Download Changes in atmospheric circulation over the North Atlantic and sea

INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 30: 558–568 (2010)
Published online 3 April 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/joc.1904
Changes in atmospheric circulation over the North Atlantic
and sea-surge variations along the Belgian coast during the
twentieth century
A. Ullmann,* and J. Monbaliu
Katholieke Universiteit Leuven, Hydraulics Laboratory, Kasteelpark Arenberg 40, Postbus 2448 B-3001 Heverlee, Belgium
ABSTRACT: Wintertime (October-to-March period) 99th percentile of sea level (sea surges) at Ostend has increased at a
rate of +3 mm/year (+1 mm/year) from 1925 to 2000. Relationships between daily sea surge at Ostend and five weather
regimes – Zonal (ZO), East Atlantic (EA), Greenland Above (GA), Blocking (BL), and Atlantic Ridge (AR) – over the
northeast Atlantic and Europe (40 ° W–40 ° E, 30° –70 ° N) are analysed during the period of 1925–2000. More than 70% of
sea surges ≥65 cm occur during the AR weather regime, ahead of low pressure travelling on a northern track from Iceland
to Scandinavia. The relationships between monthly/wintertime frequency of the AR weather regime and 99th percentile of
sea surge at Ostend tend to strengthen during the twentieth century: for example, correlation between wintertime frequencies
of AR and 99th percentile of sea surge increases from 0.21 in 1925–1950 to 0.74 in 1975–2000. This increase is associated
with the pressure rise over the near Atlantic, between (30 ° W and 15 ° W), (30 ° N and 50 ° N), leading to an increase in the
frequency of strong surge-related pressure gradient during AR days. Copyright  2009 Royal Meteorological Society
KEY WORDS
sea surge; sea-level pressure; weather regime; North Sea; Belgian coast.
Received 3 November 2008; Revised 23 February 2009; Accepted 4 March 2009
1.
Introduction
In low-lying, sandy coastal areas, vulnerability to shortterm and long-term rises in sea level is particularly
high (Nicholls and Hoozemans, 1996). Recent climate
models summarized by the Intergovernmental Panel on
Climate Change (IPCC) have predicted a significant
warming and global sea-level rise for the twenty-first
century (IPCC, 2007), which is expected to increase the
flooding risk along low-lying coasts. Several factors may
induce sea-level variations at different time scales. On
time scales longer than 1 year, regional- and global-scale
sea-level variations are associated with volume change
due to temperature and salinity variations (Gornitz et al.,
1982; Cabanes et al., 2001; Cazenave et al., 2002) and
mass change between ocean and continents, including ice
melting (Lambeck, 1990; Cabanes et al., 2001; Cazenave
and Nerem, 2004).
On time scales of less than 1 year, the predominant
forcing of sea-level variations is mostly related to atmospheric variability (Lamb, 1991; Tsimplis and Josey,
2001; Trigo and Davies, 2002; Wakelin et al., 2003;
Tsimplis et al., 2005), including extra-tropical storms
(Heyen et al., 1996; Bouligand and Pirazzoli, 1999;
Pirazzoli, 2000; Woth et al., 2006; Ullmann et al., 2008).
The atmospheric forcing leads to a sea surge defined
* Correspondence to: A. Ullmann, Katholieke Universiteit te Leuven,
Hydraulics laboratory, Kasteelpark Arenberg 40, Postbus 2448, B-3001,
Heverlee, Belgium. E-mail: [email protected]
Copyright  2009 Royal Meteorological Society
as the difference between the observed sea level and
the astronomical tide. Sea surges are thus important for
coastal impacts (i.e. flooding and erosion), especially for
low and sandy beaches such as most part of the Belgian coast. The inverse barometer effect is equivalent
to a 1-cm sea-level rise per hectopascal (hPa) of surface pressure drop and vice versa but the most important atmospheric factor are the onshore winds associated
with moving mid-latitude low-pressure systems (Pirazzoli et al., 2005; Ullmann et al., 2007) and cause water
to pile up against the coast.
Our main goal in this paper is to analyse the relationship between sea surge at the Ostend tide-gauge
station from 1925 to 2000 and atmospheric variability
over the northeast Atlantic and Europe (40 ° W–40 ° E;
30 ° N–70 ° N). In Section 3.1, we first analyse the interannual to multi-decadal variability of sea level and sea
surge at Ostend from 1925 to 2000. The atmospheric
circulation is then summarized by an objective weather
classification using a k-means algorithm (Section 3.2).
