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Chris W. Hughes

Abstract

A form of linear, barotropic potential vorticity equation is derived for an ocean with a free surface, in which only one scalar variable appears (ocean bottom pressure, or subsurface pressure). Unlike quasigeostrophic or rigid-lid derivations, the only approximation made (apart from linearization) is that changes in the circulation must be slow compared with the inertial frequency. Effects of stratification are included, but only parametrically in the sense that density is treated as a given quantity or forcing term rather than a variable.

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Chris W. Hughes and Beverly A. de Cuevas

Abstract

It is shown that wind stress curl is balanced by bottom pressure torque in a zonal integral over any strip wide enough to smooth out the effect of nonlinear terms (typically about 3° of latitude). The derivation is completely general as long as the zonal wind stress is balanced by form stress at each latitude, as is known to be the case in the ocean. This implies that viscous torques are not important in western boundary currents, their place being taken by bottom pressure torques. The prediction is confirmed in the context of a global, eddy-permitting, numerical ocean model. This link between form stress and bottom pressure torques makes it easier to consider Southern Ocean dynamics and subtropical gyre dynamics in the same conceptual framework, with topographic interactions being important in both cases.

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Richard G. Williams, Chris Wilson, and Chris W. Hughes

Abstract

Signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period, 1992–2002. In the atmosphere, there are localized regions of eddy variability referred to as storm tracks. At the entrance of the storm track the eddies grow, providing a downgradient heat flux and accelerating the mean flow eastward. At the exit and downstream of the storm track, the eddies decay and instead provide a westward acceleration. In the ocean, there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the Antarctic Circumpolar Current. Within these regions of high eddy kinetic energy, there are more localized signals of high Eady growth rate and downgradient eddy heat fluxes. As in the atmosphere, there are localized regions in the Southern Ocean where ocean eddies provide statistically significant vorticity forcing, which acts to accelerate the mean flow eastward, provide torques to shift the jet, or decelerate the mean flow. These regions of significant eddy vorticity forcing are often associated with gaps in the topography, suggesting that the ocean jets are being locally steered by topography. The eddy forcing may also act to assist in the separation of boundary currents, although the diagnostics of this study suggest that this contribution is relatively small when compared with the advection of planetary vorticity by the time-mean flow.

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Chris W. Hughes, Mike P. Meredith, and Karen J. Heywood

Abstract

It is proposed that, for periods between about 10 and 220 days, the variability in Antarctic circumpolar transport is dominated by a barotropic mode that follows f/H contours almost everywhere. Theoretical arguments are given that suggest the possible importance of this mode and show that bottom pressure to the south of the current should be a good monitor of its transport. The relevance of these arguments to eddy-resolving models is confirmed by data from the Fine Resolution Antarctic Model and the Parallel Ocean Climate Model. The models also show that it may be impossible to distinguish the large-scale barotropic variability from local baroclinic processes, given only local measurements, although this is not generally a problem to the south of the Antarctic Circumpolar Current. Comparison of bottom pressures measured in Drake Passage and subsurface pressure on the Antarctic coast, with wind stresses derived from meteorological analyses, gives results consistent with the models, showing that wind stress to the south of Drake Passage can explain most of the observed coherence between wind stresses and circumpolar transport. There is an exception to this in a narrow band of periods near 20 days for which winds farther north seem important. It is suggested that this may be due to a sensitivity of the “almost free” mode to winds at particular locations, where the current crosses f/H contours.

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Laura Jackson, Chris W. Hughes, and Richard G. Williams

Abstract

The topographical control of western boundary currents within a basin and zonal jets in a channel is investigated in terms of the potential vorticity (PV) and barotropic vorticity (BV: the curl of the depth-integrated velocity) budgets using isopycnic, adiabatic wind–driven experiments. Along the western boundary, the wind-driven transport is returned across latitude lines by the bottom pressure torque, while friction is only important in altering the PV within an isopycnic layer and in allowing a closed circulation. These contrasting balances constrain the geometry of the flow through integral relationships for the BV and PV. For both homogenous and stratified basins with sloping sidewalls, the northward subtropical jet separates from the western wall and has opposing frictional torques on either side of the jet, which cancel in a zonal integral for BV but alter the PV within a layer streamline. In a channel with partial topographic barriers, the bottom pressure torque is again important in returning wind-driven flows along western boundaries and in transferring BV from neighboring wind-driven gyres into a zonal jet. The depth-integrated flow steered by topography controls where the bottom friction alters the PV, which can lead to different PV states being attained for separate subbasins along a channel.

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Anthony Wise, Chris W. Hughes, and Jeff A. Polton

Abstract

It is our aim with this paper to investigate how the presence of a continental shelf and slope alters the relationship between interior ocean dynamics and western boundary (coastal) sea level. The assumption of a flat-bottomed basin with vertical sidewall at the coast is shown to hide the role that depth plays in the net force acting on the coast. A linear β-plane theory is then developed describing the transmission of sea level over variable depth bathymetry as analogous to the steady advection–diffusion of a thermal fluid. The parameter , relating the friction parameter r to the bathymetry depth H and width , is found to determine the contribution of interior sea level to coastal sea level, with small giving maximum penetration and large maximum insulation. In the small (infinite friction) limit the frictional boundary layer extends far offshore, and coastal sea level tends toward the vertical sidewall solution. Adding simple stratification produces exactly the same result but with reduced effective depth and hence enhanced penetration. Penetration can be further enhanced by permitting weakly nonlinear variations of thermocline depth. Wider and shallower shelves relative to the overall scales are also shown to maximize penetration for realistic values of . The theory implies that resolution of bathymetry and representation of friction can have a large impact on simulated coastal sea level, calling into question the ability of coarse-resolution models to accurately represent processes determining the dynamic coastal sea level.

