Trapped planetary (Rossby) waves observed in the Indian Ocean by satellite borne altimeters

Abstract

Using 20 years of accurately calibrated, highresolution 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.

Geostrophic adjustment on the midlatitude β plane

“…Instead, they decay monotonically with latitude from their unique single maximum located near the equatorward boundary for low modes. The relevance of the trapped-wave theory was recently confirmed, using numerical simulations (Gildor et al, 2016) and satellite observations from the Indian Ocean (De-Leon and Paldor, 2017). The trapped-wave theory is employed in this study, as it underscores the effect of β on waves in wide channels (see Sects.…”

Commonly used methods fail to detect known phase speeds of simulated signals of Sea Surface Height Anomalies

“…Explanations for these discrepancies were suggested in previous studies (see the introduction of De-Leon and Paldor, 2017b for more details) in order to bridge that gap. One of the suggestions is the trapped wave theory (Paldor, Rubin and Mariano, 2007;Paldor and Sigalov, 2008;Gildor et al, 2016) where the waves' phase speed is higher than that of harmonic waves.…”

“…However, this trapped wave theory could be applied and compared to oceanic observations only in the presence of a wall, while there are only a few such places in the world (the southern coast of Australia is the only place where we succeeded to do that, see De-Leon and Paldor, 2017b). An attempt to define a virtual boundary based on a linear fit of observed estimates have not yielded satisfactorily fit between the trapped wave theory and observations.…”

Commonly used methods fail to detect known propagation speeds of simulated signals from time–longitude (Hovmöller) diagrams

“…2.2, to the raw data or to processed data (e.g., Polito and Liu, 2003, who separated the data into tiles of different periods). The methods examined here were used in many oceanic sub-areas such as the propagation speed of Rossby waves using SSHA (e.g., Tulloch et al, 2009) or data other than SSHA (Belonenko et al, 2018;Xie et al, 2016); nearshore wave dynamics (Almar et al, 2014); eddy detection (Abernathey and Marshall, 2013;Oliveira and Polito, 2018) and intraseasonal variability from mooring (Hu et al, 2018) to name a few.…”