Inverted Barometer Research Articles

Interannual Sea Level Variability along the U.S. East Coast during the Satellite Altimetry Era: Local versus Remote Forcing

Abstract The contributions of local and remote forcings to the interannual sea level anomalies (SLAs) along the U.S. East Coast (USEC) during the satellite altimetry era from 1993 to 2019 are quantified with analytical models assisted by statistical methods. The local forcings from alongshore wind stress, sea level pressure via inverted barometer (IB) effect, and river discharges together explain 47%, 60.4%, and 66.8% of coastal sea level variance in the South Atlantic Bight (SAB), Mid-Atlantic Bight (MAB), and Gulf of Maine (GOM), respectively, with river discharges having the minimum contribution. Over a longer period of 1960–2019, the contribution of local forcings reduces significantly, with the IB effect having the minimum contribution. The remote forcings associated with open-ocean signals from the east and from the northern boundary at the Scotian coast together with the Gulf Stream (GS) variability explain 45.7%, 28.5%, and 37.7% of coastal sea level variance in the SAB, MAB, and GOM, respectively, playing a role comparable to that of local forcings in the SAB. The open-ocean sea level signals from 35° to 38°N strongly influence coastal SLAs in the SAB. The coastal SLAs in the SAB are also affected by the upstream GS strength (28°–36°N) and basin-scale wind stress curl anomaly, which is linked to the meridional shift in the downstream GS (74°–68°W). Remote forcings from the subpolar North Atlantic and wind stress curl from the Grand Banks to the Scotian coast influence coastal SLAs in the GOM and MAB via the northern boundary of the USEC at the Scotian coast.

Nonstationary Roles of Regional Forcings in Driving Low‐Frequency Sea Level Variability Along the U.S. East Coast Since the 1950s

AbstractThe nonstationary roles of regional forcings from alongshore wind stress and sea level pressure (SLP) in driving low‐frequency (interannual‐to‐decadal) sea level variability along the U.S. east coast for the 1959–2020 period are investigated. The role of regional forcings increases with time north of Cape Hatteras particularly during summer when their contributions to the variance of observed summertime coastal sea level anomalies increase by approximately 58%–87% from 1959–1989 to 1990–2020. The enhanced impact of regional forcings in recent decades results from an increase in the Inverted Barometer (IB) effect that act constructively with alongshore wind stress especially during summer, and to a lesser extent for the Gulf of Maine during fall. The summertime North Atlantic Oscillation (NAO) is largely responsible for the increased IB effect, owing to the stronger NAO‐associated low SLP anomalies centered around the Mid‐Atlantic Bight in recent decades compared to earlier decades.

Caspian Sea Level Change Observed by Satellite Altimetry

We analyze satellite altimeter observed Caspian Sea level (CSL) changes over the period January 1993 to December 2021 using the lake level series from the Hydroweb project and global sea level anomalies (SLA) grids provided by the Copernicus Marine Environment Monitoring Service (CMEMS). The two altimeter-based CSL series agree well at interannual and longer time scales, but show significantly large discrepancies at seasonal and shorter time scales. The large discrepancies are found to be introduced by the approximately inverted barometer (IB) correction applied to the CMEMS SLA over the Caspian Sea. The IB correction over the Caspian Sea or any enclosed lakes needs to be treated separately from the ocean by using the correct reference mean pressure. The actual IB effects over the Caspian Sea are significantly smaller than those applied in the CMEMS SLA grids. After applying an improved IB correction using the global mean sea level pressure fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis atmospheric model, the two CSL series agree remarkably well. Altimeter observed CSL series show a significant decreasing trend on top of strong seasonal variations. The estimated linear trends for the Hydroweb and CMEMS CSL series are −5.37 ± 0.11 and −5.40 ± 0.11 cm/yr, respectively. Annual amplitudes are 17.03 ± 1.33 vs. 15.79 ± 1.30 cm, with nearly the same phases. The CSL change shows notable acceleration in the decreasing trend since around 2005, and the estimated trends have increased to −8.86 ± 0.10 and −8.81 ± 0.10 cm/yr, respectively for the two-altimeter CSL series.

