A methodology for estimating the response of the coastal ocean to meteorological forcing: A case study in Bohai Bay

The 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 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 analysed 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)

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).

A methodology for estimating the response of the coastal ocean to meteorological forcing: A case study in the Bohai Bay

The sea level (SL) variations at the coastal ocean result from multiscale processes and are substantially contributed by the SL changes due to the meteorological forcing. In this study, a new methodology, named as IBR, is developed to estimate the response of the coastal ocean to meteorological forcing. 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 wind component at the dominant wind orientation. The dominant wind orientation is determined based on the averaged values of the magnitude squared coherences between the adjusted SL and wind at every wind orientation. The IBR is implemented to estimate the response of the coastal ocean at two stations, E1 and E2 in the Bohai Bay, China. The analysed results indicate that at both E1 and E2, the adjusted SLs are related more to the regional wind, which is the averaged value in the Bohai Bay of the 10 m wind in the ERA-Interim data, than to the local wind; the dominant regional wind orientation is 75°. 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, demonstrating that IBR with regional forcing have the best skill in estimating the response. The large deviations between the observed values and the estimated values using IBR with the regional meteorological forcing are mainly due to the remote wind, which is not considered in the IBR. This case study indicates that the IBR is a feasible and relatively effective method to estimate the response of the coastal ocean to the meteorological forcing.

Effects of Stratification on the Large-Scale Ocean Response to Barometric Pressure

Single-layer (barotropic) models have been commonly used in studies of the inverted barometer effect and the oceanic response to atmospheric pressure loading. The potential effects of stratification on this response are explored here using a general circulation model in a near-global domain with realistic coasts and bathymetry. Periodic forcing by the diurnal and semidiurnal atmospheric tides and 6-hourly stochastic forcing from weather center analyses are both examined. A global dynamic response (i.e., departures from inverted barometer behavior) is clear in the response to atmospheric tides; for stochastic forcing, the largest dynamic signals occur in shallow and semienclosed regions and at mid- and high latitudes. The influence of stratification in the dynamics is assessed by comparing surface and bottom pressure signals. Baroclinic effects are generally weak, particularly in the response to the large-scale atmospheric tides. Under stochastic forcing, largest differences between surface and bottom pressure signals reach 10%–20% of the surface signals and tend to occur in regions of enhanced topographic gradients. Bottom-intensified, localized interactions with topography seem to be involved. Enhanced baroclinicity is also seen at low latitudes, where stratification effects are also felt in the upper ocean. General implications for modeling the ocean response to high-frequency atmospheric and tidal forcing are discussed.

Low-Frequency Sea Level Variability and the Inverted Barometer Effect

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

Atmospheric Excitation of Polar Motion

Variations in the angular momentum of the atmosphere in the equatorial plane due to shifts in air mass distribution and changing winds impact the orientation of Earth so that motions of the pole occur on a broad range of time scales. The wind terms have notable diurnal fluctuations, which appear as a tidal signature. Subseasonal fluctuations of the pole are shown to be related to the atmospheric signal on scales as short as at least a week, according to a coherency analysis. Atmospheric mass fluctuations over certain regions, such as Eurasia and North America, appear to be more responsible for rapid polar motions than are those elsewhere, and may be related to known climate modes. On the other hand, atmospheric pressure fluctuations over the ocean are counteracted in large measure by a sea level response, due to an inverted barometer relationship. Ocean forcing from model results assist in narrowing the differences in the geodetic and atmospheric budgets. Efforts to assess dynamic forecasts of the atmospheric polar motion excitations have demonstrated positive skill out to at least 10 days for the mass term.