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Zhengyu Liu - One of the best experts on this subject based on the ideXlab platform.
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Thermocline Forced by Varying Ekman Pumping. Part II: Annual and Decadal Ekman Pumping
Journal of Physical Oceanography, 1993Co-Authors: Zhengyu LiuAbstract:Abstract Thermocline variability forced by zonally uniform Ekman Pumping with annual to decadal periods is investigated. Both analytical and numerical solutions are obtained by the method of characteristics. As found in Part I, there is little thermocline variability in the ventilated zone or pool zone. In contrast, strong variability may exist in the shadow zone. For annual forcings, nonlinearity is negligible. However, the linear solution is influenced substantially by the basic-state thermocline structure. As a result, local responses dominate for a shallow interface, while remote Rossby waves dominate for a deep interface. Under a strong decadal forcing, nonlinearity may become important. The time-mean thermocline in the shadow zone is shallower than the steady thermocline under the mean Ekman Pumping, particularly in the western part of a shadow zone where the mean deviation may reach the order often meters. This shallower mean thermocline is caused by the nonlinear Rossby wave.
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Thermocline Forced by Varying Ekman Pumping. Part I: Spinup and Spindown
Journal of Physical Oceanography, 1993Co-Authors: Zhengyu LiuAbstract:Abstract A two-layer planetary geostrophic model is used to investigate the thermocline variability under a suddenly changing Ekman Pumping. The effect of ventilation and the associated advection is particularly emphasized in the ventilated zone. The governing equation is a quasi-linear equation, which is solved analytically by the method of characteristics. It is found that the dynamics differs substantially between a shadow zone and a ventilated zone. In the shadow zone, the Rossby wave is the dominant mechanism to balance the Ekman Pumping. After a sudden change in the wind field, the Ekman Pumping changes rapidly, but the baroclinic Rossby wave evolves at a much slower time scale (years to decades). This mismatch of response time scale produces an imbalance in forcings and in turn results in a strong thermocline variability. However, in the ventilated zone, the cold advection replaces the Rossby wave to become the major opposing mechanism to the Ekman Pumping. After a sudden wind change, both the Ekma...
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Interannual planetary wave breaking in the presence of Ekman Pumping and mean flow
Journal of Fluid Mechanics, 1993Co-Authors: Zhengyu LiuAbstract:A two-layer planetary geostrophic model is adopted to study the breaking of planetary waves in the presence of Ekman Pumping and the associated mean flow. The governing equation for the interface is a quasi-linear equation, which is solved analytically by the method of characteristics. The waves are forced by annual or interannual upwelling or downwelling along the eastern boundary of a subtropical gyre. It is found that the time and position at which breaking occurs is mainly determined by the speed and depth of the eastern boundary perturbation, while the intensity of a breaking front is mainly determined by the amplitude of the perturbation.
Claude Frankignoul - One of the best experts on this subject based on the ideXlab platform.
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Simulated Variability of the Circulation in the North Atlantic from 1953 to 2003
Journal of Climate, 2008Co-Authors: Julie Deshayes, Claude FrankignoulAbstract:Abstract The variability of the circulation in the North Atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003. The interannual-to-decadal variability of the subpolar gyre circulation and the Meridional Overturning Circulation (MOC) is mostly influenced by the North Atlantic Oscillation (NAO). Both circulations intensified from the early 1970s to the mid-1990s and then decreased. The monthly variability of both circulations reflects the fast barotropic adjustment to NAO-related Ekman Pumping anomalies, while the interannual-to-decadal variability is due to the baroclinic adjustment to Ekman Pumping, buoyancy forcing, and dense water formation, consistent with previous studies. An original characteristic of the oceanic response to NAO is presented that relates to the spatial patterns of buoyancy and wind forcing over the North Atlantic. Anomalous Ekman Pumping associated with a positive NAO phase first induces a decrease of the southern sub...
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Variability of the Thermocline Due to a Sudden Change in the Ekman Pumping
Journal of Physical Oceanography, 2000Co-Authors: Jérôme Sirven, Claude FrankignoulAbstract:The thermocline variability caused by a sudden variation of the Ekman Pumping is studied, using a 2.5-layer geostrophic model that represents the ventilated and the shadow zones of a subtropical gyre and baroclinic Rossby wave propagation. During spinup the propagation of the first baroclinic mode induces a large deepening of the thermocline on a timescale ranging between 2 and 15 yr, depending on latitude. The propagation is similar throughout the basin, and is not influenced by the geostrophic flow, save nonlinearly through the variation in layer depth. South of the subduction line, the adjustment is completed by the second baroclinic mode. In the ventilated zone, the latter is not very active, and there are only smaller (by a factor of 5) variations of the thermocline depth primarily linked to a slight imbalance between Ekman Pumping and vertically averaged meridional advection. In the shadow zone, the second baroclinic mode plays a more important role since it primarily balances the Ekman Pumping, although the variations of the second layer depth remain smaller than in the ventilated area. Spindown induces similarly a shoaling of the thermocline, but, because of nonlinearities, lower Rossby wave speeds, and decreased advection, the adjustment is everywhere slower than during spinup.