Revisiting the Ocean’s Nonisostatic Response to 5-Day Atmospheric Loading: New Results Based on Global Bottom Pressure Records and Numerical Modeling

AbstractWe use bottom pressure records from 59 sites of the global tsunami warning system to examine the nonisostatic response of the World Ocean to surface air pressure forcing within the 4–6-day band. It is within this narrow “5-day” band that sea level fluctuations strongly depart from the isostatic inverted barometer response. Numerical simulations of the observed bottom pressures were conducted using a two-dimensional Princeton Ocean Model forced at the upper boundary by two versions of the air pressure loading: (i) an analytical version having the form of the westward propagating, 5-day Rossby–Haurwitz air pressure mode; and (ii) an observational version based on a 16-yr record of global-scale atmospheric reanalysis data with a spatial resolution of 2.5°. Simulations from the two models—consisting of barotropic standing waves of millibar amplitudes and near uniform phases in the Pacific, Atlantic, and Indian Oceans—are in close agreement and closely reproduce the observed bottom pressures. The marked similarity of the outputs from the two models and the ability of both models to accurately reproduce the seafloor pressure records indicate a pronounced dynamic response of the World Ocean to nonstationary air pressure fields resembling the theoretical Rossby–Haurwitz air pressure mode.

A Methodology for Estimating the Response of the Coastal Ocean to Meteorological Forcing: A Case Study in Bohai Bay

AbstractThe coastal ocean sea level (SL) variations result from multiscale processes and are dominated by SL changes due to meteorological forcing. In this study, a new methodology, which combines inverted barometer correction and regression analysis (IBR), is developed to estimate the coastal ocean response to meteorological forcing in shallow water. The response is taken as the combination of the static ocean response calculated using the inverted barometer formula and the dynamic ocean response estimated using the multivariable linear regression involving atmospheric pressure and the wind component in the dominant wind orientation. IBR was implemented to estimate the coastal ocean response at two stations, E1 and E2, in Bohai Bay, China. The analyzed results indicate that at both stations, the adjusted SLs are related more to the regional wind, which is the averaged value of ERA-Interim data in Bohai Bay, than to the local wind. The estimated response using IBR with the regional meteorological forcing is much closer to the observed values than other methods, including the classical inverted barometer correction, the dynamic atmospheric correction, the multivariable linear regression, and the IBR with local forcing. The deviations between the observed values and the estimated values using IBR with regional meteorological forcing can be primarily attributed to remote wind. This case study indicates that IBR is a feasible and relatively effective method to estimate the coastal ocean response to meteorological forcing in shallow water.

A 1782–1794 sea level record at Trieste (northern Adriatic)

Abstract. The physician Leonardo Vordoni recorded sea heights at Trieste from 1782 to 1794 because of his interest in studying the connections between tides and the course of diseases that he attributed to the same forces. The data, expressed in Paris feet and inches (1 ft = 12 in. = 32.4845 cm), consist of heights measured on a pole, relative to the green algae belt corresponding to the mean high water. The measurements were reported in a manuscript that was recently found in the correspondence received by Giuseppe Toaldo, an astronomer in Padua. The observations were made twice a day until June 1791 and more frequently afterwards; the data from July 1791 onwards reasonably describe both the astronomical tide and the inverted-barometer (IB) effect. The low frequency of observations and poor metadata information seriously limit the scientific value of the data set, which, therefore, has mainly a historical value. In comparisons with modern data, the amplitude of sea level variations appears rather large, as if a unit shorter than the Paris foot was used. Moreover, an anomalously large decadal trend exists, which might be due to the pole sinking into the sea floor. The sea heights were digitized and are available through SEANOE (SEA scieNtific Open data Edition; https://doi.org/10.17882/62598; Raicich, 2019a).

