Topographic Sverdrup Research Articles

Topography Effects on the Seasonal Variability of Ocean Bottom Pressure in the North Pacific Ocean

Abstract Ocean bottom pressure pB is an important oceanic variable that is dynamically related to the abyssal ocean circulation through geostrophy. In this study we examine the seasonal pB variability in the North Pacific Ocean by analyzing satellite gravimetric observations from the GRACE program and a data-assimilated ocean-state estimate from ECCOv4. The seasonal pB variability is characterized by alternations of low and high anomalies among three regions, the subpolar and subtropical basins as well as the equatorial region. A linear two-layer wind-driven model is used to examine forcing mechanisms and topographic effects on seasonal pB variations. The model control run, which uses a realistic topography, is able to simulate a basinwide seasonal pB variability that is remarkably similar to that from GRACE and ECCOv4. Since the model is driven by wind stress alone, the good model–data agreement indicates that wind stress is the leading forcing for seasonal changes in pB. An additional model simulation was conducted by setting the water depth uniformly at 5000 m. The magnitude of seasonal pB anomaly is amplified significantly in the flat-bottom simulation as compared with that in the control run. The difference can be explained in terms of the topographic Sverdrup balance. In addition, the spatial pattern of the seasonal pB variability is also profoundly affected by topography especially on continental margins, ridges, and trenches. Such differences are due to topographic effects on the propagation pathways of Rossby waves.

The Large-Scale Vorticity Balance of the Antarctic Continental Margin in a Fine-Resolution Global Simulation

ABSTRACTThe depth-integrated vorticity budget of a global, eddy-permitting ocean/sea ice simulation over the Antarctic continental margin (ACM) is diagnosed to understand the physical mechanisms implicated in meridional transport. The leading-order balance is between the torques due to lateral friction, nonlinear effects, and bottom vortex stretching, although details vary regionally. Maps of the time-averaged depth-integrated vorticity budget terms and time series of the spatially averaged, depth-integrated vorticity budget terms reveal that the flow in the Amundsen, Bellingshausen, and Weddell Seas and, to a lesser extent, in the western portion of East Antarctica, is closer to an approximate topographic Sverdrup balance (TSB) compared to other segments of the ACM. Correlation and coherence analyses further support these findings, and also show that inclusion of the vorticity tendency term in the response (the planetary vorticity advection and the bottom vortex stretching term) increases the correlation with the forcing (the vertical net stress curl), and also increases the coherence between forcing and response at high frequencies across the ACM, except for the West Antarctic Peninsula. These findings suggest that the surface stress curl, imparted by the wind and the sea ice, has the potential to contribute to the meridional, approximately cross-slope, transport to a greater extent in the Amundsen, Bellingshausen, Weddell, and part of the East Antarctic continental margin than elsewhere in the ACM.

Barotropic vorticity balance of the North Atlantic subpolar gyre in an eddy-resolving model

Abstract. The circulation in the North Atlantic subpolar gyre is complex and strongly influenced by the topography. The gyre dynamics are traditionally understood as the result of a topographic Sverdrup balance, which corresponds to a first-order balance between the planetary vorticity advection, the bottom pressure torque, and the wind stress curl. However, these dynamics have been studied mostly with non-eddy-resolving models and a crude representation of the bottom topography. Here we revisit the barotropic vorticity balance of the North Atlantic subpolar gyre using a new eddy-resolving simulation (with a grid space of ≈2 km) with topography-following vertical coordinates to better represent the mesoscale turbulence and flow–topography interactions. Our findings highlight that, locally, there is a first-order balance between the bottom pressure torque and the nonlinear terms, albeit with a high degree of cancellation between them. However, balances integrated over different regions of the gyre – shelf, slope, and interior – still highlight the important role played by nonlinearities and bottom drag curls. In particular, the Sverdrup balance cannot describe the dynamics in the interior of the gyre. The main sources of cyclonic vorticity are nonlinear terms due to eddies generated along eastern boundary currents and time-mean nonlinear terms in the northwest corner. Our results suggest that a good representation of the mesoscale activity and a good positioning of mean currents are two important conditions for a better representation of the circulation in the North Atlantic subpolar gyre.

