Inverse Barometer Research Articles

The effect of cyclones crossing the Mediterranean region on sea level anomalies on the Mediterranean Sea coast

Abstract. Large positive and negative sea level anomalies on the coast of the Mediterranean Sea are linked to intensity and position of cyclones moving along the Mediterranean storm track with dynamics involving different factors. This analysis is based on a model hindcast and considers nine coastal stations, which are representative of sea level anomalies with different magnitudes and characteristics. When a shallow water fetch is present, the wind around the cyclone centre is the main cause of positive and negative sea level anomalies, depending on its onshore or offshore direction. The inverse barometer effect produces a positive anomaly on the coast near the cyclone pressure minimum and a negative anomaly at the opposite side of the Mediterranean Sea. The latter is caused by the cross-basin mean sea level pressure gradient that is associated with the presence of a cyclone. This often coincides with the presence of an anticyclone above the station, which causes a local negative inverse barometer effect. Further, at some stations, negative sea level anomalies are reinforced by a residual water mass redistribution within the basin, which is associated with a transient response to the atmospheric pressure forcing. Though the link with the presence of a cyclone in the Mediterranean has comparable importance for positive and negative anomalies, the relation between cyclone position and intensity is stronger for the magnitude of positive events. The area of cyclogenesis, track of the central minimum and position at the time of the event differ depending on the location the sea level anomaly occurs and on its sign. The western Mediterranean is the main cyclogenesis area for both positive and negative anomalies overall. Atlantic cyclones mainly produce positive sea level anomalies in the western basin. At the easternmost stations, positive anomalies are caused by cyclogenesis in the eastern Mediterranean. North African cyclogenesis is a major source of positive anomalies on the central African coast and negative anomalies on the eastern Mediterranean and northern Aegean coasts.

Influence of sea level variability on a micro-tidal beach

Global climate change in response to buildup of human-induced greenhouse gases in the atmosphere has already resulted in several visible consequences such as an increase in ocean heat content and melting of glaciers. The relative effect of sea level and wave variability on shoreline changes along the coastline of James Town, Ghana (Gulf of Guinea), was investigated between 2013 and 2014. Data were gathered at James Town, over an 8-month period (2013–2014) from nearshore video installation, Era-Interim hindcast, and satellite altimetry. Data were evaluated over daily and monthly timescale variations. Though beach changes are not directly driven by sea level variability, its action is key in modulating waves influence. The results show that the shoreline responds in decreasing order to sea level variations (86%), waves (9%), and tidal cycles (5%) on daily bases. Wind-induced setup has limited effect on the shoreline. The observed most important component of sea level anomaly at this tropical “storm free” coast is the influence from inverse barometer. Decadal observations from satellite show that sea level anomaly is continuously rising in the Gulf of Guinea subregion with expected strong consequence for this urbanized low lying sandy coast.

Contribution of atmospheric circulation to recent off-shore sea-level variations in the Baltic Sea and the North Sea