The weather regimes are defined as persistent large-scale
flow patterns that appear repeatedly at fixed geographical
locations, and thus organize the behaviour of synoptic
systems that affect local-scale weather during several
days or consecutive weeks. The 5-cluster solution emphasizing well-known weather regimes already observed in
this area (Vautard, 1990; Michelangeli et al., 1995; Plaut
and Simonnet, 2002; Moron and Plaut, 2003) is chosen
(Section 3.2). The mean sea-surge height during each
weather regime is analysed, along with the frequency
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NORTH-ATLANTIC SEA SURGE VARIATIONS
Figure 1. Location of Ostend tide-gauge station.
of weather regimes for different sea-surge levels (Sections 3.3 and 3.4). The relationships between sea surge
and frequency of each weather regime at inter-annual to
multi-decadal time scales are then analysed (Section 3.5).
Concluding remarks are provided (Section 4).
the value of the yearly mean of daily maximum sea
level is removed from daily surge heights. The different
climatic aspects estimated this way are kept independent
as much as possible, and inter-annual variability to longterm trend of surge can be separated from ocean volume
variations (Pirazzoli et al., 2005).
2.
2.2. sea-level pressure SLP data
Data and methods
As the highest sea surges occur in winter, only the
October-to-March period (‘winter’ hereafter) is analysed
for each variable.
2.1. Sea surge
This work analyses the daily maximum time series of sealevel height measured at Ostend station along the Belgian
coast [(02° 54 E), (51° 13 N); Figure 1) for the winter
periods from October 1925 to March 2000. Sea-level
data are expressed in centimetres relatively to the same
altimetric reference, the Tweede Algemene Waterpassing
(TAW). Time (hours and minutes) of daily maximum
sea level is also available. For each daily maximum
sea level, the corresponding astronomical tide (in TAW)
has been computed in a previous study by TechnumIMDC-Alkyon (2002). The daily surge height has been
computed as the difference between the observed sea
level and the astronomical tide at the same moment. As
daily surges are derived from the daily maximum sea
level, they always correspond to surges occurring during
high tide, which is particularly interesting from an impact
perspective. The study of the inter-annual to multidecadal variability of surge height led to the decision
to separate the problem of the mean sea level (MSL)
trend and the problem of the isolated spikes (the surges)
(Pirazzoli et al., 2005; Ullmann et al., 2007). In other
words, sea-level variability associated with atmospheric
circulation (i.e. sea surges) needs to be separated from
sea-level variability associated with volume changes due
to temperature and salinity variations and mass change
between ocean and continent. Therefore, for each year,
Copyright  2009 Royal Meteorological Society
The MSL pressure available from the National Center
for Atmospheric Research (NCAR) at 1 PM UTC from
October 1, 1925 to December 31, 1939 and at noon
UTC from January 1, 1940 to March 31, 2000 has been
extracted from the NCAR web site (http://dss.ucar.edu/).
No attempt has been made to fill the missing entries in
the NCAR data or to remove the seasonal cycle. The sealevel pressure (SLP) data are not available at the exact
time of sea-surge records, but the weather regimes are
persistent large-scale patterns, and it is expected that the
one observed at noon or 1 PM is usually the same all day
long (Ullmann and Moron, 2008).
2.3. Weather classification
Weather classification has been extensively used to summarize the extra-tropical atmospheric circulation (Vautard, 1990; Michelangeli et al., 1995; Plaut and Simonnet, 2002; Ullmann and Moron, 2008). The NCAR SLP
standardized anomalies of the 93.9% of days without
missing values are weighted by the cosine of latitude
and compressed onto the 11 leading empirical orthogonal functions (EOFs) that account for 90% of the total
variance. The standard k-means algorithm (Diday and
Simon, 1976) is applied to the 11 principal components
to extract five clusters, in accordance with previous studies (Vautard, 1990; Michelangeli et al., 1995; Plaut and
Simonnet, 2002; Moron and Plaut, 2003; Ullmann and
Moron, 2008). Two hundred cluster analyses were performed with random seeds, and the classifiability index
(Michelangeli et al., 1995; Plaut and Simonnet, 2002)
that measures the average similarity within the 200 sets
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A. ULLMANN AND J. MONBALIU
Figure 2. Wintertime (October–March) 99th percentile of daily maximum (a) sea level and (b) daily sea surge at Ostend station in the period
1925–2000 (line with circle). The low-pass variations removing periods below 1/30 cycles per year superimposed as bold full line and linear
trends in dashed lines.
