Sea Level Stations Research Articles

Design of a Sea-level Tsunami Detection Network for the Gulf of Cadiz

Abstract. The devastating impact of the Sumatra tsunami of 26 December 2004, raised the question for scientists of how to forecast a tsunami threat. In 2005, the IOC-UNESCO XXIII assembly decided to implement a global tsunami warning system to cover the regions that were not yet protected, namely the Indian Ocean, the Caribbean and the North East Atlantic, the Mediterranean and connected seas (the NEAM region). Within NEAM, the Gulf of Cadiz is the more sensitive area, with an important record of devastating historical events. The objective of this paper is to present a preliminary design for a reliable tsunami detection network for the Gulf of Cadiz, based on a network of sea-level observatories. The tsunamigenic potential of this region has been revised in order to define the active tectonic structures. Tsunami hydrodynamic modeling and GIS technology have been used to identify the appropriate locations for the minimum number of sea-level stations. Results show that 3 tsunameters are required as the minimum number of stations necessary to assure an acceptable protection to the large coastal population in the Gulf of Cadiz. In addition, 29 tide gauge stations could be necessary to fully assess the effects of a tsunami along the affected coasts of Portugal, Spain and Morocco.

A Tsunami Detection and Warning-focused Sea Level Station Metadata Web Service

Currently information used to describe sea-level stations (such as location, collection and transmission capabilities, operator identification, etc.) is distributed among databases held by multiple agencies, institutions and organizations. Such information could be used to support detection and warning. However, the Indian Ocean Tsunami of 26 December, 2004 made it clear that such information is not readily accessible, is difficult to use, and is often incomplete. In addressing this issue, agencies within the Pacific region are collaborating to develop a web service to expose station metadata enabling various types of real-time data mining client applications that support decision-making and strategic planning at Tsunami Warning Centers. Because information about sea levels has a broad range of applications, integration of this information in a way that is comprehensive, and enhances its access and use, would have a tremendous impact on lives and livelihoods.

Exploiting the potential of an improved multimission altimetric data set over the coastal ocean

Until now, most satellite altimetry studies of the coastal ocean have been based on along‐track data from a single mission, whereas up to four missions were operative in 2002–2005. Here, to monitor the coastal ocean we have applied specialized corrections and dedicated processing strategies to compute a multimission data set at a mean distance of 32 km of the coast. The resulting altimetric data set is compared with sea level data from three in situ stations over a coastal zone of the northwestern Mediterranean. The mean rms difference between this data set and the sea level stations is 2.9 cm against 3.7 cm when using the AVISO altimetric product. Comparison of altimeter‐derived geostrophic velocities with a mooring also shows that the spatial and temporal variability of the surface current field is well reproduced. The agreement with in situ measurements extends to intraseasonal time scales showing a significant improvement compared to previous studies in the 50 km coastal‐band.

Coastal flooding in the Northeastern United States due to climate change

With dense population and development along its coastline, the northeastern United States is, at present, highly vulnerable to coastal flooding. At five sea level stations in the United States, from Massachusetts to New Jersey, sea level rise (SLR) trends and tidal effects were removed from the hourly sea level time series and then frequency analysis was performed on the positive remaining anomalies that represent storm surge heights. Then using eustatic SLR estimates for lower and higher greenhouse gas emissions scenarios and assumed trends in local sea level rise, new recurrence intervals were determined for future storm surges. Under the higher emissions scenario, by 2050, the elevation of the 2005 100-year event may be equaled or exceeded at least every 30 years at all sites. In more exposed US cities such as Boston, Massachusetts and Atlantic City, New Jersey, this could occur at the considerably higher frequency of every 8 years or less. Under the lower emissions scenario, by 2050, the elevation of the 2005 100-year event may be equaled or exceeded at least every 70 years at all sites. In Boston and Atlantic City, this could occur every 30 years or less.

Sea level monitoring in Africa

A review is provided of the African sea level dataset, which is limited not only in size, especially given the great length of the African coastline, but also in quality. The review is undertaken primarily from Permanent Service for Mean Sea Level (PSMSL) and Global Sea Level Observing System (GLOSS) perspectives, but the conclusions on the need for major new investments in sea level infrastructure are undoubtedly the same as would be arrived at through any other approach. Stations to be installed as part of the Ocean Data and Information Network for Africa (ODINAfrica) programme are described and a survey of currently existing and planned sea level stations in Africa is presented, together with information on where data for existing stations may be found.