Abstract. The main purpose of this study is to quantify the contribution of atmospheric factors to recent off-shore sea-level variability in the Baltic Sea and the North Sea on interannual timescales. For this purpose, we statistically analysed sea-level records from tide gauges and satellite altimetry and several climatic data sets covering the last century. Previous studies had concluded that the North Atlantic Oscillation (NAO) is the main pattern of atmospheric variability affecting sea level in the Baltic Sea and the North Sea in wintertime. However, we identify a different atmospheric circulation pattern that is more closely connected to sea-level variability than the NAO. This circulation pattern displays a link to sea level that remains stable through the 20th century, in contrast to the much more variable link between sea level and the NAO. We denote this atmospheric variability mode as the Baltic Sea and North Sea Oscillation (BANOS) index. The sea-level pressure (SLP) BANOS pattern displays an SLP dipole with centres of action located over (5° W, 45° N) and (20° E, 70° N) and this is distinct from the standard NAO SLP pattern in wintertime. In summertime, the discrepancy between the SLP BANOS and NAO patterns becomes clearer, with centres of action of the former located over (30° E, 45° N) and (20° E, 60° N). This index has a stronger connection to off-shore sea-level variability in the study area than the NAO in wintertime for the period 1993–2013, explaining locally up to 90 % of the interannual sea-level variance in winter and up to 79 % in summer. The eastern part of the Gulf of Finland is the area where the BANOS index is most sensitive to sea level in wintertime, whereas the Gulf of Riga is the most sensitive region in summertime. In the North Sea region, the maximum sea-level sensitivity to the BANOS pattern is located in the German Bight for both winter and summer seasons. We investigated, and when possible quantified, the contribution of several physical mechanisms which may explain the link between the sea-level variability and the atmospheric pattern described by the BANOS index. These mechanisms include the inverse barometer effect (IBE), freshwater balance, net energy surface flux and wind-induced water transport. We found that the most important mechanism is the IBE in both wintertime and summertime. Assuming a complete equilibration of seasonal sea level to the SLP gradients over this region, the IBE can explain up to 88 % of the sea-level variability attributed to the BANOS index in wintertime and 34 % in summertime. The net energy flux at the surface is found to be an important factor for the variation of sea level, explaining 35 % of sea-level variance in wintertime and a very small amount in summer. The freshwater flux could only explain 27 % of the variability in summertime and a negligible part in winter. In contrast to the NAO, the direct wind forcing associated with the SLP BANOS pattern does not lead to transport of water from the North Sea into the Baltic Sea in wintertime.

Observed Sea-Level Changes along the Norwegian Coast

Norway’s national sea level observing system consists of an extensive array of tide gauges, permanent GNSS stations, and lines of repeated levelling. Here, we make use of this observation system to calculate relative sea-level rates and rates corrected for glacial isostatic adjustment (GIA) along the Norwegian coast for three different periods, i.e., 1960 to 2010, 1984 to 2014, and 1993 to 2016. For all periods, the relative sea-level rates show considerable spatial variations that are largely due to differences in vertical land motion due to GIA. The variation is reduced by applying corrections for vertical land motion and associated gravitational effects on sea level. For 1960 to 2010 and 1984 to 2014, the coastal average GIA-corrected rates for Norway are 2.0 ± 0.6 mm/year and 2.2 ± 0.6 mm/year, respectively. This is close to the rate of global sea-level rise for the same periods. For the most recent period, 1993 to 2016, the GIA-corrected coastal average is 3.5 ± 0.6 mm/year and 3.2 ± 0.6 mm/year with and without inverse barometer (IB) corrections, respectively, which is significantly higher than for the two earlier periods. For 1993 to 2016, the coastal average IB-corrected rates show broad agreement with two independent sets of altimetry. This suggests that there is no systematic error in the vertical land motion corrections applied to the tide-gauge data. At the same time, altimetry does not capture the spatial variation identified in the tide-gauge records. This could be an effect of using altimetry observations off the coast instead of directly at each tide gauge. Finally, we note that, owing to natural variability in the climate system, our estimates are highly sensitive to the selected study period. For example, using a 30-year moving window, we find that the estimated rates may change by up to 1 mm/year when shifting the start epoch by only one year.

Effect of extreme ocean precipitation on sea surface elevation and storm surges

Ocean models that neglect mass and momentum contributions from precipitation can have a systematic bias in sea surface height (SSH). Here, a new rainfall scheme is introduced into the Regional Ocean Modelling System (ROMS) to incorporate the effects of precipitation mass. When precipitation is added to the sea surface, it spreads out via surface gravity waves that increase in propagation speed with increasing water depth. Over several days, the SSH increase due to the precipitation mass added creates a geostrophic adjustment, generating anticyclonic geostrophic currents around the SSH increase. The transfer of momentum from precipitation to the sea surface, or rain stress, can also be important. In the case‐study of a real tropical cyclone, Monica, passing North Australia, the effect of incorporating precipitation mass is compared with other processes affecting the storm surge: surface wind, inverse barometer effect and rain stress. The maximum SSH response is 170.6 cm for the wind effect, 61.5 cm for the inverse barometer effect, 7.5 cm for the effect of rain stress and 6.4 cm for the effect of rain mass. Each process has been shown to have different spatial influences. The effect of rain mass has a strong remote influence compared to the inverse barometer effect and the effect of rain stress. This is particularly seen in semi‐enclosed bays.

