U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
The purpose of this paper is to document the connection between intraseasonal (30-90 days) Kelvin waves in the equatorial Pacific and planetary-scale, eastward-propagating intraseasonal convection fluctuations in the tropical atmosphere (the Madden-Julian Oscillation). The connection is of interest because it suggests that the frequently observed intraseasonal waves can be viewed as a manifestation of a global phenomenon, not strictly internal to the Pacific. We then show that intraseasonal fluctuations in the ocean and atmosphere exhibit consistent low-frequency modulation associated with the annual eastward march of convection and with El Niño-Southern Oscillation (ENSO) variability. This variation of the amplitude of the intraseasonal frequency band could be one way in which the Pacific is affected by low-frequency signals originating outside the basin and may provide a mechanism whereby the Pacific feels climatic events originating in the Indian Ocean and south Asian monsoon circulation. We use the extensive TOGA-TAO buoy network (see section 2) to observe the life cycle of the Kelvin waves and the wind forcing that creates them.
A series of papers have described and diagnosed the dynamics of the intraseasonal oceanic Kelvin waves. Spillane et al. [1987] used sea level observations to show that poleward-propagating intraseasonal variability was detectable all along the coast of the Americas from California to Peru. Enfield [1987] extended this analysis and found that the source of the coastal variability was first baroclinic mode equatorial Kelvin waves forced by western Pacific winds. He noted the apparent association with atmospheric intraseasonal variability and found that the oceanic waves seemed to be best developed during the onset of the 1982-1983 El Niño but were relatively weak in the subsequent 2 years. Remarking on this interannual variation of the signal, Enfield [1987] wondered if there was any connection to the ENSO cycle. McPhaden and Taft [1988] analyzed temperature, wind, and current observations from moored buoys at 140°W and 110°W (an earlier set of the same buoy data used in the present study) and found that intraseasonal variability was prominent in subsurface temperature and zonal currents, but not in meridional currents or eastern Pacific winds, consistent with the interpretation in terms of remotely forced Kelvin waves. Johnson and McPhaden [1993a] studied the vertical and meridional structure of the Kelvin waves at 140°W, 124°W, and 110°W using frequency-domain empirical orthogonal functions (EOFs) and found important differences between the observed characteristics of the intraseasonal variability and the structures expected from linear Kelvin wave theory. These discrepancies were explained by interactions between the waves and the mean zonal current/temperature regime. In particular, intraseasonal sea surface temperature (SST) variations in the equatorial cold tongue were shown to be related to zonal advection by these Kelvin waves. Johnson and McPhaden [1993b] identified mean vertical advection as the most important effect modifying linear Kelvin propagation. In sum, the intraseasonal Kelvin waves are reasonably well described and understood, particularly once they leave the generation region in the western Pacific. The present paper extends the existing description by connecting the waves to the Madden-Julian Oscillation (MJO) in the atmosphere and follows up Enfield's [1987] query by proposing an interaction between the intraseasonal and the interannual variability of the Pacific ocean-atmosphere system.
Intraseasonal variability in the tropical atmosphere has been the subject of
many papers since Madden
and Julian [1971, 1972]
used surface pressure and upper air data collected from stations around the
tropical belt to show that these fluctuations were of global scale and had aspects
suggesting an eastward-propagating wave. The convective signal associated with
the MJO has been studied using satellite-derived outgoing longwave radiation
(OLR) data and is found to be most prominent over the warm pool regions of the
eastern hemisphere (roughly between 60°E and 180°) [Rui
and Wang, 1990]. Eastward movement of convection is clearest during
boreal winter when the warm pool extends from the Indian Ocean to the date line
and lies closest to the equator [Weickmann
et al., 1985; Lau
and Chan, 1985, 1986;
Lau
and Shen, 1988]. Migration speeds are observed to range between
There has been considerable ambiguity in the literature about the exact meaning
of "intraseasonal," with many of the early atmospheric papers citing
periods in the range of
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