Correcting GPS measurements for non-tidal loading

Non-tidal loading (NTL) deforms the earth’s surface, adding variability to the coordinates of geodetic sites. Yet, according to the IERS Conventions, there are no recommended surface-mass change models to account for NTL deformation in geodetic position time series. We investigate the NTL signal recorded at 585 GPS stations at different frequency bands, from day to years, by comparing GPS estimated displacements to modeled environmental loading. We used up-to-date and high-resolution (both temporal and spatial) models to account for NTL induced by mass changes in the atmosphere, oceans, and continental hydrology. Vertical land motions variability is reduced on average by up to 20% when correcting the series for non-tidal atmospheric and oceanic loading, employing either barotropic or baroclinic ocean models. We then focus on characterizing the ocean response to air-pressure variations, and we observe that there are no significant differences at seasonal timescales between a barotropic ocean model forced by air pressure and winds and a more classical baroclinic ocean model forced by wind, heat and freshwater fluxes. However, any of these choices further reduces the variability by 5% compared to the classical static inverted barometer ocean response. The variability of the vertical coordinate changes is further reduced by an additional 5% by also correcting for continental hydrology loading, especially at seasonal periods. For horizontal coordinate changes, the variability is reduced by less than 5% after correcting for all studied surface-mass changes.

Coastal sea level variability in the US West Coast Ocean Forecast System (WCOFS)

Sea level variability along the US West Coast is analyzed using multi-year time series records from tide gauges and a high-resolution regional ocean model, the base of the West Coast Ocean Forecast System (WCOFS). One of the metrics utilized is the frequency of occurrences when model prediction is within 0.15 m from the observed sea level, F. A target level of F = 90% is set by an operational agency. A combination of the tidal sea level from a shallow water inverse model, inverted barometer (IB) term computed using surface air pressure from a mesoscale atmospheric model, and low-pass filtered sea level from WCOFS representing the effect of coastal ocean dynamics (DYN) provides the most straightforward approach to reaching levels F>80%. The IB and DYN components each add between 5 and 15% to F. Given the importance of the DYN term bringing F closer to the operational requirement and its role as an indicator of the coastal ocean processes on scales from days to interannual, additional verification of the WCOFS subtidal sea level is provided in terms of the model-data correlation, standard deviation of the band-pass filtered (2–60 days) time series, the annual cycle amplitude, and alongshore sea level coherence in the range of 5–120-day periods. Model-data correlation in sea level increases from south to north along the US coast. The rms amplitude of model sea level variability in the 2–60-day band and its annual amplitude are weaker than observed north of 42 N, in the Pacific Northwest (PNW) coast region. The alongshore coherence amplitude and phase patterns are similar in the model and observations. Availability of the multi-year model solution allows computation and analysis of spatial maps of the coherence amplitude. For a reference location in the Southern California Bight, relatively short-period sea level motions (near 10 days) are incoherent with those north of the Santa Barbara Channel (in part, due to coastal trapped wave scattering and/or dissipation). At a range of periods around 60 days, the coastal sea level in Southern California is coherent with the sea surface height (SSH) variability over the shelf break in Oregon, Washington, and British Columbia, more than with the coastal SSH at the same latitudes.

Inverted barometer contributions to recent sea level changes along the northeast coast of North America

AbstractRegional sea level (SL) changes reflect dynamic and isostatic ocean effects. Recent works have interpreted accelerated and extreme SL changes along the northeast coast of North America primarily in terms of dynamic changes; however, dedicated study of isostatic changes related to surface atmospheric pressure loading—the inverted barometer (IB) effect—has been lacking. This investigation uses five different atmospheric pressure products to analyze the influence of the IB effect on annual mean SL from tide gauge records. The IB effect explains ∼25% of interannual SL variance and accounts for ∼50% of the magnitude of a recent extreme event of SL rise along Atlantic Canada and New England. Estimated IB effects also amount to ∼10–30% of recent multidecadal SL accelerations over the Mid‐Atlantic Bight and Southern New England. These findings reiterate the need for careful estimation and removal of isostatic effects for studies of dynamic SL.

