scholarly journals Ventilated Thermocline Strongly Affected by a Deep Mixed Layer: A Theory for Subtropical Countercurrent

1999 ◽  
Vol 29 (6) ◽  
pp. 1314-1333 ◽  
Author(s):  
Atsushi Kubokawa
Keyword(s):  
Mixed Layer ◽  
Subtropical Countercurrent ◽  
Deep Mixed Layer

scholarly journals Mixed layer variability and chlorophyll <i>a</i> biomass in the Bay of Bengal

2013 ◽  
Vol 10 (10) ◽  
pp. 16405-16452 ◽  
Author(s):  
J. Narvekar ◽  
S. Prasanna Kumar
Keyword(s):  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Chlorophyll Concentration ◽  
Short Wave ◽  
Wave Radiation ◽  
Layer Depth ◽  
Short Wave Radiation ◽  
Deep Mixed Layer ◽  
Monsoon Winds

Abstract. Mixed layer is the most variable and dynamically active part of the marine environment that couples the underlying ocean to the atmosphere and plays an important role in determining the chlorophyll concentration. In this paper we examined the seasonal variability of the mixed layer depth in the Bay of Bengal, the factors responsible for it and the coupling of mixed layer processes to the chlorophyll biomass using a suite of in situ as well as remote sensing data. The basin-wide mixed layer depth was the shallowest during spring intermonsoon, which was associated with strong themohaline stratification of the upper water column. The prevailing winds which were the weakest of all the seasons were unable to break the stratification leading to the observed shallow mixed layer. Consistent with the warm oligotrophic upper ocean, the surface chlorophyll concentrations were the least and the vertical profile of chlorophyll was characterized by a subsurface chlorophyll maximum. Similarly, during summer though the monsoon winds were the strongest they were unable to break the upper ocean haline-stratification in the northern Bay brought about by a combination of excess precipitation over evaporation and fresh water influx from rivers adjoining the Bay of Bengal. Consistent with this though the nitrate concentrations were high in the northern part of the Bay, the chlorophyll concentrations were low indicating the light limitation. In contrast, in the south, advection of high salinity waters from the Arabian Sea coupled with the westward propagating Rossby waves of annual periodicity were able to decrease stability of the upper water column and the prevailing monsoon winds were able to initiate deep mixing leading to the observed deep mixed layer. The high chlorophyll concentration observed in the south resulted from the positive wind stress curl which pumped nutrient rich subsurface waters to the euphotic zone. The southward extension of the shallow mixed layer in fall intermonsoon resulted from the advection of low salinity waters from the northern Bay combined with the secondary heating by the incoming short wave radiation. The satellite-derived chlorophyll pigment concentration during fall intermonsoon was similar to that of summer but with reduced values. The basin-wide deep mixed layer during winter resulted from a combination of reduced short wave radiation, increase in salinity and comparatively stronger winds. The mismatch between the low nitrate and comparatively higher chlorophyll biomass during winter indicated the efficacy of the limited nitrate data to adequately resolve the coupling between the mixed layer processes and the chlorophyll biomass.

Spatio-temporal variability of the thermodynamic characteristics of the marine atmospheric boundary layer (MABL) over the Indian and Southern Ocean (15oN to 70oS)

2021 ◽  
Author(s):  
Neha Salim ◽  
Harilal B Menon ◽  
Nadimpally V P Kiran Kumar
Keyword(s):  
Boundary Layer ◽  
Indian Ocean ◽  
Southern Ocean ◽  
Atmospheric Boundary Layer ◽  
Mixed Layer ◽  
Hadley Cell ◽  
Trade Wind ◽  
Marine Atmospheric Boundary Layer ◽  
Deep Mixed Layer ◽  
The Tropics

