Study of tides and sea levels at Deception and Livingston islands, Antarctica

Abstract

During the 2007–08 Spanish Antarctic campaign, two moorings of bottom pressure sensors were carried out over a ten week period. This paper presents the results of the tidal analysis from sea level records obtained at Deception and Livingston islands (South Shetland Islands, Antarctica). The main objective of this paper is to present a detailed study of the tidal characteristics at these two islands, for which statistical and harmonic analysis techniques are applied to the tidal records. A geodetic network was used to reference the pressure sensors. Geometric levelling, with an accuracy of 1 mm, allowed us to link the tidal marks with geodetic vertices located on Livingston and Deception islands. The amplitudes and phase lags obtained by harmonic analysis are compared to the harmonic constants of several coastal stations and co-tidal and co-range charts. Results show an evident influence of tides in the sea level signal, with a clear mixed semi-diurnal behaviour and a daily inequality between high and low waters. Measurements of salinity and temperature were made using electronic sensors. Results from this study showed that salinity and temperature were strongly influenced by tides. Seawater temperature varied in a manner that was consistent with the time series of residual bottom pressure.

Full text

Antarctic Science 24(2), 193–201 (2012) &Antarctic Science Ltd 2011 doi:10.1017/S095410201100068X
Study of tides and sea levels at Deception and Livingston
islands, Antarctica
JUAN VIDAL
1
, MANUEL BERROCOSO
2
and ALBERTO FERNA
´NDEZ-ROS
2
1
Centro Andaluz de Ciencia y Tecnologı
´as Marinas (CACYTMAR), Universidad de Ca
´diz, Campus Rı
´o San Pedro s/n. 11510,
Puerto Real, Ca
´diz, Spain
2
Laboratorio de Astronomı
´a, Geodesia y Cartografı
´a, Departamento de Matema
´ticas, Universidad de Ca
´diz, Facultad de Ciencias,
Campus Rı
´o San Pedro s/n. 11510, Puerto Real, Ca
´diz, Spain
juan.vidal@uca.es
Abstract: During the 2007–08 Spanish Antarctic campaign, two moorings of bottom pressure sensors were
carried out over a ten week period. This paper presents the results of the tidal analysis from sea level
records obtained at Deception and Livingston islands (South Shetland Islands, Antarctica). The main
objective of this paper is to present a detailed study of the tidal characteristics at these two islands, for
which statistical and harmonic analysis techniques are applied to the tidal records. A geodetic network was
used to reference the pressure sensors. Geometric levelling, with an accuracy of 1 mm, allowed us to link
the tidal marks with geodetic vertices located on Livingston and Deception islands. The amplitudes and
phase lags obtained by harmonic analysis are compared to the harmonic constants of several coastal stations
and co-tidal and co-range charts. Results show an evident influence of tides in the sea level signal, with a
clear mixed semi-diurnal behaviour and a daily inequality between high and low waters. Measurements of
salinity and temperature were made using electronic sensors. Results from this study showed that salinity
and temperature were strongly influenced by tides. Seawater temperature varied in a manner that was
consistent with the time series of residual bottom pressure.
Received 1 March 2010, accepted 20 July 2011, first published online 17 October 2011
Key words: harmonic constituents, seawater temperature, tidal coefficients, tidal gauge benchmark
Introduction
Tidal observations made in the Antarctic continent are sparse.
Very few records have been made over a sufficient length of
time to enable the main tidal constituents to be obtained
(SCAR 1993, King & Padman 2005). Instead, co-tidal and
co-range charts and the interpretation of satellite altimetry
data are frequently used. It is very important to obtain
series of direct sea level measurements in order to study
the propagation of tides around the Antarctic continent.
Furthermore, on the longer timescale, the oceans play an
important role in determining the Earth’s climate and its
variations, particularly in Polar Regions. Therefore, whenever
possible, the data from direct observations should be linked to
reference levels (i.e. to permanent benchmarks) to study its
possible variations. If the tidal observations are measured
based on permanent benchmarks, it is possible to study the
variations in time of the mean sea level (MSL) for different
observations in the region of study. For this purpose, the
observed water levels are based on a geodetic reference
surface, enabling them to be studied with respect to a reference
level. The aim of this paper is to study the sea level data series
obtained at Livingston and Deception islands, and to establish
an approximate mean sea level for the period of time covered.
