The role of air‐sea fluxes in Subantarctic Mode Water formation

Abstract

[1] Two hydrographic surveys and a one-dimensional mixed layer model are used to assess the role of air-sea fluxes in forming deep Subantarctic Mode Water (SAMW) mixed layers in the southeast Pacific Ocean. Forty-two SAMW mixed layers deeper than 400 m were observed north of the Subantarctic Front during the 2005 winter cruise, with the deepest mixed layers reaching 550 m. The densest, coldest, and freshest mixed layers were found in the cruise's eastern sections near 77 W. The deep SAMW mixed layers were observed concurrently with surface ocean heat loss of approximately À200 W m À2 . The heat, momentum, and precipitation flux fields of five flux products are used to force a one-dimensional KPP mixed layer model initialized with profiles from the 2006 summer cruise. The simulated winter mixed layers generated by all of the forcing products resemble Argo observations of SAMW; this agreement also validates the flux products. Mixing driven by buoyancy loss and wind forcing is strong enough to deepen the SAMW layers. Wind-driven mixing is central to SAMW formation, as model runs forced with buoyancy forcing alone produce shallow mixed layers. Air-sea fluxes indirectly influence winter SAMW properties by controlling how deeply the profiles mix. The stratification and heat content of the initial profiles determine the properties of the SAMW and the likelihood of deep mixing. Summer profiles from just upstream of Drake Passage have less heat stored between 100 and 600 m than upstream profiles, and so, with sufficiently strong winter forcing, form a cold, dense variety of SAMW.

Episodic Southern Ocean Heat Loss and Its Mixed Layer Impacts Revealed by the Farthest South Multiyear Surface Flux Mooring

“…Intense turbulent heat loss events are related to advection of cold, dry air masses mainly from the south, consistent with previous results from limited observations in the Southeast Pacific (Holte et al, 2012). Intense turbulent heat loss events are related to advection of cold, dry air masses mainly from the south, consistent with previous results from limited observations in the Southeast Pacific (Holte et al, 2012).…”

“…In each year, MLD estimates from both Argo float and mooring data show a gradual mixed layer deepening from summer through early winter; however, 2015 and 2017 have much deeper winter mixed layers than 2016 (Figure 3f ). Flanking mooring A CTDs show that the 2015 winter mixed layers exhibit the density of Southeast Pacific SAMW (26.99-27.02 g/cm 3 ) described by Holte et al (2012) and Carter et al (2014) from mid-August through early October with MLDs exceeding 500 m (Figure 3g). Even so, the overall MLDs were substantially shallower in winter 2016 compared to winter 2015 in the broader Southeast Pacific (supporting information Figure S2).…”

“…Wintertime surface ocean heat loss, driven primarily by turbulent (latent and sensible) heat flux, is the main mechanism causing SAMW formation (Cerovečki & Mazloff, 2016;Holte et al, 2012). Limited observations (Holte et al, 2017).…”

“…in the Southeast Pacific suggest that turbulent heat loss events associated with storms may be an important driver for winter mixed layer deepening (Holte et al, 2012). Hence, characterizing air-sea heat flux variability in the Southern Ocean is a key step for understanding the formation and variability of SAMW.…”

Mixing and remineralization in waters detrained from the surface into Subantarctic Mode Water and Antarctic Intermediate Water in the southeastern Pacific

“…This paper is essentially a companion study to recent analyses of this AAIW05 data set that emphasized physical and gas‐exchange processes in this region [ Sloyan et al ., ; Holte et al ., ; Hartin et al ., ]. Our primary aim is to produce reliable estimates of biogeochemical cycling in recently formed SAMW and AAIW, by taking mixing into account.…”