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