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

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

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

GOVERNING SYNOPTIC PATTERN AND DYNAMIC FACTORS

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

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 Long-term mean sea level pressure (hPa), Jul-Aug

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

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

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

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

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-zonal Cross-Section for Jul-Aug

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

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

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

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 - 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

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

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

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 1948-2004

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

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

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

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’

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

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

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

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

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

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

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

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 -

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

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

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

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’

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

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

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

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

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

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

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

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

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

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

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

‘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’

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

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)

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