Low-frequency Sea Level Variations Research Articles

Low-frequency sea-level variability in the South China Sea and its relationship to ENSO

Low-frequency sea-level variability in the South China Sea and its relationship to ENSO

Coastal sea level trends in Southern Europe

SUMMARY Low frequency sea level variability in the Mediterranean Sea and in the Atlantic Iberian coast is investigated by use of tide gauge records. The five tide gauge records that span most of the 20th century show positive trends between 1.2 and 1.5 ± 0.1 mm yr −1 and negative accelerations between −0.3 ± 0.3 and −1.5 ± 0.4 mm yr −1 century −1 . Sea level trends obtained from the 21 longest records (>35 yr) are smaller in the Mediterranean (0.3 ± 0.4 to −0.7 ± 0.3 mm yr −1 ) than in the neighbouring Atlantic sites (1.6 ± 0.5 to –1.9 ± 0.5 mm yr −1 )f or the period 1960–2000. Decadal sea level trends in the Mediterranean are not always consistent with global values, in particular for the 1990s, during which the Mediterranean has shown enhanced sea level rise of up to 5 mm yr −1 compared to the global average (mostly attributed to higher warming). The atmospheric and steric contributions to the observed sea level trends for 1960–2000 are also examined. The atmospherically induced sea level is obtained from a barotropic model forced by wind and atmospheric pressure. The atmospheric contribution accounts for 20–50 per cent of the observed yearly sea level variability and introduces negative trends of –0.2 to –0.9 mm yr −1 . The steric sea level, obtained from T and S climatologies, has negative trends ranging from −2.1 ± 0.6 to −0.1 ± 0.3 mm yr −1 . Other shorter tide gauge records (>7 yr) are used to quality check longer series and to explore their consistency with the long-term records and identify short but apparently consistent tide gauge records.

A review of sea level observations and low frequency sea-level variability in South Atlantic

Sea-level data from South Atlantic stations were quality-controlled and analysed. Eleven time series, longer than 20 years, were selected to study long-term variability. The main features of South Atlantic sea level were described by computing linear trends, mean seasonal cycles and power spectra for the selected time series. The data sparseness made it difficult to achieve a comprehensive description of the basin. The connection between sea-level variability and ENSO was studied using the composite time series obtained by merging data for Buenos Aires and Palermo, on the Argentinian side of Rio de la Plata. It was possible to recognize that sea-level variability is strongly connected with that of river discharge into Rio de la Plata, which in turn exhibits a lagged teleconnection with ENSO. The time lag between ENSO and sea-level anomalies is about 5 months.

Low-Frequency Sea Level Variability and the Inverted Barometer Effect

Abstract For a dynamical interpretation of sea level records, estimates are needed of the isostatic, or so-called inverted barometer, signals (ηib) associated with the ocean response to atmospheric loading. Seasonal and longer-period ηib signals are evaluated over the global ocean for the period 1958–2000 using monthly sea level pressure fields from two different atmospheric reanalyses. Variability and linear trends in ηib agree well for the two reanalyses in most regions but less so over the Southern Ocean, where uncertainties in ηib seem to be largest. The standard deviation of ηib ranges from <1 cm in equatorial regions to >7 cm in the regions of the Aleutian and Iceland lows and parts of the Southern and Arctic Oceans. When compared to a global tide gauge dataset, both seasonal and interannual ηib signals are found to contribute importantly to the sea level variance in many mid- and high-latitude records, with seasonal signals important as well in tropical records from India and Southeast Asia. For these records, subtracting ηib from the data can lead to changes in variance of 40% or more. Over the period of study, linear trends in ηib are mostly negative at low and midlatitudes and can cause negative biases in tide gauge estimates of global mean sea level rise that are comparable in magnitude to the effects of postglacial rebound. In agreement with previous findings, ηib signals are found to introduce anomalous behavior in local records (e.g., substantially weaker upward trends in the Mediterranean), and their removal can also reduce formal trend uncertainties. Accounting for ηib effects can be even more important when analyzing relatively short (decadal) records, such as those available from satellite altimetry.

