Mode Rossby Waves Research Articles

Zonal displacement of the western equatorial Pacific “fresh pool”

The distribution of salt in the tropical Pacific Ocean and its variability are potentially important in better understanding the El Niño Southern Oscillation (ENSO) ocean‐atmosphere coupled system. Complementary data sets (including sea surface salinity (SSS) derived from a ship‐of‐opportunity network, Geosat sea level and derived zonal surface currents, and outgoing longwave radiation‐derived precipitation, evaporation, and cruise measurements along the 165°E longitude) are used to describe and understand SSS changes in the western equatorial Pacific over several ENSO cycles. It is first shown that the 1974–1989 zonal displacements of the eastern edge of the western equatorial Pacific “fresh pool” (SSS < 35), marked by a salinity front and closely related to the eastern edge of the warm pool, were dominated by interannual variations that are highly correlated with the Southern Oscillation Index (SOI). Owing to the availability of basin‐wide Geosat surface current data, it is then shown that the eastward extension of the eastern edge of the fresh pool during the 1986–1987 El Niño, as well as the westward retreat during the 1988–1989 La Niña, were chiefly the result of zonal salt advection in the equatorial band. Although the equatorial upwelling extended toward the western Pacific during La Niña, its effectiveness in changing the near‐surface salinity (and temperature) is found questionable. Bearing in mind the uncertainty in determining open‐ocean precipitation, evaporation, and mixed‐layer depth, it appears that the net freshwater flux probably played a minor role in the zonal displacements of the eastern edge of the fresh pool, as compared to zonal salt advection. The roles of first baroclinic Kelvin and first meridional mode Rossby waves are discussed considering that they account in a large part for zonal advection.

Eigenanalysis of the two-dimensional wind-driven ocean circulation problem

A barotropic model of the wind-driven circulation in the subtropical region of the ocean is considered. A no-slip condition is specified at the coasts and slip at the fluid boundaries. Solutions are governed by two parameters: inertial boundary-layer width; and viscous boundary-layer width. Numerical computations indicate the existence of a wedge-shaped region in this two-dimensional parameter space, where three steady solutions coexist. The structure of the steady solution can be of three types: boundary-layer, recirculation and basin-filling-gyre. Compared to the case with slip conditions (Ierley and Sheremet, 1995) in the no-slip case the wedge-shaped region is displaced to higher Reynolds numbers. Linear stability analysis of solutions reveals several classes of perturbations: basin modes of Rossby waves, modes associated with the recirculation gyre, wall-trapped modes and a resonant mode. For a standard subtropical gyre wind forcing, as the Reynolds number increases, the wall-trapped mode is the first one destabilized. The resonant mode associated with disturbances on the southern side of the recirculation gyre is amplified only at larger Reynolds number, nonetheless this mode ultimately provides a stronger coupling between the mean circulation and Rossby basin modes than do the wall-trapped modes.

The effect of large-scale bottom irregularities on the dynamics of Rossby waves in the ocean

This paper discusses, in terms of the geometrical optics approximation, how large-scale bottom irregularities influence the propagation of Rossby waves in the ocean. To describe the major peculiarities of the phenomenon, a two-layer model is applied, with the depth of the upper layer being considerably smaller than that of the lower layer. However, even with the bottom topography being allowed for, the wave motion is described by two Rossby wave modes, namely, a barotropic mode and a baroclinic mode. It is demonstrated that barotropic mode transformation caused by large irregularities of the sea-floor may lead to wave interaction, resulting in their anomalous distribution.

Observation of equatorially trapped waves in the Pacific using Geosat altimeter data

A spatial integration filtering method is applied to process 4.5 years of Geosat altimeter sea level anomalies(SLA) recorded from 1 April 1985 to 17 September 1989. The integral limits are chosen as ±1.5° latitude centered at the equator and ±4° longitude centered at the each central longitude. These limits are chosen in order to eliminate the random noise and to enhance the equatorially trapped wave signals. The spatial-integration-filtered SLA time series show periodical signals with different time scales. Processing the SLA time series with a Short Time Fourier Transform (STFT) yields the time-frequency spectra of the SLA which show a frequency spectral splitting-combining phenomenon in the periods during and after the 1986–1987 El Niñio-Southern Oscillation (ENSO) event. Analysis of non-linear wave-wave interaction indicates that this phenomenon results from amplitude modulation of the short period oscillations in the 60–100 day band by a long period oscillation with a period of about 500 days. The wave components of the short period oscillations are further analysed using the equatorial wave modes to fit the meridional distribution of the SLA. The results indicate that the downwelling Kelvin wave mode before and during the peak phase of 1986–1987 ENSO and the upwelling Kelvin wave mode during the 1988 La Niña are the dominant components. The first and second Rossby wave modes play important roles for entire time series and occasionally become the dominant components. The mean phase speed of Kelvin wave mode during the 1986–1987 ENSO is 3.0 m s −1 which is 25% higher than those in non-ENSO periods.

