Abstract
Abstract. Using 20 years of accurately calibrated, high-resolution observations of sea surface height anomalies (SSHAs) by satellite borne altimeters, we show that in the Indian Ocean south of the Australian coast the low-frequency variations of SSHAs are dominated by westward propagating, trapped, i.e., non-harmonic, Rossby (Planetary) waves. Our results demonstrate that the meridional-dependent amplitudes of the SSHAs are large only within a few degrees of latitude next to the southern Australian coast while farther in the ocean they are uniformly small. This meridional variation of the SSHA signal is typical of the amplitude structure in the trapped wave theory. The westward propagation speed of the SSHA signal is analyzed by employing three different methods of estimation. Each one of these methods yields speed estimates that can vary widely between adjacent latitudes but the combination of at least two of the three methods yields much smoother variation. The estimates obtained in this manner show that the observed phase speeds at different latitudes exceed the phase speeds of harmonic Rossby (planetary) waves by 140 to 200 % (which was also reported in previous studies). In contrast, the theory of trapped Rossby (planetary) waves in a domain bounded by a wall on its equatorward side yields phase speeds that approximate more closely the observed phase speeds in the study area.
Highlights
The analysis of observations of sea surface height anomalies (SSHAs), i.e., the deviation of the sea surface height from its mean value at any given point in the ocean, was carried out since the 1990s in various parts of the world ocean by various satellite borne altimeters. Chelton and Schlax (1996), for example, analyzed the first 3 years of altimetry data collected by the TOPEX/Poseidon satellite in the world ocean, Zang and Wunsch (1999) analyzed 5 years of TOPEX/Poseidon data in the North Pacific Ocean and Osychny and Cornillon (2004) analyzed 6 years of modified TOPEX/Poseidon data in the North Atlantic Ocean
Recent studies (e.g., Chelton et al, 2007, 2011), argue that the observed SSHA features belong to mesoscale eddies and are not surface manifestations of planetary waves in the thermocline but this change of view has no effect on the estimate of the westward propagation speed since these eddies propagate westward at the phase speed of long Rossby waves (Chelton et al, 2011; O’Brien et al, 2013; Polito and Sato, 2015; see Nof, 1981, for theoretical estimate of eddy migration rate on the β-plane)
Based on the insight gained from the study of synthetic signals, an estimation of the observed phase speed is accepted here only when an isolated peak is evident in at least two of the three methods and the phase speeds that correspond to these peaks agree by better than 10 %
Summary
The analysis of observations of sea surface height anomalies (SSHAs), i.e., the deviation of the sea surface height from its mean value at any given point in the ocean, was carried out since the 1990s in various parts of the world ocean by various satellite borne altimeters. Chelton and Schlax (1996), for example, analyzed the first 3 years of altimetry data collected by the TOPEX/Poseidon satellite in the world ocean, Zang and Wunsch (1999) analyzed 5 years of TOPEX/Poseidon data in the North Pacific Ocean and Osychny and Cornillon (2004) analyzed 6 years of modified TOPEX/Poseidon data in the North Atlantic Ocean. The traditional interpretation of these SSHA observations has employed the harmonic theory of westward propagating, low frequency waves that assumes the existence of a zonal channel that bounds the north–south extent on the β-plane Under these assumptions zonally propagating wave solutions of the shallow water equations can be constructed and explicit expressions can be derived for both the zonal phase speed of the waves and the spatial structure of their amplitudes. The current study employs the available series of SSHA observations sampled on a 1/4◦ spatial grid, which are compared to the theoretical phase speeds and meridional structures of the height field using the trapped and harmonic wave theories.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