The measurements were obtained at Deception and
Livingston islands (Fig. 1, stations 1 and 2 as DECMAR
and LIVMAR, respectively) given the presence of two
Spanish Antarctic bases (Gabriel de Castilla Base and Juan
Carlos I Base, respectively). Deception and Livingston
islands are part of the South Shetland Islands group that
forms the northern boundary of the Bransfield Strait, with
the Antarctic Peninsula forming its southern boundary.
The Strait is protected from the open ocean by Smith, Snow
and Livingston islands to the north and west and by King
George Island to the north and east (Lenn et al. 2003).
Deception Island is an active volcano that has a central flooded
caldera, Port Foster, which opens onto Bransfield Strait
through a shallow and narrow sill at Neptunes Bellows
(Smith Jr et al. 2003). Livingston is the second largest island of
the South Shetland Islands, and is 60 km long and variable in
width, from 3–32 km. Johnsons Dock is a small cove located at
the south of Livingston Island near the Juan Carlos I Spanish
Antarctic Base (BEJC). It is a shallow semi-closed bay situated
near the BEJC, where a geodesic point has been installed.
Hydrographic stations and current meter moorings were
used to estimate circulation and transport in the eastern
basin of Bransfield Strait by Lo´pez et al. (1999). In addition
to hydrographic and current meter data, these authors
moored a number of sea level gauges (Aanderaa water level
recorders) at Low Island, King George and Livingston
islands (Fig. 1, stations 6, 7 and 4 respectively). In their
study, they placed emphasis on the tidal character of the
193

currents and the relative importance of tidal flow in the
general hydrodynamics of the strait.
According to these authors, tides in the Bransfield Strait
have a combination of diurnal and semi-diurnal frequencies,
with the major tidal components O1 (0.0387 cycles per hour
(cph)), K1 (0.0418 cph), M2 (0.0805 cph) and S2 (0.0833 cph).
The tides here were observed to vary from diurnal to semi-
diurnal on a fortnightly timescale, with maximum tidal ranges
between 1.7 and 2.1 m. Of these three stations, the closest to the
Livingston Island (62830'S, 60823'W) has a maximum variation
in level of 1.98 m. The M2 component is the most important in
all cases, with amplitude of 0.39 m and phase lag of 2778at the
cited station. The components K1, O1 and S2 present similar
amplitudes, of around 0.28 m (Lo´pez et al. 1994).
Garcı´a (1994) identified the major contributions to
astronomical variations in sea level at Low Island, King
George and Livingston islands. From comparisons of the
phase lags, it was shown that the M2 wave propagation is
perpendicular to the direction of the Bransfield Strait, while
the diurnal waves travel longitudinally towards the south-
west along the coast, with a NE–SW direction.
A detailed description of the propagation and
amplification of the tide at the northern side of the Antarctic
Peninsula (Gerlache Strait, Bransfield Strait and north-western
Weddell Sea) is given in a study by Dragani et al. (2004).
Based on the co-tidal and co-range charts, these authors
reported amplitudes and phase lags of the main tidal
constituents (M2, S2, O1 and K1) in the Bransfield Strait.
For their study, these authors used the results of the
harmonic analysis of series of direct sea level measurements
(Speroni et al. 2000, D’Onofrio et al. 2003). In the cited work,
the stations were located at Half Moon station on Livingston
Island (Fig. 1, station 3), and at Pendulum Cove (Pend station)
and Whalers Bay (Ball station) on Deception Island (Fig. 1a).
Sea levels were measured with a floater at Half Moon station
(HM) and a visual tide staff at the stations of Pendulum Cove
(PC) and Whalers Bay (WB). The lengths of time covered by
the records were very variable: 38 (HM), 19 (PC) and 12 days
(WB). Table I presents the harmonic constants of the principal
tides at these three stations.
The main tidal constituents can also be obtained through
tide models recently developed for this area. A high-resolution
Fig. 1. Location of tidal observations:
15Deception Island (DECMAR tidal
station), 2 5LIVMAR tidal station, 3
and 4 5Livingston Island, 5 5Jubany
tidal station, 6 5King George Island,
75Low Island, and 8 5pelagic tidal
station. a. Detailed map of Deception
Island with DECMAR, PEND and
BALL tidal stations and REGID
Geodesic Network with COLA
geodesic datum. b. Detailed map of
Johnsons Dock with LIVMAR tidal
station and BEJC geodesic datum on
Livingston Island.
Table I. Harmonic constants for the principal tidal constituents at Half Moon station on Livingston Island, and Whalers Bay and Pendulum Cove
stations on Deception Island (Dragani et al. 2004). Phase lags are referenced to Greenwich.
