Two Different Descriptions of a Thermocline in the Persian Gulf
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Abstract The Persian Gulf (PG) is a shallow sea connected to the rest of the world by the Strait of Hormuz. Temperature changes in the water column, which indicate the thermocline, are typically explained by the depth of the mixed layer and the thermocline. The thermocline is caused by a sudden decrease in temperature in the water column's subsurface layer, resulting in stratification in the PG from winter to summer. The parameters are approximated numerically through the Princeton Ocean Modeling (POM) method and compared to those determined by some CTD profiles collected in the PG. The most obvious method for approximating thermocline depth is to find the maximum negative slope \(\frac{\partial T}{\partial z}\) in a temperature profile. The method produces applied results with sufficient depth resolution and smooth temperature changes with depth. This method is a component of the Princeton Ocean Modeling (POM) framework for numerically modeling temperature variation in the water basins used in this study. The depth of the mixed layer is approximated by the surface equality temperature (Sea Surface Temperature), regardless of the thermocline approximation. The variable isotherm behavior accurately approximates the thermocline depth. Thermocline formation occurs in the PG during the summer, and this article will conclude using two methods, observational and numerical modeling.Keywords:
Mixed layer
Stratification (seeds)
Mixed layer
Entrainment (biomusicology)
Surface layer
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Intense cooling of the sea surface at intraseasonal time scales takes place in the southern tropical Indian Ocean during austral summer. Mechanisms that cause intraseasonal cooling are investigated using in situ observations by Argo floats, remote sensing data sets and simulation using a high resolution ocean general circulation model of the Indian Ocean. Temperature profiles from Argo floats within the cooling region show evidences for the entrainment of cool thermocline water into the mixed layer. The cooling events are accompanied by an increase in sea surface chlorophyll concentration which provides additional evidence for entrainment. The Indian Ocean model reproduces intraseasonal cooling events and demonstrates that mixed layer cooling originate from vertical processes at the base of the mixed layer for certain events. The air‐sea flux tends to be the dominant cooling mechanism when the thermocline is deep and mixed layer is thick whereas entrainment dominates when the thermocline is shallow.
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Mixed layer
Entrainment (biomusicology)
Stratification (seeds)
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Summertime surface heat flux and upper ocean state in 2004, 2005, and 2006 obtained from the Kuroshio Extension Observatory (KEO) buoy were investigated, focusing on the summertime preconditioning of the following winter's mixed layer. Summertime net shortwave radiation at the surface shows large year‐to‐year variations that resulted in anomalous heating in 2005 and anomalous cooling in 2006. Covariation of the surface heat flux and upper ocean stratification was found and suggests that year‐to‐year variations of summertime heat flux induce corresponding changes in the near surface stratification. Cold core rings, observed in 2006, tend to intensify both the near surface (<100 m depth) density stratification and the density stratification below the seasonal thermocline (>100 m depth). Lateral and vertical heat fluxes evaluated from the imbalance between the observed heat storage rate and the net heat flux and entrainment also have a significant role in determination of upper ocean stratification and can intensify year‐to‐year variation of the mixed layer. The physical mechanism that determines the precondition of the next winter mixed layer can change each year. In 2005, near surface stratification induced by anomalous summertime heating has a dominant role compared to deeper stratification. On the other hand, in 2006, the much deeper stratification below the seasonal thermocline (>100 m depth) associated with cold core rings contributes to make the maximum vertical density stratification.
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Buoy
Shortwave radiation
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Abstract Decadal variability of the North Pacific subtropical mode water (STMW) and its influence on the upper thermocline and mixed layer are examined in a four-dimensional variational ocean re-analysis for the Western North Pacific over 30 years (FORA–WNP30). The STMW that forms south of the Kuroshio Extension becomes thick/cold and thin/warm on decadal timescales. These variations are subducted and advected to the south, where thick (thin) STMW causes the upper thermocline to heave up (down) above the STMW, producing cold (warm) temperature anomalies at subsurface depths, with especially large anomalies at the depths of the seasonal thermocline. Temperature anomalies also appear in the mixed layer from March to November, except in September. These anomalies have the same sign as the temperature anomalies of the STMW, although they are due not to the reemergence of the STMW at the surface but to the heaving of the upper thermocline. In the FORA–WNP30, because the formation of the mixed layer temperature anomalies owes much to the increment introduced by data assimilation, the mechanism remains unclear. A heat budget analysis of the mixed layer, however, suggests the importance of entrainment and/or vertical diffusion at the base of the mixed layer for conveying temperature anomalies from the upper thermocline to the mixed layer. The STMW also affects the mixed layer depth. A thick (thin) STMW shoals (deepens) the seasonal thermocline, enhancing (weakening) stratification at depths below the mixed layer and thus hindering (favoring) mixed layer development from July to September.