Evolution of the Southern Hemisphere Subpolar Middle Atmosphere during Summer and Autumn

The evolution of zonal wind and zonal wavenumber one (wave 1) in the Southern Hemisphere subpolar middle atmosphere is described for the period December 1978 - May 1979 using temperature and ozone measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. In late December maximum zonal easterlies of approx. -70 m/s are observed at 0.1 mb, 60 deg S. A zonal flow reversal occurs during late February and westerlies subsequently increase to 60-70 m/s in the upper stratosphere by April - May. LIMS zonal winds are compared with rocketsonde measurements and nadir sounder (derived) winds for summer and autumn. Although quantitative agreement is found at stratospheric levels, substantial discrepancies are evident in the mesosphere, most likely a reflection of sampling and resolution differences in the respective datasets. Stationary and traveling wave 1 temperature disturbances (amplitudes approx. 1 - 2 K at 60 deg S) are observed by LIMS during summer. The stationary wave is confined to the lower stratosphere near the level of zero zonal- mean wind flow, whereas the traveling wave is prominent in the middle stratosphere moves west at a rate similar to the zonal-mean wind, and exhibits a vertical - meridional structure similar to a P(sub 4)(sup 1) normal mode Rossby wave. A substantial intensification of wave 1 activity occurs during autumn (amplitudes approx. 5 - 10 K), which is found to be associated with an upward-directed Eliasse - Palm flux near the subpolar tropopause level. Evidence relating wave 1 activity in the lower - middle stratosphere to the occurrence of zonal ozone perturbations of 10% - 20% amplitude is presented for summer and autumn.

Reflection of interannual Rossby waves at the maritime western boundary of the tropical Pacific

The reflection of interannual Rossby waves over a 2.7‐year period (i.e., November 1986 to August 1989) is examined in the western tropical Pacific utilizing altimetric sea level observations from the Geosat Exact Repeat Mission. In 1987 this reflection process generates upwelling Kelvin waves that tended to limit the growth of the 1986–1987 El Niño in the eastern equatorial Pacific. Two‐dimensional autocorrelation and spectral analyses verify the existence of westward traveling Rossby waves in both the equatorial and off‐equatorial domains, and eastward propagating Kelvin waves in the equatorial domain. Extended empirical orthogonal function (EEOF) analysis characterizes the time‐space evolutionary behavior of the Rossby wave reflection process, extending from the El Niño in 1986–1987 toward the anti‐El Niño in 1988–1989. Both the Philippines Archipelago in the northern hemisphere and the Solomon Archipelago/Bismarck Archipelago/New Guinea complex in the southern hemisphere appear to reflect incident Rossby waves, with maximum amplitude near 8°N and 8°S, generating Kelvin waves in the equatorial wave guide that conduct the anomalous signal eastward along the equator. This apparent reflection process is examined in terms of the linear theory, with sea level phase information in the northern hemisphere at the western boundary (130°W) expanded in terms of the Kelvin and symmetric Rossby meridional wave modes. These modes are formed as weighted sums of parabolic cylinder functions. This procedure, operating under the constraint that the reflection process occurred according to linear theory, determines the percent contribution that each incident Rossby wave mode amplitude has upon the amplitude of the reflected Kelvin wave. This percentage changed little over the evolution of the EEOF pattern. The amplitude of the first‐mode Rossby wave (i.e., mode 2) accounts for most of the Kelvin wave amplitude (i.e., 70–80%), with the higher‐mode Rossby wave amplitudes (i.e., 4, 6, and 8) contributing less (i.e., 20%, −10%, and 10%, respectively), interacting both constructively and destructively with the first.

