INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 32: 1889–1898 (2011)
Published online 2 August 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/joc.2403
Analysis of observed variability and trends in numbers of
frost days in Turkey for the period 1950–2010
Ecmel Erlata and Murat Türkeşb *
b
a The Physical Geography Division, Department of Geography, University of Ege, Bornova - İzmir, Turkey
The Physical Geography Division, Department of Geography, Çanakkale Onsekiz Mart University, Çanakkale, Turkey
ABSTRACT: This study examines the climatology of annual frost days, and analyses the size and behaviour of the longterm variability and trends in annual numbers of frost day at the 72 stations over Turkey from 1950 to 2010. The main
results are summarized as follows: (1) The annual number of frost days has evidently decreased at most of the stations
with some observed regional differences, (2) The decreasing trends are largest over the Eastern Anatolia, the Marmara
regions and along the Mediterranean coastline. The meteorological stations located in the continental northeast and the
easternmost parts of the Anatolian Peninsula, including Ardahan, Iǧdır and Van, show a negative linear trend with a rate
of four days per decade, (3) As in other regions of the Earth, changes in number of frost days are very likely associated
with changes in minimum air temperatures and increasing growing season lengths in Turkey, (4) The decreasing trends
in number of frost days also indicated considerable decadal-scale variability. This variability is very likely attributable to
the large-scale atmospheric circulation and atmospheric oscillations such as the Arctic Oscillation (AO) or North Atlantic
Oscillation (NAO) and the North Sea-Caspian Pattern, (5) Consequently, the long-winter (DJFM) composites of number
of frost days were examined for extreme phases of the AO index during the period 1950–2010 in order to assess the
influence of atmospheric oscillations on year-to-year variability in number of frost days. According to the Cramer’s tk test,
winter number of frost days tended to increase significantly during the high (positive) index AO phase, while they tended
to decrease significantly during the low (negative) index AO phase. These relationships are statistically significant at the
1% level at the majority of stations. Copyright  2011 Royal Meteorological Society
KEY WORDS
minimum air temperature; number of frost days; climate change and variability; Mann-Kendall test; Cramer’s
tk test; Turkey
Received 11 December 2010; Revised 17 May 2011; Accepted 21 January 2011
1. Introduction
According to the latest scientific assessment report of
the Intergovernmental Panel on Climate Change (IPCC,
2007), the Earth’s annual average surface (land and sea)
temperature has increased by about 0.8 ° C over the past
100 years with large regional variations. Since 1850, 11
of the 12 warmest years were detected between 1995
and 2006. Minimum (nighttime) surface air temperatures
have generally increased at a larger rate than maximum
(daytime) air temperatures, resulting in a decreased
diurnal temperature range (DTR) (Easterling et al., 1997;
Heino et al., 1999; Folland et al., 2001; Türkeş et al.,
2002). Evidence of global warming comes from warming
of the oceans, rising sea levels, glaciers melting, sea
ice retreating in the Arctic and diminished snow cover
in the Northern Hemisphere. Most of the warming in
recent decades is very likely the result of human activities
(IPCC, 2007).
* Correspondence to: Murat Türkeş, Çanakkale Onsekiz Mart University, Faculty of Sciences and Arts, Department of Geography, Physical
Geography Division, Terzioǧlu Campus, 17020 Çanakkale, Turkey.
E-mail: [email protected]
Copyright  2011 Royal Meteorological Society
Analyses of seasonal and daily temperatures showed
that since the late 1950s the most pronounced warming
has occurred in winter and spring minimum air temperatures (Easterling et al., 1997). Besides, global warming
is also associated with changing temperature distributions and frequencies of extremes (Katz and Brown,
1992). Many studies revealed a clear trend towards fewer
extremes of low temperatures in the late 20th century over
all continents (Cooter and LeDuc 1995; Horton et al.,
2001; Kunkel et al., 2004). Easterling et al. (2000) found
that for the period 1910–1998, there has been a slight
decrease in the numbers of day below freezing over the
entire USA, although there were marked regional variations in the trends. Feng and Hu (2004) showed that
there was significant decrease of annual frost days in
the western USA associated with lengthening of growing degree days, and no changes in annual frost days in
the eastern USA. The global analysis of climate extreme
indices by Frich et al. (2002) pointed out the evidence
of fewer frost days in much of the middle and high latitudes in the Northern Hemisphere during the second half
of the 20th century. Alexander et al. (2006) found significant decreases in the annual number of frost days over
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Western Europe and large parts of Russia for the period
1951–2003.