of clusters is used to select the best partition. In this way,
each day of the period 1925–2000 is classified into one of
the five clusters. The 4.6% of days, where all grid points
are not available, are classified in the cluster that minimizes the squared Euclidean distance with its centroı̈d,
considering only the available grid points. 1.2% days of
the whole sample have no SLP data at all and are not
considered in the following analyses.
3.
Results
3.1. Sea-level and sea-surge variability in the
twentieth century
Wintertime 99th percentile of daily maximum sea level
and of daily sea surge has been computed at Ostend
from 1925 to 2000. Note that 99th percentile values are
computed from a polynomial curve fit (6th order) to the
empirical cumulative distribution function (cdf). Wintertime 99th percentile of maximum sea level shows an
increasing trend of +3 mm/year, significant over 99%
level of confidence with a Student’s t-test (Figure 2(a)).
During the same period, the wintertime 99th percentile
of daily sea surge has increased at a rate of +1 mm/year
(p > 99%) and especially from ∼1955–1960 to 1980
(Figure 2(b)). Moreover, the mean wintertime (October
Copyright  2009 Royal Meteorological Society
to March) sea level in the southern North Sea and along
the Belgian coast has risen with a mean speed of ∼ +
2 mm/year during the twentieth century (Van den Eynde
et al., 2007). To summarize, long-term increase in wintertime 99th percentile of sea-level height at Ostend (i.e.
extreme sea levels) during the twentieth century could
be explained as the superposition of (1) the increase in
wintertime 99th percentile of sea surges, partly associated
with increase in onshore NW winds and storminess in the
southern North Sea, especially between 1960 and 1980
(Alexandersson et al., 2000; Weisse et al., 2005), and on
(2) the slow MSL rise, mostly linked with thermal expansion (Nerem and Mitchum, 2001; Cazenave et al., 2002).
3.2.
Weather regimes
The first weather regime (Figure 3(b)) is close to the climatological mean SLP (Figure 3(a)), with a strengthening
of the Icelandic low and a strengthening and a northeastward shift of the Azores High centred over southern
Europe. This pattern is hereafter referred to as the ‘Zonal’
(ZO) weather regime. The second pattern (Figure 3(c))
shows an anomalous high pressure shifted over continental Europe. It is referred to as an ‘East Atlantic’ regime
(EA) (Plaut and Simonnet, 2002; Ullmann and Moron,
2008). The third weather regime exhibits an anomalous
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NORTH-ATLANTIC SEA SURGE VARIATIONS
Figure 3. (a) Mean sea-level pressure (in hPa) for winter (October–March) in the period 1925–2000. Mean sea-level pressure averaged during
(b) ‘Zonal’ days, ZO; (c) ‘East-Atlantic’ days, EA; (d) ‘Greenland Above’ days, GA; (e) ‘Blocking’ days, BL and (f) ‘Atlantic Ridge’ days, AR.
anticyclone over Greenland and a deep low pressure centred around 48 ° N–50 ° N over the eastern North Atlantic
(Figure 3(d)). The flow allows synoptic perturbations to
reach Europe but on a more southerly track than usual
(Rogers, 1997). This pattern is referred to as the ‘Greenland Above’ (GA) regime. The next weather regime
(Figure 3(e)) shows typical blocking extended from Eastern Atlantic to Scandinavia and Eastern Europe and a
deep cold trough over western Europe and NW Mediterranean Sea (BL for ‘Blocking’). For this weather type,
synoptic perturbations can be shifted towards the extreme
northeast North Atlantic, but they can travel over the
Mediterranean Sea (Plaut and Simonnet, 2002). The last
pattern (Figure 3(f)) is characterized by a weak ridge over
the mid-eastern Atlantic, followed by a trough stretching from Scandinavia to Eastern Mediterranean sea. This
weather regime is called the ‘Atlantic Ridge’ (AR) regime
with reference to previous studies (Michelangeli et al.,
1995; Plaut and Simonnet, 2002; Moron and Plaut, 2003;
Copyright  2009 Royal Meteorological Society
Ullmann and Moron, 2008). These five weather regime
are almost equal to those found by Ullmann and Moron
(2008) in the period 1905–2002.