Sea level changes in the North Atlantic by solar forcing and internal variability

Sea level change due to variations in the thermohaline structure of the North Atlantic has been calculated using a coupled ocean–atmosphere model of intermediate complexity (ECBilt). Two 1000-year simulations are made, one using a constant solar forcing and one using an estimate of historic variations in solar activity. In the solar forced simulation sea level variations are a proxy for climate variations. Anomalies in sea surface temperature (SST) of the northern North Atlantic are generated by the solar radiation changes. These SST anomalies modulate the ocean thermohaline circulation (THC), affecting surface salinities in the northern North Atlantic which are subsequently advected to the deep ocean. The associated deep ocean geopotential thickness anomalies dominate sea level in the North Atlantic and are advected southwards with the overturning circulation. Sea level change in the solar forced simulation is primarily an indirect response to solar radiation changes, which modulate the THC. In the unforced run, changes in the THC affect sea level in a similar way. However, in this simulation THC variability is no longer generated by sea surface temperature variations but by sea surface salinity variations, resulting from internal climate dynamics. The present results will aid in analyses of reconstructed low-frequency sea-level variations based on proxy data.

Large-scale sea-level variations and associated atmospheric forcing in the subtropical north-east Atlantic Ocean

Abstract The large-scale sea-level variations in the subtropical north-eastern Atlantic Ocean are characterised by a predominant seasonal fluctuation. Analysis of 6 years (1993–98) of combined TOPEX/POSEIDON and ERS-1/2 altimetric data together with data of concurrent oceanic surface geophysical fields (ECMWF net heat fluxes, ECMWF winds, and Reynolds sea-surface temperature) reveals that the dominant physical forcing of sea-level variations on an annual timescale is the air–sea net heat fluxes. This local forcing generates a seasonal ocean steric response, whose amplitude (of the order of 4–5 cm) varies spatially within the studied area and temporally over the 6-year period. The residual variability is mainly characterised by a non-seasonal signal, which is observed northwards of the Azores Current (∼34°N), and its time variation resembles that of the North Atlantic Oscillation (NAO). A combination of steric effects (wind-induced heat fluxes) and wind effects (barotropic Sverdrup response) appear to explain this signal. Some evidence of a local Ekman pumping response is also found regionally, both in the seasonal and non-seasonal variations of the observed sea level. At the interannual timescale, an overall sea-level trend with a mean rate of 0.5 cm/year is detected. However, the magnitude of the trend varies locally, so that the entire sea surface appears to undergo a general tilt, which extends beyond the studied area. Superimposed on this long-term trend, a sharp temporal sea-level rise occurs in 1995, mainly between the Azores and Madeira Islands. All together, these low-frequency sea-level variations seem to co-vary with the large-scale sea-surface temperature (SST) variations. This implies a comparable evolution of the sea level with the upper-ocean thermal content, which suggests a contribution from steric effects. A weaker situation is observed in the northern area, indicating a possible low-frequency wind-forcing-enhanced contribution.

Measuring very low frequency sea level variations using satellite altimeter data

Very low frequency (VLF) sea level variations are an important indicator of global climate change, and their measurement can provide important information for determining the socioeconomic impact of sea level change on coastal land use. The prospect of measuring VLF sea level variations has been assessed using approximately 5 years of satellite altimeter data from the TOPEX/POSEIDON (T/P) mission, where synoptic mapping of the geocentric height of the ocean surface is routinely achieved with a point-to-point accuracy of better than 4 cm. The global mean sea level variations measured by T/P every 10 days have an RMS of 4 mm and a rate of change +3.1±1.3 mm/year, after accounting for an average instrument drift computed using the global tide gauge network by Mitchum (1998). A likely cause of the observed instrument drift is the microwave radiometer, which provides the water vapor delay correction, which results in a revised estimate of +2.5 mm/year if the error is assumed to be linearly related to the mean water vapor delay. Approximately half of this rise appears related to an increase in sea level that began in mid-1996, thus it is unlikely to be sustained over the long term. Estimates of sea level change over the major ocean basins reveal that the North Atlantic has risen over the T/P mission, and this is believed to be related to decadal changes in heat storage. Maps of the geographic variability of the observed sea level trends are currently dominated by ENSO variations, and thus the climate change signals cannot currently be isolated. These results suggest that T/P, when combined with tide gauge monitoring of the satellite instruments, is achieving the necessary accuracy to distinguish sea level rise caused by climate change from the natural `background' rate of sea level rise, although a longer time series is necessary to average out possible interannual and decadal variations. A longer time series will also reduce the errors in estimates of the altimeter calibration, providing an important constraint on any long-term instrument drift. Future research will focus on establishing a more realistic error budget for these measurements of global mean sea level, so that they can be put in the proper context with other observations of global climate change. In addition, the study of the spatial variability of the sea level rise signal will become increasingly important as a longer time series is collected.