&lt;p&gt;The study deals with the thermodynamic characterization of marine atmospheric boundary layer (MABL) prevailing over regions of Indian Ocean and Indian Ocean sector of Southern Ocean from 29 high-resolution radiosondes launched during the International Indian Ocean Expedition (IIOE-2) and Southern Ocean Expedition (SOE-9). IIOE-2 was conducted during December 2015 onboard ORV Sagar Nidhi during which 11 radiosondes were launched, whereas SOE-9 was conducted during January-March 2017 onboard MV SA Agulhas which had 18 radiosonde ascents. These observations spanned latitudes from ~15&lt;sup&gt;o&lt;/sup&gt;N to 70&lt;sup&gt;o&lt;/sup&gt;S having crossed three major atmospheric circulation cells: Hadley cell, Ferrell cell and Polar cell. In addition, crucial atmospheric mesoscale phenomena such as inter-tropical convergence zone (ITCZ), sub-tropical jet (STJ) and polar jet (PJ) were encountered along with several oceanic fronts. Analysis of thermodynamic structure of MABL showed large variability in the formation of atmospheric sub-layers such as surface layer, mixed layer, cloud layer and trade wind inversion layer within MABL. MABL height varied spatially from tropics and mid-latitudes (12&lt;sup&gt;o&lt;/sup&gt;N to 50&lt;sup&gt;o&lt;/sup&gt;S) to polar latitudes (60&lt;sup&gt;o&lt;/sup&gt;S to 68&lt;sup&gt;o&lt;/sup&gt;S). Deep mixed layer were found over the tropics and mid-latitudes (~700 m) while shallow mixed layer was observed over the polar latitudes (~200 m). Deep mixed layer over the tropics were attributed to intense convective mixing while shallow mixed layer over polar regions was attributed to limited convective overturning associated with negative radiation balance at the surface. Convection was negligible over mid-latitudes (43&lt;sup&gt;o&lt;/sup&gt;S to 55&lt;sup&gt;o&lt;/sup&gt;S) where most of the atmospheric mixing were forced by frontal systems where lifting of air mass was mechanically driven by high speed winds rather than by convection. The enhanced convection over the tropics was confirmed from higher values of convective available potential energy (CAPE &gt; 1000 J/kg) and large negative values of convective inhibition energy (CINE &lt; -50 J/kg). Over the mid-latitude region (43&lt;sup&gt;o&lt;/sup&gt;S to 50&lt;sup&gt;o&lt;/sup&gt;S), enhanced advection and detrainment of convection was evident with maximum values of BRN shear (~65 knots) and lowest CAPE (~4 J/kg). Over polar latitudes (~60&lt;sup&gt;o&lt;/sup&gt;S to 68&lt;sup&gt;o&lt;/sup&gt;S), minimum CAPE (~17 J/kg) and low BRN shear (~5 knots) was noticed, which indicated presence of stable boundary layer conditions. A mesoscale phenomenon (i.e., ITCZ) was witnessed at ~5.92&lt;sup&gt;o&lt;/sup&gt;S with highest CAPE ~2535.17 J/kg which signifies large convective instability resulting in strong convective updraft aiding thunderstorm activity and moderate precipitation over ITCZ. Analysis of conserved variables (CVA) revealed formation of second mixed layer (SML) structure between 12&lt;sup&gt;o&lt;/sup&gt;N and 40&lt;sup&gt;o&lt;/sup&gt;S. However, south of 40&lt;sup&gt;o&lt;/sup&gt;S this structure ceases. The characteristics of SML structure and the plausible causes for its existence are also investigated. &amp;#160;&lt;/p&gt;

Submesoscale Currents in the Subtropical Upper Ocean Observed by Long-Term High-Resolution Mooring Arrays

2021 ◽  
Vol 51 (1) ◽  
pp. 187-206
Author(s):  
Zhiwei Zhang ◽  
Xincheng Zhang ◽  
Bo Qiu ◽  
Wei Zhao ◽  
Chun Zhou ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Source Term ◽  
Dominant Role ◽  
Baroclinic Instability ◽  
Northwestern Pacific ◽  
Subtropical Countercurrent ◽  
Vertical Heat Flux ◽  
Mixed Layers ◽  
Submesoscale Currents

AbstractAlthough observational efforts have been made to detect submesoscale currents (submesoscales) in regions with deep mixed layers and/or strong mesoscale kinetic energy (KE), there have been no long-term submesoscale observations in subtropical gyres, which are characterized by moderate values of both mixed layer depths and mesoscale KE. To explore submesoscale dynamics in this oceanic regime, two nested mesoscale- and submesoscale-resolving mooring arrays were deployed in the northwestern Pacific subtropical countercurrent region during 2017–19. Based on the 2 years of data, submesoscales featuring order one Rossby numbers, large vertical velocities (with magnitude of 10–50 m day−1) and vertical heat flux, and strong ageostrophic KE are revealed in the upper 150 m. Although most of the submesoscales are surface intensified, they are found to penetrate far beneath the mixed layer. They are most energetic during strong mesoscale strain periods in the winter–spring season but are generally weak in the summer–autumn season. Energetics analysis suggests that the submesoscales receive KE from potential energy release but lose a portion of it through inverse cascade. Because this KE sink is smaller than the source term, a forward cascade must occur to balance the submesoscale KE budget, for which symmetric instability may be a candidate mechanism. By synthesizing observations and theories, we argue that the submesoscales are generated through a combination of baroclinic instability in the upper mixed and transitional layers and mesoscale strain-induced frontogenesis, among which the former should play a more dominant role in their final generation stage.