As for Europe, Heino et al. (1999) concluded that
some decrease of frost days has taken place since the
1930s due to the very strong increase in winter minimum
temperatures in northern and central Europe. Klein Tank
and Können (2003) presented a European-averaged trend
of frost days of −1.7 days/decade and 9.2 less frost days
in 1999 compared to 1946. Moberg and Jones (2005)
found statistically significant changes in both the warm
and cold tails for Tmax and Tmin, with the largest
warming in the cold tail for Tmin in winters during
1946–1999 for central and western of Europe. Bartholy
and Pongrácz (2007) found a decreasing number of cold
nights, severe cold days, frost days between 1961 and
2001.
Similar to the global and European trends, analyses
of the minimum and maximum temperatures and climate extreme indices suggests that also the climate of
the eastern Mediterranean and the Middle East regions
tended to get warmer in the second half of the 20th
century. For instance, Türkeş et al. (2002) also indicated significant warming of the minimum temperatures
in many regions of Turkey, but only weak trends in maximum temperatures for the period 1929–1999. Zhang
et al. (2005) found decreasing frost days for 52 stations
across the Middle East since the 1980s. Kostopoulou
and Jones (2005) showed decreasing trends for the frequency of cold nights in winter and especially in summer over the period 1958–2000 in the eastern Mediterranean. Rahimzadeh et al. (2009) found negative trends
for indices representing low maximum and minimum
temperature extremes in Iran since the 1970s.
However, climatology, long-term variability and trends
of annual number of frost days in Turkey have not
been examined yet. Consequently, the aim of the present
study is (1) To investigate climatology of annual frost
days, (2) To examine the magnitude and behaviour of the
variability of frost days and the likely interactions among
atmospheric circulation modes like the Arctic Oscillation
(AO) and number of frost days, and (3) To detect the
secular trends in annual number of frost days at the 72
climatological and synoptic meteorological stations over
Turkey for the period 1950–2010.
The daily minimum air temperature series were
obtained for the 72 climatological and synoptic meteorological stations of the Turkish State Meteorological
Service. Because of the lack of daily temperature observations prior to the 1950s in many stations, this analysis
focused on 1950–2010. However, in order to reach a uniform spatial coverage in other parts of Turkey, we used
8 stations covering the period 1960–2010. In addition,
we applied rules for selecting stations that were considered as having almost complete daily temperature data
(with less than 5% missing data) in that particular cold
season of the year. Quality and homogeneity controls of
the temperature series were checked with various controls and homogeneity methods, also making use of a
station history file by Türkeş et al. (2002) for 70 stations
of Turkey operated during the period 1929–1999. Adequate information on the homogeneity and other timeseries characteristics of Turkish temperature data can
be found in Türkeş et al. (2002) and Türkeş and Erlat
(2008), respectively. We have updated the dataset to 2010
for the present study. Finally, 72 temperature series are
obtained after the application of the above criteria. The
geographical distributions of studied stations are shown
on a topographic map of Turkey in Figure 1.
The coefficient of variation (CV) was used to investigate the spatial pattern of interannual variability in annual
number of frost days at the 72 stations. The statistic of CV
is computed by expressing long-term standard deviation
as a percentage of long-term average annual number of
frost days. The CV values give a general indication of the
probable percentage variation around average frost days
at the stations. Thus, relatively less dispersed variables
would have lower CVs.
