Manifestation of oceanic Rossby waves in long-term multiparametric satellite datasets

Abstract

The characteristics of westward copropagating features in satellite altimetry, sea surface temperature and ocean colour data are investigated throughout the global oceans between 50° north and 50° south for the period 1998–2008. A new ‘hybrid’ filtering approach allowing features with periods between 37 and 410days to pass polewards of 20° north and south and features with periods between 37 and 205days to pass within 20° of the equator is found to isolate features whose speed is consistent with predictions of long planetary wave theory throughout much of the oceans. Features with wavelengths between 300 and 800km and periods between 205 and 410days are shown to be a significant source of temperature and chlorophyll variability between 20° and 50° north and south, whilst within 20° of the equator, signals in the 800–1500km and 37–205days range are predominant. Between 15° and 50° north and south, observed propagation speeds are found to closely match those predicted for first-mode baroclinic Rossby waves by theory. In the equatorial Pacific and Atlantic Oceans, features with wavelengths (800–2000km), periods (15–40days) and phase speeds (20–50cms−1) characteristic of tropical instability waves are preferentially observed. The amplitude and phase relationship between copropagating signals in ocean colour and altimetry and sea surface temperature and altimetry is interpreted in terms of the horizontal (meridional) advection of mean temperature and chlorophyll gradients by Rossby waves, similar to what has recently been observed for non-linear eddies at shorter length scales. The observed phase relationships are in good agreement with the range of phases expected for horizontal advection throughout much of the oceans and consistent with the sign of the mean meridional gradients of temperature and chlorophyll. These results support the view that horizontal advection (be it from waves and/or eddies) is the dominant mode of covariation of temperature and chlorophyll with sea surface height throughout most of the oceans, although the agreement between predicted and observed amplitude and phase relationships is insufficient to completely rule out vertical mechanisms in a few regions. We believe that a ‘one size fits all’ approach for global Rossby wave studies can no longer be considered valid, and although we have modified our technique to take this into account, we advocate future work to develop a fully latitudinally dependent filtering methodology.