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Forcing of intraseasonal Kelvin waves in the equatorial Pacific

William S. Kessler and Michael J. McPhaden

Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington

Klaus M. Weickmann

Climate Diagnostics Center, NOAA, Boulder, Colorado

J. Geophys. Res., 100(C6), 10,613-10,631 (1995)
This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysical Union.

1. Introduction

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 Nio-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 Nio 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 140W and 110W (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 140W, 124W, and 110W 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 60E 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 3 and 6 m s-1, with a suggestion that movement is slowest during the intensification phase over the Indian Ocean and fastest during weakening over the central Pacific [Rui and Wang, 1990]. A deep signal in the atmospheric zonal wind also propagates eastward at up to 20 m s-1 from the regions of warm pool convection across the Pacific and into the Atlantic Ocean region [Hendon and Salby, 1995; Weickmann and Khalsa, 1990]. Thus the MJO is a global phenomenon, but it is only over the warm SST region in the eastern hemisphere that the strong surface expression (deep tropical convection and associated surface wind anomalies that force the ocean) occurs. During boreal summer the convective signal over the western Pacific tends to shift off the equator to near 10N [Knutson et al., 1986; Lau and Chan, 1986] as part of a seasonal cycle in the activity. Composite studies using global circulation data have confirmed many of the early results of Madden and Julian [1972], including the presence of large-scale, low-level westerly wind anomalies that move eastward and are maximized near or just west of the convection anomalies [Knutson and Weickmann, 1987; Rui and Wang, 1990; Hendon and Salby, 1995]. Major centers of convective activity associated with the MJO are found over the central Indian and western Pacific Oceans [Weickmann and Khalsa, 1990; Zhu and Wang, 1993] and the passage of an MJO over these regions is accompanied by westerly wind bursts, trade wind surges, tropical cyclones and supercloud clusters [Nakazawa, 1988; Sui and Lau, 1989]. We will show that the oceanic Kelvin waves represent a robust response to this multiple time and space scale forcing.

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 30-50 days, and other works finding a broadband signal with periods to 100 days. In general, the oceanic papers have tended to find the lower frequencies more prominent than do the atmospheric studies. We will argue in section 4.6 that this can be understood as a result of the response of the ocean to forcing whose fetch is similar to the wavelength of a Kelvin wave at intraseasonal frequencies. In any case it seems clear that the intraseasonal variability is not a true periodic signal but is actually composed of individual events that tend to follow one another no less than about 30 days apart [Rui and Wang, 1990]. Spectral analyses using different time periods show intraseasonal peaks varying between about 30 and about 90 days or so and emphasize the broadband nature of the phenomenon.

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