Meteorol Atmos Phys 94, 219–233 (2006)
DOI 10.1007/s00703-006-0190-y
Department of Meteorology and Climatology, Aristotelian University of Thessaloniki, Greece
The role of the subtropical jet stream during heat wave
events over north-central Greece
D. P. Brikas, T. S. Karacostas, P. J. Pennas, and A. A. Flocas
With 10 Figures
Received August 29, 2005; revised October 17, 2005; accepted January 12, 2006
Published online: July 31, 2006 # Springer-Verlag 2006
Summary
The role of the subtropical jet stream (SJ) in the occurrence
of heat waves in South Balkans and Greece is sought here.
For this purpose E CM W F grid-point data is examined,
concerning the Balkan heat wave of 5–9 July 1988, that
cost human lifes, at least in Greece.
For the city of Thessaloniki, Greece, a temperature budget
is presented, as a function of time. It turns out that the most
important heating mechanism is the adiabatic heating. Horizontal mass convergence at the maximum wind level
(200 hPa) causes descent and adiabatic heating. The convergence occurs in association with the Hadley Cell, as well as
with the right exit quadrant of an anticyclonically curved
subtropical jet streak. As air parcels that exit the above jet
streak slow down and turn anticyclonically, a strong ageostrophic wind current is established towards and to the right
of the flow direction. This ageostrophic current converges
above the northeastern Balkans. Downward ageostrophic
motion emerges from the above area of horizontal convergence and heads towards the S S W, affecting the Balkans.
From the above case study, it is concluded that intense
heat waves are favoured in the South Balkans and Greece
when the SJ is anticyclonically curved to the north of the
Balkans and a jet streak is situated to the north west of the
Balkans.
1. Introduction
The heat wave, an extreme summer weather feature, is defined as an extraordinarily hot and usually
humid period lasting for some days (Huschke,
1959). Heat waves are harmful to all living
creatures, as well as humans, especially when
they are accompanied by atmospheric pollution.
Therefore many authors have studied heat waves
worldwide (Brugge, 1991; Kunkel et al, 1996;
Campetella and Rasticucci, 1998), as well as in
Greece (Karacostas et al, 1996; Metaxas and
Kallos, 1979; Prezerakos, 1989; Repapis, 1975;
Spyrou, 2001).
The heat wave of 5–9 July 1988 (Balafoutis
and Giles, 1990; Spyrou, 2001) was the second
most severe one in Thessaloniki during the
period 1961–1990 (Gawith et al, 1999). The
Temperature-Humidity Index (T H I ) indeed exceeded the threshold (84%), above which work
efficiency rapidly reduces. It is shown here that
the heat wave under study did not affect with
high temperatures only Greece, but also the rest
of the Balkans and Southern Italy. For all the
above reasons this heat wave is worth to be
further studied in depth.
The aims of the study are: (a) to discuss quantitatively the thermodynamics and dynamics
of the heat wave episode of 5–9 July 1988 and
(b) to investigate all the physical – mainly dynamical – processes, related to the subtropical jet
stream (SJ), that can contribute to such heat wave
episodes. The study period extends from one day
before the heat wave began (July 4) to one day
after it ended (July 10). As a first picture of the
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D. P. Brikas et al
heat wave, its extent and intensity, together with
the positions of the SJ, are given in the second
section. In the third section the synoptic characteristics of the heat wave are given at the levels
of 700 and 500 hPa. In the fourth section, temperature and vorticity balances are performed at
700 hPa, in order to find out what are the most
important heating and vertical air motion mechanisms. Vertical meridional cross sections of the
atmosphere are presented in the fifth section,
showing the Hadley cell (HC) and the SJ during
the study period. The ageostrophic (AG) winds
and the mass divergence in the SJ, as well as the
vertical circulation below it, are studied in the
sixth section. In the seventh section, the findings of this paper are discussed and the conclusions are given. The data used comes from the
European Center for Medium-range Weather
Forecasts (E C M W F ). The horizontal resolution is
1.12 1.12 and there are 14 levels in the vertical, from 1000 to 70 hPa. The temporal resolution
of the data is 6-hourly. The data assimilation
scheme is described in E C M W F (1992).
2. The heat wave and the subtropical jet
The geographical extent and the intensity of the
heat wave in terms of temperature, together with
the positions of the SJ, are given in Fig. 1. On
July 4 1988 the 1000 hPa temperature exceeds
35 C only in the South Balkans. In agreement
with these not extremely high, for this time of
the year, temperatures, the SJ is absent from the
longitude of Greece, being located above the
Fig. 1. 1000 hPa temperature greater than 35 C, shaded every 3 C. Vectors
show 200 hPa winds stronger than 30 m=s
The role of the SJ during heat wave events over north-central Greece
Western Mediterranean. Two branches of the SJ
blow there: a westerly one at 35 N and a southwesterly one at 47 N. The split of the SJ is indicative of atmospheric blocking in the Central
Mediterranean just before the onset of the heat
wave episode.
