Can Rossby Waves be used as Climate Change Indicators?
Sutcliffe, Anna C. S.1; Cipollini, Paolo2; Robinson, Ian S.1
1School of Ocean and Earth Science, University of Southampton, UNITED KINGDOM; 2National Oceanography Centre, UNITED KINGDOM
Oceanic Rossby waves are capable of transporting large amounts of heat and momentum and have been linked to major climate oscillations such as the El Niño Southern Oscillation and the Meridional Overturning Circulation. These features propagate at similar speeds as large-scale eddies but at different scales; eddies' spatial scales are of the order of 100km whereas Rossby waves possess length scales of hundreds of km. The fact that they can be successfully detected in altimetry and that these records now span over 20 years, makes it possible to analyse for changes in these features over time. The comparison with NEMO sea surface height anomaly data revealed that these features are properly represented in the data, thus allowing for a longer time-series (over 40 years) to be explored.
A systematic analysis of these features in both records was performed using a using a specialized filtering technique, developed by Thomas et al. (Geophysical Research Letters, submitted), which filters to find the large-scale, non-Gaussian shaped, westward propagating signal in the data. The speeds of the output, the Rossby wave signal, were computed and analysed for trends. Results for the Pacific basin show an interesting zonal-band distribution of the trends throughout the basin, with regions of positive trends alternating with regions of negative trends. The highest positive trends are found at the eastern side of the basin at the low latitudes, with the highest values being found off the coast of Ecuador, with speeds doubling in value after a decade. A high degree of correlation was found with the NEMO data results, with speeds increasing at the same location by 50% in a decade. The similarity in results further suggests that the model adequately represents the Rossby wave signal.
The trends show a variation in the speed of the Rossby wave signal, which, for the time-series available seems to be more significant at the Rossby wave generation site. Given these waves’ importance as a mode of the ocean’s adjustment to climate, the found changes in speed shows that the Rossby waves are reacting to climate change.