Effect of Equatorial Long Waves on the North Equatorial Countercurrent

An analytical, reduced-gravity, equatorial long wave model is employed to study the effect of equatorial long waves on the North Equatorial Countercurrent(NECC).This model is first forced by an abruptly switched-on-off zonal wind stress which is distributed uniformly in the zonal direction. As the wind is turned on, an eastward current with a maximum speed at 5°N and a trough of thermocline at 3°N are generated in the western basin. These features coincide with the observed distribution of NECC. The initial intensification of this eastward current is due to the forced first meridional mode Rossby wave. The duration of intensification is related to the wind fetch and Rossby wave speed. After the forced Rossby wave has passed, the eastward current is decelerated or accelerated by a series of reflected Rossby waves generated at the eastern boundary. As the steady state is reached, the easterly wind is then turned off. The forced first meridional mode Rossby wave, now generated by wind relaxation, decelerates the eastward current and even turns the current westward, due to overshooting. Replacing the uniform wind with a linear wind, we find that the general features remain unchanged but the NECC now has larger amplitude and smaller zonal domain. The present linear wind has similar zonal distribution to the trade wind on the tropical Atlantic Ocean. Finally, the linear wind stress distribution, with a representative of time variations observed along the equator, is applied to force the model. The resultant ocean response is compared with the velocity measurement at 6°N, 28°W. General agreement is found. The equatorial long waves, especially the forced Rossby waves, have significant impact upon the NECC.

Equatorial waves in the stratosphere of Uranus

We have identified and characterized an equatorial wave in the stratosphere of Uranus through analysis of radio occultation data from Voyager 2. Our methodology relies on two physical phenomena: (1) atmospheric waves modulate the ambient background structure of a planetary atmosphere, producing a distinctive pattern of spatial variations in temperature and density; and (2) this type of spatial modulation in turn causes fluctuations in the phase and amplitude of radio signals received from an occulted spacecraft. We compared predictions of the linear theory for equatorial waves on a β-plane with observations of the stratosphere at both immersion (2.0°N, 342.2°E) and emersion (6.3°N, 197.5°E). (Note that the latitude system used here is the reverse of the IAU convention.) The observed quasi-periodic vertical variations in atmospheric density agree closely with theoretical predictions for a wave propagating vertically through the observed background structure of the stratosphere, providing compelling evidence for the presence of an atmospheric wave. The wave has an equivalent depth of 53 ± 12 m, a vertical wavelength of about one-third of a pressure scale height, and an amplitude at the 1-mbar pressure level at immersion (emersion) of 1.1 ± 0.4 K (0.5 ± 0.2 K). Within the framework of the linear wave theory, quantitative comparisons between the measurements at immersion and emersion yielded constraints on the meridional and zonal structure of the wave. The ratio of amplitudes at the two locations, 2.1 ± 0.3, is consistent with a wave confined in latitude near the equator. Moreover, the observations at immersion and emersion are correlated, suggesting a wave of planetary scale. These constraints allowed us to identify two equatorial wave modes as equally likely alternatives for explaining the observations; each retains a relatively simple spatial structure while accounting for the principal features of the data. The first is an inertia-gravity wave with westward zonal phase speed (relative to the mean zonal winds), a meridional index j = 1, and a zonal wavenumber n = 6. The second is an eastward inertia-gravity wave with j = 2 and n = 14. All Kelvin, Yanai, and Rossby wave modes are inconsistent with the observations. We expect the wave to begin breaking near the 200-μbar pressure level, resulting in an eddy diffusion coefficient of about 0.6 to 1.0 × 10 4 cm 2 sec −1, consistent with independent estimates of this parameter. Vertical transfer of momentum by the wave could accelerate the mean zonal wind at pressures less than 200 μbar by about ±10–20 cm sec −1 per planet rotation.

Fitting Dynamic Models to the Geosat Sea Level Observations in the Tropical Pacific Ocean. Part I: A Free Wave Model

Free, equatorially trapped sinusoidal wave solutions to a linear model on an equatorial beta plane are used to fit the Geosat altimetric sea level observations in the Tropical Pacific Ocean. The Kalman filter technique is used to estimate the wave amplitude and phase from the data. The estimation is performed at each time step by combining the model forecast with the observation in an optimal fashion utilizing the respective error covariances. The model error covariance is determined such that the performance of the model forecast is optimized. It is found that the dominant observed features can be described qualitatively by basin-scale Kelvin waves and the first meridional mode Rossby waves. Quantitatively, however, only 23% of the signal variance can be accounted for by this simple model.