Development of Tide–Surge Interaction Model for the Coastal Region of Bangladesh

In this paper, a two-dimensional, vertically integrated hydrodynamic model is developed taking into account entrained air bubbles during storm surges as well as incorporating inverted barometer, and river and land dynamics. The model is specifically designed for the coastal region of Bangladesh. A nested scheme method with a fine mesh scheme (FMS), capable of incorporating the complex coastline and all major offshore islands accurately, nested into a coarse mesh scheme (CMS) covering up to 15° N latitude in the Bay of Bengal is used. To incorporate the small and big offshore islands in the Meghna estuarine region with its complex coastline accurately, a very fine mesh scheme (VFMS) is again nested into the FMS. Along the northeast corner of the VFMS, the Meghna river discharge is taken into account. The coastal and island boundaries are approximated through proper stair steps. The model equations are solved by a semi-implicit finite difference technique using a staggered C-grid. A stable appropriate tidal condition over the model domain is generated by applying tidal forcing with the four major tidal constituents M2, S2, K1, and O1 along the southern open boundary of the CMS. This tidal regime is introduced as the initial state of the sea for nonlinear interaction of tide and surge. The model is applied to simulate water levels due to the interaction of tide and surge associated with the cyclones April 1991 and Aila at different coastal and island locations along the coast of Bangladesh. The results are found to be quite satisfactory with root mean square error of ∼0.50 m as calculated for both the storm events. Tests of sensitivities on water levels are carried out for air bubbles, offshore islands, river discharge, inverse barometer, and grid resolution. The presence of air bubbles increases simulated water levels a little bit in our model, and the contribution of air bubbles in increasing water level is found around 2 %. Further, water levels are found to be influenced by offshore islands, river discharge, inverse barometer as well as grid resolution.

Evaluation of an operational ocean model configuration at 1/12° spatial resolution for the Indonesian seas (NEMO2.3/INDO12) – Part 1: Ocean physics

Abstract. INDO12 is a 1/12° regional version of the NEMO physical ocean model covering the whole Indonesian EEZ (Exclusive Economic Zone). It has been developed and is now running every week in the framework of the INDESO (Infrastructure Development of Space Oceanography) project implemented by the Indonesian Ministry of Marine Affairs and Fisheries. The initial hydrographic conditions as well as open-boundary conditions are derived from the operational global ocean forecasting system at 1/4° operated by Mercator Océan. Atmospheric forcing fields (3-hourly ECMWF (European Centre for Medium-Range Weather Forecast) analyses) are used to force the regional model. INDO12 is also forced by tidal currents and elevations, and by the inverse barometer effect. The turbulent mixing induced by internal tides is taken into account through a specific parameterisation. In this study we evaluate the model skill through comparisons with various data sets including outputs of the parent model, climatologies, in situ temperature and salinity measurements, and satellite data. The biogeochemical model results assessment is presented in a companion paper (Gutknecht et al., 2015). The simulated and altimeter-derived Eddy Kinetic Energy fields display similar patterns and confirm that tides are a dominant forcing in the area. The volume transport of the Indonesian throughflow (ITF) is in good agreement with the INSTANT estimates while the transport through Luzon Strait is, on average, westward but probably too weak. Compared to satellite data, surface salinity and temperature fields display marked biases in the South China Sea. Significant water mass transformation occurs along the main routes of the ITF and compares well with observations. Vertical mixing is able to modify the South and North Pacific subtropical water-salinity maximum as seen in T–S diagrams. In spite of a few weaknesses, INDO12 proves to be able to provide a very realistic simulation of the ocean circulation and water mass transformation through the Indonesian Archipelago. Work is ongoing to reduce or eliminate the remaining problems in the second INDO12 version.

