The Dynamics of Mixed Layer Deepening during Open-Ocean Convection

AbstractOpen-ocean convection is a common phenomenon that regulates mixed layer depth and ocean ventilation in the high-latitude oceans. However, many climate model simulations overestimate mixed layer depth during open-ocean convection, resulting in excessive formation of dense water in some regions. The physical processes controlling transient mixed layer depth during open-ocean convection are examined using two different numerical models: a high-resolution, turbulence-resolving nonhydrostatic model and a large-scale hydrostatic ocean model. An isolated destabilizing buoyancy flux is imposed at the surface of both models and a quasi-equilibrium flow is allowed to develop. Mixed layer depth in the turbulence-resolving and large-scale models closely aligns with existing scaling theories. However, the large-scale model has an anomalously deep mixed layer prior to quasi-equilibrium. This transient mixed layer depth bias is a consequence of the lack of resolved turbulent convection in the model, which delays the onset of baroclinic instability. These findings suggest that in order to reduce mixed layer biases in ocean simulations, parameterizations of the connection between baroclinic instability and convection need to be addressed.

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Impact of Explicit Atmosphere–Ocean Coupling on MJO-Like Coherent Structures in Idealized Aquaplanet Simulations

Journal of the Atmospheric Sciences ◽

10.1175/jas3740.1 ◽

2006 ◽

Vol 63 (9) ◽

pp. 2289-2306 ◽

Cited By ~ 30

Author(s):

Wojciech W. Grabowski

Keyword(s):

Mixed Layer ◽

General Circulation ◽

Large Scale ◽

Mixed Layer Depth ◽

Transfer Model ◽

Quasi Equilibrium ◽

Layer Depth ◽

Oceanic Mixed Layer ◽

Ocean Coupling ◽

The Impact

Abstract This paper discusses the impact of the atmosphere–ocean coupling on the large-scale organization of tropical convection simulated by an idealized global model applying the Cloud-Resolving Convection Parameterization (CRCP; superparameterization). Because the organization resembles the Madden–Julian Oscillation (MJO), the results contribute to the debate concerning the role of atmosphere–ocean coupling in tropical intraseasonal oscillations. The modeling setup is an aquaplanet with globally uniform mean sea surface temperature (SST) of 30°C (tropics everywhere) in radiative–convective quasi equilibrium. The simulations apply an interactive radiation transfer model and a slab ocean model with a fixed oceanic mixed layer depth. Results from several 80- and 100-day-long simulations are discussed, where the only difference between the simulations is the prescribed oceanic mixed layer depth, which varied from 5 to 45 m. A simulation with a very deep oceanic mixed layer is also performed to represent constant-SST conditions. The simulations demonstrate that the interactive SST impedes the development of large-scale organization and has insignificant impact on the dynamics of mature MJO-like systems. The impediment is the result of a negative feedback between the large-scale organization of convection and SST, the convection–SST feedback. In this feedback, SST increases in regions of already suppressed convection and decreases in regions with enhanced convection, thus hindering the large-scale organization. Once developed, however, the MJO-like systems are equally strong in interactive and constant-SST simulations, and compare favorably with the observed MJO. The above impacts of the atmosphere–ocean coupling contradict the majority of previous studies using traditional general circulation models, where, typically, an enhancement of the intraseasonal signal occurs compared to prescribed-SST simulations. An explanation of this discrepancy is suggested.

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Influence of Large-Scale Initial Oceanic Mixed Layer Depth on Tropical Cyclones*

Monthly Weather Review ◽

10.1175/1520-0493(2000)129<4058:iolsio>2.0.co;2 ◽

2000 ◽

Vol 128 (12) ◽

pp. 4058-4070 ◽

Cited By ~ 23

Author(s):

Qi Mao ◽

Simon W. Chang ◽

Richard L. Pfeffer

Keyword(s):

Tropical Cyclones ◽

Mixed Layer ◽

Large Scale ◽

Mixed Layer Depth ◽

Layer Depth ◽

Oceanic Mixed Layer

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Ocean–Atmosphere Coupling in the Monsoon Intraseasonal Oscillation: A Simple Model Study