On the 5th of July temperatures rise suddenly in
Southern Italy and especially the South Balkans,
touching 41 C in Greece. At the same time the
south branch of the SJ disappears, whereas the
one in the north propagates towards the NE, possessing now an anticyclonic curvature and reaching 49 N. It is known that as the SJ shifts to the
north, the HC and tropical air masses follow the
SJ. The greatest extent of the heat wave event
occurs on the 6th of July, when temperatures
above 35 C cover almost the whole of the Balkans,
intruding even in Ukrania. In agreement with
these so far north extending high temperatures,
the SJ reaches its maximum latitude, 52 N, on
the 6th of July.
From the next day, July 7, the SJ begins to
retreat southwards and the heat wave is limited
in the South Balkans. While moving southwards,
the SJ strengthens and the jet streak moves along
the flow. As the jet streak approaches Greece, temperatures rise in most Greek regions on July 7
and 8, exceeding 44 C. The highest wind speed
of the SJ (60 m=s) occurs on the 7th of July in
Central Europe. On the 8th of July the axis of the
SJ is located at 48 N.
The intensity of the heat wave has already
decreased significantly in Greece on the 9th of
July, temperatures not exceeding 40 C anywhere.
The heat wave ends in the following day, July 10,
when the temperature is below 35 C nearly
everywhere and the SJ blows along its climatological latitude, 37 N. The SJ is slightly cyclonically curved, while a polar jet (PJ) streak
approaches from the NW. In the following days
the SJ keeps on moving to the south.
3. Description of the synoptic situation
3.1 Evolution at 700 hPa
The level of 700 hPa is chosen for the synoptic
and thermodynamic study of the heat wave. On
one hand side 700 hPa lies on the upper limit of
the boundary layer during the summer and, on
the other hand side, 700 hPa is far from topo-
221
graphic anomalies. Temperature (dotted lines)
and winds on 700 hPa can be seen in Fig. 2.
Areas where the horizontal temperature advection exceeds 30 C=100 h, are increasingly shaded
and the shading changes every 30 C=100 h.
Warm air advection (WA A ) is surrounded by
continuous and cold air advection (CAA ) by
dashed contours.
Strong WA A from the SW occurs in the
Central Mediterranean and the NW Balkans on
July 4 and 5. As a result, the temperature rises by
about 2 C in the Balkans from 4 to 5 July. It
is likely that the strong southwesterly air current
advected towards the Mediterranean part of
the Saharan Air Layer (SAL , Karyampudi and
Carlson, 1988), which is characterised by high
values of temperature and low values of vorticity.
It can indeed be seen from the wind arrows that
the North African anticyclone expands and shifts
to the Central Mediterranean and towards the
Balkans. On the 6th of July this anticyclone is
clearly centred above the Balkans extending a
ridge to Ukrania. This Balkan anticyclone has
a warm structure. The wind arrows are indeed
nearly tangential to the isotherms. The strong
southwesterly current that blows along the NW
flank of the anticyclone does not advect significantly warm air masses, as the thermal ridge is
nearly coincident with the flow ridge. The south
and east parts of Greece are affected by northeasterlies, that blow along the SE flank of the
Balkan anticyclone. In spite of the absence of
significant WA A , the temperature rises by another
2 C from 5 to 6 July in the Balkan area.
On the 7th of July the anticyclone starts retreating towards the SW, in association with the
approach of a baroclinic wave. In the wind field
this wave is visible as a veering (backing) of the
winds with time in the NW (SE) Balkans. In the
thermal field the wave is visible as a southward
propagation of the 0 C isotherm, which is therefore bold in Fig. 2. In association with the temperature drop in the NW Balkans, the cyclonic
wind shear increases there. In spite of the atmospheric wave approaching, on the 7th and 8th of
July the anticyclonic wind shear increases in
Greece, due to the approach of the SJ from the
north (see also Fig. 1). This increase of the anticyclonic shear is in agreement with a temperature
rise that took place on 7 and 8 July in most of the
Greek regions. It is interesting that this tempera-
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D. P. Brikas et al
Fig. 2. 700 hPa temperature (dotted) plotted every 1 C and labelled every 2 C. Vectors show winds at the same level.
Temperature advection absolutely greater than 30 C=100 h is increasingly shaded every 30 C=100 h. Continuous (dashed)
contours denote WA A (C A A )
The role of the SJ during heat wave events over north-central Greece
223
Fig. 3. 500 hPa relative vorticity plotted every 2 105 s1 . Bold continuous (dotted) contours denote cyclonic (anticyclonic)
circulation. Vectors show winds at the same level. Shaded areas correspond to absolute vorticity advection absolutely greater
than 2 109 s2 . Vorticity advection is plotted every 109 s2 and the continuous (dashed) contours denote P VA (N VA )
224
D. P. Brikas et al
ture rise occurs while C A A from the north takes
place, especially on the 8th of July. Note that
temperature advection weaker than 3 C=10 h
can be detected only by inference by the angle
between the wind vectors and the isotherms.
On July 9, the last day of the heat wave event,
the anticyclone weakens above Greece, too, where
the temperature drops significantly. On the 10th
of July the circulation has become slightly
cyclonic. The negative temperatures observed at
700 hPa on July 10 are associated with the appearance of the PJ near there.
3.2 Evolution at 500 hPa
The heat wave is characterised by anticyclogenesis at nearly all levels. For brevity only the anticyclogenesis at 500 hPa will be discussed here. This
is described in Fig. 3, where the vorticity (heavy
continuous and dotted contours), its advection
(shading) and the winds at 500 hPa have been
plotted. During 4–7 July the vorticity is anticyclonic in the study region, with values ranging from
2 to 6 105 s1 and the strongest anticyclonic circulation prevailing above the Balkans, to the
north of Greece. This makes sense, as the Balkans
are located directly under the southern flank of
the SJ (see Figs. 1 and 3) during this period. On
July 8 and 9 strong anticyclonic vorticity values
up to 8 105 s1 are observed above Greece,
as the jet approaches its northern border. This
increase of the anticyclonic vorticity in Greece
is not due to a significant negative vorticity advection (N VA ), as the shading in Fig. 3 reveals.
As of July 8, positive vorticity advection (PVA )
commences in Northern Greece covering nearly
the whole of the country until the 10th of July.
Cyclonic circulation prevails in the biggest part
of Greece on the 10th of July. The southward
retreat of the anticyclone is associated with the
end of the heat wave in Greece, as it emerges
from the comparison between Figs. 1 and 3.
It is interesting that despite the P VA on July 8
and 9, the anticyclonic circulation persists in
Greece during this time period. This means that
not even the sign of the vorticity budget can
be determined by the vorticity advection alone,
which stresses the need to examine the rest of the
terms of the vorticity equation. The vorticity budget is presented in the next section, together with
a temperature budget.
4. Thermodynamic characteristics
The extremely high temperatures observed in
Greece during 5–9 July were not due to WA A in
the biggest part of the country, not the anticyclogenesis accompanying the heat wave was due to
N VA , either. The above stress the need to study
the temperature and vorticity balances during the
period of interest. As Thessaloniki was severely
affected by the heat wave (Karacostas et al, 1999),
the above balances are calculated for the nearest
grid-point (22.5 E, 40.9 N) to Thessaloniki. The
results for 700 hPa are shown as a function of
time in Fig. 4. In the top panel (a) there is a
graph of the daily mean values of the terms of
the thermodynamic equation (Holton, 1992):
@
@ d
¼ Urp !
þ
;
ð1Þ
@t
@p
dt
Temperature Adiabatic Diabatic
advection
heating heating
where stands for the potential temperature,
U ¼ (u, v), the horizontal wind vector and ! for
the vertical velocity, omega.
As seen from Fig. 4a, the hottest days in
Thessaloniki were July 7 and 8, referred to as the
period of the highest intensity of the heat wave.
The period of the establishment of the heat wave
was 4–6 July, when the temperature increased by
2 C=day, to reach the highest levels. Finally,
the period of the decay of the heat wave was
9–10 July, when the temperature dropped suddenly.
WA A is weak (5 K=100 h) during the period
of the establishment of the heat wave, as has also
been seen in Fig. 2. The most important form
of heating from 5 July onwards is the adiabatic
one, becoming the sole one during the period of
the highest intensity of the heat wave, as C A A
begins then. The adiabatic heating manages to
keep the 700 hPa temperature at very high levels
(13–14 C) during the period of the highest
intensity. During the decay of the heat wave, despite the continuous – however much reduced –
adiabatic heating, the temperature drops suddenly
by 8 C, as there is strong C AA ( 35 K=100 h).
As far as the diabatic heating is concerned, this
is the response of the lowest levels to the atmospheric processes. On the 4th of July there is
strong diabatic heating (15 K=100 h) by the
ground, in association with intense mixing, realised
as intense upward motion and adiabatic cooling
( 12 K=100 h) in the boundary layer. During
The role of the SJ during heat wave events over north-central Greece
225
Fig. 4. Graphs of the terms of equation (top), the vertical velocity, !, (middle) and the terms of
the vorticity equation (bottom).
Top (a): temperature is in C,
whereas the terms of equation
are in K=100 h. Middle (b): Positive (negative) values of omega
correspond to downward (upward) motion. Bottom (c): the
relative vorticity is in s1 along
the left-hand side vertical axis
and the terms of the vorticity
equation are in s2 along the
right-hand side vertical axis.
More details in text
5–9 July there is diabatic cooling in the boundary
layer, because potentially warmer air masses are
advected either from the W–SW (WAA ), or from
higher levels (adiabatic heating). When, on the
10th of July, the adiabatic heating ceases, the
diabatic heating switches back on.
To summarise the temperature budget, the
establishment of the heat wave was due partly
to WA A , but mainly to adiabatic heating. The
achievement of the highest intensity of the heat
wave is attributed to adiabatic heating only, while
the decay of the heat wave was due to strong
CAA.
In order to explain the most important form of
heating, the adiabatic one, the vertical velocity,
!, must be studied. In Fig. 4b, we can see the
700 hPa vertical velocity at Thessaloniki, as a
function of time, for the same time period as that
in the graph of Fig. 4a. Positive (negative) values
correspond to downward (upward) motion. In
agreement with the graph of the adiabatic heating
in Fig. 4a, downward motions ranging from 2 to
7 hPa=h prevail during 5–9 July. As expected, the
changes of ! with time are, in general, in agreement with the changes of the adiabatic heating,
with subsidence (upward motions) being associated with adiabatic heating (cooling).
It is very likely that the strong subsidence,
observed during the heat wave, had its origin in
the upper troposphere. Evidence supporting this
226
D. P. Brikas et al
Fig. 5. Height–time Hovmoller diagram of the temperature departure from the climatology every 2 C (a), its advection (c),
the diabatic (b) and adiabatic heating (d) and the vertical motion ! (e) at Thessaloniki. Data is plotted every 6 hours. Date is
along the x-axis. The temperature departure is the difference between the temperature and the 1979–1998 (exc. 1994)
temperature climatology for 1–10 July. Positive (negative) departures are surrounded by solid (dotted) contours. Departures
greater than 4 C are increasingly shaded. The temperature advection and the adiabatic heating are plotted every 5 K=100 h,
whereas the diabatic heating every 10 K=100 h. Continuous (dashed) contours denote positive (negative) values. A horizontal
line is drawn along 700 hPa. ! is in hPa=h, with values absolutely greater than 1 hPa=h shaded increasingly every 1 hPa=h.
Positive values of ! denote subsidence and negative ones upward motion
can be seen in the height-time Hovmoller diagrams of Fig. 5. In the Hovmoller diagram of
the temperature departure above Thessaloniki
(left top panel), the heat wave is clearly visible
during the study period, as a warm departure
through the whole depth of the troposphere, that
reaches values greater than 10 C from 5 to 8 July
around 800 hPa. The anticyclone that prevailed
above Thessaloniki during the study period is
by 2–9 C warmer than normally in the troposphere and by 2–6 C colder than normally in
the stratosphere.
The temperature has a very well defined diurnal
cycle, which depends on the diurnal cycle of the
diabatic heating, which, in turn, depends on the
diurnal cycle of the solar radiation (figure not
shown). There is diabatic heating (cooling) by the
ground during the day (night), which maximizes
just before 12Z (00Z). This diurnal cycle is transmitted upwards with a lag. Also, the diurnal
cycle of the temperature lags the diurnal cycle
of the diabatic heating by some hours in the
low levels, this lag tending to zero upwards. Thus
the hottest (coolest) time of the day is 15Z (00Z).
The role of the SJ during heat wave events over north-central Greece
In the biggest part of the troposphere adiabatic
heating (Fig. 5d) by subsidence (Fig. 5e) takes
place, in order to compensate for the radiative
diabatic cooling occurring under clear skies. As
Rodwell and Hoskins (1996) have found for the
region of the Eastern Mediterranean, the adiabatic heating is in turn compensated for by
C A A (Fig. 5c). Because of the intense diabatic
heating by the ground, strong upward motions
have to develop, in order to mix the potential
temperature () in the vertical. This is the reason
why strong upward motions and adiabatic cooling prevail in the boundary layer (1000–700 hPa)
during most of the study period, as can be seen in
Fig. 5e and d, respectively. This boundary-layer
process is called ‘‘dry convection’’ and, in contrast with the moist (deep) convection is not associated with the formation of either rain or clouds
or latent heat release, as the ascending air mass is
too dry for condensation to occur.
It is evident from Fig. 5e that from 5 to 9 July –
and especially from 7 July – the free tropospheric
subsidence is forced to penetrate low, into the
boundary layer. At 700 hPa the downward motion
attains its maximum value, 8 hPa=h, at 12Z July 7
(Fig. 5e). The fact that the temperature rise
observed on July 7 (1.5 C) was due only to the
adiabatic heating, renders the descent observed on
this day very important from the thermodynamic
point of view. The subsidence continues to penetrate into the boundary layer the following days,
but its value at 700 hPa decreases.
In Fig. 4c (bottom graph), we can see the vorticity equation terms at Thessaloniki, at 500 hPa.
The vorticity equation used is
@
¼ Urp ð þ f Þ ð þ f ÞrU
@t
Horizontal
advection
!
Vortex
stretching
@ @u @! @v @!
þ
;
@p @p @y @p @x
|fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl}
Vertical
advection
ð2Þ
Vortex tilting
where stands for the relative vorticity.
Comparing the changes of the vertical velocity
with time (Fig. 4b) with those of the vorticity
(bold continuous line in Fig. 4c), it can be seen
that, from 5 to 9 July, the more anticyclonic
the circulation is, the stronger the subsidence is.
227
On the other hand, when on July 9 the anticyclonic circulation weakens and turns into cyclonic
on July 10, the subsidence weakens, too, turning
into upward motion on July 10.
