Wind-driven submesoscale subduction at the north Pacific subtropical front

Abstract

[1] Upper ocean observations from the north Pacific subtropical front 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 × 10−5 s−1. Relative vertical vorticity, ζ, 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 due to cooling of ∼650 W m−2 and down-front wind stress <0.5 N m−2 that generated a southward, cross-front Ekman transport of dense water over light. The resulting wind-driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude. The intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline. They were approximately 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 characterized 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 prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss, possibly resulting in symmetric instability.