Detlef Stammer

Global Characteristics of Ocean Variability Estimated from Regional TOPEX/POSEIDON Altimeter Measurements

Abstract Three years of altimetric data from the TOPEX/POSEIDON spacecraft have been used to study characteristics of eddy variability over the World Ocean. The nature of the variability and its spatial structure are characterized in terms of the geographical distribution of eddy energy, as simple approximations of observed regional frequency and wavenumber spectra, and in terms of associated eddy time and space scales of sea surface height (SSH) variability and geostrophic velocity. Emphasis is put on summarizing characteristics typical for dynamically distinct regions of the World Ocean. This effort results in an attempt to describe the observed ocean variability in terms of universal spectral relations that depend only on few mean flow parameters such as the first-mode Rossby radius of deformation. Regional peculiarities follow naturally as deviations from the fundamental frequency and wavenumber spectra presented here. Frequency spectra of both variables can be summarized by three basic types represen...

Atmospheric loading and the oceanic “inverted barometer” effect

The response of the ocean to fluctuating atmospheric pressure loads is reviewed in theory and in observations. Major theoretical issues lie primarily with oceanic boundary reflectivity and with rates of dissipation, generally. Analytical solutions for a stratified ocean show that a static (“inverted barometer”) response is anticipated at all frequencies and wavenumbers not coincident with certain dispersion curves. Nonstatic behavior of two types is predicted: zero motion of the sea surface and a resonant response. Two types of resonance emerge. The first type corresponds to barotropic modes which are long gravity waves or Rossby waves at high and low frequencies, respectively. The second type excites internal modes, either gravity waves or Rossby waves depending on frequency, but modified from a conventional resonant response by the immediate juxtaposition in frequency/wavenumber space of the rigid-lid modes. The extent to which these actual resonances are generated is obscure owing to the same uncertainties about oceanic dissipation and scattering which affect all other forced oceanic motions, especially including the tides and other barotropic motions. Zero sea surface motion is predicted at frequencies and wavenumbers corresponding to “rigid-lid” modes. Observations support the wide applicability of the static response except in the tropics and in the western boundary current extension regions; there, the signal-to-noise ratios may be inadequate. The only other known clear nonstatic response occurs near 5 days in the Pacific and South Atlantic Oceans, indicative of a low-Q resonance in the former area and a forced nonresonant response in the latter, but there are remaining problems with these interpretations.

Accuracy assessment of recent ocean tide models

Over 20 global ocean tide models have been developed since 1994, primarily as a consequence of analysis of the precise altimetric measurements from TOPEX/POSEIDON and as a result of parallel developments in numerical tidal modeling and data assimilation. This paper provides an accuracy assessment of 10 such tide models and discusses their benefits in many fields including geodesy, oceanography, and geophysics. A variety of tests indicate that all these tide models agree within 2-3 cm in the deep ocean, and they represent a significant improvement over the classical Schwiderski 1980 model by approximately 5 cm rms. As a result, two tide models were selected for the reprocessing of TOPEX/POSEIDON Geophysical Data Records in late 1995. Current ocean tide models allow an improved observation of deep ocean surface dynamic topography using satellite altimetry. Other significant contributions include theft applications in an improved orbit computation for TOPEX/POSEIDON and other geodetic satellites, to yield accurate predictions of Earth rotation excitations and improved estimates of ocean loading corrections for geodetic observatories, and to allow better separation of astronomical tides from phenomena with meteorological and geophysical origins. The largest differences between these tide models occur in shallow waters, indicating that the current models are still problematic in these areas. Future improvement of global tide models is anticipated with additional high-quality altimeter data and with advances in numerical techniques to assimilate data into high-resolution hydrodynamic models.

Steric and wind-induced changes in TOPEX/POSEIDON large-scale sea surface topography observations

Three years of TOPEX/POSEIDON altimeter data are analyzed with respect to large-scale fluctuations in sea surface height observations and underlying physical processes. In midlatitudes the most conspicuous feature in the large-scale changes in sea surface height are related to surface buoyancy fluxes, predominantly surface heat fluxes. The next prominent variability on subannual timescales is the adjustment of the ocean to varying winds stress fields in terms of planetary waves. The degree to which basin-scale sea surface height variations relative to the steric component can be described as a time-dependent Sverdrup balance is addressed for the Pacific Ocean. If a flat bottom ocean is assumed, a significant correlation of meridional mass transport variations inferred from TOPEX/POSEIDON and a simple Sverdrup model can be found in the Pacific Ocean north of 40°N, but it fails elsewhere. Numerical ocean general circulation models show a good agreement with the TOPEX/POSEIDON observations in all those components of ocean variability and allow further insight into the dynamics that govern observed sea surface height changes.

Oceanic signals in observed motions of the Earth's pole of rotation

Motion of the Earths pole of rotation relative to its crust, commonly referred to as polar motion, can be excited by a variety of geophysical mechanisms. In particular, changes in atmospheric wind and mass fields have been linked to polar motion over a wide range of timescales, but substantial discrepancies remain between the atmospheric and geodetic observations. Here we present results from a nearly global ocean model which indicate that oceanic circulation and mass-field variability play important roles in the excitation of seasonal to fortnightly polar motion. The joint oceanic and atmospheric excitation provides a better agreement with the observed polar motion than atmospheric excitation alone. Geodetic measurements may therefore be used to provide a global consistency check on the quality of simulated large-scale oceanic fields.