3.3. Sea surges and weather regimes: mean
relationship
Each day is categorized into one of the five weather
regime, and Table I displays the mean and standard deviation of sea surge at Ostend for each weather regime over
the 1925–2000 period. The highest sea surges are clearly
observed for AR, while EA is associated with the lowest sea surges (Table I). AR are potentially associated
with low pressure travelling on a northern storm track,
between Iceland and Scandinavia, that leads to strong
onshore winds in the southern part of the North Sea
(Figure 3(f)).
Nevertheless, the standard deviation is quite large,
especially for AR, implying that there is considerable
variability in sea surges relative to the mean for each
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A. ULLMANN AND J. MONBALIU
Table I. Mean and standard deviations (in cm) of daily maximum sea surges at Ostend for each weather regime (ZO,
‘zonal’; EA, ‘East-Atlantic’; GA, ‘Greenland Above’; BL,
‘Blocking’ and AR, ‘Atlantic Ridge’) and for the climatology
(i.e. the mean and standard deviation of all the days).
ZO
EA
GA
BL
AR
Climatology
Mean sea
surge (cm)
Standard deviation
of sea surge (cm)
−4.8
−13.0
−4.9
−3.4
15.8
−3.7
24.0
19.0
20.5
22.9
27.7
24.2
Table II. Column 2 (column 3): frequency of weather regimes
when sea surge at Ostend >65 cm (in all data) in the October-to-March period from 1925 to 2000.
NCAR October
1925–March 2000
ZO
EA
GA
BL
AR
% When sea
surge >65 cm
% In total
8.3
5.7
5.0
11.0
70.0
20.0
25.2
15.5
17.3
22.0
weather regime (Table I). It is interesting to reverse the
analysis and to investigate the mean frequency of each
weather regime associated with a class of sea surge at
Ostend. It is clear that the mean weather regime for AR
is associated with the highest sea surges (Table II). A
sea surge >65 cm at Ostend (i.e. the wintertime 99th
percentile of sea surges in the period 1925–2000) almost
never occurs during GA, but could rarely occur during
ZO and also BL (Table II). In summary, every AR day
is not associated with a sea surge, but a large sea surge
at Ostend occurs almost exclusively during this weather
regime.
3.4. Weather regime and sea surge during ‘Atlantic
Ridge’ days
Every weather regime describes a consistent, large-scale
mean atmospheric state but there are regional-scale
variations in the SLP pattern within each weather regime.
As shown in Section 3.3, sea surges ≥65 cm at Ostend
occur mostly during AR phases. Figure 4 shows the
mean SLP for AR weather regimes when the sea surge
<0 cm at Ostend (Figure 4(a)) and ≥65 cm at Ostend
(Figure 4(b)). The SLP patterns are, by definition, consistent at the large scale, but there are clear regional-scale
variations. In AR, sea surges ≥65 cm are associated with
a deep low pressure <995 hPa covering Scandinavia
between 55 ° N and 65 ° N and high anticyclonic conditions over the near Atlantic between 30 ° N and 50 ° N
(Figure 4(b)). A strong southwestward pressure gradient
then leads to a northwesterly flow that acts to increase
the sea surge along the Belgian coast (Figure 4(b)). However, when the sea surge <0, the deep low pressure is
centred between Iceland and British Islands and the high
pressure shows a southerly shift (Figure 4(a)). The southwestward pressure gradient on Western Europe vanishes
and shifts southward (Figure 4(a)). In summary, a strong
southwestward pressure gradient across the northern part
of western Europe in relation with a low (high) pressure
centred over Scandinavia (over the near Atlantic between
30 ° N and 50 ° N) is the main atmospheric forcing for a
rising sea surge ≥65 cm at Ostend, the frequency and
amplitude of this eastward pressure gradient being itself
modulated by the weather regime.