Albert S Fischer - One of the best experts on this subject based on the ideXlab platform.
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the upper ocean response to monsoonal forcing in the arabian sea seasonal and spatial variability
Deep-sea Research Part Ii-topical Studies in Oceanography, 2000Co-Authors: Craig M Lee, Burton H Jones, Kenneth H Brink, Albert S FischerAbstract:Abstract Observations from four towed profiler surveys undertaken between December 1994 and October 1995 examine the seasonal and spatial variability of the upper ocean response to the Monsoon cycle in the Arabian Sea. Although observed atmospheric forcing agrees well with modern climatologies, cross-basin patterns of mixed-layer depth and water properties observed in 1994–1995 are not entirely consistent with an upper-ocean response dominated by Ekman Pumping. During the winter monsoon, the mixed-layer deepens dramatically with distance offshore. Surface cooling intensifies with offshore distance, and a one-dimensional response dominated by convective overturning could explain observed wintertime mixed-layer depths. Except for waters associated with a filament extending offshore from the Omani coast, mixed-layer depths and water properties show only modest cross-basin contrasts during the Southwest Monsoon. Filament waters differ from surrounding mid-basin waters, having shallow mixed-layers and water properties similar to those of waters upwelled near the Omani coast. In September, following the Southwest Monsoon, waters within 1000 km of the Omani coast have cooled and freshened, with marked changes in stratification extending well into the pycnocline. Estimates of Ekman Pumping and wind-driven entrainment made using the Southampton Oceanographic Center 1980–1995 surface flux and the Levitus mixed-layer climatologies indicate that during the Southwest Monsoon wind-driven entrainment is considerably stronger than Ekman Pumping. Inshore of the windstress maximum, Ekman Pumping partially counters wind-driven entrainment, while offshore the two processes act together to deepen the mixed-layer. As Ekman Pumping is too weak to counter wind-driven mixed-layer deepening inshore of the windstress maximum, another mechanism must act to maintain the shallow mixed-layers seen in our observations and in climatologies. Offshore advection of coastally upwelled water offers a mechanism for maintaining upper ocean stratification that is consistent with observed changes in upper ocean water properties. Ekman upwelling will modulate wind-driven entrainment, but these results indicate that the primary mechanisms acting inshore of the windstress maximum are wind-driven mixing and horizontal advection.
Weiqing Han - One of the best experts on this subject based on the ideXlab platform.
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Observed intraseasonal thermocline variability in the Bay of Bengal
Journal of Geophysical Research: Oceans, 2013Co-Authors: M. S. Girishkumar, M. Ravichandran, Weiqing HanAbstract:The time series of temperature data obtained from moored buoys deployed at 8°N, 12°N, and 15°N along 90°E in the Bay of Bengal (BoB) shows a persistent intraseasonal variability on 30-120 day time scale in three distinct periods 30-70 day, near 90 day, and near 120 day in the thermocline region. The standard deviation of moored buoy temperature data shows that half of the variability in the thermocline region is contributed from the 30-120 day variability. The relative contribution of local Ekman Pumping velocity and remote wind forcing from equatorial Indian Ocean (EIO) to the intraseasonal thermocline variability in the BoB is examined using satellite-derived sea surface height anomaly (SSHA), wind and depths of 23° isotherm (D23, proxy for thermocline depth) derived from moored buoys temperature data. The analysis shows that large amplitude intraseasonal oscillations of thermocline - particularly the near 90 day and 120 day variability - could not be explained by local Ekman Pumping velocity alone. The SSHA, D23, and wind fields reveal that the first and second baroclinic mode Kelvin and Rossby waves, which are generated remotely by winds from the EIO and eastern BoB, can significantly influence the thermocline variability in the BoB. The near 90 day and 120 day thermocline variability is driven primarily by the variability of equatorial zonal wind stress. While the 30-70 day thermocline variability is affected most by interior Ekman Pumping over the Bay, it also appears to be influenced by zonal wind stress in the EIO and alongshore wind stress in the eastern BoB.
