Analysis of a Reconstructed Oceanic Kelvin Wave Dynamic Height Dataset for the Period 1974–2005

Abstract

Intraseasonal oceanic Kelvin waves are the dominant mode of variability in the thermocline of the equatorial Pacific. Dynamic height data from the Tropical Atmosphere Ocean (TAO) Array of buoys moored in the tropical Pacific offer a convenient grid on which to study the waves but can only be effectively applied to study basinwide wave activity since about 1988 because of insufficient data at earlier times. Kelvin wave signals are also present in sea level data from island and coastal sites from the University of Hawaii Sea Level Center, some of which are available from before 1970 and up to 2003. This work describes a technique for reconstructing equatorial dynamic height data back to 1974, by utilizing regression relationships between the TAO data and daily sea level time series from 11 stations in the tropical Pacific. The reconstructed data are analyzed for skill in approximating Kelvin wave signals when TAO data are available. Reconstructed Kelvin wave signals prior to the TAO period are then analyzed for consistency with the wind stress anomalies that are responsible for generating the waves.A regression analysis showing intraseasonal patterns of convection and winds that occur during periods of adjustment toward El Niño conditions is applied during the period 1974-87 for comparison with an earlier result calculated from TAO data for 1988-2005. Systematic changes in Kelvin wave phase speed with respect to ENSO documented for the latter period are confirmed in the earlier reconstructed dataset.

Variability of Intraseasonal Kelvin Waves in the Equatorial Pacific Ocean

“…1 are often observed in other periods based on a composite analysis using 16 yr of TAO data. This systematic behavior was also confirmed for an earlier record of island sea level data going back to 1974 (Roundy and Kiladis 2007).…”

“…They showed that the maximum Kelvin wave amplitude occurs at periods around 70 days for realistic propagating wind anomalies. Roundy and Kiladis (2006;hereafter RK06) proposed that an alternative process might be responsible for the discrepancy, which is of particular interest to the present study. They analyzed 16 yr of TOGA TAO data to describe the nature of intraseasonal Kelvin waves and their relationship to atmospheric forcing, and they frequently observed a systematic decrease in Kelvin wave phase speed over time.…”

“…In the past several years, intraseasonal Kelvin waves generated by the MJO have been extensively examined by various observational and modeling studies because of their potential importance for longer time scale climate variability tied to ENSO (e.g., Kessler et al 1995;Hendon et al 1998;McPhaden 2002;Zhang and Gottschalck 2002;Cravatte et al 2003;Roundy and Kiladis 2006). One of the unresolved issues regarding Kelvin waves associated with the MJO is that there is an apparent discrepancy of dominant periods in these processes.…”

“…While the period of the MJO is about 40-50 days (e.g., Madden and Julian 1972;Salby and Hendon 1994), the spectral peak of oceanic Kelvin waves is centered at around 70 days (e.g., Enfield 1987;McPhaden and Taft 1988;Kessler et al 1995). Recent studies provide some reconciliation of this discrepancy based on the analyses of Tropical Ocean and Global Atmosphere Tropical Atmosphere Ocean (TOGA TAO) mooring array data (Kessler et al 1995;Hendon et al 1998;Roundy and Kiladis 2006). Kessler et al (1995) offered an explanation using a patch of oscillating zonal winds in the western Pacific.…”

“…1. In this paper, as in RK06 and Roundy and Kiladis (2007), we retain the term "Kelvin wave" to describe even the fastest intraseasonal equatorial disturbances we observe, while acknowledging that the structure of these forced waves may deviate significantly from theoretical free Kelvin waves derived from the shallow water equations (Matsuno 1966;Moore and Philander 1977).…”