The horizontal vorticity advection, expressed
by the first term on the right-hand side (r.h.s.)
of Eq. (2), (thin continuous line in Fig. 4c) is
negative from 4 to 7 July, which is in agreement
with the anticyclonic circulation during this
period. However, on July 8, despite the strong
horizontal P VA , the vorticity decreases, obviously
due to vertical N VA , as the graph of the vertical
vorticity advection term (dashed line in Fig. 4c
and third term on the right-hand side of Eq. (2))
shows. It is well evident that the strong downward motion originating in the upper troposphere
is associated with anticyclogenesis. The circulation remains anticyclonic on July 9 due to the
continuing vertical N VA that takes place. On
the 10th of July vertical PVA , as well as an
increase of the horizontal P VA lead to the establishment of cyclonic circulation. Also, the tilting
term (last term on the right-hand side of Eq. (2))
is significant (dot – dashed line in Fig. 4c) during
the study period. Finally, the vortex stretching
(second term on the right-hand side of Eq. (2))
is at least an order of magnitude smaller than the
rest, as the 500 hPa is the level of non-divergence
during the summer.
In this section it has been shown that the heat
wave began due to a weak WA A at 700 hPa, accompanied by anticyclogenesis at 700 and 500 hPa.
The adiabatic heating due to subsidence was also
significant during the establishment of the heat
wave. Especially during the period of the highest
intensity of the heat wave, the role of the subsidence was outstanding as it, first, penetrated in
the boundary layer causing adiabatic heating and,
second, it was associated with vortex compressing and anticyclogenesis. In the next two sections
the dynamical processes that led to the intense
subsidence are sought.
5. Dynamical features of the Hadley cell
during the heat wave
During the summer the HC can significantly
affect the temperature of the mid-latitudes, as the
poleward edge of HC is the boundary between the
tropical and the mid-latitude air masses (Palmen
and Newton, 1969). In the HC, free tropospheric
228
D. P. Brikas et al
Fig. 6. Climatological vertical cross section of zonal wind
and temperature along 22.5 E. Latitude increases towards
the right and pressure (hPa) is plot on a logarithmic scale,
diminishing upwards. Zonal wind speed is contoured every
2 m=s. Continuous isotachs denote flow out of the page
(westerlies), whereas broken isotachs denote flow into
the page (easterlies). Temperature (dotted) is contoured
every 5 C
subsidence prevails, in general. The SJ blows
along the northern limit of the HC, at 200 hPa.
In Fig. 6, a meridional cross section along
22.5 E shows a 1979–98 climatology of the zonal
wind and the temperature. The climatological SJ
blows at 200 hPa and 37 N, with a westerly
speed of 25 m=s, whereas a very weak climatological PJ of 9 m=s blows at 300 hPa and 55 N.
Meridional cross sections along the longitude
of Greece, presented in Fig. 7, show the HC and
the SJ during the period of interest. The isotachs
show the zonal wind speed and the wind vectors
the meridional circulation (v, !) along 22.5 E.
The vertical co-ordinate is pressure on a logarithmic scale.
On the 4th of July, at the longitude of Greece,
the SJ is not a very well defined feature. There is
a westerly wind current from 32 to 42 N, just
above 200 hPa, whose zonal wind speed does not
exceed 18 m=s. This value is too low for the associated wind current to be called a jet. However,
the above zonal wind current will be called the
SJ, for convenience. In the core of the SJ the
tropopause jumps from a mid-latitude level
(200 hPa) to the north of the SJ, to a tropical
level (150 hPa) to the south of the SJ. This can
be deduced by the downward tilt of the line
‘‘PV ¼ 2.4 P V U ’’, moving from north to south
through the SJ. PV stands for the potential vorticity (Hoskins et al, 1985) and P V U for the PV
units. The value of 2.4 P V U is arbitrarily chosen
here as being representative of the tropopause.
Fig. 7. Vertical sections of the atmosphere along 22.5 E,
as in Fig. 6, but isotachs are plotted every 4 m=s and isotherms show every 4 K. Vectors depict the meridional
(v, !) circulation. The line, along which PV equals 2.4
P V U , is bold dashed
The role of the SJ during heat wave events over north-central Greece
There are two branches of the PJ on the 4th of
July, one at 52 N at 300 hPa, with a core speed of
about 25 m=s and another one outside the northern limit of the section at 400 hPa, with a core
speed greater than 28 m=s.
The meridional gradient of (dotted) shows
the fronts accompanying the jets. The subtropical
front is weak, located at 33–35 N, confined to
a layer extending from the level of the SJ down
to about 400 hPa. In contrast, the polar front is
stronger, located below the south branch of the
PJ, reaches the ground, where it extends from 42
to 52 N.