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Mode water
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Shoal
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The Mellor‐Yamada level‐2 and 2 1/2, Niiler, and Garwood one‐dimensional mixed‐layer models were compared for some simple forcing experiments and were tested by simulating changes in the mixed layer at ocean stations November and Papa for the year 1961. The ocean station simulations show that the models can be tuned to give fairly good results. However, the need to readjust model constants when changing locations suggests that there is room for improving some of the mixing parameterizations. The sensitivity of the model simulations to certain “external” parameterizations, including surface heat flux, seawater turbidity, and ambient diffusivity below the mixed layer, was investigated. The year‐long simulations were found to be very sensitive to the seawater turbidity. Increasing the turbidity from Jerlov optical type I to type III causes a maximum increase in the monthly mean SST at November of 3°C or more. The simulations are most sensitive to seawater turbidity during summer when the mixed‐layer remains shallow. For the ambient diffusivity a decrease from 0.4 to 0.01 cm 2 /s results in a maximum increase in the monthly mean SST at November and Papa of 0.5 and 1.5°C, respectively. The effects of using a constant ambient diffusivity are most noticeable in late summer when the seasonal thermocline is strongest. The effects are larger at Papa than at November because of the stronger summer seasonal thermocline at Papa. Best results at November and Papa were obtained by using the observed optical water types (I and II, respectively) and relatively small values for ambient diffusivity (less than 0.2 cm 2 /s).
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Turbidity
Forcing (mathematics)
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Abstract Submesoscale turbulence in the upper ocean consists of fronts, filaments, and vortices that have horizontal scales on the order of 100 m to 10 km. High-resolution numerical simulations have suggested that submesoscale turbulence is associated with strong vertical motion that could substantially enhance the vertical exchange between the thermocline and mixed layer, which may have an impact on marine ecosystems and climate. Theoretical, numerical, and observational work indicates that submesoscale turbulence is energized primarily by baroclinic instability in the mixed layer, which is most vigorous in winter. This study demonstrates how such mixed layer baroclinic instabilities induce vertical exchange by drawing filaments of thermocline water into the mixed layer. A scaling law is proposed for the dependence of the exchange on environmental parameters. Linear stability analysis and nonlinear simulations indicate that the exchange, quantified by how much thermocline water is entrained into the mixed layer, is proportional to the mixed layer depth, is inversely proportional to the Richardson number of the thermocline, and increases with increasing Richardson number of the mixed layer. The results imply that the tracer exchange between the thermocline and mixed layer is more efficient when the mixed layer is thicker, when the mixed layer stratification is stronger, when the lateral buoyancy gradient is stronger, and when the thermocline stratification is weaker. The scaling suggests vigorous exchange between the permanent thermocline and deep mixed layers in winter, especially in mode water formation regions. Significance Statement This study examines how instabilities in the surface layer of the ocean bring interior water up from below. This interior–surface exchange can be important for dissolved gases such as carbon dioxide and oxygen as well as nutrients fueling biological growth in the surface ocean. A scaling law is proposed for the dependence of the exchange on environmental parameters. The results of this study imply that the exchange is particularly strong if the well-mixed surface layer is thick, lateral density gradients are strong (such as at fronts), and the stratification below the surface layer is weak. These theoretical findings can be implemented in boundary layer parameterization schemes in global ocean models and improve our understanding of the marine ecosystem and how the ocean mediates climate change.
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Richardson number
Internal tide
Ocean dynamics
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A model of the ventilated thermocline consisting of three adiabatic layers surmounted by a mixed layer of finite thickness is presented. The mixed-layer depth density increase continuously northward, and these attributes of the mixed layer are specified. The effect of the mixed layer on the thermocline circulation is explicitly calculated. The mixed-layer thickness and its variation play a significant role in shifting the trajectories of the streamlines westward. The shadow zones enlarge more rapidly south of the outcrop lines. The finite mixed-layer depth and its increase northward produce shadow zones in each of the adiabatic layers of the thermocline. The augmentation of the ventilation rate, i.e., the rate at which fluid enters the thermocline from the mixed layer in excess of the Ekman pumping is directly proportional to the northward gradient of the mixed-layer potential vorticity. The enhancement is greatest in regions where the circulation is surface-intensified i.e., above the eastern shadow zones.
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Ekman transport
Circulation (fluid dynamics)
Ekman layer
Finite thickness
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Hypoxia in Lake Biwa, Japan remains a serious water environmental problem. One of the causes of hypoxia in the lake is the formation of a thermocline, which is largely affected by meteorological factors, such as (1) air temperature, (2) wind speed, and (3) precipitation. However, the effects of these three meteorological factors on the formation of the thermocline have not been clarified quantitatively. In this study, applying a three-dimensional hydrodynamic model to Lake Biwa, the effects of each of the three meteorological elements on the formation of the thermocline was quantitatively analyzed to clarify the governing factors of meteorological conditions in the formation of anoxic oxygen. Sensitivity analysis of the stratification structure in Lake Biwa was performed by changing the three meteorological factors of (1) air temperature, (2) wind speed, and (3) precipitation. As a result, the change in wind speed gives the greatest effect on the stratification structure, the change in air temperature makes the difference in the stratification structure from the surface layer to the vicinity of the thermocline, and the change in precipitation affects it less than the others.
Stratification (seeds)
Hypoxia
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Stratification (seeds)
Hypoxia
Sedimentation
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