Effects of El Niño–Southern Oscillation on sea level anomalies along the Gulf of Mexico coast

Analyses of daily sea level data show the impacts of El Niño–Southern Oscillation (ENSO) in the Gulf of Mexico (GOM). Data from three stations (St. Petersburg, Florida, Pensacola, Florida, and Galveston, Texas), all of which have at least 50 years of daily observations, are processed to identify the interannual signals. Although low frequency (interannual) signals in the sea level anomaly time series are not clearly evident, a low frequency modulation of the extreme anomaly events (upper 10% or lower 10% of the distributions) is identified. Results show that sea level variability is seasonally dependent at all stations, with maximum variability in the winter months. In the eastern GOM, low sea level events in the winter months are more frequent during El Niño (warm phase) conditions when compared to a neutral ENSO phase. This is consistent with ENSO‐related changes in the location where extratropical atmospheric low pressure systems form and in the tracks of these weather systems. The impacts of tropical systems in the summer through early fall months on coastal sea level in the GOM are shown by infrequent extreme high and low anomalies coinciding with individual storms. However, the number of storms affecting the data record from a particular sea level station is too small to confirm ENSO‐related variability. Statistical methods are employed to demonstrate a significant link between extreme sea level anomalies in the GOM and ENSO during the October to March period.

The 26 December 2004 Sumatra tsunami recorded on the coast of West Africa

Analysis of sea-level data obtained from the Atlantic Global Sea Level Observing System (GLOSS) sea-level station at Takoradi, Ghana, West Africa, clearly reveals a tsunami signal associated with the Mw = 9.3 Sumatra earthquake of 26 December 2004 in the Indian Ocean. The tsunami arrived at this location on 27 December 2004 at approximately 01:38 UTC (which is close to the expected tsunami arrival time at that site), after travelling for more than 24 hours. The first wave was negative (trough), in contrast with the South African stations where the first wave was mainly positive (crest). The dominant observed period at Takoradi was about 42 minutes. The maximum trough-to-crest wave height (41cm) was observed on 28 December at 00:15 UTC. There were two distinct tsunami 'bursts', separated in time by about 14 hours, the larger being the second burst. A small residual lowering of the sea level (∼15cm) during the tsunami and for several days afterwards, and a delayed (∼4.5 days) lowering of seawater temperature (up to ∼4.5°C), was observed, possibly indicating the presence of internal waves through the Gulf of Guinea associated with propagating tsunami waves. The prominent tsunami signal found in the Takoradi record suggests that tsunami waves could also be found at other sites off the West African coast.

Remotely sensed winds for episodic forcing of ocean models

A new method is described for forcing regional ocean models with wind stress fields derived from satellite scatterometer data. A variational technique is applied to produce regularly gridded surface wind (stress) fields in time and space using data from the NASA SeaWinds scatterometer aboard the QuikSCAT satellite. Three uniformly gridded wind stress products are produced with satellite scatterometer data for the Gulf of Mexico, one based solely on scatterometer data and the other two constrained to a background field derived from numerical weather prediction (NWP) models. These, along with winds from the Eta‐29 NWP model, are used to force a high‐resolution ocean model of the Gulf of Mexico. The four wind products are compared to National Data Buoy Center in situ observations. Ocean model data are compared to coastal sea level stations and moored acoustic Doppler current profiler data over the West Florida Shelf. The results show that the satellite products capture much of the variability at atmospheric synoptic scales and show great promise for representing energetic episodic events such as tropical cyclones. The techniques presented in this work are applicable to improving numerical ocean simulations of inaccessible or data‐sparse regions of the world.

Sea level changes at Port Stanley, Falkland Islands

Time series of sea level change at Port Stanley present a rich spectrum of variability from seiches, tides, and storm surges through to the seasonal, interannual, and secular changes of interest to ocean circulation and climate change studies. This paper emphasizes the strategic importance of the Port Stanley sea level station within a global sea level network through analysis of the decade‐length tide gauge records from Port Stanley itself, together with sets of data from a series of temporary tide gauge deployments at several other locations in East Falkland. The characteristics of seiches and of tidal, storm surge, and mean sea level variability are described, and the frequency of extreme sea level events is determined. Port Stanley sea level variability is shown to be “inverse barometer like” over a wide range of frequencies, although with a significant dependence on the local wind field and an intriguing association with the distant tropical Pacific wind field. Altimeter and tide gauge data in combination demonstrate that the sea level variability is representative of much of the Patagonian shelf and neighboring deep ocean. Consequently, Port Stanley is an ideal site to include within a global network for the purpose of altimeter calibration. The utility of Port Stanley data for monitoring variability in the Falkland‐Malvinas Current and Antarctic Circumpolar Current is explored. Port Stanley possesses one of the longest records from a Southern Hemisphere island which indicates an upward sea level secular trend (adjusted for station air pressure) of 0.70 ± 0.18 (statistical) ± 0.25 (systematic) mm/yr since 1964.