Non-linear motions of Australian geodetic stations induced by non-tidal ocean loading and the passage of tropical cyclones

We investigate daily and sub-daily non-tidal oceanic and atmospheric loading (NTOAL) in the Australian region and put an upper bound on potential site motion examining the effects of tropical cyclone Yasi that crossed the Australian coast in January/February 2011. The dynamic nature of the ocean is important, particularly for northern Australia where the long-term scatter due to daily and sub-daily oceanic changes increases by 20–55 % compared to that estimated using the inverted barometer (IB) assumption. Correcting the daily Global Positioning System (GPS) time series for NTOAL employing either a dynamic ocean model or the IB assumption leads to a reduction of up to 52 % in the weighted scatter of daily coordinate estimates. Differences between the approaches are obscured by seasonal variations in the GPS precision along the northern coast. Two compensating signals during the cyclone require modelling at high spatial and temporal resolution: uplift induced by the atmospheric depression, and subsidence induced by storm surge. The latter dominates ( $$>$$ 135 %) the combined net effect that reaches a maximum of 14 mm, and 10 mm near the closest GPS site TOW2. Here, 96 % of the displacement is reached within 15 h due to the rapid transit of cyclones and the quasi-linear nature of the coastline. Consequently, estimating sub-daily NTOAL is necessary to properly account for such a signal that can be 3.5 times larger than its daily-averaged value. We were unable to detect the deformation signal in 2-hourly GPS processing and show that seasonal noise in the Austral summer dominates and precludes GPS detection of the cyclone-related subsidence.

The oceanic response of the Turkish Straits System to an extreme drop in atmospheric pressure

AbstractMoorings across all four entry/exit sections of the Dardanelles Strait and the Bosphorus Strait simultaneously measured the response of the Turkish Straits System to the passage of a severe cyclonic storm that included an atmospheric pressure drop of more than 30 mbar in less than 48 h. The bottom pressure response at the Aegean Sea side of the Dardanelles Strait was consistent with an inverted barometer response, but the response at the other sections did not follow an inverted barometer, leading to a large bottom pressure gradient through the Turkish Straits System. Upper‐layer flow toward the Aegean Sea was reversed by the storm and flow toward the Black Sea was greatly enhanced. Bottom pressure across the Sea of Marmara peaked 6 h after the passage of the storm's minimum pressure. The response on the Dardanelles side was a combination of sea elevation and pycnocline depth rise, and the response on the Bosphorus side was an even greater sea elevation rise and a drop in pycnocline depth. The peak in bottom pressure in the Sea of Marmara was followed by another reverse in the flow through the Dardanelles Strait as flow was then directed away from the Sea of Marmara in both straits. A simple conceptual model without wind is able to explain fluctuations in bottom pressure in the Sea of Marmara to a 0.89–0.96 level of correlation. This stresses the importance of atmospheric pressure dynamics in driving the mass flux of the Turkish Strait System for extreme storms.

Crustal Deformation due to Atmospheric Pressure Loading in New Zealand

Crustal Deformation due to Atmospheric Pressure Loading in New ZealandWe investigate atmospheric pressure loading displacements in New Zealand using global and regional air-pressure data collected over a period of 50 years (1960-2009). The elastic response of the Earth to atmospheric loading is calculated by adopting mass loading Love numbers based on the parameters of the Preliminary Reference Earth Model (PREM). The ocean response to atmospheric loading is computed utilising a modified inverted barometer theory. The results reveal that the atmospheric loading vertical displacements are typically smallest along coastal regions, while gradually increasing inland with the maximum peak-to-peak displacement of 13.1 mm for this study period. In contrast, the largest horizontal displacements are found along coastal regions, where the maximum peak-to-peak displacement reaches 2.7 mm. The vertical displacements have a high spatial correlation, whereas the spatial correlation of the horizontal displacement components is much smaller. A spectral decomposition of the atmospheric loading time series shows that the signal is a broad band with most energy between 1 week and annual periods, and with a couple of peaks corresponding to approximately annual forcing and its overtones. The largest amplitudes in the atmospheric loading time series have an annual and semi-annual period.

