Wind-driven submesoscale subduction at the north Pacific subtropical front

P. J. Hosegood*, M. C. Gregg, M. H. Alford

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Upper ocean observations from the north Pacific subtropical front (STF) during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss. Prior to generation, a sharp thermohaline front was intensified by confluent flow of 1-2$\times$10$^{-5}$ s$^{-1}$. Relative vertical vorticity, $\zeta$, across a surface intensified along-front jet on the warm side of a frontal trough, was 0.5$f$. During the storm, buoyancy loss arose from cooling of $\sim$650 W m$^{-2}$ and wind stress approaching $\sim$1 N m$^{-2}$ of which a component was directed down-front. This latter wind-driven buoyancy flux due to the southwards, cross-front Ekman transport of dense water over light, was concentrated at the front and exceeded the convective buoyancy flux by an order of magnitude. The intrusions subsequently appeared both within the surface mixed layer and beneath the seasonal pycnocline immediately following the storm and were 20 m thick and horizontally elongated in the cross-frontal direction. The near-surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline, whereas the subsurface intrusions were composed of warm and saline water from the surface. The apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterised by $O$(1) Rossby and Richardson numbers, pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer, and sloping isopycnals. The intrusion properties, background environmental context, and forcing history are consistent with numerical modelling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss, possibly by symmetric instability.
Original languageEnglish
Number of pages0
JournalJournal of Geophysical Research
Volume0
Issue number0
DOIs
Publication statusPublished - 4 Sept 2013

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