Station Half Moon Pendulum Cove Whalers Bay
Instrument Floater Visual tide staff Visual tide staff
Period of observation 38 days 19 days 12 days
Tide component Phase lag (8G) Amplitude (m) Phase lag (8G) Amplitude (m) Phase lag (8G) Amplitude (m)
M2 281 0.43 280 0.46 281 0.44
S2 335 0.24 X 0.28 X 0.29
O1 49 0.28 55 0.29 48 0.29
K1 66 0.28 73 0.26 66 0.26
194 JUAN VIDAL et al.

(1/308x1/608) regional model of the Antarctic Peninsula
region and Weddell Sea has been developed by Padman et al.
(2002) and more recently described by Willmott et al. (2007).
The model calculates the main harmonic constituents for tidal
heights and flows in the narrow passages around the Peninsula
and the coastal islands. Table II shows the values obtained
using the model at Livingston and Deception islands, and the
harmonic constants of the nearest tide station PTC_4_2_33
(Smithson 1992) used in model calibration (Fig. 1, station 8).
Small differences in the amplitudes and phase lags are observed
when these results are compared with those previously obtained.
Scho¨ne et al. (1998) studied tidal gauge measurements
obtained at Jubany Station on King George Island (Fig. 1,
station 5). These authors presented the harmonic analysis
for two time series of tidal gauge records corresponding to
the periods from February–December 1996 and from
March–December 1997. A total of 12 tidal components were
presented. The M2 tidal component presents amplitudes of
0.47–0.48 m and the phase lags of 277–2788. The tidal form
factor is 0.8, calculated as:
F¼ðK1 þO1Þ=ðM2 þS2Þ:ð1Þ
Therefore, tides are described as mixed, predominantly
semi-diurnal: there are daily two high and two low waters
which show inequalities in height and time (Defant 1961).
The amplitude of the mean spring tide, calculated as
2(M
2
1S
2
), is 1.48 m.
The next section of the paper reports the oceanographic
instrumentation used and describes the data obtained during
the Spanish Antarctic Campaign of 2007–08. After describing
the data, we present the results of the data analysis of the sea
levels observed, in order to obtain the main tidal constituents
and the relative importance of the residual tide. An important
contribution of this work is the linking of these measurements
to permanent GPS benchmarks. The levelling of tidal records
with the REGID geodetic network is also presented in this
section. In the next section, we analyse the relationships
between the residual tide, seawater temperature data and
meteorological records from the station on Deception Island
to identify the non-tidal variations in sea level. Finally, we
include a discussion of these results.
Oceanographic instrumentation and data
The most serious problem for the tidal gauge station is the
freezing of the seawater. Moreover, if the sensor is
displaced by icebergs, a precise determination of the sea
level changes cannot be made. For these reasons a special
system was designed to protect the pressure sensors, and
direct measurements of the sea level were obtained by
referencing each tidal gauge survey datum to a benchmark
on land. Data were obtained using two moorings - one with
a bottom pressure sensor at 4 m at Deception Island and the
other with a bottom pressure sensor at 5 m at Livingston
Table II. Harmonic constants of the principal tidal constituents at Deception and Livingston islands obtained by Padman et al. (2002) with a numerical
model and at the PTC_4_2_23 station (Antarctic Tide Gauge Database). Phase lags are referenced to Greenwich.
Station Deception Livingston PTC_4_2_33
Location 63.008N, 60.638W 62.658N, 60.378W 62.138N, 60.688W
Source/instrument Numerical model Numerical model BPR
Resolution/period of observation 2 km 32km 2km32 km 358 days
Tide component Phase lag (8G) Amplitude (m) Phase lag (8G) Amplitude (m) Phase lag (8G) Amplitude (m)
M2 283 0.38 279 0.39 280 0.32
S2 347 0.20 342 0.20 351 0.16
N2 242 0.04 240 0.04 231 0.04
K2 347 0.06 340 0.06 352 0.05
O1 55 0.26 53 0.26 54 0.24
K1 71 0.26 68 0.25 73 0.25
P1 68 0.09 65 0.08 71 0.08
Fig. 2. A diagram of the levelling links between the marks
and GNSS (Global Navigation Satellite System) stations.
The elevation difference between the TGBM (tidal gauge
benchmark) and each of the benchmarks is determined by
accurate geodesic levelling. A perforated cylinder is used for
better reading of the instantaneous level of the sea. Scheme
for determining the zero of the tide gauge: h is the ellipsoidal
height, D is the distance from the pressure sensor to the sea
surface, H is the distance from the surface of the sea to the
land mark and L is the distance from the pressure sensor
to TGBM. This value is constant and equal to the sum of
D and H for any time.
