Parameterization of Mixed Layer Eddies. Part II: Prognosis and Impact

Abstract The authors propose a parameterization for restratification by mixed layer eddies that develop from baroclinic instabilities of ocean fronts. The parameterization is cast as an overturning streamfunction that is proportional to the product of horizontal buoyancy gradient, mixed layer depth, and inertial period. The parameterization has remarkable skill for an extremely wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. In this paper a coarse resolution prognostic model of the parameterization is compared with submesoscale mixed layer eddy resolving simulations. The parameterization proves accurate in predicting changes to the buoyancy. The climate implications of the proposed parameterization are estimated by applying the restratification scaling to observations: the mixed layer depth is estimated from climatology, and the buoyancy gradients are from satellite altimetry. The vertical fluxes are comparable to monthly mean air–sea fluxes in large areas of the ocean and suggest that restratification by mixed layer eddies is a leading order process in the upper ocean. Critical regions for ocean–atmosphere interaction, such as deep, intermediate, and mode water formation sites, are particularly affected.

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Parameterization of Mixed Layer Eddies. Part I: Theory and Diagnosis

Journal of Physical Oceanography ◽

10.1175/2007jpo3792.1 ◽

2008 ◽

Vol 38 (6) ◽

pp. 1145-1165 ◽

Cited By ~ 353

Author(s):

Baylor Fox-Kemper ◽

Raffaele Ferrari ◽

Robert Hallberg

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Baroclinic Instability ◽

Finite Amplitude ◽

Climate Impacts ◽

Surface Mixed Layer ◽

Rotation Rates ◽

Wide Range ◽

Horizontal Density Gradient ◽

Mixed Layers

Abstract Ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean. In this paper a parameterization for the restratification driven by finite-amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal. The streamfunction is proportional to the product of the horizontal density gradient, the mixed layer depth squared, and the inertial period. Hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence. In this paper the parameterization is theoretically motivated, confirmed to perform well for a wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. It is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification. Two companion papers discuss the numerical implementation and the climate impacts of this parameterization.

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Subduction over the Southern Indian Ocean in a High-Resolution Atmosphere–Ocean Coupled Model

Journal of Climate ◽

10.1175/2011jcli3888.1 ◽

2011 ◽

Vol 24 (15) ◽

pp. 3830-3849 ◽

Cited By ~ 17

Author(s):

Mei-Man Lee ◽

A. J. George Nurser ◽

I. Stevens ◽

Jean-Baptiste Sallée

Keyword(s):

Indian Ocean ◽

Mixed Layer ◽

Current System ◽

Mixed Layer Depth ◽

Coupled Model ◽

Horizontal Resolution ◽

Layer Depth ◽

Mode Water ◽

Coarse Resolution ◽

Winter Mixed Layer

Abstract This study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean–atmosphere coupled model in which the ocean component is eddy permitting. The purpose is to assess how sensitive the simulated mode water is to the horizontal resolution in the ocean by comparing with a coarse-resolution ocean coupled model. Subduction of water mass is principally set by the depth of the winter mixed layer. It is found that the path of the Agulhas Current system in the model with an eddy-permitting ocean is different from that with a coarse-resolution ocean. This results in a greater surface heat loss over the Agulhas Return Current and a deeper winter mixed layer downstream in the eddy-permitting ocean coupled model. The winter mixed layer depth in the eddy-permitting ocean compares well to the observations, whereas the winter mixed layer depth in the coarse-resolution ocean coupled model is too shallow and has the wrong spatial structure. To quantify the impacts of different winter mixed depths on the subduction, a way to diagnose local subduction is proposed that includes eddy subduction. It shows that the subduction in the eddy-permitting model is closer to the observations in terms of the magnitudes and the locations. Eddies in the eddy-permitting ocean are found to 1) increase stratification and thus oppose the densification by northward Ekman flow and 2) increase subduction locally. These effects of eddies are not well reproduced by the eddy parameterization in the coarse-resolution ocean coupled model.

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Winter Extreme Mixed Layer Depth South of the Kuroshio Extension

Journal of Climate ◽

10.1175/jcli-d-20-0119.1 ◽

2020 ◽

Vol 33 (24) ◽

pp. 10419-10436

Author(s):

Jingjie Yu ◽

Bolan Gan ◽

Zhao Jing ◽

Lixin Wu

Keyword(s):

