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SUMMER SEA BREEZE IN THE GULF OF EILAT AND ITS EFFECT
ON THE CLIMATE OF EILAT CITY
*
Hadas Saaroni∗, Eitan Maza*, Baruch Ziv**
Tel-Aviv University, Tel-Aviv, Israel; **The Open University of Israel, Tel-Aviv, Israel
Abstract
Northerly offshore winds blow persistently in Eilat. A large land - sea thermal contrast suggests the development
of southerly sea breeze there. The study indicates that sea breeze develops in 25.5% of mid-summer days; lasts
for 4-6 hours typically, with ~1/3 of the speed characterizing the prevailing northerlies. Though the maximum
temperature is lower in 'breeze' days the higher relative humidity resulting in minor relieving effect, so the heat
stress remains severe. A synoptic study reveals that in ‘breeze’ days the synoptic pattern still prevails, though the
pressure gradient weakens by over half from its value in 'non-breeze' days.
Key words: sea breeze, heat stress, Gulf of Eilat, Persian Trough
1. INTRODUCTION
Sea breeze has a large environmental implications on comfort conditions, air pollution etc (Melas 2000). The net
relieving effect of sea breeze on humans under hot conditions depends on the balance between two contradictory
contributions to heat stress: cooling, which reduces heat stress, and moisturizing, which intensifies it.
Sea breeze circulations are well developed during the summer around 30ºN due to the high radiation rate there
and the subtropical highs that causes relatively weak pressure gradients and low cloudiness (Oke 1987, Arya
2001). Significant synoptic gradient or cloudiness may hinder or even cancel the sea breeze. Arritt (1993) showed
that in order to penetrate into the land the sea breeze needs a synoptic supportive pressure gradient or a weak
gradient that won’t interfere with smaller scale wind system. Zhong and Takle (1993) showed that sea breeze
front lines enter land with a typical speed of 3-6 m/s, which is further strengthening during the day hours due to
the strengthening of the thermal gradient and acceleration. Laird et al (2001) showed in Michigan Lake that the
first-degree factor influencing the lake breeze is synoptic rather than local (thermal gradient) and found negative
correlation between the wind speed of the opposing synoptic wind and the sea breeze frequency. Melas et al.
(1995) studied the relationship between topography and synoptic pressure gradient and sea breeze development
in Athens during summer. They found that a weakening in the summer Etesian winds enable local circulations,
such as sea/land breeze, to develop. However, they found that in Athens the penetration of the Etesian winds is
the main factor in reducing heat stress and severe air pollution.
The weather conditions over the Eastern Mediterranean (EM) are highly persistent during the summer season,
particularly in July-August. The lower levels are dominated by the so-called 'Persian Trough' (Fig. 1a), a pressure
trough that extends from the Asian Monsoon through the Persian Gulf, along southern Turkey, down to the
Aegean Sea. As a result, the winds over the EM, i.e., the Etesian winds (e.g. Air Ministry 1962, Prezerakos 1984),
flow from the northwest sector. These winds veer to north over the southern and eastern parts of Israel and
strengthen, especially over the Rift Valley. This strengthening is attributed both to the ridge extending from North
Africa toward south Israel and the channeling effect over the Rift Valley (Alpert et al. 1992). Eilat is located at the
southern tip of Israel, in the Rift Valley, at the northern end of the Gulf of Eilat. The sea breeze there is expected
to blow from the south, against the synoptic forcing.
The summer weather conditions in Eilat are persistently dry and hot, with average daily maximum temperature of
39.2ºC (absolute maximum 47.2ºC) in July-August and average relative humidity of 16% at 14 LST. The daytime
surface temperatures exceed 50ºC. The resulting Heat Stress Index, HSI, (Thom 1959, Sohar 1980) in these
months is severe during 7 hours a day on the average (Bitan and Rubin 1994). The dry conditions along the entire
day suggest that sea breeze has only a minor effect on the local climate in summer. Bitan and Rubin (1994)
showed that the vast majority of the winds all year around blow from the north sector, i.e., offshore, and that
southerly winds tend to develop in Eilat during the noon hours, presumably due to sea breeze.
The average thermal contrast between the land and the Gulf in Eilat, being about 15ºC (Israel Meteorological
Service), is larger than that exists along the eastern Mediterranean coast, suggesting that Eilat should, indeed, be
subjected to sea breeze in spite of the unfavorable synoptic flow. This study documents the sea breeze in July
and August, evaluates its impact on the climatic conditions in this super-arid city and examines its relationship
with synoptic scale factors.