Tide deflection of Campbell Glacier Tongue, Antarctica, analyzed by double-differential SAR interferometry and finite element method

A glacier tongue, the terminus of a glacier projecting out from the coastline and floating over the ocean, experiences vertical deflection due to the fluctuation of ocean tide. Accurate analysis of the tidal deflection of a glacier tongue is necessary to understand the elastic properties and dynamics of glacial ice. We extracted the tidal deflection of Campbell Glacier Tongue (CGT), one of the fast-flowing glacier tongues in East Antarctica, by using 120 Double-Differential Interferometric SAR (DDInSAR) images from the 16 COSMO-SkyMed one-day tandem interferometric SAR image pairs obtained during 2010 and 2011. A map of tide deflection ratio (α-map), defined as the ratio of the tidal deflection over tide height, was generated by pixel-based linear regression between the DDInSAR-derived tidal deflection and the tidal variation predicted by various tide models such as TPXO7.1, FES2004, CATS2008a and Ross_Inv after the inverse barometer effect correction. We also generated the maps of R2, root mean square error (RMSE), and intercept (β) of the linear regression model. We confirmed that the horizontal flow of the Campbell Glacier is steady over the whole data acquisition period due to the fact that α values were close to zero over the grounded part of the glacier. Ross_Inv provided the most reliable α-map with the highest R2 (0.97) and the smallest RMSE (0.3cm) beyond the hinge zone than other tide models. The spatial variation of the tidal response of the whole CGT was clearly defined from the α-map. The grounding line retreated by 0.3–1.5km in comparison with that in 1996 defined by Rignot et al., 2011a. The ice thickness of CGT was estimated to be 397±71m from ICESat surface elevation data. The Young's modulus of CGT was found to be 0.5–1.3GPa (0.8GPa at an ice thickness of 397m) by simulating the α in the hinge zone using a 2-dimensional finite element method.

DDInSAR 기반의 빙하연구를 위한 동남극 테라노바 만의 조위모델 정밀도 평가

이중차분간섭기법(Double-Differential Interferometric SAR; DDInSAR)을 이용하여 빙하의 조위반응을 정확히 해석하기 위해서는 극지 해양에 대한 조위모델의 정밀도 평가가 수행되어야 한다. 이 연구에서는 동남극 테라노바 만에서 TPXO7.1, FES2004, CATS2008a, Ross_Inv 조위모델의 정밀도를 평가하기 위해 2년 동안 획득된 16쌍의 one-day tandem COSMO-SkyMed SAR 간섭영상으로부터 생성한 120개의 DDInSAR 영상과 수압식 파고계로 11일 동안 실측한 조위를 이용하였다. 먼저, DDInSAR 영상과 조위모델로 추출된 조위의 이중차분값(<TEX>${\Delta}\dot{T}$</TEX>)을 비교하였으며, 실측조위와 조위모델의 조위(T), 그리고 실측조위와 조위모델 조위의 24시간 차분값(<TEX>$\dot{T}$</TEX>)을 비교하였다. <TEX>${\Delta}\dot{T}$</TEX>와 T, <TEX>$\dot{T}$</TEX>의 평균제곱근오차(root mean square error; RMSE)는 조위모델의 역기압 효과(inverse barometer effect; IBE) 보정 후에 감소하였고, 이로부터 조위모델의 IBE는 필수적으로 보정되어야 함이 확인되었다. <TEX>$\dot{T}$</TEX>와 <TEX>${\Delta}\dot{T}$</TEX>는 T보다 작은 RMSE를 보였다. 이는 SAR 간섭쌍의 시간차(24시간) 동안의 차분값인 에 조위모델과 실측조위의 평균해수면 오차와 조위모델이 사용하는 조화상수 중 <TEX>$S_2$</TEX>, <TEX>$K_2$</TEX>, <TEX>$K_1$</TEX>, <TEX>$P_1$</TEX>의 오차가 상쇄되기 때문이다. 따라서 IBE가 보정된 조위모델의 <TEX>$\dot{T}$</TEX>와 <TEX>${\Delta}\dot{T}$</TEX>가 DDInSAR 기법에 사용될 수 있음을 확인하였다. 단기간 실측된 조위로는 최적의 조위모델 선정에 어려움이 있었으나, DDInSAR를 이용한 정밀도 평가에 의하면 Ross_Inv가 4.1 cm의 RMSE를 보여 테라노바 만의 DDInSAR에 가장 적합한 조위모델로 확인되었다. Accuracy assessment of tide models in polar ocean has to be performed to accurately analyze tidal response of glaciers by using Double-Differential Interferometric SAR (DDInSAR) technique. In this study, we used 120 DDInSAR images generated from 16 one-day tandem COSMO-SkyMed DInSAR pairs obtained for 2 years and in situ tide height for 11 days measured by a pressure type wave recorder to assess the accuracy of tide models such as TPXO7.1, FES2004, CATS2008a and Ross_Inv in Terra Nova Bay, East Antarctica. Firstly, we compared the double-differential tide height (<TEX>${\Delta}\dot{T}$</TEX>) for Campbell Glacier Tongue extracted from the DDInSAR images with that predicted by the tide models. Tide height (T) from in situ measurement was compared to that of the tide models. We also compared 24-hours difference of tide height (<TEX>$\dot{T}$</TEX>) from in situ tide height with that from the tide models. The root mean square error (RMSE) of <TEX>${\Delta}\dot{T}$</TEX>, T and <TEX>$\dot{T}$</TEX> decreased after the inverse barometer effect (IBE)-correction of the tide models, from which we confirmed that the IBE of tide models should be corrected requisitely. The RMSE of <TEX>$\dot{T}$</TEX> and <TEX>${\Delta}\dot{T}$</TEX> were smaller than that of T. This was because <TEX>$\dot{T}$</TEX> is the difference of tide height during temporal baseline of the DInSAR pairs (24 hours), in which the errors from mean sea level of the tide models and in situ tide, and the tide constituents of <TEX>$S_2$</TEX>, <TEX>$K_2$</TEX>, <TEX>$K_1$</TEX> and <TEX>$P_1$</TEX> used in the tide models were canceled. This confirmed that <TEX>$\dot{T}$</TEX> and <TEX>${\Delta}\dot{T}$</TEX> predicted by the IBE-corrected tide models can be used in DDInSAR technique. It was difficult to select an optimum tide model for DDInSAR in Terra Nova Bay by using in situ tide height measured in a short period. However, we could confirm that Ross_Inv is the optimum tide model as it showed the smallest RMSE of 4.1 cm by accuracy assessment using the DDInSAR images.