TIDE IN DECEPTION AND LIVINGSTON ISLANDS 195

Island. TD304 SAIV pressure sensors were used, with
additional sensors of temperature and conductivity. The
accuracy given by the pressure sensor is ± 0.01% of full-
scale, which is 10 m, thus, accuracy is ,1 mm. To convert
hydrostatic pressure into a sea level equivalent height we
used h 5(P-Pa)/rg, where P is the pressure recorded by the
tide gauge, Pa is a reference atmospheric pressure (a constant
value), g is the local acceleration due to gravity and ris the
water density. Density values are calculated with data recorded
by the temperature and conductivity sensors (Unesco 1981).
The pressure sensor measures total pressure, therefore it does
not show the static effects of atmospheric pressure variations.
A variation of the reference atmospheric pressure is equivalent
to moving the sensor reference level. We have used a
reference pressure of 990.8 mb (hPa), as reference atmospheric
pressure. This value corresponds to the average air pressure in
the King George Island region and the nearby surroundings of
the Arctowski Station during the period 1978–89 (Rakusa-
Suszczewski et al. 1992). In this way, the effective sea level
for any particular time can be corrected by adding or
subtracting the changes caused by atmospheric pressure
changes on the reference values: at low enough frequency
the correction is about -1 cm of sea surface height for every
11 mb (hPa) of atmospheric pressure (Gill 1982, Chelton &
Enfield 1986, Wunsch & Stammer 1997).
The instruments were anchored near the coast (c. 30 m
offshore) to minimize errors in the subsequent referencing
to the benchmark. The tide gauge station at Deception
Island, DECMAR, was located at Colatinas Point (unofficial
name), near Neptunes Bellows. The tidal station at Livingston
Island, LIVMAR, was located at Johnsons Dock. The
locations of the moorings are shown in Fig. 1. Tide
recordings were limited to the summer season because
weather conditions are extremely severe during the rest of
the year. The sampling interval was ten minutes and the
sampling period was from December 2007–March 2008.
For tidal observations, a land benchmark was used as the
primary reference point. These tidal gauge benchmarks
(TGBM) were well-marked points located on an exposed
rock (Unesco 1994). Another reference point was GPS
benchmarks (GPSBM) near the gauges. Control points
(Control TGBM) were also established for use in the event
of possible destruction of the TGBM points. The control
benchmarks were fixed with screws for various geodetic
observations. Using geodetic levelling, the heights of
TGBMs have been calculated relative to the heights of
nearby benchmarks COLA and BEJC, respectively. COLA
is the geodetic station closest to the tidal station DECMAR
(Deception Island, Fig. 1a). For the LIVMAR tidal station
the geodetic station BEJC at the Spanish Base Juan Carlos I
on Livingston Island (Fig. 1b) was used. A Leica Wild NA2
level was used to survey the levelling line. A diagram of the
levelling links can be seen in Fig. 2. These are the nearest
points of the REGID geodetic network (Berrocoso et al.
2006). The geodetic network consists of twelve stations on
Deception Island situated around Port Foster (Berrocoso
et al. 2008), which are provided with WGS-84 geodetic
co-ordinates with respect to the International Terrestrial
Reference Frame 2000 (Altamimi et al. 2002). Both
geodetic stations have absolute co-ordinates accurate to
1 mm (Table III). Data from GPS surveys have been
processed using Bernese (Dach et al. 2007) and data from
the IGS station OHIG, situated at the Chilean O’Higgins
Station on the Antarctic Peninsula, were used for the
adjustment of the network.
Pressure measurements were connected to fixed
benchmarks ashore. The most direct method is to install a
long staff alongside the instrument zeroed to its datum
level, and the part of the staff protruding from the water is
read from a theodolite onshore (Unesco 1988). It was very
difficult to use this method, and it was only possible for the
tidal gauge installed at Deception Island (DECMAR). The
staff had to be removed to prevent its damage by sea ice, so
no further measurements were possible during the experiment.
The alternative used was to compare simultaneous readings
of the sea level against a shore based tide staff which had
previously been levelled to a benchmark, and the pressure
sensor. The shore based tidal gauge benchmark was supported
by a network of auxiliary markers to guard against damage
and loss. At DECMAR and LIVMAR stations, sea levels were
measured with a visual tide staff and optical level. These
measurements were obtained simultaneously with the tidal
records and referenced to the TGBM. In the case of Deception
Island (DECMAR), measurements were made for one hour,
every ten minutes, in a total of 14 days distributed during the
entire period of recording of the tidal gauge. Measurements at
LIVMAR station were made at the beginning and end of the
tidal recording period (two days). The measurements were
made only on days with no wind and calm sea: no sea level
observations were made on days when the height of waves
made it difficult to read the tide staff. For these measurements
we also designed a perforated cylinder which allows a better
reading of the instantaneous level of the sea, by filtering out
small fluctuations in sea level (Fig. 2).