∗
Corresponding author address: Hadas Saaroni, Department of Geography and the Human Environment, Tel-Aviv
University, Tel-Aviv 69978, Israel; e-mail: [email protected]
2. METHODOLOGY
Eilat is located 29ºN, 36ºE, BW according to Köppen climate classification. The width of the valley in this area is
7-10 km with Eilat Mountains to the west (up to 1000 m ASL) and Edom Mountains to the east (up to 1600 m
ASL). The width of the Gulf of Eilat is 10-15 km and its length is 150 km (Fig. 1b).
a
.
N
b.
Eilat
.
Gulf
of
Eilat
Fig. 1: Long-term mean sea level pressure averaged over 1968-1996 (in hPa units, 1 hPa interval) for July-August (NCEP/NCAR
CDAS-1 archive, Kalnay et al., 1996; Kistler et al., 2001). Left corner - a satellite image of Eilat and surrounding region
The study period is July and August of the years 1994-1999. Wind speed and direction, temperature and relative
humidity were measured in Eilat airport, 2km north to the sea. Temperature and relative humidity data is available
for 1996-1999 only. Synoptic scale data is taken from the NCEP/NCAR archive (NCEP/NCAR CDAS-1 archive;
Kalnay et al. 1996, Kistler et al. 2001).
Statistical analysis was performed for the winds from the south sector, assumed to reflect sea breeze. Their effect
on temperature, humidity and heat stress was derived from comparison between 'breeze' and 'non-breeze' days.
Evaluation of the synoptic factor was done through comparison between the two groups of days.
3. RESULTS
3.1. Wind regime
Southerly winds were observed in 25.5% of the days (7.4% of the whole hours of the day), 34% of the days in
July and 16% in August. The average starting and ending times are 12:30 and 19 LST, respectively. This timing
supports the idea that the southerly winds are sea breezes. The most frequent duration is 4-6 hours, much shorter
and earlier than the sea breeze along the Mediterranean coast, which lasts for about 12 hours, from 7 to 19 LST
(Skibin and Hod 1979, Goldreich et al. 1986).
Southerly winds were observed at each of the day hours, with a maximum probability of 21% at 16 and 17 LST
and minimum of 0. % in the after midnight night hours, 3-5 LST. The daily course of the v-wind component (Fig.
2) shows that the northerly wind component in the night hours is significantly larger in the 'non-breeze' days. In
both group of days the northerly wind attains two peaks, though in different times.
3
45
Breeze
40
V-wind (ms)
1
35
-1
1
4
7
10
13
16
19
22
30
25
-3
20
15
-5
No breeze
10
-7
LST
Fig. 2: Daily course of the average southerly wind
component (v) for July August 1994 - 1999 for 'breeze'
days (dotted) and 'non-breeze' days (solid)
5
RH
Tdry
TW
HSI
Fig. 3: Average relative humidity, air temperature (Tdry), wet
bulb temp (TW), and HSI for 'breeze' (dotted) and 'non-breeze'
days (blank) for 16:30 LST. The vertical segments centered at
the top of the bars denote the STD for the respective group.
In the 'breeze' days the average wind is southerly between 13:30-19:00 LST, with maximum of 2.0 ms-1 at 16:30
LST, the same time in which the southerly winds reach their maximum frequency. Note that the maximum breeze
speed (2.0 ms-1) is still weaker than the northerly offshore winds during the majority of the day hours. The breeze
tends to be most intense when blowing from the azimuth of 190°, which coincides with the orientation of the Gulf
of Eilat.
The seasonal variation of the 'breeze' days probability indicates a pronounced decreasing trend along JulyAugust, with R=0.91. Its occurrence in August is only 0.6 that in July for the study period.
3.2 Implications on temperature, humidity and heat stress
The influence of the sea breeze is studied for two times separately: 14 LST, in which the southerly wind
component in 'breeze' days starts to dominate, and the average between 16 and 17 LST (16:30 LST, hereafter),
when the breeze attains its maximum intensity. In 14 LST on the average the 'breeze' days are cooler than the
'non-breeze' days by 1.2ºC. The wet bulb temperature and the relative humidity in 'breeze' days are higher by
0.1ºC and 2.9%, respectively, and the resulting HSI is lower by only 0.6ºC. These differences are most
pronounced at 16:30 LST (Fig 3), when 'breeze' days are cooler by 2.5ºC on the average, the wet bulb
temperature and the relative humidity are higher by 1.3ºC and 8.1%, respectively, and the HSI is lower by 0.6ºC.
