Evidence for bottom-trapped topographic Rossby waves from single moorings* 1 (original) (raw)
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Ocean Science Discussions, 2011
Tests of the new Rossby wave theories that have been developed over the past decade to account for discrepancies between theoretical wave speeds and those observed by satellite altimeters have focused primarily on the surface signature of such waves. It appears, however, that the surface signature of the waves acts only as a rather weak constraint, and that information on the vertical structure of the waves is required to better discriminate between competing theories. Due to the lack of 3-D observations, this paper uses high-resolution model data to construct realistic vertical structures of Rossby waves and compares these to structures predicted by theory. The meridional velocity of a section at 24° S in the Atlantic Ocean is pre-processed using the Radon transform to select the dominant westward signal. Normalized profiles are then constructed using three complementary methods based respectively on: (1) averaging vertical profiles of velocity, (2) diagnosing the amplitude of the Radon transform of the westward propagating signal at different depths, and (3) EOF analysis. These profiles are compared to profiles calculated using four different Rossby wave theories: standard linear theory (SLT), SLT plus mean flow, SLT plus topographic effects, and theory including mean flow and topographic effects. The model data supports the classical theoretical assumption that westward propagating signals have a well-defined vertical modal structure associated with a phase speed independent of depth, in contrast with the conclusions of a recent study using the same model. The model structures were surface intensified, with a sign reversal at depth in some regions, notably occurring at shallower depths in the East Atlantic. SLT provides a good fit to the model structures in the top 300 m, but grossly overestimates the sign reversal at depth. The addition of mean flow slightly improves the latter issue, but is too surface intensified. SLT plus topography rectifies the overestimation of the sign reversal, but overestimates the amplitude of the structure for much of the layer above the sign reversal. Combining the effects of mean flow and topography provided the best fit for the mean model profiles, although small errors at the surface and mid-depths are carried over from the individual effects of mean flow and topography, respectively. Across the section the best fitting theory varies between SLT plus topography and topography with mean flow, with, in general, SLT plus topography performing better in the east where the sign reversal is less pronounced. None of the theories could accurately reproduce the deeper sign reversals in the west. All theories performed badly at the boundaries. The generalization of this method to other latitudes, oceans, models and baroclinic modes would provide greater insight into the variability in the ocean, while better observational data would allow verification of the model findings.
Rossby Waves with Continuous Stratification and Bottom Friction
Journal of Physical Oceanography, 2018
Published observations of subinertial ocean current variability show that the vertical structure is often well described by a vertical mode that has a node of horizontal velocity at the bottom rather than the traditional node of vertical velocity. The theory of forced and free linear Rossby waves in a continuously stratified ocean with a sloping bottom and bottom friction is treated here to see if frictional effects can plausibly contribute to this phenomenon. For parameter values representative of the mesoscale, bottom dissipation by itself appears to be too weak to be an explanation, although caution is required because the present approach uses a linear model to address a nonlinear phenomenon. One novel outcome is the emergence of a short-wave, bottom-trapped, strongly damped mode that is present even with a flat bottom.
Topographic Rossby waves in a rough-bottomed ocean
Journal of Fluid Mechanics, 1973
The object is to predict the nature of small-amplitude long-period oscillations of a homogeneous rotating fluid over a ‘sea bed’ that is nowhere level. Analytically, we are limited to special choices of bottom topography, such as sinusoidal corrugations or an undulating continental slope, so long as the topographic restoring effect equals or exceeds that due to planetary curvature (the beta-effect). (Very slight topographic features, on the other hand, provide weak, resonant interactions between Rossby waves.)Integral properties of the equations, and computer experiments reported elsewhere, verify the following results found in the analytical models: typical frequencies of oscillation are [lsim ]fδ, where f is the Coriolis frequency and δ measures the fractional height of the bottom bumps; an initially imposed flow pattern of large scale will rapidly shrink in scale over severe roughness (even the simplest analytical model shows this rapid change in spatial structure with time); and...
The Transmission and Transformation of Baroclinic Rossby Waves by Topography*
Journal of Physical Oceanography, 2000
The transmission of westward propagating baroclinic Rossby waves incident on a gappy meridional barrier is studied in the context of the two-layer, quasigeostrophic model. The meridional barrier models the presence of very steep topography such as the midocean ridge system or extensive island arcs. The nature of the transmission depends strongly on the nature of the gaps in the meridional barrier. If the gaps extend throughout the depth of the fluid, the Rossby waves propagate through the barrier, as a consequence of Kelvin's theorem, with no change in vertical structure. On the other hand, if the gaps in the barrier are partial and extend only over a single layer, there is a significant transformation of the vertical structure of the wave field as it traverses the barrier. In particular, waves of baroclinic vertical structure in the model are transformed on the western side of the barrier into barotropic waves that radiate from the segment of the barrier between two such gaps. Such segments act as antennae radiating barotropic energy into the western subbasin. It is suggested that recent observations of signal enhancement of Rossby waves at the midocean ridge system in the Pacific may be related to such transformation of wave structure. The problems of free waves and forced waves in open regions and normal modes in closed basins are described.