Table III. Position (WGS-84) of the benchmarks used at Livingston (BEJC) and Deception islands (COLA). Mean sea level (MSL) in metres referenced
to the GPS benchmarks, obtained during the campaign (Preg.) and for a reference atmospheric pressure of 990 mbar (Pref.).
GNSS station X Y Z s
X
s
Y
s
Z
MSL (m)
(m) (m) (m) (m) (m) (m) Preg. Pref.
BEJC 1451089.543 -2553226.299 -5642854.597 0.003 0.003 0.006 12.33 12.36
COLA 1424578.900 -2530853.317 -5659569.913 0.003 0.003 0.009 28.83 28.85
196 JUAN VIDAL et al.

During the experiment (from December 2007–March
2008) atmospheric pressure and air temperature data from
the National Institute of Meteorology (Spain) were recorded at
the Spanish Antarctic bases Gabriel de Castilla (Deception
Island) and Juan Carlos I (Livingston Island). The air pressure
at the Deception Island station ranged from a low of 961 hPa
on 19 February 2008 to a high of 1006 hPa on 18 December
2007. The mean air pressure recorded was 988 hPa, very
similar to the value of reference pressure used. Daily averaged
air temperatures at the Deception Island station were similar
to those measured during the same period at the Livingston
Island station, with minimum temperatures of -2.28Cand
maximum of 6.28C.
Analysis of data and setting of sea levels to REGID
network
Figure 3 shows the tide records obtained by the gauges
installed at the Deception and Livingston stations. The time
series show a maximum range, defined as the largest
difference between a maximum and the following minimum,
of 2.14 m at Livingston and 2.13 m at Deception. The record
from the Livingston Island station had to be corrected due to
vertical displacement of the pressure sensor. This displacement
occurred only once and corresponded to the sensor falling
inside the structure during maintenance work. We have
calculated the difference in height when the sensor changes
from vertical to horizontal position. This difference coincides
with the step recorded in the time series.
Tidal harmonic analysis using the method of Foreman
(Foreman 1977) was performed on the tidal elevation data
to evaluate the amplitudes and phase lags for each
resolvable tidal constituent. Tidal harmonic amplitudes
and relative phases, in Universal Time zone, with their 95%
confidence intervals, were obtained using a time interval of
one hour. The P1 and K2 constituents have significant
amplitudes in the tides of the Bransfield Strait (Lo´pez et al.
1993). Since the records do not cover a sufficient length of
time to obtain P1 and K2 (Godin 1972), the amplitudes and
phase lags of P1 and K2 were inferred using the known
amplitudes and phase lags for these constituents at the
adjacent Jubany Station, where the tidal form is similar to
that of the stations under study. In Table IV, the seven most
Fig. 3. Tidal records (grey line) and residual series (bold line)
obtained at a. DECMAR station, and b. LIVMAR station.
Fig. 4. Godin-filtered time series (bold dotted line), daily
averages (bold solid line) and residual series (grey line) at
a. LIVMAR station, and b. DECMAR station.
Table IV. Harmonic constants for the principal tidal constituents at the DECMAR station on Deception Island and at the LIVMAR station on
Livingston Island. Phase lags are referenced to Greenwich. FIT represents the fraction of variance explained by the predicted tides.
DECMAR (Deception Island) LIVMAR (Livingston Island)
Location 63800'S, 60838'W 62839'S, 60822'W
Period of observation 10/12/2007–29/02/2008 13/12/2007–25/02/2008
Tide component Amplitude (m) Phase lag (8G) Amplitude (m) Phase lag (8G)
M2 0.40 281 0.42 279
S2 0.26 351 0.27 334
N2 0.05 232 0.03 230
K2 0.06 347 0.08 338
O1 0.27 53 0.25 52
P1 0.10 65 0.10 72
K1 0.29 74 0.30 72
Mm 0.04 120 0.04 104
MSf 0.07 317 0.05 347
2(M21S2) 1.32 m 1.38 m
2(k11O1) 1.14 m 1.10 m
F 0.86 0.80
FIT 0.86 0.84
TIDE IN DECEPTION AND LIVINGSTON ISLANDS 197

important tidal constituents for the Deception and Livingston
stations have been obtained by fitting a set of tidal harmonics
to the time series using the method of Foreman.
