William S. Kessler

Forcing of intraseasonal Kelvin waves in the equatorial Pacific

Ten-year time series of sea surface temperature (SST), 20°C depth, and zonal winds measured by moored buoys across the equatorial Pacific are used to define the intraseasonal (30- to 90-day period) Kelvin waves. The Kelvin waves are observed to be forced west of the date line and propagate at a speed of 2.4 m s−1, with high zonal coherence over at least 10,000 km. They form a major component of thermocline depth variability in the east-central Pacific. The intraseasonal-band variance has a low-frequency modulation both at the annual and interannual frequencies; higher amplitudes are observed in boreal fall/winter and during the onset phase of El Nino warm events. The oceanic intraseasonal variability and its low-frequency modulation are coherent with atmospheric intraseasonal variations (the Madden-Julian Oscillation (MJO)), which are known to propagate eastward into the Pacific from the Indian Ocean as part of a planetary-scale signal. The life cycle of an individual or series of MJOs is determined by a combination of factors including tropical SSTs over the warm pool regions of the Indian and Pacific Oceans and interaction with the planetary-scale atmospheric circulation. Thus the intraseasonal Kelvin waves should be taken as an aspect of a global phenomenon, not simply internal to the Pacific. The oceanic intraseasonal variability peaks at periods near 60–75 days, while the corresponding atmospheric variations have somewhat higher frequencies (35- to 60-day periods). We show that this period offset is potentially related to the zonal fetch of the wind compared to the frequency-dependent zonal wavelength of the Kelvin wave response. A simple model is formulated that suggests an ocean-atmosphere coupling by which zonal advection of SST feeds back to the atmosphere; the model duplicates the steplike advance of warm water and westerly winds across the Pacific at the onset of the El Nino of 1991–1992. The key dynamics of the model is that the atmosphere responds rapidly to the state of the ocean, but the oceans response to the atmosphere is lagged because it is an integral over the entire wind forcing history felt by the wave. This results in a nonlinear ordinary differential equation that allows a net nonzero lowfrequency ocean signal to develop from zero-mean sinusoidal forcing at intraseasonal frequencies.

Rectification of the Madden–Julian Oscillation into the ENSO Cycle

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 0.48C) in the west Pacific, and warming (about 0.18C) 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

Direct measurements of upper ocean currents and water properties across the tropical Pacific during the 1990s

Abstract Meridional sections of upper ocean zonal currents, potential temperature, and salinity are estimated at ten longitudes from 143°E to 95°W using Conductivity–Temperature–Depth and Acoustic Doppler Current Profiler data from 172 synoptic sections taken in the tropical Pacific between 138°E and 86°W, mostly in the 1990s. Data reduction is carried out in a potential isopycnal and mixed layer framework to preserve a sharp pycnocline, a mixed layer, water property extrema, and velocity extrema. Mean zonal currents, potential temperatures, and salinities are produced at each longitude. The seasonal cycles of these fields are also estimated, as well as a simple El Nino Southern Oscillation (ENSO) cycle. Zonal sections along the equator are also presented. Properties of the near-equatorial zonal currents, including transports, temperatures, and salinities, are estimated separately from the synoptic sections. The seasonal cycles of these quantities and their correlations with the Southern Oscillation Index are investigated. The work is distinguished from most existing literature in that direct estimates of zonal velocity are combined with contemporaneous temperature and salinity data, allowing trans-Pacific estimates of near-equatorial current transports and properties, including those of the northern branch of the South Equatorial Current, the New Guinea Coastal Undercurrent, and the Equatorial Undercurrent.

EOF Representations of the Madden–Julian Oscillation and Its Connection with ENSO*

Abstract Although recent El Nino 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 Nino 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 a...

Dynamic Heights and Zonal Geostrophic Transports in the Central Tropical Pacific during 1979–84

Abstract Dynamic height is calculated from XBT and surface salinity data in the central Pacific using a mean temperature–salinity (T–S) relation in the usual way below the thermocline but assuming isohaline water in the upper layer where the temperatures are isothermal. This scheme produces a better estimate of dynamic height than the use of a mean T–S relationship alone and produces significant improvements near the equator where small pressure gradients imply large geostrophic currents. During the El Nino of 1982–83, water of very low surface salinity was observed spanning the equator; this event is attributed both to extreme local rainfall and anomalous advection from the western Pacific. Geostrophic transports of the major surface currants are estimated for the period January 1979 through December 1984. The North and South Equatorial countercurrents are found to have the largest annual fluctuations, and the vertical displacements of the thermocline associated with these fluctuations are qualitatively ...

