December 4, 2007: "Explaining the global distribution of high-energy variability of the sea surface height"
Xiaopei Lin, Ocean University of China
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.