The Deception and Livingston stations have the mixed
type, mainly semi-diurnal tides, with tidal form factors (Eq. 1)
of F 50.86 and F 50.80 respectively. The range 2(M21S2),
which is 1.32 m and 1.38 m respectively for the Deception
and Livingston stations, shows the mean spring tide range.
The sum of the amplitudes of the principal constituents
M21S21K11O1 are about 1.23 m and 1.24 m, respectively.
The tidal residual, obtained as the difference between the
observed and predicted (Foreman tide fit) levels, has
maximum value of 0.13 m at Deception. In general, the
tidal residual in the records made at Livingston is similar to
that found at Deception, with a maximum amplitude of
0.18 m (Figs 3 & 4). In Fig. 4, the presence of tidal signal in
the residual series, which could not be extracted by the
harmonic analysis, can be observed.
The sensor reference levels were linked to the TGBM by
linear fitting of the instantaneous measurements of the sea
level observed by tide staff to the data obtained from the
pressure sensors (Fig. 2). In other words, the tide gauges are
referenced by matching their readings to the observations
on a TGBM installed beside the tidal station. As noted
above, the values of pressure sensors do not show the effects
of static atmospheric pressure and corrections should be made
to correlate the two measures. However, it is necessary to
assess the extent to which the meteorological conditions, and
in particular the atmospheric pressure conditions, during that
three month period were anomalous or not representative of
the mean value in this area, because this could affect the sea
level determination. The values presented in the previous
section show that the average air pressure for the campaign
was very similar to the average value over long periods of
time obtained by Rakusa-Suszczewski et al. (1992). For this
reason, we utilize the reference atmospheric pressure (990 hPa)
for the calculation of the MSL. In order to correlate the two
measures, data from the meteorological station at Deception
were used to correct sea level height variations, due to changes
in atmospheric pressure, in the visual tide staff data. According
to the inverse barometer approximation, the sea level
adjustment implies essentially negligible surface pressure
gradients and currents resulting from Pa fluctuations. The
calculated MSL is not strictly the sea level but an ‘‘inverse
barometer-corrected sea level’’ with respect to the specific
atmospheric pressure reference. Nevertheless, in a second
step, we calculated the true MSL. The process is the reverse
of that presented above. For this, sea level data were
recalculated using atmospheric pressure measured at
Deception Island. Then an adjustment is made between
the recalculated data and visual tide staff data.
The correlation coefficients between tide gauges data
and visual tide data are r
2
599.1% for Deception station
and r
2
599.8% for Livingston station. The Pvalues for
all coefficients are less than 0.001. The slopes obtained by
linear regression are 0.98 and 1.10 respectively. Linear
trends have been estimated by performing a robust linear
regression and the standard error has been used to compute
the 95% confidence intervals of the obtained trends (Fig. 5).
A datum is a base elevation used as a reference from
which to reckon heights or depths. Mean sea level as a tidal
datum is computed as a mean of hourly water level heights
observed over 18.6 years. Monthly means are generated in
the datum calculation process, and these means are used to
generate the relative local sea level trends. Shorter series
are specified in the description: monthly MSL or yearly
MSL. Unfortunately, records of tides at the two stations
cover a period of only three months. Therefore, in this
paper we define the seasonal MSL as that obtained during
the summer season. In its computation, the daily average
levels are calculated as the average of the hourly sea level
data. Table III shows the MSL referenced to the GNSS-
GPS at two stations for the period from December
2007–March 2008. The accuracy of the MSL is ± 0.04 m.
Changes in daily MSLs have been studied for three
months. Daily MSLs obtained from the residual heights at
the Deception and Livingston stations are shown in Fig. 4,
where fortnightly variations in the calculated values can be
observed. Spectral analysis of daily MSL data for ten weeks
shows periodicities of between 13 and 14 days. The Godin
filter (Godin 1972), a low-pass filter also known as tide-
killer filter (Walters & Heston 1982, Candela et al. 1989),
was used to remove short-term (less than about two days)
variations in sea level series at Deception and Livingston
stations (Fig. 4). The filtered data can be examined to
determine variations in the data that are the result of longer-
term (greater than two days) astronomical tidal variations
as well as variations resulting from non-tidal processes such
as basin inflow and meteorological patterns. Both filtered time
series exhibit subinertial variations (periods of several days to
several weeks or months, Lacombe & Richez 1982) with
timescales of c. 13–15 days. This periodicity is also observed
in the residual series of elevations, together with other higher
frequency oscillations, and may coincide with the tidal
Fig. 5. Linear fit between instantaneous measurements of sea
levels measured by the tide gauge and heights measured with
visual tide staff and optical level at a. DECMAR, and
b. LIVMAR stations.