Interannual atmospheric torque and El Niño–Southern Oscillation: Where is the polar motion signal?

In this paper, we investigate the atmospheric excitation of polar motion (PM) associated with the El Niño–Southern Oscillation (ENSO) phenomenon. ENSO effects on length‐of‐day due to changes in the axial component of atmospheric angular momentum (AAM) have long been recognized, but identification of PM excitation with ENSO‐induced equatorial AAM anomalies has proved more elusive. Here we use an appropriately modified form of the inverted barometer (IB) assumption to study ENSO‐related atmospheric torques arising from pressure loading on the Earth's ellipsoidal bulge and mountains and from frictional wind stress over the Earth's land‐ and ocean‐covered surface. The resulting dissipation torques, which accommodate adjustment of the oceanic mass distribution to time‐variable atmospheric loading, are found to be small. The ellipsoidal torques have the largest amplitude, reflecting the order‐of‐magnitude discrepancy between the height departures of the Earth's bulge (∼20 km) and its surface orography (∼2 km). Because of relatively uniform pressure covariances with the Southern Oscillation Index over the continents for the largely land‐based X component and the uniform IB response for the largely ocean‐based Y component; however, the ENSO‐related PM excitation arising from the ellipsoidal torques is reduced to an amplitude comparable with the sum of regional mountain torques from the individual continents. The largest of these are generated over Asia and Antarctica, arising from efficient coupling of ENSO‐related surface pressure anomalies with large‐scale orographic features. The geometrical mitigation of the ellipsoidal torques, classically expected to dominate equatorial AAM forcing, accounts for the lack of a detectable atmosphere‐driven polar motion response to ENSO.

Spectral and geographical variability in the oceanic response to atmospheric pressure fluctuations, as inferred from “dynamic barometer” Green's functions

A decade ago, a novel theoretical approach was developed (Dickman, 1998) for determining the dynamic response of the oceans to atmospheric pressure variations, a response nicknamed the “dynamic barometer” (DB), and the effects of that response on Earth's rotation. This approach employed a generalized, spherical harmonic ocean tide model to compute oceanic Green's functions, the oceans' fluid dynamic response to unit‐amplitude pressure forcing on various spatial and temporal scales, and then construct rotational Green's functions, representing the rotational effects of that response. When combined with the observed atmospheric pressure field, the rotational Green's functions would yield the effects of the DB on Earth's rotation. The Green's functions reflect in some way the geographical and spectral sensitivity of the oceans to atmospheric pressure forcing. We have formulated a measure of that sensitivity using a simple combination of rotational Green's functions. We find that the DB response of the oceans to atmospheric pressure forcing depends significantly on geographic location and on frequency. Compared to the inverted barometer (IB) (the traditional static model), the DB effects differ slightly at long periods but become very different at shorter periods. Among all the responses, the prograde polar motion effects are the most dynamic, with large portions of the North Atlantic and some of the North Pacific no larger than one third of IB, but most of the Southern Hemisphere oceans at least 50% greater than IB.

Patterns of atmospheric excitation functions of polar motion from high‐resolution regional sectors