The nonparametric Mann–Kendall (M-K) rank correlation test (WMO, 1966) was used to detect any possible
trend in ANFDs, and to test whether such trends are statistically significant. Before applying the test, original
observations of xi are replaced by their corresponding
ranks ki , such that each term is assigned a number ranging
from 1 to N reflecting its magnitude relative to magnitudes of all other terms. Then the P statistic is computed.
P statistic is given by
P =
N
−1
ni
(1)
i=1
2.
Data and Methods
An air frost is defined as when the air temperature in a
standard meteorological screen at a height of 2 m above
a level grass surface fails below 0 ° C (Goulter, 1981).
In this study, we used daily minimum air temperatures
equal to or below 0 ° C that are recorded in a standard
meteorological screen at a height of 2.0 m above a level
to determine the frost days. Thus, annual number of frost
days (ANFDs) is defined as the total number of days with
Tmin ≤ 0 ° C between the first frost day in autumn and the
last frost day in spring.
Copyright  2011 Royal Meteorological Society
M-K rank correlation statistic τ is derived from N and
P by the following equation
τ=
4P
−1
N(N − 1)
(2)
Distribution function of τ is the Gaussian normal for
all N larger than about 10, with an expected value of
zero and variance (τvar ) equal to
τvar =
(4N + 10)
9N(n − 1)
(3)
Int. J. Climatol. 32: 1889–1898 (2011)
ANALYSIS OF OBSERVED VARIABILITY AND TRENDS OF FROST DAYS IN TURKEY
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Figure 1. Spatial distributions of the 72 meteorological stations over a generalized physiographic map of Turkey.
and the significance test (τ )t is then written as
√
(τ )t = 0 ∓ tg τvar
(4)
where, tg is the desired probability point of the normal
distribution with a two-sided test, which is equal to 1.960
and 2.58 for the 5 and 1% levels of significance, respectively. Using a two-sided test of the normal distribution,
null hypothesis of absence of any trend in the series is
rejected for the large values of (τ )t for the desired level
of significance.
The simple least square linear regression equations
were also calculated to detect the trends rates (in
° C/decade) in the frost day series, with time as the independent variable and frost days values as the dependent
variable. The statistical significance of each estimated β
coefficient was tested using the Student’s t-test with (N2) degrees of freedom (Türkeş et al., 2002). In using a
two-tailed test of Student’s t distribution, the null hypothesis for the absence of any linear trend in the time series
is rejected for large values of t.
Standardized indices representing the AO were used
to explain the interannual frost days’ variability over
Turkey. The AO monthly index data for the period
1950–2010 was taken from the Climate Prediction Center of the National Centers for Environmental Prediction
at the NOAA/National Weather Service (http://www.cpc.
ncep.noaa.gov/products/precip/CWlink/daily ao index/
ao.shtml). In this approach, high (low) index phases
of the AO occur when it exceeds +1.0 (−1.0) standard deviations. The winter has been defined as December–January–February–March (DJFM). Composite averages of winter NFDs for the extreme winter AOI phases
were compared statistically with long-term average winter NFDs by using the Cramer’s tk test. Significance tests
Copyright  2011 Royal Meteorological Society
of the results are based on the null hypothesis of ‘no significant difference between a composite average of the
weak (strong) phase of the AOI and the long-term average of the whole period’ (Türkeş and Erlat, 2005). Test
statistic tk is distributed as Student’s t with N-2 degrees
of freedom (WMO, 1966). The null hypothesis of the test
is rejected with the two-tailed test for large values of |tk |.
Any composite NFDs average of a station is considered
as the change ‘signal’, only if the test statistic of tk computed for that station is statistically significant at the 0.05
level of significance.
3.
3.1.
