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

2011 ◽  
Vol 24 (15) ◽  
pp. 3830-3849 ◽  
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.

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

2006 ◽  
Vol 36 (7) ◽  
pp. 1365-1380 ◽  
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.

Interannual variability of the Tropical Indian Ocean mixed layer depth

2012 ◽  
Vol 40 (3-4) ◽  
pp. 743-759 ◽  
Author(s):  
M. G. Keerthi ◽  
M. Lengaigne ◽  
J. Vialard ◽  
C. de Boyer Montégut ◽  
P. M. Muraleedharan
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Tropical Indian Ocean ◽  
Ocean Mixed Layer ◽  
Layer Depth ◽  
Ocean Mixed Layer Depth

scholarly journals Observations of the Transition Layer

2009 ◽  
Vol 39 (3) ◽  
pp. 780-797 ◽  
Author(s):  
T. M. Shaun Johnston ◽  
Daniel L. Rudnick
Keyword(s):  
Layer Thickness ◽  
Mixed Layer ◽  
Potential Vorticity ◽  
Transition Layer ◽  
Mixed Layer Depth ◽  
Density Ratio ◽  
Horizontal Resolution ◽  
Surface Mixed Layer ◽  
Layer Depth ◽  
Vertical Displacements

Abstract The transition layer is the poorly understood interface between the stratified, weakly turbulent interior and the strongly turbulent surface mixed layer. The transition layer displays elevated thermohaline variance compared to the interior and maxima in current shear, vertical stratification, and potential vorticity. A database of 91 916 km or 25 426 vertical profiles of temperature and salinity from SeaSoar, a towed vehicle, is used to define the transition layer thickness. Acoustic Doppler current measurements are also used, when available. Statistics of the transition layer thickness are compared for 232 straight SeaSoar sections, which range in length from 65 to 1129 km with typical horizontal resolution of ∼4 km and vertical resolution of 8 m. Transition layer thicknesses are calculated in three groups from 1) vertical displacements of the mixed layer base and of interior isopycnals into the mixed layer; 2) the depths below the mixed layer depth of peaks in shear, stratification, and potential vorticity and their widths; and 3) the depths below or above the mixed layer depth of extrema in thermohaline variance, density ratio, and isopycnal slope. From each SeaSoar section, the authors compile either a single value or a median value for each of the above measures. Each definition yields a median transition layer thickness from 8 to 24 m below the mixed layer depth. The only exception is the median depth of the maximum isopycnal slope, which is 37 m above the mixed layer base, but its mode is 15–25 m above the mixed layer base. Although the depths of the stratification, shear, and potential vorticity peaks below the mixed layer are not correlated with the mixed layer depth, the widths of the shear and potential vorticity peaks are. Transition layer thicknesses from displacements and the full width at half maximum of the shear and potential vorticity peak give transition layer thicknesses from 0.11× to 0.22× the mean depth of the mixed layer. From individual profiles, the depth of the shear peak below the stratification peak has a median value of 6 m, which shows that momentum fluxes penetrate farther than buoyancy fluxes. A typical horizontal scale of 5–10 km for the transition layer comes from the product of the isopycnal slope and a transition layer thickness suggesting the importance of submesoscale processes in forming the transition layer. Two possible parameterizations for transition layer thickness are 1) a constant of 11–24 m below the mixed layer depth as found for the shear, stratification, potential vorticity, and thermohaline variance maxima and the density ratio extrema; and 2) a linear function of mixed layer depth as found for isopycnal displacements and the widths of the shear and potential vorticity peaks.

Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

2021 ◽  
Vol 34 (10) ◽  
pp. 3927-3953
Author(s):  
Motoki Nagura
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Surface Mixed Layer ◽  
The South ◽  
South Indian Ocean ◽  
Mode Water ◽  
Horizontal Advection ◽  
South Indian ◽  
Subantarctic Mode Water

AbstractThis study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

scholarly journals Study of the mixed layer depth variations within the north Indian Ocean using a 1-D model

2004 ◽  
Vol 109 (C8) ◽  
pp. n/a-n/a ◽  
Author(s):  
K. N. Babu ◽  
Rashmi Sharma ◽  
Neeraj Agarwal ◽  
Vijay K. Agarwal ◽  
R. A. Weller
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
North Indian Ocean ◽  
Layer Depth ◽  
The North ◽  
D Model

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

2008 ◽  
Vol 21 (20) ◽  
pp. 5254-5270 ◽  
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.