The apparent British sea slope is caused by systematic errors in the levelling-based vertical datum

The spirit-levelling–based British vertical datum (Ordnance Datum Newlyn) implies a south–north apparent slope in mean sea level of up to 53 mm deg –1 latitude, due to the datum falling on heading northwards. Although this apparent slope has been investigated since the 1960s, explanations of its origin have remained inconclusive. It has also been suggested that, rather than a slope, the British vertical datum includes a step of about 240 mm affecting all sites north of about 53°N. In either case, the British vertical datum may be of limited use for any study requiring accurate heights or changes in heights, such as testing geoid models, groundwater and hydrocarbon extraction, the calibration and validation of satellite-based digital terrain models, and the unification of vertical datums internationally. Within the last decade, however, based on an apparent reduction in the slope to only −12 mm deg –1 latitude with respect to recent geoid models, it has been claimed that the British vertical datum does provide a physically meaningful surface for use in scientific applications. In this paper, we reinvestigate the presence of apparent south–north sea slopes around Britain and reported slopes in the vertical datum, using the EGM2008 global gravitational model, together with mean sea level and GPS data from British tide gauges, GPS ellipsoidal heights of 178 fundamental benchmarks across mainland Britain, and vertical deflection observations at 192 stations. We demonstrate a south–north slope in the British vertical datum of −(20–25) mm deg –1 latitude with respect to both mean sea level (corrected for the ocean's mean dynamic topography and the inverse barometer response to atmospheric pressure loading) and the EGM2008 quasigeoid model, while EGM2008 is shown to exhibit a negligible slope of (2 ± 4) mm deg –1 with respect to mean sea level. It is clear, therefore, that the slope can only arise from systematic errors in the levelling, although we are unable to isolate their exact origin. Using an offset detection method based on a penalized likelihood maximization using the Schwarz Information Criterion, we do not detect a step in the vertical datum affecting all sites north of 53°N, but do identify regional distortions that we attribute to the inhomogeneity in both the levelling data used and the least squares adjustment procedures used to realize the datum. We conclude that the British vertical datum remains unsuitable for scientific purposes.