The possible influence of the length of a persistent
spell on the occurrence and amplitude of sea surges is
studied. Maximum sea-surge height has been averaged
for different duration of AR, expressed in consecutive
days. Sea-surge height taken on the last day only of AR’s
episodes of different duration has also been averaged
over the full analysis period 1925–2000. There is a
clear relationship between the length of the sequence
of AR and the maximum height of sea surge observed
during the sequence (Figure 5). Nevertheless, it is clear
that maximum sea-surge heights are not reached on the
last day of the sequence of AR (Figure 5). In other
words, the peak sea surge does not occur on the last
day of AR but on the day when optimum conditions
of low SLP and northwesterly winds at Ostend occur.
To summarize, a long sequence of AR weather regime
increases the occurrence probability of westward shifts of
AR’s low-pressure system, from between British Islands
Figure 4. (a) Mean sea level pressure (in hPa) when daily sea surges at Ostend <0 cm and (b) when daily sea surges at Ostend >65 cm in
‘Atlantic Ridge’, AR days, for the period 1925–2000.
Copyright  2009 Royal Meteorological Society
Int. J. Climatol. 30: 558–568 (2010)
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Figure 5. Maximum of daily sea surge at Ostend (full line) averaged for persistent spells of ‘Atlantic Ridge’ weather regime (in consecutive
days) for the period 1925–2000. The averaged surge value averaged taken on the last day of persistent spells (on consecutive days) is also
shown (dashed line).
and Iceland to Scandinavia (Figure 5), which favours sea
surges along the Belgian coast.
3.5. Weather regime and sea surges: inter-annual to
multi-decadal variability
The analysis of long-term variability uses the monthly
and wintertime 99th percentile of daily sea surge at
Ostend since 1925 to focus on the highest sea surges
that have the largest potential coastal impact. Considering the mean monthly/wintertime values leads to similar
results (not shown).
Figure 6 shows the time series of the frequency of each
weather regime with the long-term variation computed
with a recursive low-pass Butterworth filter retaining
only periods longer than 1/30 cycles per year. The longterm increase in sea surge has no real counterpart in
any of the weather regime frequencies (Figure 6(a)–(e)).
Following the relationship between sea surge at Ostend
Table III. Correlation between wintertime (October–March)
frequency of ‘Atlantic Ridge’, AR weather regime, and the
wintertime 99th percentile of daily maximum sea surge at
Ostend for the whole 1925–2000 period (columns 1–2) and
three sub-periods (columns 3–8).
1925–2000 1925–1950 1950–1975
Raw
LF
Raw
LF
Raw
LF
1975–2000
Raw
LF
AR 0.32∗ 0.42∗∗∗ 0.21 0.22 0.18 0.11 0.74∗∗∗ 0.76∗∗∗
One, two and three stars indicate the two-sided 90, 95 and 99% level
of significance according to a random-phase test (Janicot et al., 1996;
Ebisuzaki, 1997).
Copyright  2009 Royal Meteorological Society
and AR weather regimes (Section 3.3), the stationarity of
the wintertime frequency of AR days in the 1925–2000
period (Figure 6(e)) should be associated with stationary
wintertime frequency of daily sea surge. In fact, if no
changes in the occurrence of the surge-related optimum
conditions of low SLP and northwesterly winds along the
Belgian coast associated with AR weather regimes occur
during the period 1925–2000, there is no clear reason
to have an increase in surge height at Ostend. However,
as shown in Section 3.1, the wintertime 99th percentile
of sea surge at Ostend has increased in the 1925–2000
period. Such similar paradoxical long-term variability has
also been found by Ullmann and Moron (2008) between
increase in sea surges in the Mediterranean Sea and
‘Greenland Above’ weather regime.
Correlations between wintertime frequency of AR
weather regimes and 99th percentile of daily sea surge
are computed for the whole period and for three subperiods of 25 years (Table III). Correlations are given
for raw time series and low-pass filtered time series
(referred to as LF hereafter), which are residuals from
a recursive low-pass Butterworth filter retaining only
periods longer than 1/30 cycles per year to analyse longterm variability only. The correlation between LF sea
surge and AR weather regime frequencies is significant
for the full period 1925–2000 (Table III). Considering
the three sub-periods, it is interesting to note that the
correlation is significant only for the last sub-period
1975–2000 (Table III), implying that the relationship
between surges at Oostende and AR weather regime is not
stable. Figure 7 shows the running correlations between
the monthly frequency of AR weather regime and the
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A. ULLMANN AND J. MONBALIU
Figure 6. Wintertime (October–March) frequency (in %) of (a) ‘Zonal’, ZO; (b) ‘East Atlantic’, EA; (c) ‘Greenland Above’, GA; (d) ‘Blocking’,
BL and (e) ‘Atlantic Ridge’, AR days (line with circle) for the period 1925–2000, with the low-pass variations retaining periods longer than 30
cycles per year as bold line.
