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William S. Kessler

Abstract

A 14-yr time series of salinity at thermocline level was constructed from repeated meridional CTD sections (averaging about 110 days apart) spanning the equator along 165°E during 1984–97. A tongue of high salinity water extends along the isopycnal σ t = 24.5 from its surface outcrop in the southeast Pacific to 175-m depth near 5°–10°S along the section at 165°E. In the west, the tongue moves vertically with the thermocline, mostly as part of the ENSO cycle, while salinity in the tongue varied interannually over a range of 0.4 psu. Most of this variability was due to zonal advection along the isopycnal tongue, with similar changes observed at other longitudes in the west and central Pacific. Part of the interannual salinity signal can be attributed to changes in the near-zonal flow of the South Equatorial Current associated with El Niño, but a general 0.3 psu rise occurring during the 1990s (probably starting even earlier) was not apparently consistent with this explanation. An attempt was made to trace the source of these changes to surface fluxes at the outcrop, where the variation of evaporation and precipitation suggested salinity anomalies of the same magnitude as the subsurface changes. However, the implied surface salinity changes were of the wrong sign to explain the subsequent downstream subsurface variability, and therefore present observations do not demonstrate any influence of subduction of surface properties on salinity in the southern high salinity tongue.

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William S. Kessler

Abstract

Although recent El Niño events have seen the occurrence of strong intraseasonal winds apparently associated with the Madden–Julian oscillation (MJO), the usual indices of interannual variability of the MJO are uncorrelated with measures of the ENSO cycle. An EOF decomposition of intraseasonal outgoing longwave radiation and zonal wind identifies two modes of interannual variability of the MJO: a zonally stationary variation of amplitude that is unrelated to ENSO and a roughly 20°-longitude eastward extension of the MJO envelope during El Niño events. The stationary mode is represented by the first two EOFs, which form the familiar lag-correlated quadrature pair, and the eastward-extending mode is represented by the third EOF, which is usually ignored although it is statistically significant. However, the third EOF also has a systematic phase relation with the first pair, and all three should be considered as a triplet; rotating the EOFs makes the phase relation clear. The zonal shift represents about 20% of total MJO variance (which itself is about 55% of intraseasonal variance over the tropical strip). Although the eastward shift is small when compared with the global scale of the MJO, it produces a large proportional shift of MJO activity over the open Pacific, where physical interactions with ENSO processes can occur.

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William S. Kessler

Abstract

The possibility that the evolution of the ENSO phenomenon is determined by the reflection of extra-equatorial Rossby waves from the western boundary into the equatorial waveguide has been a subject of recent debate. Observations and some wind-driven models suggest an apparent continuity of off-equatorial signals and subsequent waveguide anomalies. On the other hand, coupled model results show that ENSO-like behavior can be simulated with no involvement of the extra-equatorial regions. Linear equatorial wave theory shows that significant reflection can only occur within about 8° of the equator, with a sharp fall-off in the reflectivity poleward of this latitude. Although the amplitude of the thermocline anomalies associated with observed ENSO-forced extra-equatorial Rossby waves can be large, it is the net zonal transport of these waves that is crucial to the reflectivity, and this net transport decreases rapidly as Rossby waves occur farther from the equator. The zonal geostrophic flows associated with observed extra-equatorial Rossby waves in the northern tropical Pacific do not provide a net transport that could make a significant contribution to equatorial Kelvin waves. If the extra-equatorial signals do exert an influence on the equatorial waveguide, it must be through a mechanism other than simple boundary reflection.