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Dynamics of the East India Coastal Current. 1. Analytic solutions forced by interior Ekman Pumping and local alongshore winds
Journal of Geophysical Research: Oceans, 1996Co-Authors: D. Shankar, Weiqing Han, Julian P. Mccreary, S. R. ShetyeAbstract:A linear, continuously stratified model is used to investigate how forcing by interior Ekman Pumping and local alongshore winds affects the East India Coastal Current (EICC). Solutions are found analytically to an approximate version of the equations of motion. They are obtained in a basin that resembles the actual Bay of Bengal north of 6°N and are forced by Hellerman and Rosenstein [1983] winds. The mathematical structure of the solution clearly illustrates the model physics. In response to interior Ekman Pumping, baroclinic Rossby waves are excited in the interior of the Bay; they propagate to the western boundary, where they generate a northward, coastal current if the interior circulation is anticyclonic and vice versa. In contrast, the response forced by the local alongshore winds is a coastal current that is trapped within the surface mixed layer. Consistent with the observed surface current, the model EICC flows northward along the Indian coast from March to September and equatorward along the Indian and Sri Lankan coasts from October to January. In contrast to the observations, however, the onset of northward flow along the Indian coast occurs 1–2 months late, the Sri Lankan coastal current does not reverse to flow southward during the summer, and the maximum northward transport of the model EICC is 5 Sv in May, only about half the transport estimated from hydrographic data. We conclude that, although interior Ekman Pumping and local alongshore winds have a significant impact on the EICC, other driving mechanisms must be taken into account in order to simulate the observed variability adequately.
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Dynamics of the East India Coastal Current: 2. Numerical solutions
Journal of Geophysical Research: Oceans, 1996Co-Authors: Julian P. Mccreary, Weiqing Han, D. Shankar, S. R. ShetyeAbstract:A linear, continuously stratified model is used to investigate the dynamics of the East India Coastal Current (EICC). Solutions are found numerically in a basin that resembles the Indian Ocean basin north of 29°S, and they are forced by Hellerman and Rosenstein [1983] winds. Effects due to the following four forcing mechanisms are isolated: local alongshore winds adjacent to the east coasts of India and Sri Lanka, remote alongshore winds adjacent to the northern and eastern boundaries of the Bay, remotely forced signals propagating from the equator, and interior Ekman Pumping. Each process contributes significantly to the EICC surface flow at some locations and at some times during the year. Along the Indian coast (north of 10°N), the surface EICC flows northeastward from February until September, with a strong peak in March–April and weaker flow from June to September; interior Ekman Pumping, remote alongshore winds and equatorial forcing all contribute to the springtime peak, whereas local alongshore winds are the primary driving force of the weaker summertime flow. Along Sri Lanka (south of 10°N), the surface EICC flows northward only during March and April; the absence of northward flow at other times is due to interior Ekman Pumping, which drives a strong southward current for much of the year (April to December). Along both coasts there is southward flow from October to January that is driven by interior Ekman Pumping and local alongshore winds. The EICC also has significant subsurface flow on several occasions. Along the Indian coast, there is southwestward flow extending to depths greater than 1000 m from May to July that is driven primarily by equatorial forcing. From July to September, the southwestward flow forms a shallow subsurface counterflow (a Coastal Undercurrent); its cause is primarily equatorial forcing and interior Ekman Pumping, not the local alongshore winds, as might be expected.
C. K. Chu - One of the best experts on this subject based on the ideXlab platform.
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Equilibrium Response of Ocean Deep-Water Circulation to Variations in Ekman Pumping and Deep-Water Sources
Journal of Physical Oceanography, 1992Co-Authors: F. L. Yin, I. Y. Fung, C. K. ChuAbstract:Abstract A multilayer ocean model that is physically simple and computationally efficient is developed for studies of competition and interaction among deep-water sources in determining ocean circulation. The model is essentially geostrophic and hydrostatic in the ocean interior with Rayleigh friction added in boundary-layer and equatorial regions. A stably stratified density structure is specified at static equilibrium, and cross-isopycnal mixing is parameterized as a diffusive flux. The model is forced by latitudinally varying Ekman Pumping velocities at the base of the ocean surface Ekman layer and localized deep-water sources. A four-layer version of the model has been run in a rectangular basin with 5000-m depth, extending from 65°S to 65°N latitude and covering 70 degrees of longitude. The four layers mimic the major water masses observed in the Atlantic Ocean: thermocline water, intermediate water, North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW). For forcing corresponding to the...