The vertical gradient of shows the degree of
stratification=mixing. The continental (marine)
boundary layer is well mixed (stably stratified),
due to the low-level heating (cooling) by the hot
ground (cool water). The continental boundary
Fig. 8. (K) is plotted versus pressure (hPa) above the
nearest gridpoint to Thessaloniki for 12Z July 4 (dashed)
and 12Z 7 July (continuous)
229
layer reaches 600 hPa above tropical Africa and
700 hPa above Northern Africa and Greece, lowering down polewards. In order to focus on the
vertical structure of the boundary layer above
Thessaloniki, in Fig. 8 vertical profiles of have been drawn for the nearest grid point to
Thessaloniki.
The upward motion prevailing in the African as
well as in the Balkan boundary layer is indicative
of the dry convection taking place there. Apart
from the large scale subsidence, the meridional
winds to the south of the SJ are also in agreement
with the Hadley model. Northerlies indeed prevail below 400 hPa and southerlies from 400 to
100 hPa in the HC.
On the 5th of July deep convection takes place
in the inter-tropical convergence zone (ITCZ ),
in tropical Africa, equatorward of 18 N. This
moist convective activity is accompanied by a
strengthening and poleward shift of the Hadley
circulation and the SJ. All the features of the HC,
as the vertical motions, the subtropical front and
jet and the tropopause jump across the SJ core,
become well defined from July 5 onwards.
The poleward shift of the SJ is more pronounced
on July 6, when the latter jumps to 48 N, blowing with a zonal wind speed (u) gretear than
32 m=s. The intrusion of tropical air masses in
the Balkans can also be seen in the gradual increase of the tropopause height with time. The
vertical orientation of the line u ¼ 0 at 40 N,
with westerlies to the north and easterlies to the
south, reveals the existence of a deep anticyclone
there.
On the 7th of July the axis of the SJ moves
even further north along the longitude of Greece,
reaching 51 N, attaining a zonal wind speed
being greater than 48 m=s. The tropopause reaches
150 hPa and the anticyclone further strengthens
above the South Balkans. Indicative of the deep
convection in tropical Africa is the appearance of
the tropical easterly jet (T E J ) in the top left corner of the cross sections of Fig. 7.
Another feature of the gradual establishment of
a tropical air mass in Greece is the increase of the
height of the top of the boundary layer. This can
be seen in Fig. 8, where vertical profiles of have
been plotted. On the 4th of July (dashed profile)
the top of the boundary layer is at 700 hPa, as is more or less constant with height up to there,
whereas it increases above there. Later on, on the
7th of July, when the tropical air mass is better es-
230
D. P. Brikas et al
tablished above Thessaloniki, does not increase
so fast with height above 700 hPa, which probably
means that the boundary layer extends above
there.
The SJ starts retreating towards the south on
July 8, when it blows at 48 N, shifting at 41 N
and 38 N on July 9 and 10, respectively. The
sudden jump of the tropopause from 400 hPa on
the cyclonic side of the SJ to 150 hPa on the anticyclonic one, means that the tropical air masses
are juxtaposed to the polar ones and the subtropical and polar jets have merged. Greece is on the
cold front, with continuous C A A bringing a temperature drop.
One of the most important features of the HC
for this heat wave event was the tropospheric subsidence. The subsidence contributes to the maintenance of the heat wave directly through adiabatic
heating and indirectly through anticyclogenesis
due to vertical vortex compressing. The descent
was especially strong above Greece from 7 July
onwards, as seen in the ! Hovmoller diagram of
Fig. 5e. The cross section of July 7 (Fig. 7) shows
that the descent penetrates the boundary layer of
Northern Greece (40–41 N). The good mixing
of the boundary layer means that the adiabatic
warming occurring at the top of the boundary
layer is transmitted down to the surface. This is
in agreement with the surface temperature rise
observed at 1000 hPa on July 7 (Fig. 1). It is
hypothesized that the strong subsidence, that
penetrates in the boundary layer, is not due only
to HC dynamics, but also to mass convergence
due to velocity changes in the exit of the SJ.
This possibility will be examined in the next
section.
Fig. 9. 200 hPa wind charts for 12Z 7 July. The bold continuous contours in panels a and c are isotachs of winds stronger than
30 m=s, plotted every 5 m=s. (a) Thick grey vectors: total wind stronger than 15 m=s. Thin vectors: AG wind. Horizontal
divergence (convergence) absolutely greater than 105 s1 is shaded and surrounded by thin continuous (dotted) contours.
Shading increases every 105 s1 . (b) Vectors: total wind direction, shading: s-component of AG wind. (c) Shading:
n-component of AG wind. Shading increases every 5 m=s in all panels
The role of the SJ during heat wave events over north-central Greece
231
6. Dynamical features of the subtropical jet
It has been seen above that from 7 July strong
subsidence occurred in Greece, which at 12Z
July 7 penetrated into the boundary layer of
Thessaloniki, increasing the temperature, despite
the C A A from the north (see Figs. 4a and 2).
