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Hadas Saaroni1, Baruch Ziv2, Tzvi Harpaz1, Eran Beja1 and Pinhas Alpert3
1Dep. of Geography, Tel Aviv University, Israel2The Open University of Israel3Dep. 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
OUTLINE
• Governing synoptic pattern & dynamic factors
• Circulations & tele-connections
• Analysis of extreme events
MATERIALS
Study period: Mid-summer (Jul - Aug) 1948-2006Main Data Source: NCEP-NCAR CDAS-1 archive
(Kalnay et al., 1996; Kistler et al., 2001)Air-trajectories: NOAA HYSPLIT4 Model, 1997Data processing and display:
MatLab and GrADS softwares
GOVERNING SYNOPTIC PATTERN
AND DYNAMIC FACTORS
Long-term mean 500-hPa Omega for Jul-Aug
Upper-level factor: permanent subsidence
NCAR-NCEP CDAS-1 archive
Result: minimum moisture over the N. hemisphere
Long-term mean Specific Humidity (gK/g) averaged over 500-300 hPa levels for Jul-Aug
Long-term mean sea level pressure (hPa), Jul-Aug
The Persian Trough with the NW Etesian winds
H
L
H
850 hPa temperature & wind vectors, Jul-Aug
Lower level cool advection from the Mediterranean
2024
16
12
The main dynamic factors:
Upper-level subsidence warming
Lower–level cool advection cooling
Annual 850 hPa temperature in 32.5ºN, 35ºE a. Time series of 1989 b. Total STD
0123456
Jan FebMar Apr MayJun Jul Aug Sep Oct Nov Dec
a.
b.
The balance may explain the high persistency in temp.
Correlation between p&t
Jul-Aug 1989: -0.48
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)
GOVERNING CIRCULATIONS
AND TELE-CONNECTIONS
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
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-meridional Cross-Section for
Jul-Aug
A signature of the Hadley Cell do existsS N
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)
Isentropic cross-section of wind field (440K): Jul-
Aug
A distinct circulation connecting the EM with the Asian Monsoon is well seen
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 - 1989
EM
Mid-Asia
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
Adiabatic warming overThe 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 dropsUpdraft over Mid-Asia increases
Asian Monsoon strengthens
Correlation between the vertical air velocity, at 600 hPa – India & at 150 hPa – EM (1 day lag)
Jul-Aug 1948-2004
The inter-diurnal correlation is not evident!
Some reservations concerning the Asian Monsoon – EM tele-
connection
R=-0.63 (1989)
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
ANALYSIS OF EXTREME
EVENTS
0
10
20
30
40
50
60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Length of Event (No. of days)
No. of events
Hot Cool
The ‘hot’ tail - heat waves - dominates
Hot and cool events according to their durationHot/cool day definition: Temp. exceeding 1 STD
Occurrence of ‘hot’ and ‘cool’ events (1948-2002)
1976-2002
1948-1975
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Length (No. of days)
No. of spells
Cold Hot
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Length (No. of days)
No. of spells
Cold Hot
Changes from 1948 to 2002
The ‘hot’ tail increased during the last decades
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’
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
-4.8
+5.4
In the normal days the entire MB is
‘normal’
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
17.9
28.1
22.6
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
No substantial differences in the
upper-level temperatures over
the EM
500 hPa Temp. ‘hot’ days
500 hPa Temp. ‘cool’ days
500 hPa Temp. ‘normal’ days
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’ days
500 hPa Temp. anomaly ‘normal’ days
-1.2
-0.8+0.2
The temp. difference is concentrated in the lower 3 Km
Temperature profiles for the ‘hot’, ‘cool’ and ‘normal’ days
Hot days: retreat of the Persian Trough deflects the Etesian winds & shortens its path over the sea
925 hPa GPH normal days
925 hPa GPH cool days
925 hPa GPH hot daysThe Persian Trough persists in all of
them
The difference in cool advection explains the difference in temperature
925 hPa GPH anomaly normal days
925 hPa GPH anomaly cool days
925 hPa GPH anomaly hot days
+
-
Enhanced westerly component
-
+Reduced westerly
component
700 hPa GPH normal days
Cool days: Enhanced trough over the EM
700 hPa GPH cool days
700 hPa GPH hot days
Hot days: The Subtropical High
extends over the EMThis suggests that mid-level dynamics controls lower-level
temperature
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
Surprisingly, the weakest subsidence is in the hot days!
Omega profiles for the ‘hot’,
‘cool’ and ‘normal’ days
This finding deserves further investigation
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
COOL EVENTS
All of them had common characteristics, somewhat similar
to the winter ‘Cyprus Low’
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
GPH 500 hPa, 9/7/95
500 hPa GPH anomaly
The upper-level trough seems to be the cause
for that
-8
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 HL
H
L
‘Subtropical’ events:
500 hPa GPH
500 hPa GPH anomaly
Intensification and northward expansion of the Subtropical
high
Warming over Greece and the EM eliminates the
northwesterly cool advection from the
sea
Wind & Temp. 850 hPa - 24/7/07
850 hPa Temp. anomaly
+10
Example for a ‘subtropical’
event
‘Tropical’ events:
500 hPa GPH 500 hPa GPH anomaly
Breaking of the subtropical high enables tropic penetrations by the upper level southerly winds
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
The Etesian winds veered to
easterly, implying continental hot
advection
The warm anomaly is over
the Levant
Wind & Temp. 850 hPa - 12/8/85
850 hPa Temp. anomaly
+6
‘Tropical’ events were identified according the 500 hPa relative
humidity (>30%)
‘Tropical’ events
Non-Tropical
Upper level humidity (500 hPa)
‘Subtropical’: 24 Jul 07
‘Tropical’: 13 Aug 85
‘Baroclinic’: 27 Jul 02
A dynamic ridge ahead of a pronounced
trough over the central Med.
induces intense subsidence
500 hPa GPH
500 hPa GPH anomaly
‘Baroclinic’ events:
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’
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
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
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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