In this paper regional values of equatorial components of the atmospheric excitation function of polar motion, which are proportional to the equatorial components of angular momentum (AAM), were computed at high spatial resolution in 3312 equal‐area sectors from the surface pressure fields of the NCEP‐NCAR reanalyses in order to improve our knowledge of patterns of atmospheric excitation functions of polar motion. The inverted barometer (IB) model of oceanic isostatic adjustment is applied to readjust the effective atmospheric pressure fields. To identify the influence of different geographic regions of the atmosphere on the excitation of polar motion, correlations and covariances between these regional excitations values and either the global geodetic excitation function, determined from the geodetic observational data, or the atmospheric excitation functions, equivalent to the sum of such excitations in all regions, were computed in different spectral bands, from subseasonal to interannual period ones. The covariances show the regional contribution to the global excitation, whereas the correlations show an overall similar relationship. Furthermore, these analyses determine regional sources of polar motion excitation by surface pressure variations in the atmosphere in different spectral bands. The broad regions over Eurasia and North America were identified as principal atmospheric excitation sources for polar motion. The results revealed that atmospheric excitation over land regions in the northern middle latitudes, especially over Asia, is exceptionally strong, a point generally noted in previous studies, though without identifying detailed features. Over the Himalayan Mountains the covariance of local with global atmospheric excitation in the annual band forms a small minimum surrounded by larger values in a ring‐like formation. For other terms, maxima of atmospheric excitation have a longitudinal formation mostly across the Eurasian continent. The atmospheric excitation of polar motion over the ocean is weak when subject to the IB correction, but it contains nevertheless a clear annual signal and is highly correlated with the annual oscillation of the global geodetic excitation function of polar motion.

Relation between daily variations in sea level and meteorological forcing in Sancti Petri Channel (SW Spain)

Sea level series were recorded from October 2006 to January 2007 at two different sites in a shallow-water estuary of SW Spain. Results showed a very significant temporal variability in daily sea level. Moreover, 85% of the variance was explained by atmospheric pressure variability, obtaining a relation between sea level and atmospheric pressure of around –2 cm hPa–1. A more detailed study showed the existence of two different types of oscillations in daily sea level: the first, with periods of 10 days, was related to the inverted barometer (IB) effect associated with synoptic-scale atmospheric pressure oscillations (IB factor of around 1.4 ± 0.1 cm hPa–1), and the second, with periods of more than 10 days, was related to pressure changes in the planetaryscale atmospheric waves. Even though the high barometric behaviour of the 10-day period was partially related to the local winds, the shallowness of the study area invalidated the assumption of a static IB effect (–1 cm hPa–1). Rather, it was the dynamic IB effect that explained the displacement of the IB factor relative to the static assumption.

High-frequency non-tidal ocean loading effects on surface gravity measurements

SUMMARY We model atmospheric and non-tidal oceanic loading effects on surface gravity variations, using global surface pressure field provided by the European Centre for Medium-range Weather Forecasts (ECMWF), and sea surface height from the Toulouse Hydrodynamic Unstructured Grid Ocean model (HUGO-m) barotropic ocean model. We show the improvement in terms of reduction of variance of 15 different superconducting gravimeters of the worldwide Global Geodynamics Project (GGP) network, compared to the classical inverted barometer assumption. We also study two storm surges over the Western European Shelf in 2000 and 2003. We compare the HUGO-m sea surface height variations to various tide gauges measurements as well as the induced loading effects to the computations of Fratepietro et al., using the Proudman Storm Surge model, for the Membach (Belgium) station. The agreement between modelled ocean loading and gravity observations is largely improved when using a global atmospheric loading correction, compared to the classical local approach. The remaining discrepancies are mainly due to hydrological loading contributions.

Comparing the sea level response to pressure and wind forcing of two barotropic models: Validation with tide gauge and altimetry data