198 JUAN VIDAL et al.

constituent Mf (13.66 days), which is close to the MSf
component (14.76 days) but cannot be differentiated by
harmonic analysis, since the tidal series are not long enough.
An amplitude of 4.4 cm for Mf component has been obtained
by Scho¨ne et al. (1998) for Jubany Station. According to these
authors, the MSf component has an amplitude less than 2 cm
for this station. The remaining variations are studied in the
next section.
Water temperature and residual sea level
Mean sea level variability is the result of the various
forcing functions such as air pressure, wind stress, ocean
circulation and fluctuations in the heat and salt content of
the ocean. The relationships between residual sea level and
forces due to air pressure, air temperature and local wind
have been examined.
Wind speeds measured at the Deception station were
generally in the range 2–15 m s
-1
with direction predominantly
from the south-west or the north-east. During the experiment,
recorded sea levels do not show changes associated with the
wind measurement variables. On the other hand, the influence
of atmospheric pressure in the residual series should be weak,
because the atmospheric pressure variations are not registered
by the pressure sensor. The influence of the atmospheric
pressure is estimated by using the inverted barometric
approximation. In order to examine this influence, correlation
coefficients between residual sea level and barometric
approximations have been calculated. The correlation
coefficients obtained are r
2
50.01% (P,0.05) at Deception,
and r
2
50.05% (P,0.05) at Livingston. However, variations
in atmospheric pressure on the sea surface heights are of great
importance. The air pressure variations in these regions can
produce changes in sea levels of over 40 cm. Figure 6 shows
the variations of atmospheric pressure recorded during the
campaign at the Deception station.
Additionally, we analysed the hourly averaged temperature
at the terrestrial station on Deception Island over the tidal gauge
period. Weather conditions at Deception and Livingston were
highly variable although a slight increase in temperature at both
tide stations during the experiment has been observed,
coinciding with summer.
In Antarctic coastal zones, the meteoric water, mostly in
the form of glacial ice melt, is the dominant freshwater
source, accounting for up to 5% of the near surface ocean
during the summer. Seasonality is significant, with warmest
waters occurring during the summer and high salinities and
freezing-point temperatures during the winter (Meredith
et al. 2008). Water temperature presents seasonal and
diurnal variability but the seasonal cycle is dominant in
temperature series at Port Foster. Water temperatures are
minimal in September and begin to rise as solar forcing
increases (Lenn et al. 2003).
The series of seawater temperatures recorded at the
Deception and Livingston stations during the experiment have
been analysed. These series show a positive trend (warming)
from December–February (Fig. 6). A series analysis shows that
the correlation between them is over 79%. A slight drop in
temperatures is observed from February–March at Livingston
station. It is also significant that the series obtained at Deception
presents greater temperature fluctuations. Hydrothermal heating
in Port Foster may accentuate temperature fluctuations. During
the first weeks of December there are significantly higher inputs
of freshwater from melting glaciers and snow on the islands.
This freshwater input should produce changes in general
circulation within Port Foster, especially in the surface currents,
but it is not known if the meltwater can cause a rise in mean sea
level in semi-enclosed bays. Lenn et al. (2003) found that water
in Port Foster has a mean residence time of 2.4 years and 1%
volume exchange occurs over each tidal cycle. According to
these authors, Port Foster water mass characteristics differ
significantly from Bransfield Strait water mass characteristics.
Johnsons Dock also has a low rate of water exchange, but this
could not be quantified.
At temperatures near to freezing point, seawater density
depends almost entirely upon salinity (Unesco 1981, Fofonoff
& Millard 1983, Pilson 1998). Significant changes have
Fig. 6. a. Time series of residual bottom pressure at DECMAR
Station, b. seawater temperature, in grey line at DECMAR
station and in bold line at LIVMAR station, c. salinity at
LIVMAR station, and d. air pressure at DECMAR.
Fig. 7. Low frequency variations of seawater temperatures
at a. DECMAR station, and b. LIVMAR station. c. Godin-
filtered bottom pressure at DECMAR.
TIDE IN DECEPTION AND LIVINGSTON ISLANDS 199

also been observed in the time series of salinity data during
the experiment (Fig. 6). The salinity decreased from
December–February. The time series of salinity shows
large variations probably due to contributions of freshwater
from glaciers. However, at shallow depths, ,5 m, the
density changes (changes of 2 or 3 sigma-t units) cannot
explain variations in levels greater than ,1 cm.
