Summertime increases in upper-ocean stratification and mixed-layer depth

AbstractThe Argo dataset is used to study the winter upper-ocean conditions in the northeastern subtropical (NEA) Atlantic during 2006–12. During late winter 2010, the mixed layer depth is abnormally shallow and a negative anomaly of density-compensated salinity, the so-called spiciness, is generated in the permanent pycnocline. This is primarily explained by unusual weak air–sea buoyancy flux during the late winter 2010, in contrast with the five other studied winters. Particularly deep mixed layers and strong spiciness anomalies are observed during late winter 2012. The 2010 winter conditions appear to be related to historically low North Atlantic Oscillation (NAO) and high tropical North Atlantic index (TNA). Interannual variability of the eastern subtropical mixed layer is further investigated using a simple 1D bulk model of mean temperature and salinity linear profiles, based on turbulent kinetic energy conservation in the upper-ocean layer, and forced only with seasonal air–sea buoyancy forcing corresponding to fall–winter 2006–12. It suggests that year-to-year variability of the winter convective mixing driven by atmospheric buoyancy flux is able to generate interannual variability of both late winter mixed layer depth and spiciness in a strongly compensated layer at the base of the mixed layer and in the permanent pycnocline.

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Effects of high-frequency wind sampling on simulated mixed layer depth and upper ocean temperature

Journal of Geophysical Research Atmospheres ◽

10.1029/2004jc002746 ◽

2005 ◽

Vol 110 (C5) ◽

Cited By ~ 13

Author(s):

Tong Lee

Keyword(s):

Mixed Layer ◽

High Frequency ◽

Mixed Layer Depth ◽

Upper Ocean ◽

Ocean Temperature ◽

Layer Depth

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The Annual Cycle of Upper-Ocean Potential Vorticity and its Relationship to Submesoscale Instabilities

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0099.1 ◽

2020 ◽

Author(s):

Xiaolong Yu ◽

Alberto C. Naveira Garabato ◽

Adrian P. Martin ◽

David P. Marshall

Keyword(s):

Mixed Layer ◽

Potential Vorticity ◽

Gravitational Instability ◽

Mixed Layer Depth ◽

Upper Ocean ◽

Physical Processes ◽

Layer Depth ◽

Order Of Magnitude ◽

Mixed Layers

AbstractThe evolution of upper-ocean potential vorticity (PV) over a full year in a typical mid-ocean area of the Northeast Atlantic is examined using submesoscale- and mesoscale-resolving hydrographic and velocity measurements from a mooring array. A PV budget framework is applied to quantitatively document the competing physical processes responsible for deepening and shoaling the mixed layer. The observations reveal a distinct seasonal cycle in upper-ocean PV, characterized by frequent occurrences of negative PV within deep (up to about 350 m) mixed layers in winter to mid spring, and positive PV beneath shallow (mostly less than 50 m) mixed layers during the remainder of the year. The cumulative positive and negative subinertial changes in the mixed layer depth, which are largely unaccounted for by advective contributions, exceed the deepest mixed layer by one order of magnitude, suggesting that mixed layer depth is shaped by the competing effects of de-stratifying and re-stratifying processes. Deep mixed layers are attributed to persistent atmospheric cooling in winter to mid spring, which triggers gravitational instability leading to mixed layer deepening. However, on shorter time scales of days, conditions favourable to symmetric instability often occur as winds intermittently align with transient frontal flows. The ensuing submesoscale frontal instabilities are found to fundamentally alter upper-ocean turbulent convection, and limit the deepening of the mixed layer in the winter-to-mid-spring period. These results emphasize the key role of submesoscale frontal instabilities in determining the seasonal evolution of the mixed layer in the open ocean.

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The Annual Range of Southern Hemisphere SST: Comparison with Surface Heating and Possible Reasons for the High-Latitude Falloff*

Journal of Climate ◽

10.1175/2009jcli3154.1 ◽

2010 ◽

Vol 23 (8) ◽

pp. 1994-2009 ◽

Cited By ~ 1

Author(s):

A. M. Chiodi ◽

D. E. Harrison

Keyword(s):

Heat Flux ◽

Southern Hemisphere ◽

Mixed Layer ◽

Surface Heat Flux ◽

Seasonal Cycle ◽

Mixed Layer Depth ◽

Surface Heat ◽

Upper Ocean ◽

Layer Depth ◽

Annual Range

Abstract Globally, the seasonal cycle is the largest single component of observed sea surface temperature (SST) variability, yet it is still not fully understood. Herein, the degree to which the structure of the seasonal cycle of Southern Hemisphere SST can be explained by the present understanding of surface fluxes and upper-ocean physics is examined. It has long been known that the annual range of Southern Hemisphere SST is largest in the midlatitudes, despite the fact that the annual range of net surface heat flux peaks well poleward of the SST peak. The reasons for this discrepancy (“falloff of the annual range of SST”) are determined here through analysis of net surface heat flux estimates, observed SST, and mixed layer depth data, and results from experiments using two different one-dimensional ocean models. Results show that (i) the classical explanations for the structure of the annual range of SST in the Southern Hemisphere are incomplete, (ii) current estimates of surface heat flux and mixed layer depth can be used to accurately reproduce the observed annual range of SST, and (iii) the prognostic mixed layer models used here often fail to adequately reproduce the seasonal cycle at higher latitudes, despite performing remarkably well in other regions. This suggests that more work is necessary to understand the changes of upper-ocean dynamics that occur with latitude.

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