The role of heating, winds, and topography on sea level changes in the North Atlantic

AbstractSeasonal and interannual‐to‐decadal variations of large‐scale altimetric sea surface height (SSH) owing to surface heating and wind forcing in the presence of topography are investigated using simplified models. The dominant forcing mechanisms are time scale dependent. On the seasonal time scale, locally forced thermosteric height explains most of the SSH variance north of 18°N. First‐mode linear long baroclinic Rossby waves forced by changes in the winds and eastern boundary conditions explain most of the variance between 10°N and 15°N and are also important east of Greenland. On interannual‐to‐decadal time scales, local thermosteric height remains important at several locations in the middle and high latitudes. A topographic Sverdrup response explains interannual‐to‐decadal SSH between 53°N and 63°N east of Greenland. Farther south, the linear Rossby wave model explains SSH variations on interannual‐to‐decadal time scales between 30°N and 50°N from mid‐basin to the eastern boundary. Propagation of the eastern boundary condition into the interior dominates the interannual‐to‐decadal SSH signals south of 30°N. The effect from NAO‐related heat flux on SSH is small, but forcing the topographic Sverdrup models with NAO‐regressed winds gives slightly better agreement with the observed SSH in the subpolar gyre on interannual‐to‐decadal time scales than using the full winds.

Response of North Atlantic Ocean Circulation to Atmospheric Weather Regimes

Abstract A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO−), and positive NAO (NAO+) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO+ causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO− induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO+ and to spin down following persistent AR and NAO− conditions. These responses are driven by changes in deep water formation in the Labrador Sea.

Response of the northern North Atlantic and Arctic oceans to a sudden change of the North Atlantic Oscillation

The influence of a switch from a long‐time high North Atlantic Oscillation (NAO+) state to an enduring NAO− situation is investigated with a coupled sea ice–ocean model. We compare the response to a sustained shift to NAO− conditions with that to a short‐lived transition to NAO− and back to NAO+ forcing. Observed changes in sea ice and oceanic variables between high and low NAO states are well captured in the model response. The ocean circulation adjustment includes a fast barotropic anomaly, accompanied by an enhancement of the meridional overturning and the northward heat transport at 48°N. The slow response contains a substantial decrease of the northward heat transport, which is caused by a reduction of the transport of subpolar and subtropical gyres. After a delay of around 2 years, the oceanic heat transport reacts to counteract the meridional temperature gradient imposed by the surface forcing. The propagation time of baroclinic Rossby waves is irrelevant for the delay between the forcing and the oceanic response. Rather, the strength of the subpolar gyres is determined by the rapid spin‐up of a topographic Sverdrup circulation and the subsequent slower changes through the JEBAR (Joint Effect of Baroclinicity And Relief). In an experiment where only the wind stress is changed to NAO− conditions, the adjustment is dampened strongly by the NAO+ thermohaline forcing. The gyres decrease slower than in the NAO− case. Comparison with an experiment where only the wind stress is changed to NAO− conditions indicates that the thermohaline forcing is most important. The heat fluxes counteract the wind stress forcing such that the meridional overturning and the northward heat transport decrease only slightly compared to the control run.

Causes of large‐scale sea level variations in the Southern Ocean: Analyses of sea level and a barotropic model

We analyze a decade of sea surface height (SSH) measurements in the Southern Ocean from the TOPEX/Poseidon and ERS altimeters, with a focus on the variability at timescales <2 years. Among the different processes contributing to large‐scale SSH variations, the barotropic response to the winds dominates poleward of 50°S, while thermosteric processes dominate equatorward, except for resonant basins for the barotropic modes and regions of intense eddy activity. A finite element barotropic model has been developed to analyze the vorticity budget. The SSH from the model agrees well with observations. The leading barotropic mode, which is annular and is confined near Antarctica, is responsible for most of the barotropic circumpolar transport. It is coherent with the zonally integrated eastward wind stress consistent with a free mode response. Although previously evidenced in bottom pressure data, this mode is only partially seen in altimeter data because of ice coverage. It nevertheless distinctly appears above the Pacific ridges where it expands meridionally up to midlatitudes. In the rest of the domain, several regions coherent with the local wind stress curl are found. These are regions isolated by f/H contours, mostly deep basins. An analysis of the vorticity budget shows that, generally, topographic Sverdrup balance is the leading process for periods ≥50 days, but in some regions (resonant basins), diffusive and nonstationary terms are important. A model experiment shows that transients redistribute energy along f/H waveguides, contributing to drain resonant regions, as was hypothesized in previous works.

