Abstract
Many physical phenomena in the ocean involve interactions between water masses of different temperatures and salinities at boundaries. Of particular interest is the characterisation of finescale structure at the marginal interaction zones of these boundaries, where the structure is either destroyed by mixing or formed by stratification. Using high-resolution seismic reflection imaging, we present observations of temporal changes at the leading edge of an interface between sub-thermocline layers in the Panama Basin. By studying time-lapse images of a seismic reflector between two water boundaries with subtle differences, we provide empirical constraints on how stratified layers evolve. The leading edge of this reflector, which is characterised by a gradual lateral decrease in vertical temperature contrast (|Delta T|), increases in length over ~3 days coupled with an increase in |Delta T|. A critical mixing state, in which turbulent diffusion is gradually replaced by double-diffusion as the dominant mixing process, is thus revealed.
Highlights
Background advection Expansion Lengthening Expansion LengtheningDuration (h) Velocity 9.6 ± 1.212.5 m over a period of 3 days, of d5T×z=1d0t−%4 s3− ́110À8 °C (Table m−1 s−1. 1), theGiven that horizontal uz is on the temperature gradient that would be required to account fully for the estimated dTz/dt would be 6 × 10−5 °C m−1, or a temperature slope of 6 °C in 100 km, which is grossly unrealistic
The stratification here favours the double-diffusive process of salt fingering as the density ratio Rρ % 5 (Fig. 1; Supplementary Fig. 2; Table 1)
By checking the water parcel advection of both mean current and tidal current during the seismic acquisition, we argue that the reflector’s tip was repeatedly mapped within an uncertainty of 95% confidence interval while the uncertainty by tidal excursion is insignificant (Supplementary Fig. 6). After correction for this movement caused by the mean current, considerable spatial offsets of the trackable reflection tips still exists (Fig. 5; Table 1). To account for this offset we propose that the observed interface is lengthening at rates estimated at 3.4 ± 1.2 cm/s and 4.6 ± 1.2 cm/s from the stacked section and Amplitude Versus Offset (AVO) data, respectively (Table 2)
Summary
Background advection Expansion Lengthening Expansion LengtheningDuration (h) Velocity (cm/s) 9.6 ± 1.212.5 m (the typical thickness of the interface; Table 1) over a period of 3 days (the time separation between seismic lines; Table order1), of d5T×z=1d0t−%4 s3− ́110À8 °C (Table m−1 s−1. 1), theGiven that horizontal uz is on the temperature gradient that would be required to account fully for the estimated dTz/dt would be 6 × 10−5 °C m−1, or a temperature slope of 6 °C in 100 km, which is grossly unrealistic. Whether in a relatively smooth or extremely sharp gradient regime, attract much attention but direct observation normally suffers from sparse sampling[22]. This can be problematic to ascertain whether the same step/. Interface is being detected and to distinguish whether the interface change is due to growth or advection[10,23]. This difficulty in tracing finescale variations in time and space means much of the understanding on the development of thermoclines is by numerical simulation[7].
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