monthly 99th percentile of daily sea surge at Ostend
over periods of 60 months (i.e. 10 years) from 1925 to
2000. A monthly time scale is chosen here to increase
the sample size. The correlations between the monthly
99th percentile of daily sea surge and frequencies of AR
significantly increase at the end of the twentieth century,
especially from about 1965 (Figure 7; Table III).
Copyright  2009 Royal Meteorological Society
The variation in the relationship between sea surge and
weather regime frequencies could be explained by several
factors. A first possible factor is that low-frequency
variability in running correlations, particularly between
indices of inter-annual modes of climatic variability
(like between our 99th percentile value of surge and
frequency of AR), may be affected by pure stochastic
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Figure 7. Running correlation (on 60-month segments) between monthly (October–March) 99th percentile of daily sea surge at Ostend and the
monthly frequency of ‘Atlantic Ridge’ weather regime for the 1925–2000 period.
processes (Gershunov et al., 2002). A statistical test
is performed for measuring the probability to achieve
the deviations amongst the three correlations computed
on sub-periods (Table III). Raw and LF time series of
wintertime frequencies of weather regimes and 99th
percentiles of daily sea surge are randomly permuted by
pairs 1000 times, and the correlations are recomputed on
the permuted pairs using the same three sequences as
those shown in Table III. The computed correlations are
then sorted in ascending order. The standard deviation
of the correlations between raw and LF time series of
wintertime frequency of AR and 99th percentiles of
daily sea surges is outperformed by less than 1% of the
permuted pairs in the 1975–2000 period.
The variation in the correlation between sea surge and
AR weather regime frequency could also be associated
with atmospheric changes. Such a change has been also
observed in other parts of northern Europe using other
data and methods (Wakelin et al., 2003; Jevrejeva et al.,
2005). We observed that the highest sea surges are associated with a strong southwestward pressure gradient
between near Atlantic [(30 ° W–15 ° W), (30 ° N–50 ° N)]
and Scandinavia [(15 ° E–30 ° E), (55 ° N–65 ° N)] (Figure
4(b)). Figure 8 displays the low-pass filtered evolution,
retaining periods longer than 1/30 cycles per year of some
of the components of this SLP pattern during AR days
only. Figure 8(b) shows a clear increase in the SLP over
the near Atlantic between 30 ° N and 50 ° N, particularly
from ∼1960 to 1980, synchronous with the increase in
the Azores high and the positive deviation of the North
Atlantic Oscillation shown in previous studies (Hurrell
and van Loon, 1997; Machel et al., 1998). The linear
trend computed on the whole available period shows a
mean rate of +0.026 hPa/year with over 99% confidence
according to a Student’s t-test. The behaviour of the SLP
over Scandinavia stays statistically stationary in spite
of an important multi-decadal variability (Figure 8(a)).
The wintertime frequency of daily barometric gradient
between both sectors clearly increases since ∼1960 at
Copyright  2009 Royal Meteorological Society
a rate of +0.12%/year with a level of confidence over
99% (Figure 8(c)), synchronous with the SLP rise over
the near Atlantic between 30 ° N and 50 ° N (Figure 8(b)).
The wintertime frequency of such daily barometric gradient is significantly lower in the period 1925–1975 than
in the period 1975–2000, synchronous with the weakest
correlation between the wintertime frequency of AR days
and the wintertime 99th percentile of daily sea surge at
Ostend (Table III).
These changes could modify the sensitivity of sea
surges’ variability along the Belgian North Sea coast to
the variation in the weather regime frequency. This is
illustrated in Figure 9. The pair of monthly frequency
of AR and 99th percentile of daily sea surge on the
running 10-year period is ordered according to the
frequency of AR. The means, corresponding to the
lower (i.e. less AR than 10-year mean) and upper (i.e.
more AR than 10-year mean) halves are then computed.