The Mean Summertime Circulation along Australia's Southern Shelves: A Numerical Study

Abstract For the first time, a high-resolution regional model is developed for the slope and shelf circulation within the Great Australian Bight and for the Gulfs region of South Australia. The results indicate the extent, nature, and dynamical interaction of a variety of circulation features that are most likely to be important for the region. In particular, the positive wind stress curl south of Australia leads to an equatorward Sverdrup transport in the deep ocean, westward Flinders Current along the slope, and upwelling of the (600 m) deep permanent thermocline. The wind stress curl also leads to a seaward topographic Sverdrup transport within the wide sloping shelf of the bight and results in an anticyclonic gyre that is intensified off the Eyre Peninsula and reduced in magnitude by the joint effect of baroclinicity and topographic relief. In the western half of the bight, the seaward surface Ekman and topographic transports are shown to converge with the onshore component of the Flinders Current lea...

A Simple Method for Diagnosing the Bottom Current Field of the World’s Oceans

Abstract A simple diagnostic method presented here uses the already well-known dynamics for large-scale, time-averaged circulation (i.e., depth-integrated vorticity balance or topographic Sverdrup balance) and takes into account the material conservation of bottom density. It yields a unique solution from prescribed climatological datasets of water density, wind stress, and bottom topography, which guarantees the conservation of total transport. Unlike previous diagnostic models, this method is based on local calculations and does not require any spatial integration, thus avoiding the cumbersome encounter with planetary islands (where there are closed contours of f/h, with f being the Coriolis parameter and h the bottom depth). Comparison with previous work in the North Atlantic, including other diagnostic and inverse solutions, shows that this method compares reasonably well with generally accepted large-scale abyssal circulation patterns, and yields results that are comparable to previously suggested so...

Contributions of wind forcing, waves, and surface heating to sea surface height observations in the Pacific Ocean

The dominant processes affecting sea surface height (SSH) variability observed by the TOPEX/Poseidon altimeter vary regionally in the Pacific; baroclinic Rossby waves, equatorially trapped Kelvin waves, steric response to seasonal heating, and the response to wind stress curl forcing are all important. The steric response to surface heating dominates seasonal SSH variability in the subpolar gyre and the eastern subtropical gyre. South of the Kuroshio Extension and south of 20°N in the eastern Pacific, the dominant contribution to SSH is from near‐annual period Rossby waves. To quantify the wave energy, observed SSH was assimilated into a kinematic model of westward propagating waves. These waves account for >70% of SSH variance between 10°S and 10°N but only ∼30% between 10°N and 30°N. Although wave energy in the eastern Pacific is correlated with SSH anomalies at the equator, the much larger wave energy in the western Pacific is correlated with wind stress curl, suggesting that the Rossby waves there are locally forced. In addition to these planetary waves, the ocean response to wind forcing via Ekman pumping is observed in several places, specifically in the North Equatorial Current. A quasi‐steady topographic Sverdrup balance is detectable over most of the North Pacific at latitudes as low as 10–15°N, as well as in the South Pacific, where it is seen north of 50°S. The decomposition of the SSH signal into propagating waves, an Ekman pumping response, and Sverdrup transport is consistent with the results from an isopycnal numerical model.

A note on the barotropic response of sea level to time‐dependent wind forcing

This study examines the extent to which sea level variations at periods between 30 days and 1 year and spatial scales greater than 1000 km can be described by the wind‐driven linear barotropic vorticity dynamics. The TOPEX/POSEIDON altimetric observations of sea level and the wind products of the National Meteorological Center are used as the database for the study. Each term of the linear barotropic vorticity equation was evaluated by averaging over regions of 10°×10°. In most of the open ocean the result of the analysis suggests that the sea level variabilities at the scales considered cannot be fully described by the equation; the apparent net vorticity change is more than what can be explained by the local wind stress curl. In the few regions where the wind stress curl is strong enough to balance the vorticity budget, predominantly in the northeast Pacific and the southeast Pacific, the balance is basically achieved in terms of the time‐dependent topographic Sverdrup relation, namely, the balance between the advection of the planetary vorticity plus the topography‐induced vorticity and the forcing by the wind stress curl.

