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M. J. McPhaden and A. E. Gill

Abstract

We develop a linear, reduced-gravity model with two active layers above a deep, resting layer to examine the scattering of equatorial Kelvin waves from meridional submarine ridges. Model ridges, idealized as infinitely long in the meridional direction and infinitesimally thin in the zonal direction, completely obstruct flow in the lower active layer. The equatorial long-wave approximation is made, which restricts the class of motions considered to nondispersive Kelvin and Rossby waves in each of two internal modes. Thus, coastal and topographically trapped phenomena are filtered out, but variability far from the ridge is accurately modeled.

The scattering of Kelvin wave energy depends on two parameters, r = H 0/H 1 and γ = (ρ1 − ρ0)/(ρ2 − ρ1), where H 0 and H 1 are the equilibrium thicknesses of the upper and lower active layers, respectively, and ρ0≲ρ1≲ρ2 are the layer densities. Incident first internal mode Kelvin waves are little affected by a deep ridge (i.e., for large r) and strongly reflected by a shallow ridge (i.e., for small r). Second internal mode Kelvin waves behave in an opposite sense, being more strongly reflected by a deep ridge for example. Strong stratification typical of the tropics, corresponding to large &gamma, decouples the near surface from the deep ocean, enhancing the transmissivity of the first mode and the reflectivity of the second mode.

Potentially observable topographic effects in the wake of low-mode Kelvin fronts include enhanced eddying in the far field west of ridges, enhanced vertical shear of zonal flows over and east of ridges, and changes in the depth and intensity of the thermocline across ridges. Weak eddying may also be generated to the east of ridges in the form of boundary trapped currents.

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M. J. McPhaden, J. A. Proehl, and L. M. Rothstein

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.

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M. J. McPhaden, J. A. Proehl, and L. M. Rothstein

Abstract

We investigate the interaction of baroclinic equatorial Kelvin waves with realistically sheared background zonal currents in a linearized, continuously stratified model. The background flows are in thermal wind balance and allowed to vary continuously in the meridional and vertical directions. The governing equations for long-wave perturbations that are harmonic in time and longitude are reduced to a second-order partial differential equation for pressure on the meridional plane, then solved numerically in finite difference form.

Our results indicate that the presence of a background jet can significantly modify the structures and dispersion characteristics of baroclinic Kelvin waves, depending on the speed and the spatial scales of the jet relative to those of the waves. The lowest baroclinic mode, which has a zonal phase speed much larger than typical mean flows, is the least structurally modified and Doppler-shifted. Higher baroclinic modes am more noticeably Doppler-shifted and experience structural changes that can be qualitatively understood in terms of local changes in the phase speed of the wave relative to the mean flow. On the other hand, waves are strongly damped and absorbed by the background flow in the vicinity of a critical surface where the zonal phase speed equals the background flow speed. This result suggests that it may not be possible to set up energetic high vertical modes in the equatorial ocean as predicted by linear theory without mean currents. This, in turn, could account for the fact that recent measurements show a dominance of low vertical-mode energy in the deep ocean on seasonal time scales.

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S. P. Hayes, M. J. McPhaden, and J. M. Wallace

Abstract

Temporal correlations between near-equatorial surface wind and sea-surface temperatures (SST) at 11°W in the eastern Pacific Ocean are investigated using data from an array of moored sensors between 5°N and 5°S. The signature of tropical instability waves with periods of 20–30 days is apparent in time series of SST and both the meridional and zonal wind components. Results indicate the existence of a band of pronounced horizontal divergence in the surface wind field associated with the large meridional SST gradient (equatorial front) normally located just north of the equator. Perturbations of the equatorial front by the instability waves induce fluctuations in the overlying winds. Evidence of the air-sea coupling is stronger in time series of the meridional gradients of wind and SST than between time series of the variables themselves. The meridional differencing serves as a high-pass filter in the space domain, which removes planetary-scale wind fluctuations that are unrelated to the local SST perturbations. The wind fluctuations observed in association with tropical instability waves are on the order of 1–2 m s−1.

These results indicate that SST variability on weekly to monthly time scales forces perturbations in the surface wind field. It is suggested that the principal coupling mechanism in this region is the modification of the atmospheric boundary layer stratification. Over the equatorial cold SST tongue the vertical wind shear within the lowest 100 m of the atmosphere is strong and the surface winds are conspicuously weak. As the air flows northward across the equatorial front the boundary layer becomes destabilized, momentum is mixed downward, and the surface winds increase.