Oceanic Equatorial Waves and the 1991–93 El Niño

Abstract Equatorial Kelvin and Rossby waves associated with the 1991–93 El Nino warm event were detected in temperature observations made by the Tropical Atmosphere-Ocean (TAO) buoy array. Intraseasonal Kelvin waves were a prominent part of equatorial thermocline depth variability and were well represented by a simple model consisting only of first- and second-mode baroclinic Kelvin waves. The second mode was essential to properly represent the observed amplitude. Thermocline depth variability at 5°N and 5°S was dominated by annual and interannual Rossby waves, which were found to have been largely wind forced in midbasin, with little if any signal associated with eastern boundary reflection. An evaluation of the Wyrtki buildup hypothesis and the delayed oscillator hypothesis in connection with the 1991–93 events showed that a long lag (about two years) occurred between the arrival of the downwelling signal in the west and the subsequent initiation of El Nino., this was considerably longer than suggested ...

Air-Sea Interaction over the Eastern Pacific Warm Pool: Gap Winds, Thermocline Dome, and Atmospheric Convection*

High-resolution satellite observations are used to investigate air‐sea interaction over the eastern Pacific warm pool. In winter, strong wind jets develop over the Gulfs of Tehuantepec, Papagayo, and Panama, accelerated by the pressure gradients between the Atlantic and Pacific across narrow passes of Central American cordillera. Patches of cold sea surface temperatures (SSTs) and high chlorophyll develop under these wind jets as a result of increased turbulent heat flux from the ocean and enhanced mixing across the base of the ocean mixed layer. Despite a large decrease in SST (exceeding 38C in seasonal means), the cold patches associated with the Tehuantepec and Papagayo jets do not have an obvious effect on local atmospheric convection in winter since the intertropical convergence zone (ITCZ) is located farther south. The cold patch of the Panama jet to the south, on the other hand, cuts through the winter ITCZ and breaks it into two parts. A pronounced thermocline dome develops west of the Gulf of Papagayo, with the 208C isotherm only 30 m deep throughout the year. In summer when the Panama jet disappears and the other two wind jets weaken, SST is 0.58C lower over this Costa Rica Dome than the background. This cold spot reduces local precipitation by half, punching a hole of 500 km in diameter in the summer ITCZ. The dome underlies a patch of open-ocean high chlorophyll. This thermocline dome is an ocean dynamic response to the positive wind curls south of the Papagayo jet, which is optimally oriented to excite ocean Rossby waves that remotely affect the ocean to the west. The meridionally oriented Tehuantepec and Panama jets, by contrast, only influence the local thermocline depth with few remote effects on SST and the atmosphere. The orographical-triggered air‐sea interaction described here is a good benchmark for testing high-resolution climate models now under development.

The 1991–1993 El Niño in the central Pacific

Abstract The 1991–1993 El Nino event is described using data from the TOGA-TAO buoy network, concentrating on variability at 140°W where a full suite of temperature, current, and surface meteorological observations were made. The daily time series furnished by the buoy array brings out the conspicuous importance of remotely forced intraseasonal variability in the form of equatorial baroclinic Kelvin waves during the evolution of the 1991–1993 El Nino. Notable variations along 140°W included a major weakening of the Equatorial Undercurrent in late 1991 to early 1992, and a reduction in intensity of monthly period tropical instability waves during the latter part of 1991 compared to previous non-El Nino years. The North Equatorial Countercurrent showed no major signal due to this El Nino, in contrast to earlier warm events. Although anomalies of the South Equatorial Current spanning the equator were in an eastward sense during the height of the event, the result of these changes was that near-surface flow across 140°W between 5°S and 5°N was close to zero, so there was apparently no large eastward transport of surface water past 140°W into the eastern equatorial Pacific. The relative phasing of anomalies of thermocline depth, equatorial undercurrent speed and SST during the warm event of 1991–1992 was somewhat similar to that seen during the 1986–1987 El Nino, although the earlier event was followed by a strong cold (La Nina) event whereas the recent one was not. Uniquely among modern El Nino events, after the 1991–1992 episode appeared to end with a reappearance of the equatorial cold tongue in mid-1992, a second SST warming in the eastern Pacific occurred in early 1993. The present data set is inadequate to fully diagnose the mechanisms of SST change at 140°W, but time series of insolation, air-sea temperature difference and humidity show that local air-sea heat flux variations (either radiative or turbulent) were probably not the primary cause of the SST changes during the El Nino. Similarly, although horizontal advective mechanisms were important contributors to SST signals at certain times, these alone did not account for the major warming and cooling events. Instead, the largest SST variations of the 1991–1992 El Nino can be ascribed to upwelling variations, and a simple parameterization for this process is presented. The major warming and cooling events in the central/eastern equatorial Pacific occurred nearly simultaneously over a wide longitudinal range, indicating that oceanic wave processes could not have been the sole source of the changes.

Pacific western boundary currents and their roles in climate

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents—in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow—has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.

The Annual Wind-driven Rossby Wave in the Subthermocline Equatorial Pacific

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 i...