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Anomalous Characteristics of Equatorial Thermohaline Finestructure

John M. ToolePacific Marine Environmental Laboratory, NOAA, Seattle, WA 98105

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Abstract

Finestructure observations gathered in the equatorial Pacific are discussed. The measurements consist of a 48 h CTD time series within the salinity front at the northern edge of the Equatorial Undercurrent at 110°W. The water column at depths shallower than 150 m at this site was dominated by intense inter-leaving. The vertical temperature and salinity gradient spectra from this depth range are strongly peaked at a wavelength of 20 m and the temperature-salinity anomalies are coherent and in phase at this scale. Yet, unlike the finestructure observed in fronts at higher latitudes, the finestructure is not density-compensating on any scale on account of an excess of temperature variance. Rather than interpret this equatorial finestructure as resulting from nearly isopycnal differential advection, it is suggested that the features may be the signature of low-frequency internal waves where lateral displacements are reflected in the salinity profiles and lateral and vertical displacements induce the temperature variability. If so, lateral mixing across the Equatorial Undercurrent may involve an interaction between internal waves and salt fingering as well as double-diffusively driven intrusions.

Abstract

Finestructure observations gathered in the equatorial Pacific are discussed. The measurements consist of a 48 h CTD time series within the salinity front at the northern edge of the Equatorial Undercurrent at 110°W. The water column at depths shallower than 150 m at this site was dominated by intense inter-leaving. The vertical temperature and salinity gradient spectra from this depth range are strongly peaked at a wavelength of 20 m and the temperature-salinity anomalies are coherent and in phase at this scale. Yet, unlike the finestructure observed in fronts at higher latitudes, the finestructure is not density-compensating on any scale on account of an excess of temperature variance. Rather than interpret this equatorial finestructure as resulting from nearly isopycnal differential advection, it is suggested that the features may be the signature of low-frequency internal waves where lateral displacements are reflected in the salinity profiles and lateral and vertical displacements induce the temperature variability. If so, lateral mixing across the Equatorial Undercurrent may involve an interaction between internal waves and salt fingering as well as double-diffusively driven intrusions.

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