Note that at 16:30 LST, when the sea breeze is the strongest, though the temperature and humidity differences
are larger, the difference in HSI remains relatively negligible due to the relative cancellation between the cooling
and moisturizing effects. In spite of the breeze, severe HSI still exists in Eilat during both 'breeze' days, for 10
hours, as in the ‘non-breeze’ days. It is interesting to note that in the ‘breeze’ days the period in which severe
conditions prevail lag that found in the ‘non-breeze’ days by one hour.
3.3 Synoptic scale factors
An inspection of the NCEP/NCAR data for the study period indicates that in the vast majority of both 'breeze' and
'non-breeze' days the synoptic situation is similar, i.e., the Persian Trough. In only 5 of the 95 'breeze' days the
synoptic scale pressure gradient supported the sea breeze. In these 5 days the wind could not be considered
pure breeze, though it is assumed that the breeze had a considerable contribution during the day hours.
Several synoptic-scale differences were found between the 'breeze' and the 'non-breeze' days. The most
significant is the deepening of the Mediterranean part of the Persian Trough further toward the west during
'breeze' days, where as no significant difference is found over Saudi Arabia, to the east of Eilat. In the upperlevels a deeper semi-permanent upper-level trough over the eastern Mediterranean is noted in the 'breeze' days,
expressed in a lowering of 17 and 20 m in the geopotential height at the 700 and 500 hPa levels, respectively,
over the study region.
The difference noted at the sea level implies a backing of the northwesterly Etesian winds over Israel and a
weakening of the west-east pressure gradient over the Gulf of Eilat. The effect of this difference on the sea
breeze was estimated by sea level pressure difference between 32.5ºN, 32.5ºE and 27.5ºN, 35ºE grid points,
located to the northwest and the southeast of Eilat, respectively. The average daily difference for 'breeze' days
was 1.2 hPa, whereas for 'non-breeze' days it was 2.7 hPa, representing the northerly synoptic-scale wind. It's
role is also expressed by a correlation of R=-0.43 found between the duration of the sea breeze and the pressure
difference in the 'breeze' days.
4. SUMMARY AND DISCUSSIONS
The sea breeze in Eilat in the mid-summer months, July-August, was examined for the years 1994-1999. Sea
breeze should be expected there due to the large sea-land temperature contrast in the summer season, in spite of
the suppressing synoptic-scale persistent northerly winds, further enhanced by topographic effect of the Rift
Valley. Southerly winds, which were observed in 95 out of the 372 days, were regarded as a manifestation of sea
breeze and were analyzed.
When occurs, the sea breeze starts around 13 LST and lasts for about 4-6 hours, with maximum speed of ~ 2 m
/s, which are 1/3 of that of the prevailing northerly, wind there. A comparison with the sea breeze near the
Mediterranean coast (Skibin and Hod, 1979) shows that the starting time of the sea breeze in Eilat lags the former
by 5 hours and that the Red Sea breeze lasts half the time characterizing the Mediterranean Sea breeze. These
differences stem from the substantial difference in the role of synoptic-scale forcing, which supports the sea
breeze along the Mediterranean coastline in the summer season, but suppresses it at Eilat. The synoptic analysis
reveals that in 95% of the 'breeze' days the lower level synoptic conditions remained similar to the long-term
situation, except for a decrease in the regional northwest-southeast pressure drop, which is reduced by 38% with
respect to the 'non-breeze' days.
A seasonal decrease in the frequency of 'breeze' days along July and August was noted. The inhibiting effect of
the pressure gradient over the region on the development of sea breeze indicates a seasonal gradual and
consistent increase in the pressure gradient over the region from the last decade of July to August by over 25%.
The seasonal decrease in solar radiation, of 7.2% between July and August (Bitan and Rubin, 1994), also
explains the decreasing trend in 'breeze' days occurrence.
It was found that the sea breeze has only a minor relieving effect on heat stress in the noon hours, being only
0.6°C lower than in the 'non-breeze' days. This is due to the cancellation between the small temperature drop and
the small rise in relative humidity. It is worth noting that since wind speed has a relieving effect (Givoni, 1991), the
fact that the wind speed in the sea breeze in Eilat is lower than that of the prevailing northerly winds, the
subjective sensation in ‘breeze’ days may be even worse. Another important factor in Eilat concerns a house
cooling system, the “desert coolers”, which cools the indoor air by evaporating water, and which efficiency
depends on the difference between dry and wet bulb temperatures. Since this difference is reduced by the sea
breeze, the breeze hours may become oppressive in houses where this cooling device is used. It is worth noting
that the average duration of severe heat stress for the study period 1994-1999, i.e., 10 hours, is larger by 3 hours
than the duration reported by Bitan and Rubin (1994) based on the period 1963 - 1983. This can express a
combined effect of increasing urban heat island and a regional warming trend found for the summer (Saaroni et
al., 2003).
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