Missing ozone at high altitude: Comparison of in situ and satellite data

An experiment was designed to measure ozone at a site on the Andes Mountains, near La Paz, Bolivia, at a height of 3420 m, with the objective of investigating the ozone reduction seen over the mountains and the increased UV‐B radiation. A Brewer spectrophotometer was used for surface measurements of the vertical ozone column and UV‐B radiation; the spatial ozone variation around the mountain site was obtained using Total Ozone Mapping Spectrometer (TOMS) data. The ozone column values show a decrease over the mountain region, which maximizes at about 20 Dobson units, in comparison to the values in the Pacific regions, to the west, and the continental values to the east, at about the same latitude. To investigate the reason for this missing ozone, the vertical ozone distribution was measured using electrochemical concentration cell ozonesondes. The ozone profiles were integrated downward to sea level. It is shown that this integral corresponds to the missing ozone over the mountain area. The strongest consequence of the reduced atmosphere at high altitude on the UV‐B intensities is not the reduction of the ozone column but that of the Rayleigh‐ scattering effect. The absence of scattering considerably increases the UV‐B radiation at high altitudes. For example, the ozone effect on the ratios of UV‐B intensities between La Paz and Natal, a sea level station, is only 1.15, while for the Rayleigh‐scattering reduction, the ratio between stations is 1.75, at 306.3 nm. Measured UV‐B intensities at both sites show La Paz/Natal ratios of this order of magnitude.

Use of the wavelet transform on hydro-meteorological data

The advantages of utilising wavelet analysis to study the dependence of sea level variability on meteorological parameters are demonstrated on data collected at a sea level station on the northwestern coast of Malta. The strength of the method lies in the fact that time information is conserved and signal variability can be resolved up to levels which would equivalently require a much longer data set with Fourier transform methods. The validity of the wavelet transform is verified in its potential to decompose and identify in detail the synoptic variability of a parameter, as well as to follow the inter-dependence of more parameters by a quantitative comparison based on a correlation of their wavelet expressions. The results of discrete wavelet analysis exhibit significant differences in the temporal development of the wavelet decompositions of residual sea level. This is even more evident in the air pressure field for which the variability is principally explained by wavelet decompositions corresponding to central frequencies of 0.56 and 0.28 cpd. The reduced activity in barometric pressure fluctuations during summer is particularly evident at these two decompositions. This work also furnishes a contribution to studies concerning the validity of the inverted barometer effect. The position and extent of the frequency range at which the local sea level variation can be approximated by the inverted barometer effect is found to depend on the amplitude of the air pressure fluctuations. During the stronger pressure variations, the inverted correlation to the sea level occurs at practically the full range of synoptic frequencies and even lower.

Wind induced sea level variability in the Red Sea

Analysis of TOPEX/Poseidon altimetry data for the Red Sea shows a large seasonal sea level signal in agreement with the few available coastal sea level stations. The results are compared with the predictions from a simple model that balances the sea surface elevation against the wind stress. The winter sea level field in the central and northern Red Sea is shown to be a response to the convergent wind pattern associated with the Indian Monsoon, consisting of NNW winds in the northern Red Sea and SSE winds in the south. There is a very good agreement between the model and observations, indicating that the basic mechanism responsible for the variability of the sea surface height inside the Red Sea on seasonal time scales is the wind pattern.

Sea surface slope as an estimator of the Kuroshio volume transport east of Taiwan

The Kuroshio volume transport (KVT) at 24.5°N near Taiwan was directly measured for the first time during the period from September 1994 to May 1996 through the channel between Taiwan and Iriomote, Japan. Adjusted sea level difference anomaly (SLDA) across the Kuroshio is examined as a proxy for long‐term monitoring of KVT. Regression analyses were performed on the directly measured KVT and SLDA between Ishigaki and sea level stations on the Taiwan side (Keelung, Kenfang and Suao). The best estimators of KVT are: (1) KVT1=0.32×SLDA1 (Ishigaki‐Suao) + 20.85 Sυ±2.3 Sυ for daily KVT and (2) KVT10=0.37 × SLDA10 (Ishigaki‐Suao) + 20.87 Sυ±1.4 Sυ for 10‐day low‐passed KVT.

