Seasonal Sea Level Variability Research Articles

Determining sea-level rise in the Caribbean: A shift from temperature to mass control

Tropical Small Island Developing States (SIDS), such as those in the Caribbean, are among the most vulnerable to the impacts of climate change, most notably sea-level rise. The current sea-level rise in the Caribbean is 3.40 ± 0.3 mm/year (1993–2019), which is similar to the 3.25 ± 0.4 mm/year global mean sea-level (GMSL) rise (1993–2018). Throughout the year, Caribbean seasonal sea-level variability is found to respond to sea surface temperature variability. Over the past few decades, the trend in Caribbean Sea-level rise is also found to be variable. Satellite altimetry and steric sea-level records of the Caribbean region reveal a shift in the late 2003-early 2004, which separates two distinct periods of sea-level rise. Thermal expansion dominates the sea-level trend from 1993–2003. Following this period, there is an increased trend in sea-level rise, with a dominance of mass changes from 2004–2019, as confirmed by GRACE data. During this period, the sea-level trend is 6.15 ± 0.5 mm/year, which is 67% faster than the most recent estimates of global mean sea-level rise provided by the Intergovernmental Panel on Climate Change (3.69 ± 0.5 mm/year for the period 2006–2018). Despite its reduced importance, increasing temperatures contribute greatly to sea-level rise in the Caribbean region through thermal expansion of ocean water, hence there is a need to limit the current trend of global warming.

Arctic sea level variability from high-resolution model simulations and implications for the Arctic observing system

Abstract. Two high-resolution model simulations are used to investigate the spatiotemporal variability of the Arctic Ocean sea level. The model simulations reveal barotropic sea level variability at periods of < 30 d, which is strongly captured by bottom pressure observations. The seasonal sea level variability is driven by volume exchanges with the Pacific and Atlantic oceans and the redistribution of the water by the wind. Halosteric effects due to river runoff and evaporation minus precipitation ice melting/formation also contribute in the marginal seas and seasonal sea ice extent regions. In the central Arctic Ocean, especially the Canadian Basin, the decadal halosteric effect dominates sea level variability. The study confirms that satellite altimetric observations and Gravity Recovery and Climate Experiment (GRACE) could infer the total freshwater content changes in the Canadian Basin at periods longer than 1 year, but they are unable to depict the seasonal and subseasonal freshwater content changes. The increasing number of profiles seems to capture freshwater content changes since 2007, encouraging further data synthesis work with a more complicated interpolation method. Further, in situ hydrographic observations should be enhanced to reveal the freshwater budget and close the gaps between satellite altimetry and GRACE, especially in the marginal seas.

Atmospherically forced sea-level variability in western Hudson Bay, Canada

Abstract. In recent years, significant trends toward earlier breakup and later freeze-up of sea ice in Hudson Bay have led to a considerable increase in shipping activity through the Port of Churchill, which is located in western Hudson Bay and is the only deep-water ocean port in the province of Manitoba. Therefore, understanding sea-level variability at the port is an urgent issue crucial for safe navigation and coastal infrastructure. Using tidal gauge data from the port along with an atmospheric reanalysis and Churchill River discharge, we assess environmental factors impacting synoptic to seasonal variability of sea level at Churchill. An atmospheric vorticity index used to describe the wind forcing was found to correlate with sea level at Churchill. Statistical analyses show that, in contrast to earlier studies, local discharge from the Churchill River can only explain up to 5 % of the sea-level variability. The cyclonic wind forcing contributes from 22 % during the ice-covered winter–spring season to 30 % during the ice-free summer–fall season due to cyclone-induced storm surges generated along the coast. Multiple regression analysis revealed that wind forcing and local river discharge combined can explain up to 32 % of the sea-level variability at Churchill. Our analysis further revealed that the seasonal cycle of sea level at Churchill appears to be impacted by the seasonal cycle in atmospheric circulation rather than by the seasonal cycle in local discharge from the Churchill River, particularly post-construction of the Churchill River diversion in 1977. Sea level at Churchill shows positive anomalies for September–November compared to June–August. This seasonal difference was also revealed for the entire Hudson Bay coast using satellite-derived sea-level altimetry. This anomaly was associated with enhanced cyclonic atmospheric circulation during fall, reaching a maximum in November, which forced storm surges along the coast. Complete sea-ice cover during winter impedes momentum transfer from wind stress to the water column, reducing the impact of wind forcing on sea-level variability. Expanding our observations to the bay-wide scale, we confirmed the process of wind-driven sea-level variability with (i) tidal-gauge data from eastern Hudson Bay and (ii) satellite altimetry measurements. Ultimately, we find that cyclonic winds generate sea-level rise along the western and eastern coasts of Hudson Bay at the synoptic and seasonal timescales, suggesting an amplification of the bay-wide cyclonic geostrophic circulation in fall (October–November), when cyclonic vorticity is enhanced, and Hudson Bay is ice-free.