Rossby Wave Scattering by a Meridional Line Barrier in an Infinitely Long Zonal Channel

The interaction of a prescribed channel mode Rossby wave with a meridional line barrier situated in an infinitely long, zonally aligned channel is considered. An analytical solution for the scattered field is presented for a barrier that spans half the channel width. The solution is obtained using the Wiener-Hopf technique. For other barrier widths the scattered field is determined by numerically solving the Rossby wave equation. In all cases the scattered field is described in terms of propagating cross-channel eigenmodes, together with an infinite number of evanescent modes. The solutions are interpreted in terms of the energy density and energy flux associated with each scattered field eigenmode. Mode one dominates the energy balance in the scattered field for most barrier widths in response to a mode-one incident wave. However, for a particular range of barrier widths the second cross-channel mode is found to be the most energetic. The energy density of each scattered field mode is larger on the eastern side of the barrier than on the west. The implication of the model results for Rossby wave propagation in the Southern Ocean are examined.

The Slow Sea Surface Temperature Mode and the Fast-Wave Limit: Analytic Theory for Tropical Interannual Oscillations and Experiments in a Hybrid Coupled Model

A modified shallow water model with simplified mixed layer dynamics and a sea surface temperature (SST) equation is employed to gain a theoretical understanding of the modes and mechanisms of coupled air-sea interaction in the tropics. Approximations suggested by a scaling analysis are used to obtain analytic results for the eigenmodes of the system. A slow time scale, unstable eigenmode associated with the time derivative of the SST equation is suggested to be important in giving rise to interannual oscillations. This slow SST mode is not necessarily linked to conventional equatorial oceanic wave modes. A useful limit of this mode is explored in which the wave speed of uncoupled oceanic wave modes is fast compared to the time scales that arise from the coupling. This is referred to as the fast-wave limit. The dispersion relationship in this limit is used to present a number of coupled feedback mechanisms, which contribute simultaneously to the instability of the SST mode. It is suggested that interannual oscillations observed in a hybrid coupled general circulation model (HGCM) are related to the slow SST mode. A method of testing applicability of the fast-wave limit in any coupled model through distorted physics experiments is presented. Such experiments with the HGCM are employed to demonstrate that the fast-wave limit is quite a good approximation for interannual oscillations at moderate coupling. It is shown that the time delay associated with oceanic wave propagation across the basin is not essential to the existence of interannual coupled oscillations. Asymptotic expressions are also derived for the eigenvalues of coupled Rossby and Kelvin wave modes in the simple model. The manner in which various coupling mechanisms affect the stability of these modes is discussed and the results are used to explain the behavior of a secondary bifurcation found in the HGCM in terms of coupled Kelvin wave instability. For coupled Rossby and Kelvin modes, various coupling mechanisms oppose one another, suggesting that instability of these modes will be less robust to changes of model parameters and basic state than that of the SST mode, in which all coupling mechanisms tend to give growth.

On the reflection and transmission of low‐frequency energy at the irregular western Pacific Ocean boundary