Results and Discussion
Climatology of annual frost days
In Turkey, the spatial pattern of the average ANFDs is
geographically highly variable due to the continentality,
distance to the sea, altitude and orographic/topographic
characteristics. The average ANFDs is smaller than
10 days following the southern coastal belt of Turkey
from İskenderun station on the eastern Mediterranean to
İzmir station on the Aegean region (Figure 2(a)). For
example the average ANFDs is below one day at the
stations on the southward-middle Mediterranean coast
such as Alanya and Anamur. Similar small numbers of
frost days are detected east of the Ordu station along
the eastern Black Sea coasts. The average ANFDs varies
between 20 and 50 days over the coastal areas of the
Marmara region, the interior part of the Aegean region
and in southeastern Anatolia following the Syrian border
(Figure 2(a)). In winter, these regions are mainly influenced by the northeast Atlantic originated mid-latitude
depressions and the Mediterranean depressions with the
prevailing westerly flows in winter (Türkeş, 1998; Karaca
et al., 2000; Trigo et al., 2002, etc.). Cold air advection
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0 50 100 150 200 250
(b)
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Long-term averages of the
annual numbers of frost days
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Coefficients of variation
(in per cent) for annual
numbers of frost days
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km
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Figure 2. Geographical distributions of (a) long-term averages of the annual numbers of frost day, and (b) coefficients of variation (CV%) for
annual number of frost days. This figure is available in colour online at wileyonlinelibrary.com/journal/joc
is short lived in a post-frontal situation, while more
northerly circulations either from the anticyclones or
cyclones and associated cold and dry air masses have
long-term influences over the temperature modifications
on the northern and western coastal regions of Turkey
(Türkeş, 1998). In some winters, for example, cold polar
air advection accompanied by cold sectors of depressions
or continental polar air masses originated in an anticyclone over Eastern Europe, the Balkans and Siberia generate freezing produce freezing temperatures (Katsoulis
et al., 1998).
The ANFDs are of 60 days in the continental Thrace
sub-region in northwestern Turkey due to the increased
continentality. The ANFDs show a sharp increase
towards the interior and eastern parts of the country
Copyright  2011 Royal Meteorological Society
where the altitude often exceeds 1000 metres. In central
Anatolia, ANFDs are between 80 and 130 (Figure 2(a)).
The largest ANFDs reach up to 180 days/year across the
mountainous parts of eastern and northeastern Turkey.
Such can be explained by thermally originated highpressure systems extending from central Asia in winter. More frost days occur when high pressure dominates on the monthly time scale in association with clear
skies and lower nighttime minimum temperatures. In
addition, radiation frosts frequently occur during domination of the high-pressure centres (anticyclones) over
central and eastern Anatolia regions in autumn, winter
and early spring. This affects, especially, isolated valleys and depressions where cold nocturnal air generates
temperature inversions.
Int. J. Climatol. 32: 1889–1898 (2011)
ANALYSIS OF OBSERVED VARIABILITY AND TRENDS OF FROST DAYS IN TURKEY
The geographical distribution pattern of the variability
in annual frost days was investigated by using the
coefficient of variation (CV). CVs of the ANFDs reveal
the increasing variability from the northeast towards
the south. The CV over Turkey reaches from 6.9% at
Ardahan station in the northeasternmost part of Anatolia
to 322% at Anamur station on the Mediterranean coast
(Figure 2(b)). Year-to-year variability of the ANFDs
decreases from the coastal and southern regions to the
northeastern and interior regions of the country. The
lowest year-to-year variability is evident at the stations
of the eastern and northeastern parts of the Anatolian
Peninsula (Figure 2(b)).
3.2. Variations and trends in annual numbers of frost
days
Although an overall decreasing trend in NFDs is detected
for a majority of the stations for the period 1950–2010,
ANFDs is highly variable. Owing to this fact, some distinct periods can be defined in the time series: from 1950
to 1959 the annual number of frost days showed a remarkably high interannual and decadal variability (Figure 3).
