December 4, 2007: "Explaining the global distribution of high-energy variability of the sea surface height"
Xiaopei Lin, Ocean University of China
Abstract:
The
global observation of sea surface height (SSH) by satellite altimeters in the
last 14 years reveals interesting characteristics of oceanic variability. At
any given latitude in either hemisphere and in all oceanic basins, there is a
peak of energy spectrum for high-frequency variability (with a time period
shorter than the semi-seasonal cycle). The frequency associated with this peak
spectrum decreases from the tropics to high-latitudes. For instance, a 30-day
period variability dominates at both 7 oN and 7 oS, while
120-day period fluctuations peak at 28o in latitude. Our study finds
that the frequency of the peak spectrum at any latitude is nearly identical to
the frequency of those Rossby waves with a zero group velocity - the speed at
which oceanic energy propagates. Rossby waves play the most important role in
oceanic response to atmospheric forcing. There are two distinct groups of
Rossby waves; long waves transmit the energy from the atmospherically-forced regions
toward the western oceanic boundaries and short waves push the energy eastward.
These two waves merge at a frequency or wavelength at which the energy
propagation becomes stagnant (zero group velocity). At this critical frequency (or
at the wavelength that is determined by this critical frequency), the energy
from the atmospheric forcing may accumulates in the ocean interior, and thus
results in a spectrum peak.