Factors Controlling Seasonal Variability of Sea Level in the Western Gulf of

Daily means sea level has been computed from hourly observations at Aden and Djibouti for the period of 2011–2014 to study seasonal variability in sea level in the western Gulf of Aden.

Wind forcing of sea level variability in a tropical coral reef area in the western Gulf of Mexico

Sea level, water temperature, and wind stress data were obtained from the Western Gulf of Mexico to elucidate the relationship between sea level changes and meteorological variables in a tropical coral reef system. Sea level and water temperature data were measured with a series of acoustic Doppler current profilers anchored at a depth of approximately 13 m in the Veracruz Reef System. The barometric and wind stress data were obtained from an automatic coastal weather station. Principal Component Analysis was applied to the series to determine the relative influence of the different meteorological variables on sea level. The seasonal variability of sea level is evident in the year-long data (September 2008 to April 2010) with a thermal expansion during the summer. Barometric pressure was found to be the second strongest forcing mechanism of sea level variability and not the first one. Opposite to what would be expected in protected coastal areas where the wind forcing mechanism on sea level may not be as important. The local winds were the main forcing mechanism of the sea level variability from March to September, while the meridional winds were more important than local winds from October to February.

Modelling seasonal circulation and thermohaline structure of the Caspian Sea

Abstract. The wind- and buoyancy-driven seasonal circulation of the Caspian Sea is investigated for a better understanding of its basin-wide and mesoscale dynamics, mixing and transport. The model successfully reproduces the following basic elements of the circulation: the southward-flowing current systems along the eastern and western coasts, the upwelling along the eastern coast, the cyclonic circulation in the Middle Caspian Sea (MCS), especially in winter, and the cyclonic and anticyclonic cells of circulation in the South Caspian Sea (SCS). The observed seasonal variability of sea level and sea surface temperature (SST) is well reproduced. Mesoscale structures, not always evident from hydrographic observations, are identified.

Seasonal sea level variations in the gulf of Cadiz continental shelf from in-situ measurements and satellite altimetry