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Lewis M. Rothstein, Michael J. McPhaden, and Jeffrey A. Proehl

Abstract

A numerical model is designed to study the effects of the strong, near-surface associated with the equatorial current system on energy transmission of time-periodic equatorial waves into the deep mean. The present paper is confined to long wavelength, low-frequency Kelvin waves forced by a longitudinally confined patch of zonal wind. Energy transmission into the deep ocean is investigated as a function of mean current shear amplitude and geometry and the forcing frequency.

Solutions form well-defined beams of energy that radiate energy eastward and vertically toward the deep ocean in the absence of mean flow. However, the presence of critical surfaces associated with mean currents inhibits low-frequency energy from reaching the deep ocean. For a given zonal wavenumber, longitudinal propagation through mean currents will be less inhibited as the frequency increases (phase speed increases). When the mean current amplitude is large enough, the beam encounters multiple critical surfaces (i.e., critical surfaces for different wavenumber components of the beam) where significant and momentum can take place with the men currents via Reynolds stress transfers. Work against the dominant vertical shear is the dominant wave energy loss for the case of a mean South Equatorial Current–Equatorial Undercurrent system, illustrating the need for high vertical resolution in equatorial ocean models.

The model also describes the possible induction of a mean zonal acceleration as well as a mean meridional circulation. Eliassen-Palm fluxes are used to diagnose these dynamics. The presence of critical surfaces result in mean field accelerations on the equator above the core of the Equatorial Undercurrent. Implications of these results with regard to observations in the equatorial waveguide are discussed.

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R-C. Lien, E. A. D'Asaro, and M. J. McPhaden

Abstract

In the shear stratified flow below the surface mixed layer in the central equatorial Pacific, energetic near-N (buoyancy frequency) internal waves and turbulence mixing were observed by the combination of a Lagrangian neutrally buoyant float and Eulerian mooring sensors. The turbulence kinetic energy dissipation rate ϵ and the thermal variance diffusion rate χ were inferred from Lagrangian frequency spectral levels of vertical acceleration and thermal change rate, respectively, in the turbulence inertial subrange. Variables exhibiting a nighttime enhancement include the vertical velocity variance (dominated by near-N waves), ϵ, and χ. Observed high levels of turbulence mixing in this low-Ri (Richardson number) layer, the so-called deep-cycle layer, are consistent with previous microstructure measurements. The Lagrangian float encountered a shear instability event. Near-N waves grew exponentially with a 1-h timescale followed by enhanced turbulence kinetic energy and strong dissipation rate. The event supports the scenario that in the deep-cycle layer shear instability may induce growing internal waves that break into turbulence. Superimposed on few large shear-instability events were background westward-propagating near-N waves. The floats' ability to monitor turbulence mixing and internal waves was demonstrated by comparison with previous microstructure measurements and with Eulerian measurements.

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Caihong Wen, Arun Kumar, Yan Xue, and M. J. McPhaden

Abstract

The characteristics of El Niño–Southern Oscillation (ENSO) variability have experienced notable changes since the late 1990s, including a breakdown of the zonal mean upper-ocean heat content as a precursor for ENSO. These changes also initiated a debate on the role of thermocline variations on the development of ENSO events since the beginning of the twenty-first century. In this study, the connection between thermocline variations and El Niño and La Niña events is examined separately for the 1980–98 and 1999–2012 periods. The analysis highlights the important role of thermocline variations in modulating ENSO evolutions in both periods. It is found that thermocline variation averaged in the central tropical Pacific, including both equatorial and off-equatorial regions, is a good precursor for ENSO evolutions before and after 1999, while the traditional basinwide mean of equatorial thermocline variation is a good precursor only before 1999. The new precursor, including both high-frequency variability in equatorial regions and low-frequency variability in off-equatorial regions, is found to be indicative of multiyear persistent warm and cold conditions in the tropical Pacific. Further, it is found that the strength of the subtropical cells (STCs) interior mass transport in both hemispheres increased rapidly around the late 1990s. It is proposed that the strengthened STC interior transports provide a pathway for the enhanced influence of off-equatorial thermocline variations on the development of ENSO events after 1999.

