1. 2nd ESF MedCLIVAR workshop, October 8-10, 2007
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. Long-term mean 500-hPa Omega for Jul-Aug
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 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
Long-term mean sea level pressure (hPa), Jul-Aug

8. 850 hPa temperature & wind vectors, Jul-Aug
Lower level cool advection from the Mediterranean 20 24 16 12
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.
Annual 850 hPa temperature in 32.5ºN, 35ºE a. Time series of b. Total STD

11. The lower-level advection dominates the inter-diurnal temp. variations
The pressure gradient Cyprus-Egypt reflects the advection effectiveness
T (K/day)
P (hPa/day)
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. 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
Long-term mean Vertical-zonal Cross-Section for Jul-Aug

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

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)
168h back-trajectories for a typical summer day

17. Isentropic cross-section of wind field (440K): Jul-Aug
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
The EM subsidence is highly correlated (r = -0.63) with ascendance over Mid Asia, with 1 day lag
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. The inter-diurnal variations in horizontal & vertical advections are negatively correlated (- 0.37)
vertical advection
Horizontal advection
Contribution of horizontal & vertical advections to the 850-hPa daily temperature in the EM for Jul-Aug 1989

20. Asian Monsoon strengthens
Proposed mechanism balancing the temperature variations
(Ziv et al. 2004)
Asian Monsoon strengthens
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 EM increases
TEMPERATURE IS BALANCED

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

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
Changes from 1948 to 2002

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

28. Similar pattern, except for a difference in the temperature gradient
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. Horizontal projection
Lower- mid-levels cool advection is weakest in hot days
Horizontal projection
No differences in upper-levels
View from south
Back-trajectories for the groups of the ‘hot’, ‘cool’ and ‘normal’ days

30. No substantial differences in the upper-level temperatures over the EM
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. In both ‘hot’ and ‘cool’ days – negative anomalies is found in the EM,
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. The Persian Trough persists in all of them
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. Enhanced westerly component Reduced westerly component
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. Cool days: Enhanced trough over the EM
700 hPa GPH normal days
700 hPa GPH cool days
Cool days: Enhanced trough over the EM
700 hPa GPH hot days
Hot days: The Subtropical High extends over the EM
This suggests that mid-level dynamics controls lower-level temperature

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

37. Omega profiles for the ‘hot’, ‘cool’ and ‘normal’ days
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. Typical cool event The cool tongue is to the northwest
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. The upper-level trough seems to be the cause for that
GPH 500 hPa, 9/7/95
The upper-level trough seems to be the cause for that -8
500 hPa GPH anomaly

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

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

44. Example for a ‘subtropical’ event
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. Example for a ‘tropical’ event
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. The warm anomaly is over the Levant
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%)
Non-Tropical

49. Upper level humidity (500 hPa)
‘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

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

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
Otherwise, the lower-level advection dominates the inter-diurnal temperature variations
The EM is tele-connected to the Asian Monsoon, South Europe and the eastern African Monsoon (Hadley Circulation)

54. CONCLUDING REMARKS (2)
Extreme events result from upper-level synoptic factors, but the thermal processes are confined to the lower-levels
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:
‘Subtropical’ events: elimination of the cool air source by subsidence
‘Tropical’ events: break of the Etesian winds
‘Baroclinic’ events: increased subsidence