Barotropic currents and vorticity in the central North Pacific Ocean during summer 1987 determined from long‐range reciprocal acoustic transmissions

Large‐scale depth‐integrated currents and relative vorticity were measured in the central North Pacific Ocean during summer 1987 using long‐range reciprocal acoustic transmissions between transceivers in a triangle approximately 1000 km on a side. Inverse techniques were used to estimate the depth‐averaged (barotropic) current bihourly at 4‐day intervals from differential travel times. Tidal constituent amplitudes and phases found from the acoustically determined currents agree with those found from current meters and with the tidal models of Schwiderski (1980) and Cartwright et al. (1992), providing confirmation that the tomographically derived barotropic currents are correct within the expected uncertainties. The estimated low‐frequency, large‐scale currents are compared with depth‐averaged currents determined by point measurements using current meters and bottom‐mounted electrometers. Meridional and zonal currents are calculated using the topographic Sverdrup balance with the Fleet Numerical Oceanography Center wind field. The measured time derivative of the areally averaged relative vorticity is shown to be insignificant to the Sverdrup balance. Currents and vorticity calculated using the Sverdrup balance are an order of magnitude smaller than the observations. The magnitude and variability of the large‐scale currents and vorticity determined from the Semtner and Chervin (1988) eddy‐resolving model of ocean circulation are similar to the direct measurements.

Barotropic currents, vorticity, and tides in the northcentral Pacific in summer 1987 determined from long‐range acoustic transmissions

Large‐scale depth‐integrated currents, relative vorticity, and tides were measured in the northcentral Pacific Ocean during summer 1987 using long‐range reciprocal acoustic transmissions between transceivers in a triangle approximately 1000 km on a side. Tidal harmonic constants found from the acoustically determined currents agree with those found from current meters and with the tidal models of Schwiderski [E. W. Schwiderski, Mar. Geod. 3, 161–255 (1980)] and Cartwright et al. [D. E. Cartwright et al., NASA Tech. Mem. 104578 (1992)]. Sum travel times were used to calculate the baroclinic tide isotach displacement. A significant part of the derived internal tide is deterministic. Currents are calculated using the topographic Sverdrup balance with the Fleet Numerical Oceanography Center wind field. The measured time derivative of the areal‐averaged relative vorticity is insignificant to the Sverdrup balance. Currents and vorticity calculated using the Sverdrup balance are an order of magnitude smaller than the observations. The magnitude and variability of the large‐scale currents and vorticity determined from the Semtner–Chervin eddy‐resolving model of ocean circulation [A. J. Semtner and R. M. Chervin, J. Geophys. Res. 93, 15 502–15 522 (1988)] are similar to the direct measurements. [Work supported by NSF and ONR.]

The barotropic response of the subpolar North Pacific to stochastic wind forcing

In a recent observational study, Koblinsky et al. (1989) attempted to demonstrate that the topographic Sverdrup balance can account for mesoscale current fluctuations with periods between 10 and 100 days in the abyssal North Pacific. However, the topographic Sverdrup balance proved difficult to verify systematically over the North Pacific. In this paper the conditions required to obtain the topographic Sverdrup balance are investigated through numerical experiments with a stochastically forced, barotropic, quasi‐geostrophic numerical model of the subpolar North Pacific. In the numerical experiments the topographic Sverdrup balance emerges from area averages of the terms in the vorticity equation, but only for periods longer than about 40 days when the averages are taken over 4°×4° region. This period increases when averages are taken over progressively smaller regions. An examination of vorticity balances at individual points along sections through the model domain indicates that the topographic Sverdrup balance is not obtained at individual points, even for very long period motions. The possibility of verifying topographic Sverdrup dynamics with oceanic data is considered. It is argued that topographically generated small scales in the velocity field reduce the likelihood that the topographic Sverdrup balance can be systematically observed in velocity measurements from individual current meter moorings. The results suggest that the vorticity balance is complex and that topographic Sverdrup dynamics cannot be expected to provide an adequate explanation for the seasonal modulation of the abyssal eddy kinetic energy that is observed with moored instrumentation in the North Pacific.

