In both ‘hot’ and ‘cool’ days – negative anomalies is found in the EM, BUT their locations are different -1.2

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

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

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

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

  36. Contribution of vertical motion Contribution of horizontal advection 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

  37. Surprisingly, the weakest subsidence is in the hot days! Omega profiles for the ‘hot’, ‘cool’ and ‘normal’ 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. Wind&Temp. 850 hPa, 9/7/95 Typical cool event increased Etesian winds combined with cold surge in the Aegean Sea Temp. anomaly The cool tongue is to the northwest -8

  41. GPH 500 hPa, 9/7/95 The upper-level trough seems to be the cause for that -8 500 hPa GPH anomaly

  42. H H L H L H 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

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

  44. Wind & Temp. 850 hPa - 24/7/07 Example for a ‘subtropical’ event 850 hPa Temp. anomaly +10 Warming over Greece and the EM eliminates the northwesterly cool advection from the sea

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

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

  47. Wind & Temp. 850 hPa - 12/8/85 The Etesian winds veered to easterly, implying continental hot advection 850 hPa Temp. anomaly +6 The warm anomaly is over the Levant

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

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

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

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