For instance, the winter of 1953–1954 was characterized by highest positive anomalies for most of the stations. On the other hand, in the winter of 1954–1955,
higher mean temperature conditions caused very small
ANFDs over most of Turkey. Examination of decadal
variability is a positive anomaly between 1950 and 1960
with considerable variability, a strong positive anomaly
1953–1954 and distinct reversal with a strong negative
anomaly 1954–1955. During the 1960s, ANFDs were
characterized by negative anomalies at the majority of the
stations (Figure 3). After this period, the decadal anomaly
showed a modest trend towards more frost days, and
particularly the winters of 1991–1992 and 1992–1993
were abnormally cold at many stations. The decrease of
ANFDs is evident for the period 2000–2010 and largest
since the 1950s, reflecting stronger warming during the
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first decade of this century. The significant decreasing
trends in ANFDs at many stations seem to be mainly
driven by significantly increased minimum temperatures
in Turkey after 1992–1993 (Türkeş et al., 2002). It is
worth noticing that 2009–2010 was the warmest winter in Turkey with highest negative anomalies of ANFDs
over the whole study period from 1950 to 2010.
The numbers of frost day are highly correlated with
minimum winter temperature in Turkey. Generally, the
long-term and year-to-year variability of minimum winter temperatures and frost day numbers are controlled
by the large-scale atmospheric circulation and atmospheric oscillation patterns such as the Arctic Oscillation
(AO) or North Atlantic Oscillation (NAO) and North
Sea–Caspian Pattern (NCP) (Thompson and Wallace,
1998). Previous studies indicated significant negative correlations between year-to-year variability of winter mean
temperatures in Turkey, the winter North Atlantic Oscillation Index (NAOI) and the Arctic Oscillation Index
(AOI). The results show that the influence of the AO on
the winter temperature is greater than that of the NAO
(Türkeş and Erlat, 2008, 2009). Because of this, in the
study, the variations of winter NFDs have been evaluated in connection with the high (positive)/low (negative)
index phases of winter AO variability. Winter NFDs tend
to decrease significantly during low index AO phase and
tend to increase significantly during high index winter AO
phases. Composite winter NFDs computed for the lowindex phase of the winter AO indicate negative anomalies
at all stations of Turkey (Figure 4(a)). Cramer’s tk test
shows that composite winter NFDs averages corresponding to the low index AO phase are significantly lower
in comparison with long-term winter NFDs averages at
50 of 72 stations, 41 of which are at the 0.01 level
(Figure 4(a)). A strong reduction of NFDs appears to be
concentrated inner parts of the Anatolian Peninsula. On
the other hand, composite winter NFDs negative anomalies are weaker at some stations mainly located in the
Figure 3. Inter-annual and inter-decadal variations in numbers of frost days averaged 72 stations over Turkey during the period 1950/1951 to
2009/2010 relative to the long-term average. This figure is available in colour online at wileyonlinelibrary.com/journal/joc
Copyright  2011 Royal Meteorological Society
Int. J. Climatol. 32: 1889–1898 (2011)
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Figure 4. Geographical distributions of the composite winter NFDs characterized with the resultant anomalies calculated from the significance
test of the Cramer’s methodology, (a) during the low (negative), and (b) the high (positive) winter index years of the Arctic Oscillation. The
filled inverse triangle (bold plus) symbol shows the significantly lower (higher) than long-term average mean NFDs at the 0.05 level, according
to the Cramer’s tk test. This figure is available in colour online at wileyonlinelibrary.com/journal/joc
coastal belt of the Mediterraenean, Agean and Marmara
regions. The high interannual variability at these regions
are considered as the main factors that cause weakening
of the associations between winter NFDs and the AOI.
During the low-index phase of the AO, the geopotential height at 500-hPa level is anomalously low over the
northeast Atlantic, Europe and the Mediterranean region
(Erlat and Türkeş, 2008). This circulation pattern controls the advection of warm air masses from the Atlantic
Ocean to the Mediterranean basin and Turkey.
Contrarily, composite NFDs anomalies during the high
index AO phase are characterized by higher than longterm average numbers at all stations for the winter
Copyright  2011 Royal Meteorological Society
months except at five stations on the Mediterranean coast.