Observations of wind‐forced deep ocean currents in the North Pacific

This study examines the atmospheric response of the deep ocean using 200 instrument years of moored current measurements from across the breadth of the mid‐latitude North Pacific. We have found evidence of a seasonal modulation in the ocean eddy kinetic energy beneath the thermocline at several locations north of 35°N. This modulation is often in phase with the local atmospheric forcing function. At frequencies between 0.1 and 0.01 cpd, correlations between the local wind stress curl and the deep currents have been found. For a few locations the local forced response is parallel to the potential vorticity gradient (∇f/H) and consistent in amplitude with a local topographic Sverdrup balance. Nonlocal forcing in the form of flow along isolines of potential vorticity has been estimated, and it is comparable to the observed flow at some locations, but the modeled flow is not correlated with the observations. Throughout the mid‐latitude North Pacific the bottom slope tends to enhance the β effect. This suggests that topography narrows the available bandwidth for forced barotropic Rossby waves, facilitating a quasi‐steady topographic Sverdrup response over most of the basin. However, the enhanced β effect reduces the magnitude of the ocean's response to wind forcing. Nonlocally forced Sverdrup currents (flow along isolines of f/H) are the dominant component, but are only of the order of 1 cm s−1. Locally forced Sverdrup currents (flow across isolines of f/H) are typically of the order of 0.1 cm s−1 rms and are not commonly measured. Hence these wind‐forced variations are a background level signal in the mid‐latitude eddy kinetic energy. In a few isolated areas the bottom slope reduces the β effect. In these regions the local Sverdrup response is amplified and has been observed.

Stochastically forced current fluctuations in vertical shear and over topography

The effects of constant vertical shear of zonal velocity, uniform meridional bottom slope, and a deep meridional wall (representing the Mid‐Atlantic Ridge) on the response of a linear two‐layer quasi‐geostrophic ocean to stochastic wind stress curl forcing are investigated. The model is motivated by discrepancies in energy levels and distributions between recent observations and a similar model without shear or topography that predicted remote coherences relatively well. The results indicate that the presence of mean shear can significantly enhance the wind‐generation of surface‐intensified oceanic kinetic energy at frequencies above the zero‐shear cutoff for free baroclinic waves. The primary effect of the sloping topography is enhanced barotropic response near the barotropic topographic Sverdrup resonance, where topographic and planetary potential vorticity gradients cancel. The wall inhibits the barotropic response and increases coherences on the mooring side of the wall. Remote coherences for 49‐day period fluctuations are due to long barotropic waves as in the previous model, so the present model only partially reconciles the previous discrepancies.

Wind‐forced changes in Labrador Current transport

Three years of current data from the offshore branch of the Labrador Current are examined. The meters were at least 400 m deep (to avoid being struck by icebergs) and positioned over the 1000‐m isobath on the eastern flank of Hamilton Bank. The currents are predominantly directed along isobath. At periods of 1–8 days the motion is bottom‐intensified; at longer periods (10–40 days) it is essentially independent of depth. One of the objectives of this paper is to determine whether or not the 10‐ to 40‐day fluctuations, which are O(5 cm s−1), can be explained by large‐scale wind forcing over the North Atlantic. Monthly variations in the transport of the North Atlantic are calculated by means of the topographic Sverdrup relationship. The Sverdrup return flow along the coast of Labrador, −〈ψw〉, is found to have a well‐defined seasonal cycle with a range of 6 Sv. The maximum southward transport generally occurs in February, which is also the month when the standard deviation of (ψw) is a maximum (9 Sv). If we assume that the monthly changes of the return flow are barotropic and confined to a boundary current of (say) 100 km width and 1 km depth (the mooring depth), then 1 Sv corresponds to a speed of 1 cm s−1. We would then expect the large‐scale wind over the North Atlantic to cause the flow along the Labrador coast to vary with a seasonal range of 6 cm s−1 and exhibit monthly variations in winter of ∼9 cm s−1. However, a comparison of the (ψw) and current records shows no similarity. We therefore speculate, with some corroborative evidence from an analysis of Nain sea level, that these energetic variations in the (topographic) Sverdrup return flow along the Labrador Coast probably occur in deeper water, offshore of the current meter mooring. The cause of the barotropic current variations remains unclear. It is possible that they do not reflect genuine transport changes but are due to a meandering of the Labrador Current. Further analysis, and perhaps measurements of bottom pressure, are required to explore this possibility.