Rui M. Ponte

Decadal Trends in Sea Level Patterns: 1993–2004

Abstract Estimates of regional patterns of global sea level change are obtained from a 1° horizontal resolution general circulation model constrained by least squares to about 100 million ocean observations and many more meteorological estimates during the period 1993–2004. The data include not only altimetric variability, but most of the modern hydrography, Argo float profiles, sea surface temperature, and other observations. Spatial-mean trends in altimetric data are explicitly suppressed to isolate global average long-term changes required by the in situ data alone. On large scales, some regions display strong signals although few individual points have statistically significant trends. In the regional patterns, thermal, salinity, and mass redistribution contributions are all important, showing that regional sea level change is tied directly to the general circulation. Contributions below about 900 m are significant, but not dominant, and are expected to grow with time as the abyssal ocean shifts. Esti...

Causes for Contemporary Regional Sea Level Changes

Regional sea level changes can deviate substantially from those of the global mean, can vary on a broad range of timescales, and in some regions can even lead to a reversal of long-term global mean sea level trends. The underlying causes are associated with dynamic variations in the ocean circulation as part of climate modes of variability and with an isostatic adjustment of Earths crust to past and ongoing changes in polar ice masses and continental water storage. Relative to the coastline, sea level is also affected by processes such as earthquakes and anthropogenically induced subsidence. Present-day regional sea level changes appear to be caused primarily by natural climate variability. However, the imprint of anthropogenic effects on regional sea level-whether due to changes in the atmospheric forcing or to mass variations in the system-will grow with time as climate change progresses, and toward the end of the twenty-first century, regional sea level patterns will be a superposition of climate variability modes and natural and anthropogenically induced static sea level patterns. Attribution and predictions of ongoing and future sea level changes require an expanded and sustained climate observing system.

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.

A Preliminary Model Study of the Large-Scale Seasonal Cycle in Bottom Pressure Over the Global Ocean

Output from the primitive equation model of Semtner and Chervin [1992] is used to examine the seasonal cycle in bottom pressure p b over the global ocean. Effects of the volume-conserving formulation of the model on the calculation of p b are considered. The estimated seasonal, large-scale p b signals have amplitudes ranging from less than 1 cm over most of the deep ocean to several centimeters over shallow, boundary regions. Variability generally increases toward the western sides of the basins and is also larger in some Southern Ocean regions. An oscillation between subtropical and higher latitudes in the North Pacific is clear. Comparison with barotropic simulations indicates that, on basin scales, seasonal p b variability is related to barotropic dynamics and the seasonal cycle in Ekman pumping and results from a small, net residual in mass divergence from the balance between Ekman and Sverdrup flows.

Variability in a homogeneous global ocean forced by barometric pressure

Abstract The nature of the oceanic response to pressure loading is explored using a constant-density, shallow-water numerical model driven by atmospheric pressure fields from the European Centre for Medium Range Weather Forecasts. The model has realistic bottom topography and coastlines and is run for 1 year (1986) on a global domain. Meridional gradients in mean sea-level are generally large (10–20 cm over 20–30°), particularly in high southern latitudes. Sea-level variability is strong in mid- and high latitudes (typical standard deviations of 10–15 cm), but weakens towards the equator. Results indicate a significant contribution of pressure-driven fluctuations to the observed large-scale sea-level variability in mid- and high latitudes, away from western boundary regions. Pressure-induced velocity signals are, in contrast, generally small compared with other types of variability. The validity of the inverted barometer approximation is found to be strongly dependent on frequency and geographical location. Globally, the approximation is not reliable for periods shorter than approximately 2 days, but failure at longer periods occurs over extensive regions (e.g. the tropical Atlantic and Pacific, and the Southern Ocean). Nonisostatic contributions to the sea-level variability are substantial in many areas, including the tropics, the high-latitude North Atlantic, the Gulf of Mexico, and several other boundary regions. The dynamical signals are partly associated with the excitation of several high-frequency normal modes. Some of these features have a spatial structure and period very similar to normal modes calculated by Platzman and collaborators. Their presence in the model indicates that atmospheric pressure forcing is a possible mechanism for normal mode excitation.

Sea Level Response to Pressure Forcing in a Barotropic Numerical Model

Abstract A barotropic shallow-water model is used to study the large-scale sea level response to realistic barometric forcing at periods ranging from 1 day to 1 year. Results are presented from coarse resolution “open” ocean experiments (i.e., no shallow continental shelf regions or marginal seas) with coastal geometries and bottom topography representative of the North Atlantic and Pacific basins. The validity of the inverted barometer (IB) approximation is examined in detail, including nonlocal effects which result from taking into account the constant volume of the ocean. These effects are found to be important at low latitudes, where a considerable part of the sea level variability is related to pressure forcing over higher latitudes. Root-mean-square deviations from an IB response in the range of 1–3 cm are typical, with most of the variance occurring at high frequencies. Basin-averaged estimates yield IB deviations of only a few percent at time scales longer than 1 week increasing to 5%–20% over the...