Temporal changes in eddy energy of the oceans

Insight into the sources of eddy energy in the ocean can be obtained by studying the degree and nature of its temporal changes. As a preliminary to a dynamical and modeling discussion, we provide a description of the changes in variability on the annual and interannual time scales as seen in the TOPEX/POSEIDON altimeter data and in current meter records with a focus on the North Atlantic and North Pacific Oceans. The patterns of change and corresponding ones in the wind stress are globally very variable and intricate, with few easy generalizations possible. Over most of the subtropical oceans and along major mean fronts, seasonal variations of the eddy energy are negligible. There are, however, regions that show a pronounced annual cycle in eddy energy, notably the northeastern Pacific, the northern and eastern North Atlantic, as well as the tropical oceans. In those locations a strong correlation of a time-varying altimetric eddy kinetic energy on annual and longer periods with wind stress forcing is found, and trends present in near-surface eddy kinetic energy can be related to drifts in meteorological wind stress fields (mean and storm tracks) over the four-year TOPEX/POSEIDON record. While the seasonal cycles in mooring data are found generally to agree with altimetry, the only statistically significant evidence for interannual trends in current meter data appears in the long duration eastern Atlantic site mooring near 33°N, 22°W.

The role of variable wind forcing in generating eddy energy in the North Atlantic

Seasonal changes in eddy energy are used to investigate the role of high-frequency wind forcing in generating eddy kinetic energy in the oceans. To this end, we analyze two experiments of an eddy-permitting model of the North Atlantic driven by daily and monthly mean wind stress fields, and compare results with corresponding changes in the variance of the wind fields, and related results from previous studies using altimeter and current meter data. With daily wind-stress forcing the model is found to be in general agreement with altimetric observations and reveal a complex pattern of temporal changes in variability over the North Atlantic. Observations and the model indicate enhanced levels of eddy energy during winter months over several areas of the northern and, particularly northeastern North Atlantic. Since the wind-generated variability is primarily barotropic, its signal can be detected mostly in the low-energy regions of the northern and north-eastern North Atlantic, which are remote from baroclinically unstable currents. There the winter-to-summer difference in simulated eddy kinetic energy caused by the variable wind forcing is <0.5 cm2 s2 between 30° and 55°N, and is 1–3 cm2 s2 north of 55°N. Seasonal changes in kinetic energy are insignificant along the path of the North Atlantic current and south of about 30°N. The weak depth dependence of the seasonal changes in eddy energy implies that the relative importance of wind-generated eddy energy is maximum at depth where the general (baroclinic) variability level is low. Accordingly, a significant correlation is found between the seasonal cycle in the variance of wind stress and the seasonal cycle in eddy energy over a substantially wider area than near the surface, notably across the entire eastern North Atlantic between the North Atlantic Current and the North Equatorial Current.

Impact of initialization procedures on the predictive skill of a coupled ocean-atmosphere model and related mechanisms for predictability

The sensitivity of the predictive skill of a decadal climate prediction system is investigated with respect to details of the initialization procedure. For this purpose, the coupled ocean-atmosphere UCLA/MITgcm climate model is initialized using the following three different initialization approaches: full state initialization (FSI), anomaly initialization (AI) and full state initialization employing heat flux and freshwater flux corrections (FC). The ocean initial conditions are provided by the GECCO state estimate (the German contribution to Estimating the Circulation and Climate of the Ocean project), from which ensembles of decadal hindcasts are started every 5 years from 1961 to 2001. To evaluate the performance of the initialized hindcasts, they are compared with the persistence forecasts and an ensemble of twentieth-century simulations (un-initialized hindcasts). The study is divided in two main parts. In the first part the primary focus is to estimate the predictive skill for differently initialized hindcasts of sea surface temperature (SST), sea surface height (SSH) and the Atlantic meridional overturning circulation (AMOC). The predictive skill for SST, SSH and AMOC is assessed against the GECCO synthesis using anomaly correlation coefficient and root-mean-squared-error skill score. In regions with a deep mixed layer the predictive skill for SST anomalies remains significant for up to a decade in the FC experiment. By contrast, FSI shows less persistent skill in the North Atlantic and AI does not show high skill in the extratropical Southern Hemisphere, but appears to be more skillful in the tropics. In the extratropics, the improved skill is related to the ability of the FC initialization method to better represent the mixed layer depth, and the highest skill occurs during wintertime. The correlation skill for the spatially averaged North Atlantic SSH hindcasts remains significant up to a decade only for FC. The North Atlantic MOC initialized hindcasts show high correlation values in the first pentad while correlation remains significant in the following pentad too for FSI and FC. Overall, for the current setup, the FC approach appears to lead to the best results, followed by the FSI and AI procedures. An extended analysis of predictive skill for SSH hindcasts from different initialization experiments is performed in the second part. The analysis employs a method that allows to distinguish different contributions to steric SSH changes, namely those related to density changes imposed by temperature or salinity anomalies beneath the mixed layer (thermosteric and halosteric heave terms) and those related to processes of density compensated temperature–salinity changes (spice term). The spice and heave contributions in the mixed layer are not separated (mixed layer term). The patterns of the predictive skill suggest significant improvement of initialization which is related to the thermosteric heave term (in the subtropical Pacific, western North Atlantic), thermosteric mixed layer term (in the subtropical Atlantic) and spice term (in the eastern and subpolar North Atlantic and Southern Ocean). These contributions imply useful predictive skill as they occur in