The western boundary of the tropical Pacific is not continuous, and leakage of low‐frequency energy from the Pacific to the Indian Ocean is possible. At low frequencies, equatorial Kelvin and Rossby waves have very large east‐west scales compared with the east‐west scale of the land masses in the region. Consequently, these land masses may be treated as islands that are infinitesimally thin in the east‐west direction. By generalizing previous theory for a single island, the leakage and multiple reflection of low‐frequency energy through the seven major “islands” forming the boundary of the western Pacific can be studied. The results obtained depend on continuity of mass and large‐scale balances and not on the details of nonlinear and/or frictional flow near island eastern boundaries. The major results are as follows: (1) When a mode 1 low‐frequency Rossby wave is reflected at the discontinuous western Pacific boundary, the eastward reflected Kelvin wave energy flux is about one third of the incoming energy flux, or about two thirds ofthat expected for a solid meridional wall. In other words, the reflected Kelvin wave amplitude is 83% of that which would be reflected from a solid meridional wall. The reflection mainly occurs from the Indonesia/Borneo/Asia land mass and very little energy gets into the Indian Ocean. (2) Sea levels in the western equatorial Pacific and on the western boundaries of the major western Pacific land masses should be in phase and of a similar amplitude. In particular, in‐phase interannual sea levels should occur along Australia's western coast and be highly correlated with sea levels in the western equatorial Pacific. Sea level data at Truk Island and on Australia's western coastline confirm this prediction. (3) The interannual exchange of water between the Pacific and Indian oceans due to interannual oscillations in the Pacific is about 6 Sv (1 Sv = 106 m3 s−1), is highly correlated with El Niño/Southern Oscillation (ENSO) events, and bears definite phase relationships to sea level stations at locations in the western Pacific. For example, for a high sea level at Darwin, the transport is westward from the Pacific Ocean into the Indian Ocean. It seems that interannual transport may be approximately monitored using coastal sea level. (4) Strong narrow low‐frequency currents are predicted to occur westward of some island tips. (5) Interannual Indian Ocean Kelvin wave signals are largely blocked at the Indonesia/Borneo/Asia land mass. (6) In terms of net transport, reflection, and transmission properties, the seven island Pacific western boundary can be reduced to a simpler two‐island problem, in which one island is the Indonesia/Borneo/Asia land mass and the other New Guinea/Australia. (7) The comparatively small Halmahera Sea between Halmahera and New Guinea is dynamically important. Closing this sea for the incident first meridional mode Rossby wave case reduces the interannual transport between the Pacific and Indian Oceans by 40%.

Baroclinic Modons as Prototypes for Atmospheric Blocking

Abstract A two-layer quasi-geostrophic channel model on a β-plane is used to investigate the properties of dipole eddies which may be relevant models for atmospheric blocking. It is shown that quasi-stationary equivalent barotropic dipole eddies, similar to the 1½-layer(reduced gravity) “modons” of Stern, can be resonantly excited in “realistic” westerly zonal wind conditions, including vertical shear, and that these eddies are persistent on lifetimes which are comparable with those of blocking. These eddies cannot persist indefinitely due to interactions with stationary Rossby wave modes. The decay of these eddies into Rossby waves is studied under varying zonal flow conditions and it is found that the decay rate increases as the group velocity of the stationary Rossby waves increase. The decay rate is also found to be sensitive to the closeness of the channel walls which can restrict the development of the radiation field. Meanwhile the loss of fluid (and hence potential vorticity) from the vortices is ...

Transmission and reflection of the Rossby waves in a stratified ocean with bottom topography

Transmission and reflection coefficients are calculated for Rossby waves incident on a bottom topography with constant slope in a continuously stratified ocean. The characteristics of the coefficients are interpreted in terms of the quasigeostrophic waves on the slope. In the parameter range where only the barotropic Rossby waves can propagate in the region outside the slope, the bottom trapped wave plays the same role as the topographic Rossby wave in a homogeneous ocean, and hence the transmission is weak unless phase matching takes place. When both of the barotropic and baroclinic Rossby waves can propagate outside the slope, the total transmission can be strong. The bottom trapped wave affects the transmission and reflection, and it leads to the possibility that the Rossby wave is transmitted as a mode different from the incident mode. When the number of the wavy modes on the slope is smaller than that of the Rossby wave modes outside the slope, strong reflection occurs.

Normal-Mode Rossby Waves Observed in the Wavenumber 1–5 Geopotential Fields of the Stratosphere and Troposphere

Abstract Daily global geopotential height fields have been analyzed for zonally propagating, planetary scale structures with periods in the range of 4 to 30 days. The geopotentials extend from the 850- to 2-mb levels and 85°N to 85°S latitudes, and describe four Northern Hemisphere winters. The analyses indicate that variations in geopotential are dominated by westward moving waves that resemble normal mode Rossby waves. Structures are determined which correspond to the firs symmetric modes of zonal wavenumbers 1–5, the second and antisymmetric modes of wavenumbers 1–3, the second symmetric modes of wavenumbers 1–3, and perhaps the third antisymmetric mode of wavenumber 1. Mode structures are found to agree in general with the predictions of theory and numerical models. Only the first symmetric modes are found to maintain their symmetry throughout the troposphere and lower stratosphere. Other modes are found to resemble their predicted Hough function structures in the troposphere but, to varying degree, n...