Ocean Tides from TOPEX/Poseidon Sea Level Data

Abstract Tidal constants are computed from TOPEX/Poseidon sea level data in the northeast Pacific Ocean for the period of September 1992–December 1997. The method used is harmonic analysis. Tidal constituents are also calculated at track crossover locations, where twice as many observations are available. The crossover constituents are compared with nearest alongtrack constituents and with constituents at 57 pelagic sea level stations in the northeast Pacific. Examples of small-scale features in the alongtrack constituents are presented. These include a strong evidence of internal M2 tides propagating away from the Aleutian Islands and of K1 and O1 shelf waves in the northern Gulf of Alaska. Good agreement was found between the shelf waves observed in altimetry data and those calculated by a numerical tidal model.

Dynamics of Remotely Forced Intraseasonal Oscillations off the Western Coast of South America

Abstract Six years of sea level observations at six locations along the South American coast from Peru to Chile were analyzed at intraseasonal frequencies (periodicity 1–2 months). A high-resolution, low-frequency numerical coastal model, having realistic shelf and slope topography and bottom friction and forced by an equatorial signal, was used to analyze the intraseasonal coastal dynamics at these latitudes. The following results were obtained. Lagged correlations between sea level stations imply a sea level poleward propagation of 250 km day−1. The frictionless vertical wall theory poleward phase speed was considerably less at 216 km day−1. Model runs with realistic bottom friction and shelf and slope bottom topography gave poleward phase speeds much closer to that observed. For typical parameters the phase speed was 253 km day−1. Both bottom friction and topography significantly affect the propagation speed. Past work has shown that the sea level amplitude should grow alongshore like |f| (f = Coriolis...

Seasonal variations in the atmospheric O2/N2 ratio in relation to the kinetics of air‐sea gas exchange

Observations of seasonal variations in the atmospheric O2/N2 ratio are reported at nine baseline sites in the northern and southern hemispheres. Concurrent CO2 measurements are used to correct for the effects of land biotic exchanges of O2 on the O2/N2 cycles thus allowing the residual component of the cycles due to oceanic exchanges of O2 and N2 to be calculated. The residual oceanic cycles in the northern hemisphere are nearly diametrically out of phase with the cycles in the southern hemisphere. The maxima in both hemispheres occur in summer. In both hemispheres, the middle‐latitude sea level stations show the cycles with largest amplitudes and earliest phasing. Somewhat smaller amplitudes are observed at the high‐latitude stations, and much smaller amplitudes are observed at the tropical stations. A model for simulating the oceanic component of the atmospheric O2/N2 cycles is presented consisting of the TM2 atmospheric tracer transport model [Heimann, 1995] driven at the lower boundary by O2 fluxes derived from observed O2 saturation anomalies in surface waters and by N2 fluxes derived from the net air‐sea heat flux. The model is optimized to fit the observed atmospheric O2/N2 cycles by adjusting the air‐sea gas‐exchange velocity, which relates O2 anomaly to O2 flux. The optimum fit corresponds to spatially and temporally averaged exchange velocities of 24±6 cm/hr for the oceans north of 31°N and 29±12 cm/hr for the oceans south of 31° S. These velocities agree to within the uncertainties with the gas‐exchange velocities expected from the Wanninkhof [1992] formulation of the air‐sea gas‐exchange velocity combined with European Centre for Medium‐Range Weather Forecasts winds [Gibson et al., 1997] but are larger than the exchange velocities expected from the Liss and Merlivat [1986] relation using the same winds. The results imply that the gas‐exchange velocity for O2, like that of CO2, may be enhanced in the open ocean by processes that were not systematically accounted for in the experiments used to derive the Liss and Merlivat relation.

Mean sea surface topography in the Baltic Sea and its transition area to the North Sea: A geodetic solution and comparisons with oceanographic models