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.

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.

A simple treatment of very low frequency wind‐driven sea level variations

It is shown that very low frequency sea level variations can be calculated from the sum of the steric or density effect and the wind‐driven setup derived from barotropic dynamics, provided that a weak slope criterion is met. This result is shown to be consistent with a recent two‐layer formulation for wind‐driven shelf motions.

Low-Frequency Sea Level Variability in the Northeastern Mediterranean

Abstract Analyses of time series of one year of sea level data from 11 stations in the northeastern Mediterranean (Adriatic, Ionian and Aegean seas) are presented in terms of local atmospheric pressure forcing. The study focuses on low-frequency oscillations with time scales ranging from one day to several weeks, but shorter than the seasonal scale. The summer and winter seasons are analyzed separately. An empirical orthogonal function analysis of both sea level and atmospheric pressure resulted in a separation of lower frequency oscillations of planetary-wave time scale (expressed by the first mode) from the higher frequency synoptic time scale variability (included in the second mode). The first mode is related to the in-phase sea level or atmospheric pressure variations of the entire area, while the second mode represent variations for which the Adriatic Sea is out of phase with both the Ionian and Aegean seas. Only those first two modes represent a regionally coherent signal subtracting more than 90% ...

Low frequency sea level variability in the vicinity of the East River tidal strait

The East River is a narrow tidal strait connecting Long Island Sound with the lower Hudson Estuary; the Sound and Estuary both communicate with the New York Bight. Subtidal fluctuations in sea level difference between the ends of the strait which are coherent with barotropic flow through the strait are produced mainly by direct setup in the Sound and Estuary and by sea level fluctuations (primarily wind forced) in the Bight. Approximately 70% of the variance in subtidal sea level difference can be accounted for by the two components of wind stress. The direction of dominant wind forcing (wind stress direction producing maximum response) varies considerably with frequency. Response functions relating sea level difference to wind stress components computed for 128 day series for sea level and winds allow us to interpret the direction of dominant forcing in terms of the relative contributions of direct setup and sea level fluctuations in the Bight.

The nontidal flow in the Providence River of Narragansett Bay: a stochastic approach to estuarine circulation : WEISBERG R. H., 1976 J. phys. Oceanogr., 6 (5): 721–734

The large-scale sea-level variations in the subtropical north-eastern Atlantic Ocean are characterised by a predominant seasonal fluctuation. Analysis of 6 years (1993–98) of combined TOPEX/POSEIDON and ERS-1/2 altimetric data together with data of concurrent oceanic surface geophysical fields (ECMWF net heat fluxes, ECMWF winds, and Reynolds sea-surface temperature) reveals that the dominant physical forcing of sea-level variations on an annual timescale is the air–sea net heat fluxes. This local forcing generates a seasonal ocean steric response, whose amplitude (of the order of 4–5 cm) varies spatially within the studied area and temporally over the 6-year period. The residual variability is mainly characterised by a non-seasonal signal, which is observed northwards of the Azores Current (∼34°N), and its time variation resembles that of the North Atlantic Oscillation (NAO). A combination of steric effects (wind-induced heat fluxes) and wind effects (barotropic Sverdrup response) appear to explain this signal. Some evidence of a local Ekman pumping response is also found regionally, both in the seasonal and non-seasonal variations of the observed sea level. At the interannual timescale, an overall sea-level trend with a mean rate of 0.5 cm/year is detected. However, the magnitude of the trend varies locally, so that the entire sea surface appears to undergo a general tilt, which extends beyond the studied area. Superimposed on this long-term trend, a sharp temporal sea-level rise occurs in 1995, mainly between the Azores and Madeira Islands. All together, these low-frequency sea-level variations seem to co-vary with the large-scale sea-surface temperature (SST) variations. This implies a comparable evolution of the sea level with the upper-ocean thermal content, which suggests a contribution from steric effects. A weaker situation is observed in the northern area, indicating a possible low-frequency wind-forcing-enhanced contribution.