A new approximation for the dynamics of topographic Rossby waves
A B S T R A C T A new theory of non-harmonic topographic Rossby waves over a slowly varying bottom depth of arbitrary, 1-D, profile is developed based on the linearised shallow water equations on the f-plane. The theory yields explicit approximate expressions for the phase speed and non-harmonic cross-slope structure of waves. Analytical expressions are derived in both Cartesian and Polar coordinates by letting the frequency vary in the cross-shelf direction and are verified by comparing them with the numerical results obtained by running an ocean general circulation model (the MITgcm). The proposed approximation may be suitable for studying open ocean and coastal shelf wave dynamics.
Synergistic observations of Rossby waves
2000
Rossby waves are an important part of the ocean's response to changes in atmospheric stimuli, and provide a mechanism by which the west of ocean basins learns of changes in the east. In the past decade many papers have discussed their sea surface height signal in the altimetric record, showing that Rossby waves are common in all ocean basins and drawing attention to the discrepancy between observations and theory. Here we examine the signature of Rossby waves from a number of different sensors and discuss the extra information gained from such synergistic observations.
Analysis of remotely forced oceanic Rossby waves off California
Journal of Geophysical Research, 1991
Complex empirical orthQIOnal function (CEOP) analysis is used to investigate the coastal Kelvin wave driven Rossby wave response in the northeast Pacific. Using CEOF analysis, a spatial stmcture function is obtained from model upper layer thickness data. The model is a nonlinear, reduced gravity model of the northeast Pacific forced by coastal Kelvin waves originatinl in the equatorial Pacific. The spatial stmcture function is used to extract the interannual Rossby wave response from observed 300-m-depth temperature anomalies. The observed Rossby wave signal is termed the projection mode. Rossby wavelike feanues observed in the projection mode are onIer 1(MX) km long with most periods ranging between 2 and 4 years. The wave numbeR and frequencies found are consistent with Rossby d~cs. The mean phase speed of the Rossby wavelike features within the projection mode is 1.3 cm s-1, in agreement with the theoretical Rossby wave phase speed at 400N. Large amplitude nearshore and decreasing amplitude away from shore suggests nearshore generation of these waves. An important source of sea level variability along the east coast of North America at periods of 2-4 years was identified by Pares-Sierra and O'Brien (1989) as poleward propaga~n~ Kelvin waves. Since the Rossby wavelike features observed in the projection mode have a ~ty of their periods raDging between 2 and 4 yean, their forcing can be attributed to long-period Kelvin waves. Spectral comparisons between the nearshore values in the projection mode and coastal sea level show greater than 90% coherence in the period band 3-4.4 years. The high coherence between coastal sea level variations and the projection mode shows that there is a strong correlation between the Rossby wavelike features within the projection mode and coastal Kelvin wave propaption. It is concluded that the Rossby waves within the projection mode are forced by coastal Kelvin wave propagation. The projection mode accounts for 47.5% of the variance in the 300-m-depth temperature anomalies. The implication of this result is that the physical mechanism of the numerical model, i.e., Rossby waves excited by coastal Kelvin wave propagation, accounts for 47.5% of the variance in the observed 300-m depth temperature anomalies.
Properties of Rossby Waves in the North Atlantic Estimated from Satellite Data
Journal of Physical Oceanography, 2004
This study uses satellite observations of sea surface height (SSH) to detect westward-propagating anomalies, presumably baroclinic Rossby waves, in the North Atlantic and to estimate their period, wavelength, amplitude, and phase speed. Detection involved a nonlinear fit of the theoretical dispersion relation for Rossby waves to the time-longitude spectrum at a given latitude. Estimates of period, wavelength, and phase speed resulted directly from the detection process. Based on these, a filter was designed and applied to extract the Rossby wave signal from the data. This allowed a mapping of the spatial variability of the Rossby wave amplitude for the North Atlantic. Results showed the familiar larger speed of observed Rossby waves relative to that expected from theory, with the largest differences occurring at shorter periods. The data also show that the dominant Rossby waves, those with periods that are less than annual, propagated with almost uniform speed in the western part of the North Atlantic between 30Њ and 40ЊN. In agreement with previous studies, the amplitude of the Rossby wave field was higher in the western part of the North Atlantic than in the eastern part. This is often attributed to the influence of the Mid-Atlantic Ridge. By contrast, this study, through an analysis of the wave spatial structure, suggests that the source of the baroclinic Rossby waves at midlatitudes in the western North Atlantic is located southeast of the Grand Banks where the Gulf Stream and the deep western boundary current interact with the Newfoundland Ridge. The spatial structure of the waves in the eastern North Atlantic is consistent with the formation of these waves along the basin's eastern boundary.
On long baroclinic Rossby waves in the tropical North Atlantic observed from profiling floats
Journal of Geophysical Research, 2007
1] Argo float data (subsurface tracks and temperature profiles collected from March 2004 through May 2005) are used to detect signatures of long Rossby waves in the velocity of the currents at 1000-m depth and temperature, between the ocean surface and 950 m, in the zonal band of 4°N-24°N in the tropical North Atlantic. Different types of long Rossby waves (with the characteristic scales between 1000 and 2500 km) are identified in the western [west of the Mid-Atlantic Ridge (MAR)] and eastern [east of the MAR] subbasins.