The mean frequency of AR is almost stationary in
the 1925–2000 period (not shown, but mean of upper
and mean of lower half of %AR shown in Figure 9
are already almost stationary). The long-term increase
in the mean 99th percentiles of sea surge for both
samples is associated with the increase already observed
(Figure 2(b)). The mean 99th percentile of daily sea
surge associated with the frequent AR [AR(+)] increases
from ∼1960 to 1980, synchronous with the pressure
rise over the near Atlantic between (30 ° W–15 ° W),
(30 ° N–50 ° N) shown before. During the same period,
the mean 99th percentile of sea surge associated with
the rare AR [AR(−)] is almost stationary (Figure 9).
It helps to understand how the pressure rise over the
near Atlantic could increase the sensitivity of sea surges
when AR is more frequent than the mean conditions.
This asymmetric evolution (Figure 9) contributes to the
increase in the correlation between winter AR frequency
and 99th percentile of daily sea surge (Table III and
Figure 7).
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A. ULLMANN AND J. MONBALIU
Figure 8. (a) Low-pass filtered (retaining periods longer than 1/30 cycles per year) wintertime (October–March) mean sea level pressure (SLP)
averaged over (15° –30 ° E), (55° –65 ° N), and over (b) (30° –15 ° W), (30 ° N–50 ° N) on the 1925–2000 period. The mean and the frequency
are computed on days belonging to ‘Atlantic Ridge’ only.(c) Low-pass filtered frequency in ‘Atlantic Ridge’ days of SLP >1020 hPa over
(30 ° E–15 ° E), (30 ° N–50 ° N) associated with SLP <1000 hPa over (15 ° E–30 ° E), (55 ° N–65 ° N).
4.
Conclusion
From 1925 to 2000, wintertime (October-to-March
period) 99th percentile of daily maximum sea level
at Ostend shows an increasing trend of +3 mm/year
(Figure 2(a)). During the same period, the wintertime
Copyright  2009 Royal Meteorological Society
99th percentile of daily sea-surge height has risen at
a rate of +1 mm/year (Figure 2(b)). The relationship
between daily sea surge at the Ostend tide gauge station
and five weather regimes over the northeast Atlantic and
Europe is analysed over the 1925–2000 period. Overall,
AR is associated with the highest sea surges, while EA
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567
Figure 9. Mean of the upper (line with circle) and lower (line with square) halves of monthly frequency of ‘Atlantic Ridge’ (AR) days on
running 60-month segments with the associated mean 99th percentile of daily sea surge at Ostend for the 1925–2000 period. Full (dashed) line
gives the surge level when AR is more (less) frequent than the running 60-month mean.
is associated with the lowest ones (Table I). More than
70% of sea surges ≥65 cm occur during AR weather
regimes, ahead of low pressure travelling on a northern track from Iceland to Scandinavia associated with
strong onshore surface winds over the Belgian coast.
Nevertheless, the occurrence of this weather regime is not
necessarily associated with a high sea surge. In particular, the sea surge <0 cm when the low-pressure system in
AR stays stationary between British Islands and Iceland
(Figure 4(a)). These features emphasize regional-scale,
SLP differences within each weather regime. High sea
surges are systematically associated with a strong southwestward pressure gradient across Western Europe that
leads to a northwesterly onshore flow and low SLP over
the Belgian coast.
Sea surges’ heights at Ostend show an increase from
1925 to 2000, which is apparently not related to slow
variations in the frequency of AR weather regimes.
Beyond this long-term rise, there is an increase in
the correlation between 99th percentiles of daily sea
surge at Ostend and frequency of AR weather regimes
(Figure 7 and Table III). This variation does not seem
to be explained by pure stochastic processes. Possible
factors could be long-term changes in the mean SLP
field. For example, the long-term increase in the surface pressure over the near Atlantic, between Spain
and British Islands (Figure 8(b)), and the frequency of
a high southwestward pressure gradient over western
Europe (Figure 8(c)) could induce a stronger sensitivity of the sea surge to the AR weather regime at the
end of the twentieth century when it occurs frequently
Copyright  2009 Royal Meteorological Society
(Figure 9). This slow change needs to be considered in
any climate change study using future large-scale atmospheric conditions to infer regional-scale and local-scale
variations (i.e Von Storch and Reichardt, 1997), such
as sea surges and extreme sea levels along the Belgian
coast.
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