Long-term time series (1997–2008) of gridded multi-mission near shore altimeter data and in-situ tide gauge records have been analyzed to investigate the seasonal variability of sea level along the Gulf of Cadiz continental shelf and the Strait of Gibraltar. The contribution of different forcing mechanisms including atmospheric pressure, wind, steric anomalies and river runoff was also addressed. Comparison between altimeter and ground-truth data showed that the former can be a valuable tool to study the sea level seasonal cycle near the coast as well as to detect anomalous data from neighboring tide gauges. Overall, both sets of time series presented a similar mean seasonal cycle along the coast, dominated by the annual component and, to a lesser extent, by the semiannual one. Low-frequency atmospheric pressure and steric anomalies were the main forcing factors affecting the sea level seasonal cycle, their effect diminishing toward the Strait of Gibraltar. The combined effect of both forcing factors accounted for 55–61%/62–63% of the observed sea level variance at the tide gauge stations/ altimeter points located outside the Strait of Gibraltar and for only 27%/48% at those placed within the Strait. The influence of local wind was small in all stations except for the westernmost tide gauges. The effect of large river discharges was notable at the tide gauge located within the Guadalquivir river mouth, with 18% of the pressure-adjusted sea level variance explained, and negligible (< 0.1%) at the one placed within the Tinto–Odiel rivers mouth, indicating that such effects only need to be taken into account when dealing with tide gauges located within major river estuaries. Moreover, the effect of river runoff seems to be confined within a small area near the river mouth, thus not affecting the altimeter measurements. A small but significant influence of the wintertime North Atlantic Oscillation on sea level was observed, its effect being mainly reflected through changes in the atmospheric pressure, wind and river discharge.

The Kuroshio Extension: a leading mechanism for the seasonal sea-level variability along the west coast of Japan

Sea level changes coherently along the two coasts of Japan on the seasonal timescale. Archiving, validation, and interpretation of satellite oceanographic altimetry data and ocean general circulation model for the Earth Simulator results indicate that the variation propagates clockwise from Japan’s east coast through the Tsushima Strait into the Japan/East Sea (JES) and then northward along the west coast. In this study, we hypothesize and test numerically that the sea-level variability along the west coast of Japan is remotely forced by the Kuroshio Extension (KE) off the east coast. Topographic Rossby waves and boundary Kelvin waves facilitate the connection. Our 3D Princeton Ocean Model when forced by observed wind stress reproduces well the seasonal changes in the vicinity of JES. Two additional experiments were conducted to examine the relative roles of remote forcing and local forcing. The sea-level variability inside the JES was dramatically reduced when the Tsushima Strait is blocked in one experiment. The removal of the local forcing, in another experiment, has little effect on the JES variability. Both experiments support our hypothesis that the open-ocean forcing, possibly through the KE variability, is the leading forcing mechanism for sea-level change along the west coast of Japan.

ENSO and seasonal sea-level variability – A diagnostic discussion for the U.S.-Affiliated Pacific Islands

The El Nino-Southern Oscillation (ENSO) climate cycle is the basis for this paper, aimed at providing a diagnostic outlook on seasonal sea-level variability (i.e. anomalies with respect to the Climatology) for the U.S.-Affiliated Pacific Islands (USAPI).

Atmospheric forcing on the seasonal variability of sea level at Cochin, southwest coast of India

The seasonal cycles of some atmospheric parameters at Cochin (southwest coast of India) have been studied with a specific emphasis on the role played by them in forcing the seasonal sea level. Equatorward along-shore wind stress as well as equatorward volume transport by coastal currents along the Indian peninsula could play an important role in the sea level low during the premonsoon and southwest monsoon seasons. During postmonsoon season, along-shore wind stress plays no major role in the high sea level whereas this could be due to the poleward volume transport by the coastal along-shore currents. Atmospheric pressure and river discharge do not seem to influence much the sea level during the southwest monsoon period, even though the river discharge during that period is considerable. The sea level was minimal during the southwest monsoon season, when the river discharge was at its annual maximum. The difference between the seasonal march of observed and pressure corrected sea level (CSL) was not significant for the study region. Harmonic analysis of the climatological data on the various parameters revealed that air temperature is the only parameter with a dominance of the semi-annual over the annual cycle. Cross-shore wind stress indicated strong interannual variability whereas relative density showed strong seasonal variability. The climatological seasonal cycles of CSL at eight other tide gauge stations along the west coast of the Indian subcontinent are also examined, to assess the role of various forcings on the seasonal sea level cycle. The signatures of El Nino-Southern Oscillation (ENSO) phenomenon could be seen in some of the parameters (SST, air temperature, atmospheric pressure, along-shore wind stress, relative density and sea level). The signature of ENSO was particularly strong in the case of atmospheric pressure followed by relative density, the variance accounted by the relationship being 47% and 16%, respectively.