Coherent regions with increased winter NFDs dominate
mainly over the central and western parts of the Anatolian
Peninsula. Cramer’s tk test indicates that above-average
frost days for the high-index AO phase are significant
at 42 of 72 stations, 20 of which are at the 0.01 level
(Figure 4(b)). During the high-index phase of the AO, a
strong anticyclonic anomaly circulation dominates over
the northeast Atlantic and Europe, including Turkey and
the Mediterranean basin. This induces dry and cold
advection from the polar and sub-polar regions across
the Black Sea, Turkey and the eastern Mediterranean
basin. Prevailing advection of dry, cold and stable polar
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ANALYSIS OF OBSERVED VARIABILITY AND TRENDS OF FROST DAYS IN TURKEY
air masses from the higher latitudes associated with this
AO variability pattern results in spatially coherent cold
signals in Turkey (Erlat and Türkeş, 2008; Türkeş and
Erlat, 2008).
The results suggest that, interannual and decadal variability in Turkey’s winter NFDs are mostly controlled
by the high-/low-index AO conditions. For instance,
with respect to the winter frost days, the most coherent widespread and cold conditions in Turkey occurred
in the winter of 1991–1992 which was the highest winter NFDs at 31 stations for the period under examination. This situation was closely linked to the positive
anomaly conditions of the AOI dominating in this year
(1992 AOI is +1.07) and a cooling effect of Pinatubo’s
volcanic eruption in 1991. Similarly, coherent largescale cold conditions in Turkey occurred during the
high-index AO phase in winters of 1953–1954 (AOI,
+0.18), 1975–1976 (AOI, +0.89) and 1992–1993 (AOI,
+1.52). On the other hand, the lowest number of winter frost days appeared during the strongest AO event
in 2009–2010, with the extreme low-index values of
−2.67. Accordingly pronounced negative anomalies at
NFDs at majority of Turkish stations in winters such
as 1969–1970 (AOI −1.92), 1954–1955 (AOI, −0.93)
and 1965–1966 (AOI, −1.35) are also linked to the
extreme low-index AO winters. This is very likely linked
to circulation pattern implying the advection of warm
air masses from the Atlantic Ocean to the Mediterranean basin and Turkey (Türkeş and Erlat, 2005, 2008),
when anomalously humid/temperate conditions occurred
over Turkey contributing to strong decreases in frost
days.
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The Mann-Kendall test statistics show that ANFDs
are generally characterized by a decreasing trend over
Turkey. Resultant Mann-Kendall test statistics showed
that annual frost days are mostly characterized with
a general decreasing trend over much of Turkey. The
analysis suggests a downward trend in ANFDs at 51
out of 72 stations over the study period (Figure 5).
For the study period 1950–2010, statistically significant
decreasing trends are detected at 16 stations, 9 of which
are at the 0.01 significance level. The annual decreasing
trends are highest over the continental mountainously
eastern Anatolia and the Marmara regions, and along
the Mediterranean coastline. The analysis of ordinary
parametric linear trends for the meteorological stations
located in the continental northeast and easternmost
parts of the country such as Ardahan, Iǧdır and Van
reveal a negative trend of −4 days/decade, which would
mean a decrease of 4 days in annual frost events over
a decade. Similarly, negative trends range from −2.1
(Adapazarı) to −3.5 days/decade (Gaziantep) for stations
in the Marmara and the Mediterranean regions of Turkey
(Figure 6).
