No abstract

No abstract

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.

Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle Long baroclinic Rossby waves are potentially important in the adjustment of the tropical Pacific pycnocline to both annual and interannual wind stress curl fluctuations. Evidence for such waves is found in variations of the depth of the 20'C isotherm in the northern tropical Pacific during 1970-1987.A total of 199,067 bathythermograph profiles have been compiled from the archives of several countries; the data coverage is dense enough that westward propagating events may be observed with a minimum of zonal interpolation. After extensive quality control, 20'C depths were gridded with a resolution of 2' latitude, 5' longitude, and bimonths; statistical parameters of the data were estimated. A simple model of low-frequency quasi-geostrophic pycnocline variability allows the physical processes of Ekman pumping, the radiation of long (nondispersive) Rossby waves due to such pumping in midbasin, and the radiation of long Rossby waves from the observed eastern boundary pycnocline depth fluctuations. Although the wind stress curl has very little zonal variability at the annual period in the northern tropical Pacific, an annual fluctuation of 20'C depth propagates westward as a long Rossby wave near 5'N and 14'-18'N in agreement with the model hindcast. Near the thermocline ridge at 10'N, however, the annual cycle across the basin is dominated by local Ekman pumping. The wave-dominated variability at 5'N weakens the annual cycle of geostrophic transport of the North Equatorial Countercurrent in the western Pacific. E1 Nifio events are associated with westerly wind anomalies concentrated in the central equatorial Pacific; upwelling wind stress curl is generated in the extraequatorial tropics by these westerlies. Long upwelling Rossby waves forced by this curl pattern were observed to raise the western Pacific thertoocline well outside the equatorial waveguide in the later stages of E1 Nifios, consistent with the simple long-wave model. The observations suggest that although simple reflection of the long Rossby waves from the western boundary is not the major process affecting subsequent development on the equator, it is likely that the extraequatorial waves play some role in setting the timescale of ENSO events. 1983; White and Saur, 1981, 1983; White et al., 1982, 1985; with hindcasts derived assuming either local Ekman pumping lnoue et al., 1987; Pazan et al., 1986; Pazan and White, 1987; alone or including the radiation of low-frequency Rossby Graham and White, 1988] have sought to interpret BT obser-waves. Section 5 focuses on the role of long extraequatorial vations in the extraequatorial north Pacific in terms of similar Rossby waves in the evolution of E1 Nifio. Waves due to two simple long Rossby wave dynamics, often using the linear principal forcing sources are identified: upwelling long reduced-gravity model developed by Busalacchi and O'Brien Rossby waves generated in midbasin by the large-scale wind stress curl pattern...

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 Niño 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 Niño of 1991–1992. The key dynamics of the model is that the atmosphere responds rapidly to the state of the ocean, but the ocean's 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.

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 0.4ЊC) in the west Pacific, and warming (about 0.1ЊC) 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.

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 20Њ-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.

After early ideas that saw El Niños as isolated events, the advent of coupled models brought the conception of ENSO as a cycle in which each phase led to the next in a self‐sustained oscillation. Twenty‐two years of observations that represent the El Niño and La Niña peaks (east Pacific SST) and the memory of the system (zonal mean warm water volume) suggest a distinct break in the cycle, in which the coupled system is able to remain in a weak La Niña state for up to two years, so that memory of previous influences would be lost. Similarly, while the amplitude of anomalies persists from the onset of a warm event through its termination, there is no such persistence across the La Niña break. These observations suggest that El Niños are in fact event‐like disturbances to a stable basic state, requiring an initiating impulse not contained in the dynamics of the cycle itself.