Stirring and Mixing in the Wake of Angaur and Peleliu Islands, Palau

Abstract

Temperature or salinity variability on isopycnals is referred to as spice, which acts as a tracer in the absence of mixing. The westward North Equatorial Current and eastward North Equatorial Countercurrent are incident upon Palau, where the meridional extent of the topography exceeds 100 km and the thermohaline structure on the up‐ and downstream sides of the islands can be distinct. Westward flow of high salinity water past two islands at the south point of Palau injects a spice anomaly into the wake in the form of a 15 km wide jet with changes of salinity of 0.1 psu across the headland and the jet. With three repeated surveys, several terms in the spice balance are estimated for the jet. Horizontal diffusivity has a typical value of about 11 m2 s−1 for 15‐km length scales, while the vertical diffusivity is about 10−5 m2 s−1 from the balance, with similar maximum estimates for the latter from a shear‐based parameterization. Typical vertical diffusivities at this latitude are about 10−6 m2 s−1, which is the area mean from the shear‐based parameterization. Amidst this typical background diffusion, the spice variance increases linearly in the wake over three successive surveys. The surveys occurred over similar but not identical areas, following the shifting flow. The spice anomaly is detected 100–200 km downstream and is consistent with the mesoscale flow obtained from altimetric absolute geostrophic currents. Spice anomalies are a useful way of tracing flows encountering abrupt topography.

Stirring and Mixing in the Wake of Velasco Reef, Palau

“…Period 1 coincides with the peak of the 2015/2016 El Niño when in the western North Pacific the NECC transitions from an eastward to the westward anomaly (i.e., the NECC flow decreases; Johnston & Merrifield, 2000). During this time, mesoscale recirculations/eddies bring southern (i.e., NECC) waters to Velasco (Figure 6a; Schönau et al, 2019) with concomitant S(σ θ ) contrast between the western and eastern sides of Palau (similar to Johnston [2022]). The T-S properties associated with the southern waters are either advected or contained within a closed eddy.…”

“…The Mindanao Eddy is centered near 129°E, 8°N and also brings NECC waters northward to the west of Palau. Episodic excursions of NECC waters northward are noted at other times (Figure 6; Johnston, 2022; Schönau et al., 2019). However, for our study, the higher S on line E and the usual westward flow between lines W and E are well established in our means and time series (Figures 3–5).…”

“…T and S profiles were obtained using a Seabird 911+ conductivity‐temperature‐depth (CTD) instrument mounted on SeaSoar, an undulating body that is towed from the ship at 4 m s −1 (Johnston, 2022; Johnston & Ou, 2021; Rudnick, 1996; Rudnick & Luyten, 1996). SeaSoar sampled the upper 400 m with a complete dive cycle taking ∼10 min.…”

“…Eddies with Ro of $$\mathcal{O}$$(10) and radii of $$\mathcal{O}$$(1 km) are found within several kilometers of the topography (Johnston, MacKinnon, et al., 2019). However, farther downstream streamers of either tracers or elevated negative Ro are found in models and observations more often than distinct submesoscale eddies, which is likely due to instability and mixing (Johnston, 2022; Johnston, MacKinnon, et al., 2019; Simmons et al., 2019; White & Helfrich, 2013). The last quadrant is examined with observations from coincident high‐frequency radar and model results (Merrifield et al., 2019).…”

“…S ( σ θ ) depends on the water mass history, which includes topographic effects on the west‐/eastward NEC/NECC that are often located at the north/south end of Palau. Due to the excursions of the NECC and NEC on either side of Palau, there can be considerable contrasts in S ( σ θ ) between the up‐ and downstream sides (Johnston, 2022), with S changes of ∼0.2 psu over 5–10 km (Figure 2). For example, the mean flow from altimetry shows a recirculation of the NECC to the east of Palau (Figure 1b), which can bring different waters into contact.…”