Low (web) Quality - BALTEX

201
Projections of eastern Mediterranean climate change simulated in a
transient RCM experiment with RegCM3
Simon Krichak, Pinhas Alpert and Pavel Kunin
Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv, Israel
1. Introduction
The climate conditions of the eastern Mediterranean (EM
hereafter) are characterized by changeable rainy weather
with moderate temperatures during the cool seasons and dry
and hot weather during summers. The climate conditions fit
the "Mediterranean" type of the Koppen and Geiger's (1936)
classification and are strongly determined by the region's
location in the zone dominated by polar front activity in
winter and that of subtropical high-pressure system during
summer. The EM region’s climate is influenced by the
effects of annual variations in the area of the Asian monsoon
(Bedi et al., 1976; Rodwell and Hoskins, 1996). During the
cool season the EM climate is also affected by specific
synoptic processes associated with interaction of the uppertroposphere
jet stream with the regional terrain over
northern Africa (Krichak et al., 1997). Winters in the EM
are characterized by passing disturbances known as Cyprus
cyclones and between them intrusions of high pressure
systems and polar air masses (Alpert et al. 1990, Saaroni et
al. 1996, Levin and Saaroni, 1999). In the summer the EM is
under the influence of a quasi-stationary system, a Persian
Gulf trough, an extension of the Asian monsoon, and in the
southern parts, and extension of the Azores anticyclone. The
system is characterized by westerly winds and a diurnal
breeze mechanism and is limited to an average height of
about 1000 m. Above it is dominated by the subtropical
anticyclone causing subsidence and almost absolute absence
of rainfall (Bitan and Saaroni, 1992) These conditions are
permanent during mid-June to mid-September. During
intermediate seasons there are several different synoptic
mechanisms acting. Among those are Sharav cyclones and
Red Sea Trough systems. The thermal low pressure systems
known as the north-African (Sharav) cyclone, move fast
along the north-African shore area, especially during spring.
They cause very high temperatures and low humidity when
the EM is ahead of and below the warm sector of cyclone
(Alpert and Ziv, 1989, Levin and Saaroni 1999). The Red
Sea trough (RST) is common in both spring (less typical)
and autumn (Tsvieli and Zangvil 2005; Ziv et al. 2005). The
RST is an extension of the African monsoon trough.
Intensity of the associated with the trough weather processes
is significantly determined by the position of the intertropical
convective zone (ICTZ). During October and April
the ITCZ is located approximately at 15ºN-20ºN (along the
0ºE). The RST is a low-level trough extending from the Red
Sea northward. The air-low in the EM depends on the
location of the trough’s axis (Dayan, 1986). The EM
weather conditions associated with the RST systems are
usually dry. Intense EM precipitation events are typical
however in the cases of development of Active Red Sea
Troughs, with lower level RST accompanied by extending
southward upper-level trough (Kahana et al. 2002).eral
biases in results of simulation of the EM climate have been
noted (Evans et al., 2004; Krichak et al. 2007). Among
those a negative bias in representation of the regional
precipitation was also noted. (Evans et al., 2004; Giorgi et
al., 2004; Krichak et al., 2007).
The analysis was focused on optimization of the climate
simulation results over the region centered over the Israeli
part of the EM region and realization of a transient
regional climate change simulation experiment.
2. Justification of model configuration
Synoptic processes over a large geographic area from the
Atlantic to the east-Europe and Middle East participate in
formation of the EM climate. Intense EM rainy periods are
usually associated with powerful synoptic developments
and cold-air intrusions taking place over a large zone. Ten
climate simulation experiments driven from the lateral
boundaries by the NCAR/NCEP Reanalysis data are
realized to understand the role of model domain
configuration in the RCM over the region. Sensitivity of
the model results over the area to the positioning of the
lateral boundaries of the domain is evaluated. An optimal
configuration of the domain is finally selected.
3. Transient climate change RCM experiment
Climate change trends over the EM region are simulated
in a regional climate simulation experiment with the ICTP
RegCM3 model (Pal et al. 2007) driven from the lateral
boundaries by results of ECHAM5/MPI-OM1 (Roeckner
et al. 2003) transient climate simulation from 1960 to 2060
(SRES A1B emission scenario after 2001). The trends
projected for the southern part of the eastern
Mediterranean region include a 20-30% precipitation drop
during winter (the period with major rains take place) and
its rise during autumn (20-25%) and spring (5%); also
projected is a 1.5-2.0°C mean seasonal 2m air temperature
rise during winter and spring, somewhat lower one (1.0–
1.5°C) in summer and a non-significant (0.3°C)
temperature rise during autumn from 1961-1990 to 2021-
2050. The climate change signal is not uniform over the
region however. Notable mean seasonal precipitation rises
from the current climate conditions are projected over the
southern part of the region during winter and summer,
whereas a drop in summer rains is projected for the northeastern
part of the EM. During winter and summer seasons
the warming process seems to be mainly controlled by the
Middle East effects – leading to maximum air temperature
changes over eastern part of the region. During spring
however the zone with the maximum warming is found
over the northern part of the EM. During autumn the
major zone with the maximum warming up is found over
the Mediterranean Sea area. The transient RCM simulation
experiment projects significant decadal oscillations of the
EM climate parameters during the coming decades
however which may be indicating a time-lagged
contribution of climate change processes over remote
areas. The roles of several possible external effects are
evaluated.