Journal of Climate ◽

10.1175/2008jcli2305.1 ◽

2008 ◽

Vol 21 (20) ◽

pp. 5254-5270 ◽

Cited By ~ 29

Author(s):

Gilles Bellon ◽

Adam H. Sobel ◽

Jerome Vialard

Keyword(s):

Mixed Layer ◽

Bay Of Bengal ◽

Mixed Layer Depth ◽

Intraseasonal Oscillation ◽

Thermal Inertia ◽

Coupled Model ◽

Boreal Winter ◽

Quasi Equilibrium ◽

Layer Depth ◽

Ocean Atmosphere

Abstract A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the effect of ocean–atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model consists of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled to a simple, linear model of the upper ocean. This model has one unstable eigenmode with a period in the 30–60-day range and a structure similar to the observed northward-propagating intraseasonal oscillation in the Bay of Bengal/west Pacific sector. The ocean–atmosphere coupling is shown to have little impact on either the growth rate or latitudinal structure of the atmospheric oscillation, but it reduces the oscillation’s period by a quarter. At latitudes corresponding to the north of the Indian Ocean, the sea surface temperature (SST) anomalies lead the precipitation anomalies by a quarter of a period, similarly to what has been observed in the Bay of Bengal. The mixed layer depth is in phase opposition to the SST: a monsoon break corresponds to both a warming and a shoaling of the mixed layer. This behavior results from the similarity between the patterns of the predominant processes: wind-induced surface heat flux and wind stirring. The instability of the seasonal monsoon flow is sensitive to the seasonal mixed layer depth: the oscillation is damped when the oceanic mixed layer is thin (about 10 m deep or thinner), as in previous experiments with several models aimed at addressing the boreal winter Madden–Julian oscillation. This suggests that the weak thermal inertia of land might explain the minima of intraseasonal variance observed over the Asian continent.

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Modeling submerged biofouled microplastics and their vertical trajectories

10.5194/bg-2021-236 ◽

2021 ◽

Author(s):

Reint Fischer ◽

Delphine Lobelle ◽

Merel Kooi ◽

Albert Koelmans ◽

Victor Onink ◽

...

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Euphotic Zone ◽

Equatorial Pacific ◽

Open Ocean ◽

Subtropical Gyre ◽

Layer Depth ◽

High Biological Activity ◽

Near Surface

Abstract. The fate of (micro)plastic particles in the open ocean is controlled by physical and biological processes. Here, we model the effects of biofouling on the subsurface vertical distribution of spherical, virtual plastic particles with radii of 0.01–1 mm. For the physics, four vertical velocity terms are included: advection, wind-driven mixing, tidally induced mixing, and the sinking velocity of the biofouled particle. For the biology, we simulate the attachment, growth and loss of algae on particles. We track 10,000 particles for one year in three different regions with distinct biological and physical properties: the low productivity region of the North Pacific Subtropical Gyre, the high productivity region of the Equatorial Pacific and the high mixing region of the Southern Ocean. The growth of biofilm mass in the euphotic zone and loss of mass below the euphotic zone result in the oscillatory behaviour of particles, where the larger (0.1–1.0 mm) particles have much shorter average oscillation lengths (< 10 days; 90th percentile) than the smaller (0.01–0.1 mm) particles (up to 130 days; 90th percentile). A subsurface maximum concentration occurs just below the mixed layer depth (around 30 m) in the Equatorial Pacific, which is most pronounced for larger particles (0.1–1.0 mm). This occurs since particles become neutrally buoyant when the processes affecting the settling velocity of the particle and the motion of the ocean are in equilibrium. Seasonal effects in the subtropical gyre result in particles sinking below the mixed layer depth only during spring blooms, but otherwise remaining within the mixed layer. The strong winds and deepest average mixed layer depth in the Southern Ocean (400 m) result in the deepest redistribution of particles (> 5000 m). Our results show that the vertical movement of particles is mainly affected by physical (wind-induced mixing) processes within the mixed layer and biological (biofilm) dynamics below the mixed layer. Furthermore, positively buoyant particles with radii of 0.01–1.0 mm can sink far below the euphotic zone and mixed layer in regions with high near-surface mixing or high biological activity. This work can easily be coupled to other models to simulate open-ocean biofouling dynamics, in order to reach a better understanding of where ocean (micro)plastic ends up.