Some aspects of the seasonal sea level variations around Spain

Tide gauge stations are used to investigate the seasonal variability of sea level around Spain, which exhibits a well‐defined annual cycle and a small but distinguishable semiannual cycle in all stations. The contribution of different sources to observed sea level has been analyzed. The effect of wind and atmospheric pressure has been explored, analyzing sea level residuals computed with a barotropic ocean model forced by the meteorological fields. Sea level corrected for meteorological forcing contains only an annual signal, indicating that the semiannual cycle in the observed sea levels is mainly forced by these meteorological agents. Additional data sets (sea surface temperature satellite images and climatologies) are used to show that the annual cycle is related to the seasonal steric anomaly, which accounts for ∼60% of the cycle amplitude. The lack of agreement between the annual phases of sea level and steric anomaly cycles for the stations located in the continental margin of the Atlantic Ocean suggests the existence of other effects in the observed sea levels related to seasonal changes in ocean circulation. On the contrary, open‐ocean stations of the Canary Islands have sea level annual cycles phase locked with the steric anomaly cycles, although amplitudes differ significantly from one station to the other. This fact is interpreted as the local response to large‐scale seasonal fluctuations of the Canary Current. Sea level cycle in stations within the semienclosed Mediterranean Sea suggests the existence of a hydrological cycle that must include a seasonal barotropic signal through the Strait of Gibraltar.

Exchange through the Bab el Mandab

During the last 10 years there have been a number of important developments in the understanding of the exchange flow in the Bab el Mandab. New and improved estimates of the fluxes of mass, heat and salt through the Strait have been made directly from oceanographic measurements and indirectly from air–sea flux data. Hydraulic theory has been applied to explain the seasonal variations in the exchange flow, and in particular the three-layer summer circulation. The effects of wind stress, friction, and seasonal sea-level variability on hydraulic control are addressed in this paper. Analyses of oceanographic data have failed to provide direct evidence of hydraulic control, but they have highlighted the importance of tidal scale variability and differences between real continuously stratified fluids and idealised layer flows often used in hydraulic theory. Other studies have investigated the coupling between the flow in the Strait and the circulation within the Red Sea.

Seasonal circulation on the western shelf of the Gulf of Mexico using a high‐resolution numerical model

The seasonal circulation on the western shelf of the Gulf of Mexico is studied using a high‐resolution numerical simulation, historical hydrographic data, sea level data, and satellite images. Three regions are distinguished, the Tamaulipas‐Veracruz (TAVE) shelf, the Louisiana‐Texas (LATEX) shelf, and the western Campeche Bank. On the TAVE shelf there is a swift reversal of the along‐shelf current, downcoast from September to March and upcoast from May to August when there is upwelling due to offshore Ekman transport. Circulation on the western Campeche Bank is upcoast throughout the year. The LATEX shelf has a cyclonic circulation, except during summer months when the flow is eastward. During spring‐summer the upcoast current on the TAVE shelf reaches the southern Texas shelf where it encounters a downcoast coastal current favoring offshore transports. In the fall‐winter, the downcoast current reaches the southern Bay of Campeche where it meets an opposing along‐shelf current, generating seasonal offshore transports. During fall and winter, cool low‐salinity water from the Mississippi and Atchafalaya Rivers is advected westward along the LATEX shelf onto the TAVE shelf, developing along‐shelf fronts and temperature inversions commonly observed over the outer shelf and shelf break. The main forcing over the western shelf of the gulf is the along‐coast wind stress component. The existence of the cross‐shelf transports in the confluence regions is supported by chlorophyll a data. Up to 80% of the seasonal sea level variability is explained by the along shelf currents and the low‐frequency variability of the atmospheric sea level pressure.