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William S. Kessler

Abstract

Historical XBT data are used to construct a mean climatology of the three-dimensional geostrophic circulation in the northeast tropical Pacific (southwest of Mexico and Central America) and are diagnosed based on linear dynamics forced with satellite scatterometer winds. Unlike the familiar central tropical Pacific, where the zonal scales are very large and the wind forcing nearly a function of latitude alone, the North Pacific east of about 120°W is strongly influenced by wind jets blowing through gaps in the Central American cordillera. The curl imposed by these wind jets imprints on the ocean, producing a distinctive pattern of thermocline topography and geostrophic currents that are consistent with the Sverdrup balance. Notably, the weakening of the North Equatorial Countercurrent near 110°W is due to the wind forcing. Given the observed stratification and wind stress curl, planetary vorticity conservation also determines the distribution of vertical velocity in the region, with about 3.5 Sv (Sv ≡ 106 m3 s−1) of upwelling through the base of the thermocline under the Costa Rica Dome. This upwelling is associated with stretching of the water column under the dome, which thereby causes the northern “Subsurface Counter Current” (SSCC or Tsuchiya Jet) to turn away from the equator; about half the transport of the SSCC upwells through the thermocline via this mechanism. This may be part of the process by which intermediate-depth water, flowing into the Pacific from the south, is brought to the surface and into the Northern Hemisphere.

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William S. Kessler and Sophie Cravatte

Abstract

Historical section data extending to 1985 are used to estimate the interannual variability of transport entering the Coral Sea between New Caledonia and the Solomon Islands. Typical magnitudes of this variability are ±5–8 Sv (Sv ≡ 106 m3 s−1) in the 0–400-m layer relative to 400 m, and ±8–12 Sv in the 0–2000-m layer relative to 2000 m, on a mean of close to −30 Sv (relative to 2000 m). Transport increases a few months after an El Niño event and decreases following a La Niña. Interannual transport variability is well simulated by a reduced-gravity long Rossby wave model. Vigorous westward-propagating mesoscale eddies can yield substantial aliasing on individual ship or glider surveys. Since transport variability is surface intensified and well correlated with satellite-derived surface geostrophic currents, a simple index of South Equatorial Current transport based on satellite altimetry is developed.

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William S. Kessler and Lionel Gourdeau

Abstract

An ocean GCM, interpreted in light of linear models and sparse observations, is used to diagnose the dynamics of the annual cycle of circulation in the western boundary current system of the southwest Pacific Ocean. The simple structure of annual wind stress curl over the South Pacific produces a large region of uniformly phased, stationary thermocline depth anomalies such that the western subtropical gyre spins up and down during the year, directing flow anomalies alternately toward and away from the boundary at its northern end, near 10°S. The response of the western boundary currents is to redistribute these anomalies northward toward the equator and southward to the subtropical gyre, a redistribution that is determined principally by linear Rossby processes, not boundary dynamics. When the subtropical gyre and South Equatorial Current (SEC) are strong (in the second half of the year), the result is both increased equatorward transport of the New Guinea Coastal Current and poleward transport anomalies along the entire Australian coast. Because of this opposite phasing of boundary current anomalies across 10°S, annual migration of the bifurcation point of the total SEC, near 18°S in the mean, has no significance regarding variability of transport from subtropics to equator.

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William S. Kessler and Richard Kleeman

Abstract

An ocean general circulation model, forced with idealized, purely oscillating wind stresses over the western equatorial Pacific similar to those observed during the Madden–Julian oscillation (MJO), developed rectified low-frequency anomalies in SST and zonal currents, compared to a run in which the forcing was climatological. The rectification in SST resulted from increased evaporation under stronger than normal winds of either sign, from correlated intraseasonal oscillations in both vertical temperature gradient and upwelling speed forced by the winds, and from zonal advection due to nonlinearly generated equatorial currents. The net rectified signature produced by the MJO-like wind stresses was SST cooling (about 0.4°C) in the west Pacific, and warming (about 0.1°C) in the central Pacific, tending to flatten the background zonal SST gradient. It is hypothesized that, in a coupled system, such a pattern of SST anomalies would spawn additional westerly wind anomalies as a result of SST-induced changes in the low-level zonal pressure gradient. This was tested in an intermediate coupled model initialized to 1 January 1997, preceding the 1997–98 El Niño. On its own, the model hindcast a relatively weak warm event, but when the effect of the rectified SST pattern was imposed, a coupled response produced the hypothesized additional westerlies and the hindcast El Niño became about 50% stronger (measured by east Pacific SST anomalies), suggesting that the MJO can interact constructively with the ENSO cycle. This implies that developing the capacity to predict, if not individual MJO events, then the conditions that affect their amplitude, may enhance predictability of the strength of oncoming El Niños.