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Rory J. Bingham, Keith Haines, and Chris W. Hughes

Abstract

In principle the global mean geostrophic surface circulation of the ocean can be diagnosed by subtracting a geoid from a mean sea surface (MSS). However, because the resulting mean dynamic topography (MDT) is approximately two orders of magnitude smaller than either of the constituent surfaces, and because the geoid is most naturally expressed as a spectral model while the MSS is a gridded product, in practice complications arise. Two algorithms for combining MSS and satellite-derived geoid data to determine the ocean’s mean dynamic topography (MDT) are considered in this paper: a pointwise approach, whereby the gridded geoid height field is subtracted from the gridded MSS; and a spectral approach, whereby the spherical harmonic coefficients of the geoid are subtracted from an equivalent set of coefficients representing the MSS, from which the gridded MDT is then obtained. The essential difference is that with the latter approach the MSS is truncated, a form of filtering, just as with the geoid. This ensures that errors of omission resulting from the truncation of the geoid, which are small in comparison to the geoid but large in comparison to the MDT, are matched, and therefore negated, by similar errors of omission in the MSS. The MDTs produced by both methods require additional filtering. However, the spectral MDT requires less filtering to remove noise, and therefore it retains more oceanographic information than its pointwise equivalent. The spectral method also results in a more realistic MDT at coastlines.

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Chris W. Hughes, Shane Elipot, Miguel Ángel Morales Maqueda, and John W. Loder

Abstract

Measurements of ocean bottom pressure, particularly on the continental slope, make an efficient means of monitoring large-scale integrals of the ocean circulation. However, direct pressure measurements are limited to monitoring relatively short time scales (compared to the deployment period) because of problems with sensor drift. Measurements are used from the northwest Atlantic continental slope, as part of the Rapid Climate Change (RAPID)–West Atlantic Variability Experiment, to demonstrate that the drift problem can be overcome by using near-boundary measurements of density and velocity to reconstruct bottom pressure differences with accuracy better than 1 cm of water (100 Pa). This accuracy permits the measurement of changes in the zonally integrated flow, below and relative to 1100 m, to an accuracy of 1 Sv (1 Sv ≡ 106 m3 s−1) or better. The technique employs the “stepping method,” a generalization of hydrostatic balance for sloping paths that uses geostrophic current measurements to reconstruct the horizontal component of the pressure gradient.

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Anthony Wise, Chris W. Hughes, Jeff A. Polton, and John M. Huthnance

ABSTRACT

Coastal trapped waves (CTWs) carry the ocean’s response to changes in forcing along boundaries and are important mechanisms in the context of coastal sea level and the meridional overturning circulation. Motivated by the western boundary response to high-latitude and open-ocean variability, we use a linear, barotropic model to investigate how the latitude dependence of the Coriolis parameter (β effect), bottom topography, and bottom friction modify the evolution of western boundary CTWs and sea level. For annual and longer period waves, the boundary response is characterized by modified shelf waves and a new class of leaky slope waves that propagate alongshore, typically at an order slower than shelf waves, and radiate short Rossby waves into the interior. Energy is not only transmitted equatorward along the slope, but also eastward into the interior, leading to the dissipation of energy locally and offshore. The β effect and friction result in shelf and slope waves that decay alongshore in the direction of the equator, decreasing the extent to which high-latitude variability affects lower latitudes and increasing the penetration of open-ocean variability onto the shelf—narrower continental shelves and larger friction coefficients increase this penetration. The theory is compared with observations of sea level along the North American east coast and qualitatively reproduces the southward displacement and amplitude attenuation of coastal sea level relative to the open ocean. The implications are that the β effect, topography, and friction are important in determining where along the coast sea level variability hot spots occur.

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David A Williams, David M Schultz, Kevin J Horsburgh, and Chris W Hughes

Abstract

Meteotsunamis are shallow-water waves that, despite often being small (~ 0.3 m), can cause damage, injuries and fatalities due to relatively strong currents (> 1 m s−1). Previous case studies, modelling and localised climatologies have indicated that dangerous meteotsunamis can occur across northwest Europe. Using 71 tide gauges across northwest Europe between 2010–2017, a regional climatology was made to understand the typical sizes, times and atmospheric systems that generate meteotsunamis. A total of 349 meteotsunamis (54.0 meteotsunamis per year) were identified with 0.27–0.40 m median wave heights. The largest waves (~ 1 m high) were measured in France and the Republic of Ireland. Most meteotsunamis were identified in winter (43–59%), and the fewest identified meteotsunamis occurred in either spring or summer (0–15%). There was a weak diurnal signal, with most meteotsunami identifications between 1200–1859 UTC (30%) and fewest between 0000–0659 UTC (23%). Radar-derived precipitation was used to identify and classify the morphologies of mesoscale precipitating weather systems occurring within 6 h of each meteotsunami. Most mesoscale atmospheric systems were quasi-linear systems (46%) or open-cellular convection (33%), with some non-linear clusters (17%) and a few isolated cells (4%). These systems occurred under westerly geostrophic flow, with Proudman resonance possible in 43 out of 45 selected meteotsunamis. Because most meteotsunamis occur on cold winter days, with precipitation, and in large tides, wintertime meteotsunamis may be missed by eyewitnesses, helping to explain why previous observationally-based case studies of meteotsunamis are documented predominantly in summer.

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