In order to reveal the role of the upper troposphere in the development of the strong downward motions, the 200 hPa horizontal divergence
(shading) and AG winds for 12Z 7 July have
been plotted in Fig. 9a. The SJ is anticyclonically
curved to the north of Greece, as the grey arrows
illustrate. As the isotachs (bold contours) show, the
SJ streak (60 m=s) is to the NW of the Balkans,
at 12 E, 48 N. In the region of the jet exit, near
the NE Balkans, two conceptual AG wind currents are illustrated: a radial one, towards the
right of the flow (r), due to the air masses deceleration and a tangential one towards the direction
of the flow (t), due to the anticyclonic curvature
of the SJ. As a vector sum of the above two
vectors, a strong AG wind results towards the
direction of the flow and to the right of it (Keyser
and Shapiro, 1986; Moore and Van Knowe,
1992). This AG wind current converges above
the NE Balkans, which explains the horizontal
convergence with a maximum of 4 105 s1
in the NE Balkans. The position of Thessaloniki
is denoted by ‘‘TH’’ for reference.
In order to clearly show that the strong AG
wind current above the NE Balkans is the vector
sum of two AG wind currents, the two components of the AG wind in the natural coordinate
system (see Holton, 1992) have been plotted in
Fig. 9b and c. The parallel to the flow, s-component
of the AG wind, has been plotted in Fig. 9b and
the one normal to the flow, n-component, in
Fig. 9c. s (n) AG component is due to the curvature (acceleration) of the flow. It can indeed be
seen that, due to the strong anticyclonic curvature of the SJ, positive values of s, with a maximum of 20 m=s, prevail near the northern border
of the Balkans (Fig. 9b). On the other hand, as
the flow decelerates, negative values of n, with a
minimum of 10 m=s, prevail in the Northern
Balkans. The arrows in Fig. 9b and c show the
direction of the AG wind currents related to the
parallel (s) and normal (n) components of the AG
wind, respectively. The convergence area seen in
Fig. 9a is associated with the vector sum of the
Fig. 10. Vertical cross section of the atmopshere for 12Z
7 July, along line AB, shown in Fig. 9a. Wind exiting the
section in right angles is in bold isotachs. Shading shows
horizontal convergence stronger than 1.5 105 s1 . Vectors show the AG circulation, that is parallel to the plane of
the section. Thessaloniki is denoted by ‘‘TH’’ for reference
conceptual AG wind currents denoted by t and r.
This convergence area is denoted by ‘‘C’’ in both
Fig. 9b and c.
In order to see how subsidence is transmitted
from the above described horizontal convergence
area, to the boundary layer of Thessaloniki, where
the adiabatic heating took place (see Sect. 4), in
Fig. 10 a cross section is plotted along line AB
(see Fig. 9a) for 12Z 7 July. The continuous contours are the isotachs of the wind that is normal
to the section. The dotted lines show and the
vectors depict the AG circulation (vag, w), that is
parallel to the section. In the cross section of
Fig. 9 the SJ blows at 200 hPa with a speed
greater than 40 m=s. In agreement with thermal
wind balance concepts, there is a zone of strong
negative meridional temperature gradient below
the SJ. This zone tilts towards the north with
height. Above the Balkans there is an anticyclonic PV anomaly, as revealed by the local hump
on the line ‘‘PV ¼ 2.4 PVU ’’. The anticyclonic
shear that prevails at nearly all levels is indicative
of the deep anticyclone above the Balkans. The
horizontal convergence maximum observed above
the NE Balkans in Fig. 9a is confined within a
thin layer through the core of the SJ (Fig. 10).
Strong descent occurs below the horizontal
convergence area at the maximum wind level.
One of the descending air currents, the tilted
one, highlighted with a grey arrow, reaches the
232
D. P. Brikas et al
boundary layer of Thessaloniki. The associated
adiabatic heating is so strong, that, even acting
alone, manages to intensify the heat wave, as
seen in Sect. 4.
7. Conclusions
The following conclusions can be drawn from
the description of the thermodynamic and dynamic processes that took place during the heat
wave of 5–9 July 1988. The heat wave was associated with a strengthening and a poleward shift
of the SJ in the study area and mainly to the NW
of the Balkans. The SJ reached 50 N, while its
climatological latitude is 37 N. Since the SJ is
the limit between the tropical and the midlatitude air masses, any poleward shift of the SJ
is accompanied by heat waves.
The poleward shift and strengthening of the
SJ is associated with a strengthening of the rest
of the Hadley circulation and particularly the
deep convective branch in the ITCZ and the
T E J . Mo and Rasmusson (1993) have indeed
found a link between the ITCZ and the SJ.
Enhanced deep convective activity in the ITCZ
can enhance the poleward outflow at upper levels
in the HC, shift the SJ polewards and, via angular
momentum conservation, increase the SJ wind
speed.
During the heat wave under study, the anomalous subsidence brought by the HC and the SJ,
led to the formation of a warm anticyclone above
the Central Mediterranean and the Balkans. Initially, the temperature reached the heat wave levels
due to adiabatic heating in the above anticyclone,
as well as due to WA A from the SW along the
NW flank of the anticyclone. The adiabatic heating was from the beginning more important than
the WA A , which was limited to some areas only.
