Dynamics of a Standing Meander of the Subantarctic Front Diagnosed from Satellite Altimetry and Along-Stream Anomalies of Temperature and Salinity

Abstract

Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot-spots, characterised by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient-wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance.

Physical Drivers of Biogeochemical Variability in the Polar Front Meander

“…Meijer et al. (2022) linked these subsurface relationships to patterns of divergence in surface velocities from a gradient wind balance applied to SSH. The ageostrophic component of the flow reveals deceleration of the current in the trough and acceleration in the crest of the meander.…”

“…These studies show that rotation of the horizontal current drives anomalies in temperature and salinity and vertical motion along isopycnals such that, for the southern hemisphere, there is downwelling leading out of a meander crest and into the trough and upwelling leading out of the trough and into the next crest. Meijer et al (2022) linked these subsurface relationships to patterns of divergence in surface velocities from a gradient wind balance applied to SSH. The ageostrophic component of the flow reveals deceleration of the current in the trough and acceleration in the crest of the meander.…”

“…The result is convergence of the flow leading into a trough and divergence leading out of a trough and into a crest. From their detailed hydrographic survey, Meijer et al (2022) linked along-front variations in water mass properties to show that upwelling along isopycnals is associated with divergence at the sea surface. Applying this theory to our study, we would expect to see divergence and upwelling leading into the crest near 150°E and leading out of the trough and into the next crest, which is where we see the strongest blooms.…”

Characteristics and Trends of the Campbell Plateau Meander in the Southern Ocean: 1993-2020

“…We also identify the monthly position of four key standing peaks and four troughs of the meander and estimate the monthly amplitude in two regions (Table 1; Figure 3). We define the peaks as the southernmost points (farthest distance from the equator) and troughs as the northernmost (in closest proximity to the equator) points of the meander's trajectory (Newton, 1959;Meijer et al, 2022) for each month, within the manually-defined longitude ranges (Table 1). These peaks and troughs are consistently identifiable over the 1993-2020 period and are labelled Pk 1 to Pk 4 and Tr 1 to Tr 4 from west to east (Figure 3).…”

“…Compared with quieter downstream regions, Southern Ocean standing meanders regions stand out with larger lateral buoyancy gradients in mixed layer, increased variability in mixed layer depth, and show signs of stronger ocean mixing (Thompson & Naveira Garabato, 2014;Langlais et al, 2017) While studies have been undertaken to assess the response of the Antarctic Circumpolar Current fronts to climate change, less work has focused on meanders and their trends. A majority of meander studies have looked at dynamic mechanisms, energy transport, and their role in the Southern Ocean system (e.g., Thompson and Sallée (2012); Chapman et al (2015); Barthel et al (2017); Youngs et al (2017); Barthel et al (2022); Meijer et al (2022); X. Zhang et al (2022); Cyriac et al (2023)).…”