Inter‐annual to decadal sea‐level variability in the coastal zones of the Norwegian and Siberian Seas: The role of atmospheric forcing

Inter‐annual to decadal sea‐level variations from tide gauge records in the coastal zones of the Norwegian and Siberian Seas are examined for the period 1950–2010 using a combination of hydrographic observations, wind data, and theory. We identify two large areas of highly coherent sea‐level variability: one that includes the Norwegian, Barents, and Kara Seas, and another one that includes the Laptev, East Siberian, and Chukchi Seas. We provide evidence of a new contribution to the sea‐level variability along the Norwegian coast associated with the poleward propagation of sea‐level fluctuations along the eastern boundary of the North Atlantic. When this propagating signal is combined with the local wind, we are able to explain over 70% of the variance along the Norwegian coast. The steric component explains ~61% of the sea‐level (corrected for the inverse barometer) variability along the Norwegian coast. The high coherency between the sea level along the Norwegian coast and that in the Barents and Kara Seas suggests that part of the Norwegian signal propagates further north into these regions. We introduce an atmospheric vorticity index that explains much of the sea‐level variability in the Laptev, East Siberian, and Chukchi Seas with correlations ranging from 0.73 to 0.81. In the East Siberian Sea, we identify a sea‐level increase of ~22 cm between 2000 and 2003, which is partly explained by the vorticity index, and a decline of ~15 cm after 2003, which we relate to the strengthening of the Beaufort Gyre.

The effect of the NAO on sea level and on mass changes in the Mediterranean Sea

Sea level in the Mediterranean Sea over the period 1993–2011 is studied on the basis of altimetry, temperature, and salinity data and gravity measurements from Gravity Recovery and Climate Experiment (GRACE) (2002–2010). An observed increase in sea level corresponds to a linear sea level trend of 3.0 ± 0.5 mm/yr dominated by the increase in the oceanic mass in the basin. The increase in sea level does not, however, take place linearly but over two 2–3 year periods, each contributing 2–3 cm of sea level. Variability in the basin sea level and its mass component is dominated by the winter North Atlantic Oscillation (NAO). The NAO influence on sea level is primarily linked with atmospheric pressure changes and local wind field changes. However, neither the inverse barometer correction nor a barotropic sea level model forced by atmospheric pressure and wind can remove fully the NAO influence on the basin sea level. Thus, a third contributing mechanism linked with the NAO is suggested. During winter 2010, a low NAO index caused a basin sea level increase of 12 cm which was almost wholly due to mass changes and is evidenced by GRACE. About 8 cm of the observed sea level change can be accounted for as due to atmospheric pressure and wind changes. The residual 4 cm of sea level change is caused by the newly identified contribution. The physical mechanisms that may be responsible for this additional contribution are discussed.

Regional and local scale atmospheric forcing upon sea level along the coast of SW Europe

ABSTRACTSea level height variability along the coast of southwestern Europe is analyzed in relation with local and regional scale atmospheric forcing mechanisms. A circulation type (CP) catalogue, derived from a combination of principal component analysis and cluster analysis was used to characterize the regional scale atmospheric circulation, whereas the local scale was highlighted through the analysis of hourly wind records during extreme events. A large portion of common anomalous sea level variability at 6‐hourly time frame can be explained by the effect of one CP, depicting a deep extra‐tropical disturbance, close to the Iberian Peninsula, which represents the dominance of the inverse barometer mechanism. Other CPs can induce occasionally similar effects, but showing a spatially restricted scope, in relationship with the location of the main centres of action. Embedded into this synoptic framework, a local forcing, related to both meteorological processes (wind set up) and coastal oceanographic features (depth of the continental shelf) usually control the timing and less frequently, the magnitude of the surges. The decreasing trend observed in the magnitude of the sea level residuals derived from a hindcast model as well as some spatial differences (more intense trend in the upper percentiles along the northern Spain and Mediterranean coasts) can also be explained by long‐term changes in the frequency, and specially, in the internal variability of the CPs, which might be linked to the recent positive phase and eastward displacement of the southern node of the North Atlantic Oscillation. An overall increase in sea level pressure, detected in all CPs, has been accompanied by significant increases in types characterized by anticyclones over the Iberian Peninsula and decreases of northerly flows, related to a reduction of the number of cyclones passing through the Gulf of Biscay and the Western Mediterranean Basin.