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Mesoscale and Submesoscale Effects on Mixed Layer Depth in the Southern Ocean

Journal of Physical Oceanography ◽

10.1175/jpo-d-17-0034.1 ◽

2017 ◽

Vol 47 (9) ◽

pp. 2173-2188 ◽

Cited By ~ 16

Author(s):

S. D. Bachman ◽

J. R. Taylor ◽

K. A. Adams ◽

P. J. Hosegood

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Large Scale ◽

Mixed Layer Depth ◽

Surface Mixed Layer ◽

Grid Spacing ◽

Layer Depth ◽

State Estimate ◽

Nested Models ◽

Significant Difference

AbstractSubmesoscale dynamics play a key role in setting the stratification of the ocean surface mixed layer and mediating air–sea exchange, making them especially relevant to anthropogenic carbon uptake and primary productivity in the Southern Ocean. In this paper, a series of offline-nested numerical simulations is used to study submesoscale flow in the Drake Passage and Scotia Sea regions of the Southern Ocean. These simulations are initialized from an ocean state estimate for late April 2015, with the intent to simulate features observed during the Surface Mixed Layer at Submesoscales (SMILES) research cruise, which occurred at that time and location. The nested models are downscaled from the original state estimate resolution of 1/12° and grid spacing of about 8 km, culminating in a submesoscale-resolving model with a resolution of 1/192° and grid spacing of about 500 m. The submesoscale eddy field is found to be highly spatially variable, with pronounced hot spots of submesoscale activity. These areas of high submesoscale activity correspond to a significant difference in the 30-day average mixed layer depth between the 1/12° and 1/192° simulations. Regions of large vertical velocities in the mixed layer correspond with high mesoscale strain rather than large . It is found that is well correlated with the mesoscale density gradient but weakly correlated with both the mesoscale kinetic energy and strain. This has implications for the development of submesoscale eddy parameterizations that are sensitive to the character of the large-scale flow.

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Mixed layer depth calculation in deep convection regions in ocean numerical models

Ocean Modelling ◽

10.1016/j.ocemod.2017.10.007 ◽

2017 ◽

Vol 120 ◽

pp. 60-78 ◽

Cited By ~ 17

Author(s):

Peggy Courtois ◽

Xianmin Hu ◽

Clark Pennelly ◽

Paul Spence ◽

Paul G. Myers

Keyword(s):

Mixed Layer ◽

Numerical Models ◽

Deep Convection ◽

Mixed Layer Depth ◽

Layer Depth

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Global open-ocean biomes: mean and temporal variability

Earth System Science Data ◽

10.5194/essd-6-273-2014 ◽

2014 ◽

Vol 6 (2) ◽

pp. 273-284 ◽

Cited By ~ 52

Author(s):

A. R. Fay ◽

G. A. McKinley

Keyword(s):

Large Scale ◽

Mixed Layer Depth ◽

Open Ocean ◽

Sea Surface ◽

Data Sets ◽

Layer Depth ◽

Future Studies ◽

Chl A ◽

Complex Boundaries ◽

Ice Fraction

Abstract. Large-scale studies of ocean biogeochemistry and carbon cycling have often partitioned the ocean into regions along lines of latitude and longitude despite the fact that spatially more complex boundaries would be closer to the true biogeography of the ocean. Herein, we define 17 open-ocean biomes classified from four observational data sets: sea surface temperature (SST), spring/summer chlorophyll a concentrations (Chl a), ice fraction, and maximum mixed layer depth (maxMLD) on a 1° × 1° grid (available at doi:10.1594/PANGAEA.828650). By considering interannual variability for each input, we create dynamic ocean biome boundaries that shift annually between 1998 and 2010. Additionally we create a core biome map, which includes only the grid cells that do not change biome assignment across the 13 years of the time-varying biomes. These biomes can be used in future studies to distinguish large-scale ocean regions based on biogeochemical function.

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