The tidal period fluctuations are removed from the
hourly water temperature records by using the tide-killer
filter (Thompson 1983). The resulting series show the low
frequency variations in temperature (Fig. 7). Correlation
coefficients between low-pass filtered sea level records and
filtered water temperature series have been calculated. The
correlation coefficients obtained are r
2
565% (P,0.05)
at Deception and r
2
559% (P,0.05) at Livingston. If
the filtered sea levels and the water temperature series at
DECMAR are correlated by removing the first week, the
correlation rises to 68% and 59% respectively. During this
first week, Neptunes Bellows (Deception) were closed due to
the presence of field ice on the seawater. The series of filtered
air temperature was not correlated with the previous series.
Conclusions
Antarctica is a region where the data on tides are scarce.
Long data series of this sort are extremely valuable to tidal
scientists. Two tidal records of more than two months
duration were obtained at Livingston and Deception islands
during the 2007–08 Spanish Antarctic campaign. Harmonic
analysis has been used to obtain the amplitudes and phase
lags of the most energetic tidal constituents. The tidal
regime is mixed, mainly semi-diurnal. The fortnightly tides
are quite large with a range of 10–12 cm. The analysis of
the tidal residual series shows the presence of periodicities
around 13 days and there are indeed variations in daily
mean sea level due to fortnightly tides. These periodicities
could not be extracted by harmonic analysis due to the
insufficient record length.
An important conclusion is the lack of tidal records
referenced to permanent benchmarks. One of the essential
objectives of this campaign was to estimate the MSL to
establish the reference height for the islands. By referencing
the bottom pressure sensors to the benchmarks, it was possible
to calculate MSL relative to a precise levelling network for
the period analysed. Although the MSL shows variations at
low frequencies and these computations are supposed to give
only indications as to the approximate behaviour of this MSL,
these results could be very useful for the geodesic and
oceanographic studies being conducted in the area. This work
is only the first of a series of tide studies using long data series
linked to GPS benchmarks.
In order to explain the long-period sea level changes,
atmospheric pressure, wind velocity and seawater temperature
were included in the regression model. Although the sea level
data have been low-pass filtered and adjusted sea level for
barometric pressure effects, there are still pronounced
fluctuations in the filtered sea level signal. In addition to the
fortnightly variability, the tidal residual series reveals a rise in
sea levels highly correlated with seawater temperature.
However, in shallow water the thermal expansion is too
small to explain significant changes in sea surface height.
There are several possible reasons for these longer period
variations. First, it is possible that water temperature
variations (greater than two days) could indicate changes in
sea ice along the east coast of the Antarctic Peninsula. In this
case, this correlation also provides a means of assessing the
possible influence of Bransfield basin waters on the sea levels
through periodic changes in ocean hydrography. Second, it
should be noted that this work has not been able to study the
annual and interannual variations. Additionally, variations in
the filtered series can be caused by nonlinear interactions of
tidal components that generate long period oscillations. Third,
an increase in sea levels may be associated with freshwater
input from snowmelt and rainfall. The sea level changes at
Deception Island and Johnsons Dock are limited by the
exchange flow through a shallow and narrow sill. Deception
Island had a thick covering of snow, accumulated during the
winter, and this was melting during the summer months. In a
partially enclosed body of water like that of Deception Island,
these freshwater inputs can cause an elevation in the level of
the sea. Moreover, the snowmelt has a strong effect on the
seawater temperature and salinity. The presence of glaciers like
the Johnsons Glacier could produce the same effect. These
contributions, coupled with poor water exchange with open
sea, could explain why the water mass characteristics in Port
Foster and Bransfield Strait differ significantly. It is therefore
important to study how the thawing of glaciers and the
accumulated snow affect the average sea levels.
Acknowledgements
The work described in this article was made possible by the
financial support to the project: Investigaciones geode´sicas,
geofı´sicas y teledeteccio´n en la Isla Decepcio´nysuentorno
(Penı´nsula Anta´rtica, Islas Shetland Del Sur) (Cgl2005-
07589-C03-01/Ant) from the Spanish Ministry of Science
and Technology, through the National Program of Antarctic
Research of Natural Resources. We would also like to thank
the RV Las Palmas crew and the members of the Spanish
Antarctic Station Gabriel de Castilla and the Spanish
Antarctic Station Juan Carlos I for their collaboration during
the surveying campaigns. The authors thank two anonymous
referees for helpful comments on earlier versions of this
paper.
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