scholarly journals A Mechanism of Mixed Layer Formation in the Indo–Western Pacific Southern Ocean: Preconditioning by an Eddy-Driven Jet-Scale Overturning Circulation

2017 ◽  
Vol 47 (11) ◽  
pp. 2755-2772 ◽  
Author(s):  
Qian Li ◽  
Sukyoung Lee
Keyword(s):  
Southern Ocean ◽  
Mixed Layer ◽  
Model Simulation ◽  
Mixed Layer Depth ◽  
Warm Season ◽  
Western Pacific ◽  
Overturning Circulation ◽  
Winter Mixed Layer ◽  
Deep Mixed Layer ◽  
Ekman Advection

AbstractThe formation of a narrow band of the deep winter mixed layer (hereinafter “mixed layer wedge”) in the Indo–western Pacific Southern Ocean is examined using an eddy-resolving Parallel Ocean Program (POP) model simulation. The mixed layer wedge starts to deepen in June, centered at 47.5°S, with a meridional scale of only ~2° latitude. Its center is located ~1° north of the model’s Subantarctic Front (SAF). The Argo-based observed mixed layer is similarly narrow and occurs adjacent to the observed SAF. In the small sector of 130°–142°E, where the SAF is persistent and the mixed layer is deepest, the formation of the narrow mixed layer wedge coincides with destratification underneath the mixed layer. This destratification can be attributed primarily to the downwelling branch of a jet-scale overturning circulation (JSOC). The JSOC, which was reported in an earlier study by the authors, is driven by eddy momentum flux convergence and is therefore thermally indirect: its descending branch occurs on the warmer equatorward flank of the SAF, promoting destratification during the warm season. The model-generated net air–sea heat flux reveals a similar wedge-like feature, indicating that the flux contributes to the mixed layer depth wedge, but again this feature is preconditioned by the JSOC. Ekman advection contributes to the formation of the mixed layer, but it occurs farther north of the region where the mixed layer initially deepens. These findings suggest that the eddy-driven JSOC associated with the SAF plays an important role in initiating the narrow, deep mixed layer wedge that forms north of the SAF.

scholarly journals Mixed layer variability and chlorophyll <i>a</i> biomass in the Bay of Bengal

2014 ◽  
Vol 11 (14) ◽  
pp. 3819-3843 ◽  
Author(s):  
J. Narvekar ◽  
S. Prasanna Kumar
Keyword(s):  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Remote Sensing Data ◽  
Ekman Pumping ◽  
Layer Depth ◽  
Basin Scale ◽  
Deep Mixed Layer ◽  
Northern Bay Of Bengal

Abstract. The mixed layer is the most variable and dynamically active part of the marine environment that couples the underlying ocean to the atmosphere and plays an important role in determining the oceanic primary productivity. We examined the basin-scale processes controlling the seasonal variability of mixed layer depth in the Bay of Bengal and its association with chlorophyll using a suite of in situ as well as remote sensing data. A coupling between mixed layer depth and chlorophyll was seen during spring intermonsoon and summer monsoon, but for different reasons. In spring intermonsoon the temperature-dominated stratification and associated shallow mixed layer makes the upper waters of the Bay of Bengal nutrient depleted and oligotrophic. In summer, although the salinity-dominated stratification in the northern Bay of Bengal shallows the mixed layer, the nutrient input from adjoining rivers enhance the surface chlorophyll. This enhancement is confined only to the surface layer and with increase in depth, the chlorophyll biomass decreases rapidly due to reduction in sunlight by suspended sediment. In the south, advection of high salinity waters from the Arabian Sea and westward propagating Rossby waves from the eastern Bay of Bengal led to the formation of deep mixed layer. In contrast, in the Indo–Sri Lanka region, the shallow mixed layer and nutrient enrichment driven by upwelling and Ekman pumping resulted in chlorophyll enhancement. The mismatch between the nitrate and chlorophyll indicated the inadequacy of present data to fully unravel its coupling to mixed layer processes.