TOWARDS A PROBABILISTIC FLOOD BASE MODEL FOR CARIBBEAN COASTS

An estimate of the total water level setup at the coast is examined at a monthly resolution to assess the base combined contribution from wind, wave and pressure setup in the absence of cyclonic activity. A stochastic approach is used to estimate the total values of water level setup from the three parameters and the results indicate that while the total coastal water level setup closely follows the wave setup due to a difference of an order of magnitude of two with the wind and pressure setup from the inverse barometer effect, the latter two parameters have a greater impact during the first seven months of the year - the dry season with an approximate two month overlap into the rainy season. Using the generated moments from monthly combined probability density functions, we show that the average coastal setup from the three driving forces show a general decreasing trend from the highest coastal setup months of December, January and February.

Pressure tide gauge records from the Atlantic shelf off Tierra del Fuego, southernmost South America

Based on pressure tide gauge observations at three sites off the Atlantic coast of Tierra del Fuego main island, time series spanning one to seven months of bottom pressure and sea-level variations are derived and analysed to reveal the major driving mechanisms. Ocean tides account for 99.5% of the total energy of the sea-level variations. The amplitudes and phases of a comprehensive set of tidal constituents resulting from a harmonic tidal analysis are presented. Exceptionally large shallow-water tides are identified. The second largest contribution is due to the local inverse barometer model accounting for up to 65% of the variance of the tide residual sea-level variations. Close to the shore a significant topographic modulation of the sea-level variations is revealed. The in situ observations are compared with six recent global ocean tide models, official tide tables, and sea-surface heights derived from satellite altimetry data. The amplitudes and phases predicted by the ocean tide models for the semidiurnal and diurnal constituents agree with those derived from our tide gauge records on average within 2cm and 5°, respectively. In the time domain the tidal signal represented by the models deviates typically by a few decimetres from that extracted from our records. Absolute altimeter biases were determined for the Jason-2, Jason-1 extended mission, and Envisat satellite altimeters. Relative sea-level variations are represented by the altimetry data with an accuracy of the order of 5cm.

Understanding the patterns of biological response to physical forcing in the Alborán Sea (western Mediterranean)

[1] A singular value decomposition (SVD) analysis was performed to determine the coupled modes of variability of satellite surface chlorophyll (CHL) and absolute dynamic topography (ADT) data for the Alboran Sea (western Mediterranean) during a 12-year period (1998–2009). From this analysis we have been able to detect features of the Alboran Sea dynamics that had not been recognized in previous research on the primary production of this basin. We have found that the two leading SVD modes represent more than 97% of the total squared covariance. The first mode is associated with the inverse barometer and its impact on chlorophyll distribution, whereas the second physical mode can explain the distribution of eutrophic and oligotrophic waters in the Alboran Sea. The results also confirm that the phytoplankton in the basin is very tightly controlled by meteorological and physical processes, via the advection of properties through the Strait, by the influence of Atlantic Jet intensity, which controls the gyres of the basin, and by the activation of up- or down-welling coastal processes by wind.