Drivers of oceanic exchange through the Indonesian Throughflow since the late 1800s – a synthesis of coral δ18O and high-resolution ocean models

2020 ◽  
Author(s):  
Sujata Murty ◽  
Caroline Ummenhofer ◽  
Markus Scheinert ◽  
Erik Behrens ◽  
Arne Biastoch ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Heat Content ◽  
Phase Changes ◽  
Boreal Winter ◽  
Indonesian Throughflow ◽  
Layer Depth ◽  
Ocean Variability ◽  
The Indian Ocean

<p>The Indonesian Throughflow (ITF) serves as an important oceanic teleconnection for Indo-Pacific climate, altering heat and buoyancy transport from the Pacific to the Indian Ocean. Equatorial Pacific wind forcing transmitted through the ITF impacts interannual to interdecadal Indian Ocean thermocline depth and heat content, with implications for preconditioning Indian Ocean Dipole events. Yet the modulation of Indian Ocean thermal properties at seasonal timescales is still poorly understood. Here we synthesize coral δ<sup>18</sup>O records, instrumental indices (El Niño Southern Oscillation (ENSO), Asian Monsoon), and simulated ocean variability (sea surface salinity (SSS) and temperature (SST), heat content, mixed layer depth) from state-of-the-art NEMO ocean model hindcasts to explore drivers of seasonal to multi-decadal variability. All coral sites are located within main ITF pathways and are influenced by monsoon-driven, buoyant South China Sea (SCS) surface waters during boreal winter that obstruct surface ITF flow and reduce heat transport to the Indian Ocean. Makassar and Lombok Strait coral δ<sup>18</sup>O co-varies with simulated SSS, subsurface heat content anomalies (50-350m) and mixed layer depth at the coral sites and in the eastern Indian Ocean. At decadal timescales, simulated boreal winter ocean variability at the coral sites additionally indicates a potential intensification of the SCS buoyancy plug from the mid- to late-20<sup>th</sup> century. Notably, the variability in these coral and model responses reveals sensitivity to phase changes in the Interdecadal Pacific Oscillation and the East Asian Winter Monsoon. These results collectively suggest that the paleoproxy records are capturing important features of regional hydrography and Indo-Pacific exchange, including responses to regional monsoon variability. Such proxy-model comparison is critical for understanding the drivers of variability related to changes in ITF oceanic teleconnections over the 19<sup>th</sup> and 20<sup>th</sup> centuries.</p>

scholarly journals Effects of the Non-breaking Surface Wave-induced Vertical Mixing on Winter Mixed Layer Depth in Subtropical Regions

2018 ◽  
Vol 123 (4) ◽  
pp. 2934-2944 ◽  
Author(s):  
Siyu Chen ◽  
Fangli Qiao ◽  
Chuanjiang Huang ◽  
Zhenya Song
Keyword(s):  
Surface Wave ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Layer Depth ◽  
Winter Mixed Layer ◽  
Wave Induced

Decadal Variation in Winter Mixed Layer Depth South of the Kuroshio Extension and Its Influence on Winter Mixed Layer Temperature

2016 ◽  
Vol 29 (3) ◽  
pp. 1237-1252 ◽  
Author(s):  
Shusaku Sugimoto ◽  
Shin’ichiro Kako
Keyword(s):  
Mixed Layer ◽  
North Pacific ◽  
Kuroshio Extension ◽  
Mixed Layer Depth ◽  
Positive Anomaly ◽  
Surface Cooling ◽  
Layer Depth ◽  
Mixed Layer Temperature ◽  
Winter Mixed Layer ◽  
The Kuroshio

Abstract The long-term behavior of the wintertime mixed layer depth (MLD) and mixed layer temperature (MLT) are investigated in a region south of the Kuroshio Extension (KE) (30°–37°N, 141°–155°E), an area of the North Pacific subtropical gyre where the deepest MLD occurs, using historical temperature profiles of 1968–2014. Both the MLD and MLT in March have low-frequency variations, which show significant decadal (~10 yr) variations after the late 1980s. Observational data and simulation outputs from a one-dimensional turbulent closure model reveal that surface cooling is the main control on winter MLD in the late 1970s and 1980s, whereas there is a change in the strength of subsurface stratification is the main control after ~1990. In the latter period, a weak (strong) subsurface stratification is caused by a straight path (convoluted path) of the KE and by a deepening (shallowing) of the main thermocline depth due to oceanic Rossby waves formed as a result of positive (negative) anomalies of wind stress curl associated with a southward (northward) movement of the Aleutian low in the central North Pacific. During deeper (shallower) periods of winter MLD, the strong (weak) vertical entrainment process, resulting from a rapid (slow) deepening of the mixed layer (ML) in January and February, forms a negative (positive) anomaly of temperature tendency. Consequently, the decadal variations in wintertime MLT are formed.

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