scholarly journals Interannual to Decadal Variations of Submesoscale Motions around the North Pacific Subtropical Countercurrent

Fluids ◽  
2020 ◽  
Vol 5 (3) ◽  
pp. 116
Author(s):  
Hideharu Sasaki ◽  
Bo Qiu ◽  
Patrice Klein ◽  
Yoshikazu Sasai ◽  
Masami Nonaka
Keyword(s):  
Mixed Layer ◽  
Decadal Variability ◽  
Mixed Layer Depth ◽  
Northwestern Pacific ◽  
The North ◽  
Subtropical Countercurrent ◽  
The Pacific ◽  
Decadal Variations ◽  
The Impact ◽  
The North Pacific

The outputs from a submesoscale permitting hindcast simulation from 1990 to 2016 are used to investigate the interannual to decadal variations of submesoscale motions. The region we focus on is the subtropical Northwestern Pacific including the subtropical countercurrent. The submesoscale kinetic energy (KE) is characterized by strong interannual and decadal variability, displaying larger magnitudes in 1996, 2003, and 2015, and smaller magnitudes in 1999, 2009, 2010, and 2016. These variations are partially explained by those of the available potential energy (APE) release at submesoscale driven by mixed layer instability in winter. Indeed, this APE release depends on the mixed layer depth and horizontal buoyancy gradient, both of them modulated with the Pacific Decadal Oscillation (PDO). As a result of the inverse KE cascade, the submesoscale KE variability possibly leads to interannual to decadal variations of the mesoscale KE (eddy KE (EKE)). These results show that submesoscale motions are a possible pathway to explain the impact associated with the PDO on the decadal EKE variability. The winter APE release estimated from the Argo float observations varies synchronously with that in the simulation on the interannual time scales, which suggests the observation capability to diagnose the submesoscale KE variability.

TEMPERATURE VARIABILITY AT SENUNU BAY, WEST SUMBAWA

2013 ◽  
Vol 5 (2) ◽  
Author(s):  
Syamsul Hidayat ◽  
Mulia Purba ◽  
Jorina Waworuntu
Keyword(s):  
Layer Thickness ◽  
Mixed Layer ◽  
Cross Correlation ◽  
Sea Surface ◽  
Thermocline Depth ◽  
Kelvin Wave ◽  
The West ◽  
Sea Bottom ◽  
Annual Period ◽  
Regional Processes

The purposes of this study were to determine the variability of temperature and its relation to regional processes in the Senunu Bay. The result showed clear vertical stratifications i.e., mixed layer thickness about 39-119 m with isotherm of 27°C, thermocline layer thickness about 83-204 m with isotherm of 14–26°C, and  the deeper layer from the thermocline lower limit to the sea bottom with isotherm <13°C. Temperature and the thickness of each layers varied with season in which during the Northwest Monsoon the temperature was warmer and the mixed layer was thicker than those during Southeast Monsoon. During Southeast Monsoon, the thermocline layer rose  about 24 m. The 2001, 2006, and 2009 (weak La Nina years),  the Indonesia Throughflow (ITF) carried warmer water, deepening thermocline depth and reducing upwelling strength.  In 2003 and 2008 thickening of mixed layer occurred in transition season  was believed  associated with the  arrival of Kelvin Wave from the west. In 2002 and 2004 (weak El Nino period,) ITF carries colder water shallowing thermocline depth and enhancing upwelling strength. In 2007 was believed to be related with positive IODM where the sea surface temperature were decreasing due to intensification of southeast wind which induced strong upwelling. The temperature spectral density of mixed layer and thermocline was influenced by annual, semi-annual, intra-annual and inter-annual period fluctuations. The cross-correlation between wind and temperature showed significant value in the annual period.  Keywords: temperature, thermocline, variability, ENSO, IODM.

DEMINERALIZATION OF WATER WITH MIXED-LAYER ION-EXCHANGERS

2013 ◽  
Vol 12 (1) ◽  
pp. 137-145 ◽  
Author(s):  
Iuliana Rogoveanu Radosavlevici ◽  
Dan Niculae Robescu
Keyword(s):  
Mixed Layer ◽  
Ion Exchangers
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