Further evidence of normal mode Rossby waves

Further observational evidence of normal mode Rossby waves with higher meridional mode numbers is presented with the aid of global data from the troposphere to the stratosphere over the period November 1979 through April 1986.

An estimate of equatorial wave energy flux at 9‐ to 90‐day periods in the central Pacific

Deep fluctuations in current along the equator in the Central Pacific are dominated by coherent structures which correspond closely to narrow‐band (in wave number) propagating equatorial waves. Currents were measured roughly at 1500 and 3000 m depths at 5 moorings between 144 and 148°W from January 1981 to March 1983 as part of the Pacific Equatorial Ocean Dynamics (PEQUOD) program. In each frequency band resolved, a single complex empirical orthogonal function accounts for half to three quarters of the observed variance in either zonal or meridional current. Dispersion for equatorial first meridional Rossby and Rossby gravity waves is consistent with the observed vertical‐zonal coherence structure. The observations indicate that energy flux is westward and downward in long first meridional mode Rossby waves at periods 45 days and longer and eastward and downward in short first meridional mode Rossby waves and Rossby‐gravity waves at periods 30 days and shorter. A local minimum in energy flux occurs at periods corresponding to a maximum in upper ocean meridional current energy contributed by tropical instability waves. Total vertical flux (energy transport per unit zonal distance) across the 9‐ to 90‐day period range is 2.5 kW/m.

On the Structure of Low-Frequency Equatorial Waves

Abstract We examine the effects of realistically sheared mean currents on low-frequency, low baroclinic mode equatorial Kelvin and Rossby waves. Mean flows can induce significant small-scale zonal velocity fluctuations near the surface, can reduce pressure and velocity variations near the surface relative to those at depth, and can shift zero crossings by O(100 m). Thus the structure of dynamical modes is different than that given by the traditional model decomposition for waves in a resting ocean.

An Eddy-Resolving Numerical Model of the Ventilated Thermocline: Time Dependence

A primitive equation, eddy-resolving numerical model is used to study the inherent time scales of variability in the subtropical ocean, assuming temporally constant surface forcing. Three primary scales arise: mesoscale variability of roughly 50-day period, zonally elongated barotropic bands of 1.1-year period, and basin-scale undulations of approximately 4-year period. The latter are identified as first baroclinic mode Rossby waves, associated with bursts of ventilation at isopycnal outcrops. As a result, the equatorward transport required by Sverdrup theory in the subtropics occurs not as a broad, sluggish drift throughout the interior, but as a succession of more intense flows that slowly propagate westward. The zonally elongated bands agree in characteristics to those predicted from theory of homogeneous turbulence. Eddy energy that is generated by baroclinic instability leaks from baroclinic to barotropic mode and thereafter, due to rotational constraints, seeks low zonal wavenumbers. The group velocity of the zonal bands is such that they tend to concentrate in the western interior of the subtropical gyre. The resulting anisotropy in eddy energy produces stronger zonal than meridional mixing by eddy processes in the model.

Quasigeostrophic energetics of open ocean regions

We present a method for local energy and vorticity analysis (EVA) of open regions of oceanic flow governed by quasigeostrophic dynamics. The purpose is to infer from real and simulated data sets the physics of synoptic/mesoscale processes, and to identify general signatures of such processes. We first derive, via a Rossby number expansion, the form of the local conservation law for quasigeostrophic energy density in terms of the geostrophic pressure field. We relate the quasigeostrophic terms to their more general form and also identify the different local ageostrophic contributions to the pressure work flux divergences. Analysis methods include time series of maps of terms, space-time integral time series, and schematic open region diagrams. Rossby wave and normal mode barotropic and baroclinic instability processes are studied in open regions, and local conversion/transport properties are defined. It is found that the instability process is indicated by both Reynolds-stress-like terms ( ΔF ϰ , ΔF A ) and ageostrophic pressure work divergence ( ΔF π a, δƒ π a ). The process of local growth of energy is indicated by the local growth of asymmetries in the divergence terms. The application of EVA to real data situations which are made self-consistent by quasigeostrophic filtering is introduced. Real data initialization of a quasigeostrophic dynamical model provides the required dynamical interpolation procedure. Finally an eddy merger event captured during a successful dynamical forecast in the California Current region (Robinson et al.) is described and interpreted via EVA.