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12.14.10: Potential density and stratification during wh01a and wh02a

The potential density σ (referenced at the surface) during the two WHOTS deployment wh01a and wh02a is shown in Fig. 1. In both cases, there are high-frequency (< 10 days) events of large intrusion of deep water into the upper layers (days 150-225 during wh01a and days 175-225 during wh02a). These intrusions are amongst the largest of the two records. Assuming that these features are propagating zonally and estimating their speed of propagation using AVISO SSH (8.46 cm/s±1.8 cm/s), I estimate their scale to be between 17 and 80 km (see this note). These events may be reminiscent of the similar events reported by Johnson et al. (Nature, 2010) using ARGO data (see this note where these data are plotted).

So far, I have not found a strong link between these and the intensity of horizontal stirring (estimated using Finite-Size Lyapunov Exponents; see this note). However, we see in these records that these events occur during Spring when the stratification in the upper layers is weak. Figs. 2 and 3 emphasize this points by comparing time series of σ anomaly (defined as the variability in σ for periods less than 1 month) over 100-150 m depth range and that of the maximum of stratification. According to numerical simulations performed by Patrice Klein, filamentation is reduced when the stratification is increased. This might be what we see here in the observations but it would be nice to confirm it in other WHOTS data or in ARGO data.


Figure 1: σ during (a) wh01a and (b) wh02a.


Figure 2: (a) Buoyancy frequency, (b) its maximum in the vertical and (c) σ anomaly (high-pass filtered, periods less than 1 month) averaged between 100 and 150 m during wh01a.


Figure 3: Same as in Fig. 2 but during wh02a.