A three‐dimensional model of the mean and seasonal circulation of the Gulf of California

The mean and seasonal circulation of the Gulf of California is simulated with a three‐dimensional numerical model, forced by the Pacific Ocean through specifying the sea level, the temperature, and the salinity fields at its entrance. At the sea surface, the wind and the heat and freshwater fluxes are specified. The model reproduces the observed mean and seasonal variability of sea level, of heat and salt balances, and the sea surface temperature (SST) climatology. It also reproduces the general surface circulation of the northern gulf, which consists of a seasonally reversing basin‐wide gyre. It is found that tides and heat fluxes are both indispensable to reproduce the spatial structure and temporal evolution of the SST. Tides provide the mixing to upwell the cooler subsurface waters in the large islands area, and the heating from the surface raises the SST. The general circulation of the southern part of the gulf is due to the wind and the Pacific forcing. In the Ekman layer, two periods of anticyclonic circulation and one cyclonic per year occur; below this layer, two cyclonic periods develop. In the northern part, the tides play an important role in producing mean residual currents, and both, tides and winds, compete against the Pacific forcing to produce a cyclonic and anticyclonic circulation once a year. Contrary to conclusions of previous studies, the thermohaline circulation is found to be unimportant in the gulf.

Seasonal Variability of Sea Level and Sea-surface Temperature on the North-east Coast of New Zealand

Low-frequency, seasonal variations in sea level and sea-surface temperature (SST) for the north-east coast of the North Island, New Zealand were investigated using mainly multivariate analyses in the frequency domain over a 20-year period (1973–92). The dominant influence on the annual cycle of sea level (mean amplitude=378mm) is associated with thermo-steric sea-level adjustments, which explains 50–80% of the variance in the annual frequency band. Sea levels generally peak at the end of April (austral autumn), lagging the SST cycle by around 2 months. The inclusion of secondary forcing variables (barometric pressure and alongshore wind stress) in the multivariate analysis increases the proportion of the variance explained to 70–90+%. Thermo-steric adjustments in sea level almost completely mask the inverted-barometer effect at annual and longer timescales. The response of sea level to wind stress (alongshore) is also anticorrelated to its response to barometric pressure, thereby appearing to reduce the magnitude of the barometric factor below 10mmhPa−1. These latter factors, along with changes in oceanic current patterns and seasonal coupling of El Niño–Southern Oscillation effects, cause secondary effects on seasonal sea-level variability.

Non-tidal aliasing in seasonal sea-level variability and annual Rossby waves as observed by satellite altimetry

Abstract. It is becoming well known that aliasing associated with ocean tides could be a major source of systematic error in altimeter sea-level measurements, due to asynoptic sampling and imperfect tide modelling. However, it has been shown that signals of non-tidal origin may also contribute significantly to the observed aliasing. In this paper, numerical simulations are performed to demonstrate the full aliasing potential associated with altimeter observations of seasonal sea-level variability and annual Rossby waves. Our results indicate that ignorance of non-tidal aliasing may lead to the possibility of underestimating the total aliasing and misinterpreting or overlooking existing geophysical phenomena. Therefore, it is argued that an entire aliasing picture should be kept in mind when satellite altimeter data are analysed.

Non-tidal aliasing in seasonal sea-level variability and annual Rossby waves as observed by satellite altimetry

Non-tidal aliasing in seasonal sea-level variability and annual Rossby waves as observed by satellite altimetry

ATSR sea surface temperature data in a global analysis with TOPEX/POSEIDON altimetry

Along Track Scanning Radiometer (ATSR) data from the ERS 1 satellite mission are used in a global analysis of the surface temperature of the oceans. The data are the low resolution 0.5° by 0.5° average temperatures and cover about 24 months. At global scales a significant seasonal variability is found. On each of the hemispheres the surface temperatures reach their maximum after summer heating. The seasonal sea level variability, as observed from TOPEX/POSEIDON, reaches its maximum 1.1–1.4 months later.