Forcing of intraseasonal Kelvin waves in the equatorial Pacific
Kessler, W.S., M.J. McPhaden and K.M. Weickmann, 1995
Journal of Geophysical Research, 100, 10613-10631.
Ten-year time series of SST, 20[degrees]C depth and zonal winds measured
by moored buoys across the equatorial Pacific are used to define the
intraseasonal (30-90 day period) Kelvin waves. The Kelvin waves are observed
to be forced west of the dateline, and propagate at a speed of 2.4 m/s, 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 ENSO
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) measured by outgoing longwave
radiation (OLR), 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 (35-60 day periods).
We show this period offset to be 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 which suggests an ocean-atmosphere coupling by
which zonal advection of SST feeds back to the atmosphere; the model duplicates
the step-like advance of warm water and westerly winds across the Pacific at
the onset of the El Nino of 1991-92. 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 non-linear ordinary
differential equation which allows a net non-zero low-frequency ocean signal
to develop from zero-mean sinusoidal forcing at intraseasonal frequencies.
Download this paper as it appeared in JGR in pdf format (7.2 Mb)
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA
