Multiple Oscillatory Modes of the Argentine Basin. Part I: Statistical Analysis

Abstract

Observations of the sea surface height in the Argentine Basin indicate that strong variability occurs on a time scale of 20Ϫ30 days. The aim of this study is to determine the physical processes responsible for this variability. First, results are presented from two statistical techniques applied to a decade of altimetric data. A complex empirical orthogonal function (CEOF) analysis identifies the recently discovered dipole mode as the dominant mode of variability. A principal oscillation pattern (POP) analysis confirms the existence of this mode, which has a period of 25 days. The second CEOF displays a propagating pattern in the northern Argentine Basin, plus a rotating dipole in the southwest corner. The POP analysis identifies both patterns as individual modes, with periods of 30 and 20 days, respectively. Second, the barotropic normal modes of the Argentine Basin are studied, using a shallow-water model capturing the full bathymetry of the basin. Coherences between the spatial patterns of these modes and altimeter data suggest that several of the basin modes are involved in the observed variability. This analysis implies that the 20-day mode detected by recent bottom-pressure measurements is a true barotropic mode. However, the 25-day variability, as found in altimeter data, cannot be directly attributed to the excitation of a free Rossby basin mode. This study indicates that the results of several apparently conflicting observations of the flow variability in the Argentine Basin can be reconciled by assuming that multiple basin modes are involved.

Multiple Oscillatory Modes of the Argentine Basin. Part II: The Spectral Origin of Basin Modes

“…In the first part of this study (Weijer et al 2007, hereinafter Part I), statistical analyses of altimeter data suggested that more than one mode of variability may be present in the Argentine Basin. This conclusion may reconcile several studies reporting on modes of variability with conflicting periods (Weatherly 1993;Fu et al 2001;Hughes et al 2007).…”

Stability of large-scale oceanic flows and the importance of non-local effects

“…Possible regions of interest where these results may apply, are semi-enclosed basins and marginal seas, where variability in the form of high-frequency barotropic oscillations suggestive of Rossby basin modes has been observed (Warren et al, 2002;Weijer et al, 2007a;Fu et al, 2001;Stanev and Rachev, 1999).…”

“…For a basin the size of the Atlantic or the Pacific Ocean however the periods of the barotropic Rossby basin modes are too short (O(15days) or less) to be resolved by the altimetry data -the Topex/Poseidon has a 20-day Nyquist period, (Le Traon and Morrow, 2000). However, barotropic oscillations consistent with Rossby basin modes have been observed in basins semi-enclosed by bathymetry (Warren et al, 2002;Fu et al, 2001;Weijer et al, 2007a) and marginal seas (Stanev and Rachev, 1999).…”

“…Examples of such systems are marginal seas and semi-enclosed by bathymetry basins such as the Mascarene basin (Warren et al, 2002) and the Argentine basin (Fu et al, 2001;Weijer et al, 2007a), where barotropic basin modes have been observed.…”

“…Part of it is due to the intense eddy and frontal activity resulting from confluent currents, part of it has been identified in several measurements coming from altimetry (Fu et al, 2001), current meters (Weatherly, 1993) and pressure gauges (Hughes et al, 2007), as a barotropic oscillation with a 20-30 days period. Most recently, different statistical techniques applied to a decade of weekly SSH maps suggests that actually multiple basin modes are present in the Argentine basin (Weijer et al, 2007a). Matching the real ocean basin oscillations with their theoretical counterpart is challenging since they are highly distorted by the bottom bathymetry.…”