This is because, after the heat wave was established, the anticyclone moved to the NW of
Greece and the winds became NE. The NE wind
direction is indicative of the Hadley cell and tropical air masses. The CAA by the northeasterlies
tends to compensate for the adiabatic heating.
The heights of the tropopause and the top of
the boundary layer increase as the tropical air
masses gradually prevail. It is likely that the
strong southwesterlies in the Central Mediterranean advected the SAL up to Greece during the
heat wave under study. However, to prove this,
isentropic PV charts, Saharan dust concentration
measurements and even air parcel trajectories are
required.
When the SJ began moving southwards and
strengthening on July 7, the SJ streak reached
the Balkans. Thus Greece was located under
the right exit quadrant of a northwesterly jet
streak. Due to the flow deceleration a conceptual
AG wind current had to develop towards the SW
(Murray and Daniels, 1953). On the other hand,
the curvature in the exit of the SJ was strongly
anticyclonic. So, another conceptual AG wind
current, stronger than the latter, developed
towards the SE (Beebe and Bates, 1955). The
vector sum of the above two AG air currents is
a strong northerly AG wind current. Due to the
associated horizontal convergence, strong descent developed. A descending branch reached
the boundary layer in Greece and caused strong
adiabatic heating that intensified the heat wave.
Despite the C A A , the heat wave was maintained
by the intense adiabatic heating until the 8th of
July. It was then that strong C AA established
cooler mid-latitude air masses, first in Northern
Greece and afterwards in the whole country.
Generalising the results, strong ageostrophic
currents of the kind described above, are expected
to be a general feature of the SJ around the world
and trigger heat waves elsewhere.
References
Beebe RG, Bates FC (1955) A mechanism for assisting in
the release of convective instability. Mon Wea Rev 83:
1–10
Brugge R (1991) The record-breaking heat wave of 1–4
August 1990 over England and Wales. Weather 46:
2–10
Campetella C, Rasticucci M (1998) Synoptic analysis of an
extreme heat wave over Argentina in March 1980. Meteor
Appl 5: 217–226
Carlson TN, Prospero JM (1972) The large scale movement
of Saharan air outbreaks over the northern Equatorial
Atlantic. J Appl Meteor 11: 283–297
ECMWF (1992) ECMWF data assimilation scheme.
Research Manual 1, Scientific documentation. ECMWF
Meteorol Bull M1.5=3
Gawith MJ, Downing TE, Karacostas TS (1999) Heat waves
in a changing climate. Climate, change and risk (Downing
TE, Olsthoorn AJ, Tol RSJ, eds). Routledge, 407 pp
Giles BD, Balafoutis ChJ (1990) The Greek heatwaves of
1987 and 1988. Int J Climatol 10: 505–517
Holton JR (1992) An introduction to dynamic meteorology.
New York: Academic Press, 511 pp
The role of the SJ during heat wave events over north-central Greece
Hoskins BJ, McIntyre ME, Robertson AW (1985) On the use
and significance of isentropic potential vorticity maps.
Q J R Meteorol Soc 111: 877–946
Huschke RE (1959) Glossary of meteorology. Amer Meteor
Soc, USA, 638 pp
Karacostas TS, Pennas PJ, Mitas CM (1996) On the development of a regression model for the identification of
heat wave events in Thessaloniki, Greece. Proc. 14th
Int. Congress of Biometeorology, Ljubljana, Slovenia,
pp 80–86
Keyser DA, Shapiro MA (1986) A review of the structure
and dynamics of upper level frontal zones. Mon Wea Rev
114: 452–499
Krishnamurti TN (1961) The subtropical jet stream of
winter. J Meteorol 18: 172–191
Kunkel KE, Changnon SA, Reinke BC, Arritt RW (1996)
The July 1995 heat wave in the mid-west: A climatic
perspective and critical weather factors. Bull Amer
Meteor Soc 77: 1507–1518
Metaxas D, Kallos G (1979) Heat waves from a synoptic
point of view. Technical Rep 14, Ioannina, Greece
233
Moore JT, Van Knowe GE (1992) The effect on jet-streak
curvature on kinematic fields. Mon Wea Rev 120:
2429–2441
Murray R, Daniels SM (1953) Transverse flow at entrance and
exit to jet stream. Quart J Roy Meteor Soc 79: 236–241
Palmen E, Newton CW (1969) Atmospheric circulation
systems. New York: Academic Press, 603 pp
Prezerakos NG (1989) A contribution to the study of the
extreme heat wave over the South Balkans in July 1987.
Meteorol Atmos Phys 41: 261–271
Repapis Ch (1975) On the warm invasions in the lower
troposphere in Greece. PhD dissertation, University of
Ioannina, Greece
Spyrou K (2001) Synoptic and dynamic study of the
phenomenon of the heat wave in the wide Greek region.
PhD dissertation, Dept. of Meteorology and Climatology,
Aristotle University of Thessaloniki, Greece, 285 pp
Corresponding author’s address: Dimitris P. Brikas, 70
Feidiou St., Ilioupolis 16341, Athens, Greece (E-mail:
[email protected])