Heat Flux ◽

Mixed Layer ◽

Kuroshio Extension ◽

Mixed Layer Depth ◽

Surface Cooling ◽

Layer Depth ◽

Mode Water ◽

Surface Warming ◽

Mechanical Stirring ◽

The Kuroshio

AbstractChange in the extratropical wintertime-mean mixed layer has been widely studied, given its importance to both physical and biogeochemical processes. With a focus on the south of the Kuroshio Extension region where the mixed layer is deepest in March, this study shows that variation of the synoptic-scale extreme mixed layer depth (MLD) is a better precursor than the monthly mean (or nonextreme) MLD for change in the subtropical mode water formation in spring, based on the NCEP Climate Forecast System Reanalysis (1979–2010). It is found that the extreme MLD events are attributable to the accumulation of excessive surface cooling driven by the synoptic storms that characterize cold-air outbreaks. Particularly, the difference between the extreme and nonextreme MLD is primarily related to differences in the cumulative synoptic heat flux anomalies, while a change in the preconditioning upper-ocean stratification contributes almost equally to both cases. Relative contributions of oceanic and atmospheric forcing to the interannual variation of the extreme MLD are quantified using a bulk mixed layer model. Results show comparable contributions: the preconditioning stratification change accounts for ~44% of total variance of the extreme MLD, whereas the convective mixing by surface heat flux and the mechanical stirring by wind stress account for ~35% and ~13%, respectively. In addition, both the reanalysis and observational data reveal that the extreme and nonextreme MLD has been shallowed significantly during 1979–2010, which is accounted for by the strengthened stratification due to the enhanced ocean surface warming by the Kuroshio heat transport.

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Role of the Seasonal Cycle in the Subduction Rates of Upper–Southern Ocean Waters

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-060.1 ◽

2013 ◽

Vol 43 (6) ◽

pp. 1096-1113 ◽

Cited By ~ 8

Author(s):

Eun Young Kwon ◽

Stephanie M. Downes ◽

Jorge L. Sarmiento ◽

Riccardo Farneti ◽

Curtis Deutsch

Keyword(s):

Exchange Rate ◽

Southern Ocean ◽

Layer Thickness ◽

Mixed Layer ◽

Temporal Change ◽

Mixed Layer Depth ◽

Layer Depth ◽

Mode Water ◽

Annual Cycles ◽

Mode Waters

Abstract A kinematic approach is used to diagnose the subduction rates of upper–Southern Ocean waters across seasonally migrating density outcrops at the base of the mixed layer. From an Eulerian viewpoint, the term representing the temporal change in the mixed layer depth (which is labeled as the temporal induction in this study; i.e., Stemp = ∂h/∂t where h is the mixed layer thickness, and t is time) vanishes over several annual cycles. Following seasonally migrating density outcrops, however, the temporal induction is attributed partly to the temporal change in the mixed layer thickness averaged over a density outcrop following its seasonally varying position and partly to the lateral movement of the outcrop position intersecting the sloping mixed layer base. Neither the temporal induction following an outcrop nor its integral over the outcrop area vanishes over several annual cycles. Instead, the seasonal eddy subduction, which arises primarily because of the subannual correlations between the seasonal cycles of the mixed layer depth and the outcrop area, explains the key mechanism by which mode waters are transferred from the mixed layer to the underlying pycnocline. The time-mean exchange rate of waters across the base of the mixed layer is substantially different from the exchange rate of waters across the fixed winter mixed layer base in mode water density classes. Nearly 40% of the newly formed Southern Ocean mode waters appear to be diapycnally transformed within the seasonal pycnocline before either being subducted into the main pycnocline or entrained back to the mixed layer through lighter density classes.

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Decadal Variability in the Formation of the North Pacific Subtropical Mode Water: Oceanic versus Atmospheric Control

Journal of Physical Oceanography ◽

10.1175/jpo2918.1 ◽

2006 ◽

Vol 36 (7) ◽

pp. 1365-1380 ◽

Cited By ~ 101

Author(s):

Bo Qiu ◽

Shuiming Chen

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Dynamic State ◽

Late Winter ◽

Layer Depth ◽

Mode Water ◽

The North ◽

Winter Mixed Layer ◽

Recirculation Gyre ◽

The North Pacific

Abstract In situ temperature and altimetrically derived sea surface height data are used to investigate the low-frequency variations in the formation of the North Pacific Ocean Subtropical Mode Water (STMW) over the past 12 yr. Inside the Kuroshio Extension (KE) recirculation gyre where STMW forms, the dominant signal is characterized by a gradual thinning in the late winter mixed layer depth and in the 16°–18°C thermostad layer from 1993 to 1999 and a subsequent steady thickening of these features after 2000. This same decadal signal is also seen in the low-potential-vorticity (PV) STMW layer in the interior subtropical gyre south of the recirculation gyre. By analyzing the air–sea flux data from the NCEP–NCAR reanalysis project, little correlation is found between the decadal STMW signal and the year-to-year changes in the cumulative wintertime surface cooling. In contrast, the decadal signal is found to be closely related to variability in the dynamic state of the KE system. Specifically, STMW formation is reduced when the KE path is in a variable state, during which time high regional eddy variability infuses high-PV KE water into the recirculation gyre, increasing the upper-ocean stratification and hindering the development of a deep winter mixed layer. A stable KE path, on the other hand, favors the maintenance of a weak stratification, leading to a deep winter mixed layer and formation of a thick STMW layer. The relative importance of the surface air–sea flux forcing versus the preconditioning stratification in controlling the variations in the late winter mixed layer depth is quantified using both a simple upper-ocean heat conservation model and a bulk mixed layer model. The majority of the variance (∼80%) is found to be due to the stratification changes controlled by the dynamic state of the KE system.

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