A consistent height system designed for comparisons between geodesy and oceanography is presented for the Baltic Sea area. Mean sea surface topography is computed geodetically in this height system at 42 reliable long‐term sea level stations, connected by high‐precision levelings, along the coasts of the Baltic Sea, the Kattegat, the Skagerrak, and the adjacent part of the North Sea. Based on this, a map is drawn of the mean sea surface topography in the Baltic Sea and its transition area to the North Sea. The general agreement between our geodetic solution and recent oceanographic models turns out to be excellent; the discrepancies rarely exceed 2–3 cm. Hence the mean sea surface topography in the Baltic Sea area to a dominating extent is caused by the distribution of salinity. There are two main characteristics of the sea surface topography found here. First, there is a continuous increase of the sea surface height from the North Sea into the Baltic Sea, the height difference between the inner part of the Gulf of Bothnia and the Skagerrak amounting to 35–40 cm. The main reason behind this is the considerable difference in salinity, close to the maximum possible one. Second, there is a steep sea level gradient in the border zone between the Kattegat and the Skagerrak, reaching 2 cm per 10 km. This reflects the salinity front there, separating the brackish Baltic Sea water from the saline North Sea water, and the associated Baltic current. A local maximum in the sea surface can be seen in the Oslo Fiord, reflecting an accumulation of low‐salinity water there. An area where there is still no oceanographic model agreeing sufficiently with the geodetic solution is the Belt Sea/southwestern Kattegat.

A consistent map of the postglacial uplift of Fennoscandia

ABSTRACTA consistent map of the recent postglacial rebound of Fennoscandia is constructed on the basis of sea‐level records, lake‐level records and repeated high‐precision levellings.The uplift rates calculated from the sea‐level series form a consistent framework of the map. The sea‐level stations used are 56 reliable stations in the Baltic Sea and adjacent waters with series spanning 60 years or more, many of them about 100 years. Using a reference station in the Baltic Sea and another one outside the Baltic, all results are reduced to a common time span, the 100‐year‐period 1892–1991, in order to eliminate oceanographic changes. Inland, uplift differences are obtained from the repeated national levellings and, in four of the large lakes, from long water level series in pairs. The levellings, however, yield less accurate land uplift values than the sea‐level and lake‐level data.The resultant map shows a fairly smooth phenomenon, with a maximum apparent uplift in the Gulf of Bothnia of 9.0 mm yr‐1. The standard error is typically 0.2 mm yr‐I close to the sea level stations, larger inland.Finally the pattern of the present uplift as determined here is observed to be very similar to that of the past uplift as determined from ancient shore‐lines of the Litorina Sea. However, the ratio between the past uplift and the present uplift rate tends to increase somewhat towards the uplift centre. This might reflect a non‐uniform mantle viscosity. Also, the uplift maximum seems to have migrated towards NNE.

Global sea level observing system (GLOSS)

The Intergovernmental Oceanographic Commission (IOC) was founded in 1960 as a functionally autonomous body within UNESCO and now includes 124 member states. The IOC promotes marine scientific investigations and related ocean services with a view to learning more about the nature and resources of the oceans and coastal areas. In 1985, the Intergovernmental Oceanographic Commission (IOC) initiated the Global Sea Level Observing System (GLOSS) as an international system to provide high‐quality standardized sea level data from a global network of sea level stations. Some 300 sea level stations maintained by 80 countries have been selected as a GLOSS network to observe large‐scale sea level variations of global implications and to serve as the framework for other regional and scientific programs as well as national practical applications. This article provides a description of the GLOSS and the progress in achieving its objectives, observational network, data exchange, present and new technology for sea level ...

Long-term sea-level variations in the central Great Barrier Reef

Low frequency sea-level variations and associated geostrophic currents in the central Great Barrier Reef (GBR) region near Townsville are studied using optimally-lagged multivariate regression. The analyses show that pressure-adjusted coastal sea levels and mid-shelf geostrophic currents are influenced predominantly by local along-shelf wind stress at the weather time-scale, and by climatic variables, such as atmospheric pressure and temperature, at seasonal and inter-annual time-scales. These forcing variables can specify sea levels over annual and inter-annual time-scales with a forecasting skill of 0.53 and 0.22, respectively (where 1.0 is perfect skill). Associated along-shelf geostrophic currents can be forecast with a skill of 0.57 over an annual time scale. If, instead, absolute coastal sea levels or offshore sea-level differences are used to specify the along-shelf geostrophic current, the forecasting skill is 0.75. A characteristic El Niño/Southern Oscillation (ENSO) response is detected for time periods up to 25 years in monthly sea-level both at Townsville and at western Pacific island sea-level stations. This spatially coherent response varies in intensity and phase within the Coral Sea. Sea-level differences show a pattern which characterizes known features of the large-scale circulation of the Coral Sea. These very low frequency sea-level variations in the Coral Sea must be taken into account to obtain accurate predictions of along-shelf geostrophic current variations on seasonal and inter-annual time scales. Regression analysis and a diagnostic river plume model show that the influence of the major rivers can produce sea-level changes due to buoyancy of order 5 cm. The corresponding errors in geostrophic velocities estimated using pressure-adjusted Townsville sea-level data alone are of order 5 cm s −1 rms.