Atmospheric correction of satellite altimetry observations and sea-level variability in the NE Atlantic

Satellite altimetry provides continuous and spatially regular measurements of the height of the sea surface. Sea level responds to density changes of the water, to mass changes, due to addition or reduction of water mass, and to changes in the atmosphere above it. The present study examines the influence of atmospheric effects on sea-level variability in the North-East Atlantic. The association between the height of the sea surface and the North Atlantic Oscillation (NAO) is investigated by considering different sets of altimetry measurements for which the atmospheric effects have been handled differently. Altimetry data not corrected for atmospheric effects are strongly anti-correlated with the state of the NAO, reflecting the hydrostatic response of sea-level to the NAO pressure dipole. The application of an atmospheric correction to satellite altimetry observations in the NE Atlantic decreases variability of the height time series by more than 70% and reduces the amplitude of the seasonal cycle by ∼5cm. Altimetry data for which atmospheric effects are removed via an inverse barometer correction show a non-negligible correlation with the NAO index at some locations suggesting further indirect non-hydrostatic influences of the state of the NAO on sea level variability.

Toward the predictability of meteotsunamis in the Balearic Sea using regional nested atmosphere and ocean models

[1] Meteotsunamis are oceanic waves that possess tsunami-like characteristics but are meteorological in origin. In the western Mediterranean, travelling atmospheric pressure oscillations generate these long oceanic surface waves that can become amplified and produce strong seiche oscillations inside harbors. We analyze a June 2006 meteotsunami event in Ciutadella harbor (Menorca Island, Spain), studying numerically the phenomenon during its full life cycle, from the early atmospheric stages to the atmosphere-ocean resonant phase and the final highly amplified harbor oscillation. The Weather Research Forecast (WRF) atmospheric model adequately reproduces the development of a convective nucleus and also reproduces the induced atmospheric pressure oscillations moving at a speed of 27 m/s. The oceanic response is studied using the Regional Ocean Modeling System (ROMS), forced by the WRF pressure field. It shows an inverse barometer wave front in the open ocean progressively amplified through resonant interactions in the different shelf and coastal regions. The predictive capability of this new WRF/ROMS modeling approach is then discussed.

The 154-year record of sea level at San Francisco: extracting the long-term trend, recent changes, and other tidbits

A data adaptive method called ensemble empirical mode decomposition (EEMD) is used to examine the 154-year record of monthly sea level at San Francisco. The mode that is lowest in frequency corresponds to the long-term trend. The next highest mode corresponds to an oscillation with a period of ~100 years and may be related to solar variability. When this mode is combined with the long-term trend, the rate of increase in sea level starts to decrease by ~1980. The next lower mode corresponds to interdecadal time scales and thus includes the Pacific Decadal Oscillation. When combined with the two lower modes, sea level itself starts to decrease by the mid-1990s. These results are consistent with the most recent results from the intergovernmental panel on climate change (IPCC), and may be the first obtained from a tidal record. Prior to conducting EEMD, corrections for glacial isostatic adjustment (GIA) and the inverse barometer (IB) effect were applied. The effect of applying the GIA correction was relatively small, but the IB correction reduced the slope of the long-term trend in sea level by almost 15%. This reduction is due to a long-term increase in the variance of sea level pressure. To determine if the 10–15 year ENSO modulation cycle could be detected from the decomposition we first compared the envelope from the mode associated with ENSO, with the two adjacent modes that were lower in frequency. Spectral analysis revealed no significant maxima in the ENSO mode envelope, but a major peak in the spectrum for the two adjacent modes, with a period of 12.8 years. This is consistent with a local response to El Nino warming for the ENSO mode, but a non-local response for the two adjacent modes. A similar analysis was performed for the Southern Oscillation Index and a spectral maximum was found between 12 and 16 years, consistent with our non-local interpretation of the previous two modes.

On the contributions of atmospheric pressure and wind to daily sea level in the northern Adriatic Sea

Daily sea level variability in the Adriatic Sea is studied from different data sets using Empirical Orthogonal Functions, in connection with atmospheric pressure and wind stress. The first mode explains 56–69% of total variance and consists of uniform sea level variability all over the basin, correlated with atmospheric pressure through the inverse barometer effect. The second mode explains 13–16% of variance and accounts for an along-basin sea level gradient, which is correlated with the meridional wind stress component. The first two Principal Components are used as proxies to pressure- and wind-induced components of storm surges in the northern Adriatic. The analysis of the frequency of the most intense events in the 1957–2005 period shows that the wind contribution to storm surges has decreased, while no significant trends are found in the contribution of atmospheric pressure.