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William S. Kessler and Julian P. McCreary

Abstract

The annual cycle of temperature in the subthermocline equatorial Pacific is studied using a new compilation of historical hydrographic profiles. The observations have several characteristics suggestive of a vertically propagating, first meridional mode (l=1) long-wavelength Rossby wave: phase lines that slope downward from east to west indicative of upward and westward phase propagation amplitude maxima parallel to phase lines, and nearly symmetric off-equatorial maxima of annual amplitude. Estimates of zonal wavenumber, vertical wavenumber, and the location of maxima of isotherm displacements are consistent with those of the l = 1 Rossby wave.

A solution to a linear continuously stratified model, driven by a version of the observed annual wind field, confirms this interpretation. The solution is dominated by a vertically propagating, l = 1 Rossby wave. The wave is generated primarily by the westward-propagating component of the equatorial zonal wind field; it carries energy along WKB ray paths into the deep ocean. Both amplitude and phase of the model density field agree well with the observations. There are, however, two prominent differences between the observations and the solution: first, in the solution a boundary-reflected l = 3 Rossby wave is present in the deep eastern Pacific but is apparently absent in the data; second, the model solution is nearly symmetric about the equator, while the observations are symmetric in phase but have larger amplitude in the Northern Hemisphere.

Thus, efficient vertical propagation of Rossby wave energy through the thermocline into the deep ocean appears to be an important oceanic process. The lack of this process in single active-layer models may explain the unrealistically high amplitudes of off-equatorial variability that are produced in them, since such models necessarily trap all energy in the surface layer.

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Renellys C. Perez and William S. Kessler

Abstract

The shallow tropical cells (TCs) in the central equatorial Pacific Ocean are characterized by strong equatorial upwelling, near-surface wind-driven poleward flow, downwelling near the cold tongue boundaries, and equatorward flow below the surface mixed layer. Meridional and vertical velocity fluctuations associated with tropical instability waves (TIWs) in the central equatorial Pacific are much larger than those associated with the TCs and can modify the background circulation. OGCM experiments are used to simulate the spinup of the cells along 140°W in response to perturbed trade winds during various phases of the annual cycle. Equatorially modified versions of geostrophy and Ekman theory, and zonal filtering, are used to isolate the large-zonal-scale wind-driven response. Weakening of the trade winds in any season rapidly weakens the TCs, decreases the zonal current shear, and reduces the amplitude and propagation speed of the TIWs. In boreal fall and winter, when the background TCs and TIWs are seasonally strong, the ocean response is equatorially asymmetric (stronger flows north of the equator) and there is evidence of rectification by the modified TIWs onto the TCs. The linear equatorially modified Ekman solutions largely explain the meridional structure and temporal evolution of the anomalous ageostrophic response in the TCs. In fall and winter, however, deviations from the modified Ekman solutions were attributed to interactions with the background TCs and TIWs. An observing system able to quantify the relative contributions of these two processes to the seasonally varying equatorial asymmetry of background circulation would require fine meridional and temporal sampling.

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Meghan F. Cronin and William S. Kessler

Abstract

Near-surface shear in the Pacific cold tongue front at 2°N, 140°W was measured using a set of five moored current meters between 5 and 25 m for nine months during 2004–05. Mean near-surface currents were strongly westward and only weakly northward (∼3 cm s−1). Mean near-surface shear was primarily westward and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was subtracted from the observed shear, the residual ageostrophic currents relative to 25 m were northward and had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a vertically uniform front. According to this “frontal Ekman” model, the ageostrophic Ekman spiral is forced by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A diurnal jet was observed that was on average 12 cm s−1 stronger at 5 m than at 25 m, even though daytime stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night than daytime and decreases with depth. A “generalized Ekman” model is also developed that assumes that viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the features of the observed mean shear and is valid both in frontal regions and at the equator.

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