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M. J. McPhaden, S. P. Hayus, L. J. Mangum, and J. M. Toole

Abstract

We describe variability in the western Pacific Ocean during the 1986–87 El Niño/Southern Oscillation (ENSO) event, with emphasis on time series measurements of currents, temperature, sea level and winds near the equator at 165°E. Zonal winds were anomalously westerly from mid-1986 to late 1987 and were punctuated by 2–10 m s−1 episodes of westerlies lasting about 10 days to 2 months. Zonal current in the upper 100-m surface layer responded to these wind variations typically within a week, in some cases with speeds exceeding 100 cm s−1 to the east. Zonal current variations in the thermocline below 100 m were generally less coherent with the local wires than currents near the surface. They were also generally less variable, although the Equation Undercurrent disappeared for 3–4 weeks in October-November 1987 at a time when the normal eastward directed zonal pressure gradient force reversed along the equator. Periods of intense and prolonged eastward flow in the surface layer were associated with a decrease in sea level by 10–20 cm at the end of 1986 and in May-August, 1987. Similarly, significant westward flow near the surface and in the thermocline in September-November 1987 was accompanied by rising sea level and a westward migration from the date line of surface waters >30°C. These results suggest that wind-driven zonal currents at the equator were important in the evolution of the mass and heat balance of the western Pacific during the 1986–87 ENSO, Conversely, meridional wind stress and meridional velocity energy levels at periods longer than 100 days on the equator were 5–10 times weaker than in the zonal direction and less obviously related to the evolution of the 1986–87 ENSO.

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J. N. Moum, D. Hebert, C. A. Paulson, D. R. Caldwell, M. J. McPhaden, and H. Peters

Abstract

Appearing in this issue of the Journal of Physical Oceanography are three papers that present new observations of a distinct, narrow band, and diurnally varying signal in temperature records obtained in the low Richardson number shear flow above the core of the equatorial undercurrent. Moored data suggest that the intrinsic frequency of the signal is near the local buoyancy frequency, while towed data indicate that the horizontal wavelength in the zonal direction is 150–250 m. Coincident microstructure profiling shows that this signal is associated with bursts of turbulent mixing, it seems that this narrowband signal represents the signature of instabilities that ultimately cause the turbulence observed in the equatorial thermocline. Common problems in interpreting the physics behind the signature are discussed here.

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William S. Kessler, M. C. Spillane, Michael J. McPhaden, and D. E. Harrison

Abstract

The highly temporally resolved time series from the Tropical Atmosphere-Ocean moored buoy array are used to evaluate the scales of thermal variability in the upper equatorial Pacific. The TAO array consists of nearly 70 deep-ocean moorings arranged nominally 15° longitude and 2°–3° latitude apart across the equatorial Pacific. The bulk of the data from the array consists of daily averages telemetered in real time, with some records up to 15 years long. However, at several sites more finely resolved data exist, in some cases with resolution of 1 minute. These data form the basis for spectral decomposition spanning virtually all scales of variability from the Brunt-Väiälä frequency to the El Niño-Southern Oscillation timescale. The spectra are used to define the signal to noise ratio as a function of sample rate and frequency, and to investigate the effects of aliasing that results from sparser sampling, such as ship-based observational techniques. The results show that the signal to noise ratio is larger in the east, mostly because the low-frequency signals are larger there. The noise level for SST varies by as much as a factor of 10 among the locations studied, while noise in thermocline depth is relatively more homogeneous over the region. In general, noise due to aliased high-frequency variability increases by roughly a factor of 10 as the sample rate decreases from daily to 100-day sampling. The highly resolved spectra suggest a somewhat more optimistic estimate of overall signal-to-noise ratios for typical ship of opportunity (VOS) XBT sampling (generally about 2) than had been found in previous studies using sparser data. Time scales were estimated for various filtered versions of the time series by integration of the autocorrelation functions. For high-passed data (periods longer than about 150 days removed), the timescale is about 5 days for both surface and subsurface temperatures everywhere in the region. Conversely, for low-passed data (the annual cycle and periods shorter than 150 days removed), the timescale is roughly 100 days. Horizontal space scales were estimated from cross-correlations among the buoys. Zonal scales of low-frequency SST variations along the equator were half the width of the Pacific, larger than those of thermocline depth (about 30°–40° longitude). In the cast, meridional scales of low-frequency SST were large (greater than about 15° latitude), associated with the coherent waxing and waning of the equatorial cold tongue, whereas in the west these scales were shorter. Thermocline depth variations had meridional scales associated with the equatorial waves, particularly in the east. Spatial scale estimates reported here are generally consistent with those found from the VOS datasets when the ENSO signals in the records of each dataset are taken into account. However, if signals with periods of 1 to 2 months are to be properly sampled, then sampling scales of 1°–2° latitude by 8°–10° longitude, with a 5-day timescale, are needed.

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