The ANFDs show statistically insignificant increasing trends at 21 stations across Turkey. Positive trends
in the ANFDs are found mainly for the continental
inner-region stations of the country (Figure 5). Increasing trends are significant only at five stations (Isparta,
Burdur, Çankırı, Çorum and Karaman). These increased
numbers of frost day in the inner region stations of country may be attributed to local or micro-topographical
features and local-scale circulation influenced these stations. Almost all stations showing increasing trend in
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-1 to -0.17
-0.17 to 0
0 to 0.17
0.17 to 1
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Mann-Kendall
rank correlation
coefficents
km
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40°
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Figure 5. Spatial distribution of the Mann-Kendall rank correlation coefficients over Turkey, which is calculated for annual number of frost days
for the period 1949/1950 to 2009/2010. Here, the downward triangles represent negative and plus represent positive trends, respectively. Solid
triangles and plus represent the statistically significant trends at the 5% level of significance, which are displayed in the legend. This figure is
available in colour online at wileyonlinelibrary.com/journal/joc
Copyright  2011 Royal Meteorological Society
Int. J. Climatol. 32: 1889–1898 (2011)
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Figure 6. Inter-annual variations and long-term trends in the time series of annual numbers of frost day for some stations. Solid line represents
linear trends for the period 1949/1950 to 2009/2010.
frost days except Diyarbakır and Trabzon are located in
tectonically formed depressions, basins or valley bottom
(Figure 1). This physiographical (tectonics, topographic,
etc.) features, especially under the specific meteorological conditions accompanied with clear skies and weak
winds associated with weak pressure gradients and strong
high-pressure systems, may cause the occurrence of local
strong nighttime radiative surface cooling and a formation
of cold pools in the valleys and depression bases
4.
Conclusions and Outlook
The main conclusions of the study are summarized as
follows:
(1) Results of the study showed that for the 1950–2010
period there has been a general decrease in the
annual numbers of frost days at most of the stations over Turkey, although some regional differences existed in trends. The decrease of annual numbers of frost day indicates warming of the nighttime
temperatures, especially in winter and spring, confirmed by the increase of minimum air temperatures
Copyright  2011 Royal Meteorological Society
due to the human-induced (or enhanced greenhouse
gas-induced) climate change and urban heat island
effects. On the other hand, some stations showed
a weak increasing tendency in frost day numbers.
Spatial patterns of relative variations of frost days
are also indicative of regional-scale atmospheric circulation changes that affect variability of nighttime
minimum air temperatures.
(2) The decreasing trends in frost day numbers are not
all uniform during the study period and showed considerable decadal-scale variability. This variability
is very likely attributable to the large-scale atmospheric circulation and atmospheric oscillations such
as the Arctic Oscillation (Erlat and Türkeş, 2008;
Türkeş and Erlat, 2008) or North Atlantic Oscillation (Tatli et al, 2005) and the North Sea–Caspian
Pattern (Kutiel et al, 2002; Kutiel and Türkeş, 2005;
Gündüz and Özsoy, 2005; Tatli, 2007, etc.). Especially, the results obtained from composite analysis
provide clear evidence that the extreme phases of
the AOI have a impact significant on NFDs throughout winter season. Winter NFDs tended to increase
significantly during the high index AO phase, while
Int. J. Climatol. 32: 1889–1898 (2011)
ANALYSIS OF OBSERVED VARIABILITY AND TRENDS OF FROST DAYS IN TURKEY
they tended to decrease significantly during the low
index AO phase. According to Cramer’s tk test, lower
(higher) than long-term average NFDs during the low
(high) phase of the AO winter index are significant
at 50 (42) of the 72 stations. Spatial coherence with
the change signals is more characteristic for the continental middle regions of Turkey.
(3) Global coupled climate model simulations show,
with the general increases of nighttime minimum
air temperatures, that the number of frost days will
be fewer almost globally, but there will be greatest
decreases over the western parts of the continents
(Dai et al., 2001; Meehl et al., 2004).
(4) By considering the conclusion of Meehl et al. (2004),
we would also suggest for Turkey that changes in
frost day numbers have been very likely associated with changes in minimum air temperatures that
could affect growing season length in Turkey. Consequently, further detailed studies on the severity and
numbers of the frost events and the first (earliest)
dates of the frost events in autumn and the last (latest) dates of the frost events in spring shall reveal the
best changes.
Acknowledgement
We are grateful to both the anonymous referees for
their valuable suggestions and constructive comments
that have greatly improved our study.
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