Understanding the relation between wind‐ and pressure‐driven sea level variability

Sea surface adjustment to combined wind and pressure forcing is examined using numerical solutions to the shallow water equations. The experiments use coastal geometry and bottom topography representative of the North Atlantic and are forced by realistic barometric pressure and wind stress fields. The response to pressure is essentially static or close to the inverted barometer solution at periods longer than a few days and dominates the sea level variability, with wind-driven sea level signals being relatively small. With regard to the dynamic signals, wind-driven fluctuations dominate at long periods, as expected from quasi-geostrophic theory. Pressure becomes more important than wind stress as a source of dynamic signals only at periods shorter than approximately three days. Wind- and pressure-driven sea level fluctuations are anticorrelated over most regions. Hence regressions of sea level on barometric pressure yield coefficients generally smaller than expected for the inverted barometer response known to be the case in the model. In the regions of significant wind-pressure correlation effects, to infer the correct pressure response using statistical methods, input fields must include winds as well as pressure. Because of the nonlocal character of the wind response, multivariate statistical models with local wind driving as input are not very successful. Inclusion of nonlocal wind variability over extensive regions is necessary to extract the correct pressure response. Implications of these results to the interpretation of sea level observations are discussed.

Impact of self-attraction and loading on the annual cycle in sea level

The annual exchange of water between the continents and oceans is observed by GPS, gravimetry, and altimetry. However, the global average amplitude of this annual cycle (observed amplitude of ∼8 mm) is not representative of the effects that would be observed at individual tide gauges or at ocean bottom pressure recorders because of self-attraction and loading effects (SAL). In this paper, we examine the spatial variation of sea level change caused by the three main components that load the Earth and contribute to the water cycle: hydrology (including snow), the atmosphere, and the dynamic ocean. The SAL effects cause annual amplitudes at tide gauges (modeled here with a global average of ∼9 mm) to vary from less than 2 mm to more than 18 mm. We find a variance reduction (global average of 3 to 4%) after removing the modeled time series from a global set of tide gauges. We conclude that SAL effects are significant in places (e.g., the south central Pacific and coastal regions in Southeast Asia and west central Africa) and should be considered when interpreting these data sets and using them to constrain ocean circulation models.

Regional analysis of the inverted barometer effect over the global ocean using TOPEX/POSEIDON data and model results

Crossover sea level differences from almost 5 years of TOPEX/POSEIDON data are regressed against corresponding differences in atmospheric pressure (−Δpa), and the regression coefficient (α) is examined for deviations from the value of ∼1 cm/mbar expected under a pure inverted barometer (IB) signal. Only crossovers within each 10-day repeat cycle are used. We focus on variability at the shortest periods where non-IB response is more likely. Results indicate a marked tendency to have α < 1 cm/mbar, with values in the general range of 0.8–1 cm/mbar in middle and high latitudes and 0.4–0.8 cm/mbar in the tropics. Effects of errors in Δpa and altimeter data seem small to explain all of the observed IB deviations, which imply then positive correlations between Δpa and dynamic sea level Δη′ (sea level adjusted for an IB signal). A simple constant-density ocean model is used to help interpret the regression results. Only effects of winds and pa on Δη′ are considered. Model-based α estimates agree qualitatively with the data estimates. On the basis of the model results, wind-driven Δη′ signals contribute importantly to the spatial variability of α observed at middle and high latitudes. This is particularly evident in the Southern Ocean, where strongest wind-driven effects on α coincide with regions of anomalous ambient potential vorticity gradients and different vorticity dynamics. In contrast, the observed decrease in α toward low latitudes is due to a true dynamic response to pa, mostly at periods shorter than 10 days. The stronger non-IB signals suggest a response closer to resonance and may be due to the richer resonance spectrum in the tropics. The non-IB signals are largely remotely driven and therefore weakly correlated with local pa. Thus regression results tend to underestimate the importance of the nonisostatic response in the tropics but also at higher latitudes.

Role of ocean currents and bottom pressure variability on seasonal polar motion

Changes in the ocean angular momentum (OAM) components about the equatorial axes, either due to fluctuations in currents or bottom pressure (mass redistribution), can induce movements of the Earths pole of rotation, commonly referred to as polar motion or wobble. Output from a 1° resolution ocean model is used to calculate the effective equatorial OAM functions χ1O and χ2O, corresponding to polar motion excitation about the equatorial axis pointing to the Greenwich and 90°E meridians, respectively. Time series of χO are combined with similar atmospheric series χA, computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyses, to interpret the observed low-frequency polar motion excitation for the period 1985–1996. Results indicate that the oceans are a very important excitation source for the Chandler (∼433 days), annual, and semiannual wobbles, providing for much better amplitude and phase agreement with the observed excitation at these periods, in comparison with what is obtained when only the atmosphere is considered. Both oceanic mass and motion terms are found to be important but with mass signals having somewhat larger amplitudes. The role of regional variability in ocean currents and bottom pressure in contributing to χO signals is quantified. Midlatitude regions (∼30°–70°) figure prominently as places of strong local oceanic excitation signals. The North Pacific basin is found to be generally important for χ1O excitation, while the Southern Ocean is important for both χ1O and χ2O. The largest positive covariances of local with global χ1O signals occur in the Kuroshio region near the western boundary of the North Pacific for χ1O and southwest of Australia for χ2O.