The sea level output from two barotropic models, Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe (HIPOCAS) and Dynamic Atmospheric Correction (DAC), are compared and evaluated on the basis of coastal tide gauges (TG) and TOPEX/POSEIDON measurements in the Mediterranean Sea and the NE Atlantic Ocean for the period 1993–2001. Both models reduce the observed sea level variance more than the classical inverted barometer correction. However, differences between the models arise for different regions and frequency bands. In coastal areas, Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe (HIPOCAS) reproduces observed (TG recorded) sea level better than DAC (residual variance of 70.81 ± 0.69 cm2 versus 74.05 ± 0.68 cm2). This is particularly true in the Atlantic Iberian coasts (49.58 ± 1.09 cm2 versus 68.53 ± 1.12 cm2), where HIPOCAS is able to reproduce a wind‐generated signal probably linked with seasonal upwelling. The exception is the northern Adriatic, where HIPOCAS gives higher residual variance than DAC (118.80 ± 0.60 cm2 versus 107.15 ± 0.60 cm2). At low frequencies (T > 20 days) the atmospherically induced coastal sea level is better reproduced by HIPOCAS in the entire domain (23.43 ± 0.34 cm2 versus 32.35 ± 0.33 cm2). At high frequencies (T < 20 days), DAC and HIPOCAS perform on average similarly (37 ± 0.5 cm2). In the open ocean, both corrections provide equivalent results (60 ± 5 cm2 residual altimeter variance). Our general recommendation would be to use either DAC or HIPOCAS for the correction of altimetry, and to use HIPOCAS for coastal studies aiming at separating the atmospheric contribution to sea level variability from the steric and mass contributions.

Response of Regional Sea Level to Atmospheric Pressure Loading in a Climate Change Scenario

Abstract The regional response of the global ocean to low-frequency changes in atmospheric pressure loading, ηib, is analyzed as it occurs in the Max Planck Institute for Meteorology (MPI-M) coupled ocean–atmosphere climate model in response to increased atmospheric CO2 concentrations. Results suggest that long-term changes in ηib can lead to increases in high-latitude sea level by up to 5 and 10 cm, respectively, after doubling and quadrupling the atmospheric CO2 content. At low latitudes, sea level will decrease simultaneously between 2 and 4 cm through the combined effects of changes in the atmospheric circulation and through the increase of its moisture content. In subpolar regions, associated rates of sea level increase are of the order of 0.4–0.6 mm yr−1 for quadrupled atmospheric CO2 concentrations, while in mid- and low latitudes, sea level will decrease at a rate of 0.2 mm yr−1. Differences between doubling and quadrupling CO2 concentrations indicate regionally dependent nonlinearities in the changing climate system. The analysis suggests that in some regions (including the coasts of northern Europe) low-frequency ηib changes could be as large as 10%–20% of a global sea level increase anticipated over the next 100 yr. While not being a dominant effect, amplitudes of long-period ηib changes are large enough to be included in future estimates of climate-related regional sea level change. Increasing the vertically integrated atmospheric CO2 content by 4 and 12 kg m−2 (on global average), in response to doubling and quadrupling atmospheric CO2 concentrations, suggests associated reductions of global sea level by 0.6 and 1.7 cm, respectively. The differences between two different model solutions are significant, especially in the Southern Ocean, where they show significantly different atmospheric mass and pressure distributions, and at low latitudes, where differences resemble the contribution of increased moisture content added to the inverted-barometer (IB) effect in the MPI-M solution.

Dynamic and regression modeling of ocean variability in the tide‐gauge record at seasonal and longer periods

Comparison of monthly mean tide‐gauge time series to corresponding model time series based on a static inverted barometer (IB) for pressure‐driven fluctuations and a ocean general circulation model (OM) reveals that the combined model successfully reproduces seasonal and interannual changes in relative sea level at many stations. Removal of the OM and IB from the tide‐gauge record produces residual time series with a mean global variance reduction of 53%. The OM is mis‐scaled for certain regions, and 68% of the residual time series contain a significant seasonal variability after removal of the OM and IB from the tide‐gauge data. Including OM admittance parameters and seasonal coefficients in a regression model for each station, with IB also removed, produces residual time series with mean global variance reduction of 71%. Examination of the regional improvement in variance caused by scaling the OM, including seasonal terms, or both, indicates weakness in the model at predicting sea‐level variation for constricted ocean regions. The model is particularly effective at reproducing sea‐level variation for stations in North America, Europe, and Japan. The RMS residual for many stations in these areas is 25–35 mm. The production of “cleaner” tide‐gauge time series, with oceanographic variability removed, is important for future analysis of nonsecular and regionally differing sea‐level variations. Understanding the ocean model's strengths and weaknesses will allow for future improvements of the model.