Overflows and upper ocean interaction : a mechanism for the Azores Current

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2006The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows are a major source of the dense water of the global deep ocean, originating from only a few marginal seas. They enter the open ocean as dense gravity currents down a continental slope and play a crucial role in the deep ocean circulation. To understand the dynamics of these overflows, previous studies simplified their dynamics by treating the overlying ocean as inactive. This simplification may be a first approximation for the overflow but not for the overlying ocean. The Mediterranean overflow, for example, entrains about 2 Sv of overlying Atlantic water when it enters the Atlantic through Gibraltar Strait. The upper ocean must balance the mass loss and vortex stretching associated with entrainment. Thus for the upper ocean, overflows represent a localized region of intense mass and PV forcing. The simulations in this study show that in the upper layer, entrainment forces a cyclonic circulation along bathymetric contours. This is a topographic β-plume and its transport depends on the entrainment region size and the topographic slope. Baroclinic instability also develops and creates eddy thickness flux to the in-shore direction, forcing a double gyre topographic β-plume near the strait due to eddy PV flux convergence on the in-shore side of the continental slope and divergence on the offshore side. When the upper oceanic response to overflows is examined specifically for the Mediterranean overflow, the upper ocean is found to establish two trans-Atlantic zonal jets, analogous to the Azores current and the Azores Counter current. These two zonal jets are an extension of the topographic β-plume driven by the overflow. Because the eddies in the steep slope region near Cape St. Vincent drive a mean flow across the slope, the topographic β-plume connects to the Atlantic Ocean to become a basin scale flow. This thesis shows that overflows can induce a significant circulation in the upper ocean, and for the Mediterranean overflow, this circulation is a basin scale flow.This work was supported by the National Science Foundation Grant OCE-0424741

Eddy dynamics of ß plumes

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2003The importance of eddies and nonlinearities in ß-plume dynamics in the deep ocean was investigated using reduced gravity models of the deep ocean forced by a small region of cross isopycnal transport in the interior. The effect of topography on ß-plumes was also examined by placing a Gaussian bump in the forcing region. Despite the fact that the mean flow is weak in the deep ocean interior, it was found that the nonlinearity and instabilities are still important for realistic parameter and forcing values. The flow was dominated by eddies and was remarkably different from what would be expected from a linear solution.This study was supported by Woods Hole Oceanographic Institution Academic Programs Office Fellowship and National Science Foundation Grant 89542700

Overflows and upper ocean interactions : a mechanism for the Azores current

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 155-162).The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows are a major source of the dense water of the global deep ocean, originating from only a few marginal seas. They enter the open ocean as dense gravity currents down a continental slope and play a crucial role in the deep ocean circulation. To understand the dynamics of these overflows, previous studies simplified their dynamics by treating the overlying ocean as inactive. This simplification may be a first approximation for the overflow but not for the overlying ocean. The Mediterranean overflow, for example, entrains about 2 Sv of overlying Atlantic water when it enters the Atlantic through Gibraltar Strait. The upper ocean must balance the mass loss and vortex stretching associated with entrainment. Thus for the upper ocean, overflows represent a localized region of intense mass and PV forcing. The simulations in this study show that in the upper layer, entrainment forces a cyclonic circulation along bathymetric contours. This is a topographic [beta]-plume and its transport depends on the entrainment region size and the topographic slope.(cont.) Baroclinic instability also develops and creates eddy thickness flux to the in-shore direction, forcing a double gyre topographic [beta]-plume near the strait due to eddy PV flux convergence on the in-shore side of the continental slope and divergence on the offshore side. When the upper oceanic response to overflows is examined specifically for the Mediterranean overflow, the upper ocean is found to establish two trans-Atlantic zonal jets, analogous to the Azores current and the Azores Counter current. These two zonal jets are an extension of the topographic [beta]-plume driven by the overflow. Because the eddies in the steep slope region near Cape St. Vincent drive a mean flow across the slope, the topographic [beta]-plume connects to the Atlantic Ocean to become a basin scale flow. This thesis shows that overflows can induce a significant circulation in the upper ocean, and for the Mediterranean overflow, this circulation is a basin scale flow.by Shinichiro Kida.Ph.D

Marginal sea overflows and the upper ocean interaction

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 39 (2009): 387-403, doi:10.1175/2008JPO3934.1.Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume. The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more. Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentum flux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone. Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger.SK’s support during the time of his Ph.D. research in the MIT/WHOI Joint Program was provided by the National Science Foundation through Grant OCE04-24741. JP and JY have also received support from the Climate Process Team on Gravity Current Entrainment, NSF Grant OCE-0611530. JY has also been supported by NSF Grant OCE-0351055

The annual cycle of the Japan Sea throughflow

Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 23–39, doi:10.1175/JPO-D-15-0075.1.The mechanism responsible for the annual cycle of the flow through the straits of the Japan Sea is investigated using a two-layer model. Observations show maximum throughflow from summer to fall and minimum in winter, occurring synchronously at the three major straits: Tsushima, Tsugaru, and Soya Straits. This study finds the subpolar winds located to the north of Japan as the leading forcing agent, which first affects the Soya Strait rather than the Tsushima or Tsugaru Straits. The subpolar winds generate baroclinic Kelvin waves along the coastlines of the subpolar gyre, affect the sea surface height at the Soya Strait, and modify the flow through the strait. This causes barotropic adjustment to occur inside the Japan Sea and thus affect the flow at the Tsugaru and Tsushima Straits almost synchronously. The barotropic adjustment mechanism explains well why the observations show a similar annual cycle at the three straits. The annual cycle at the Tsugaru Strait is further shown to be weaker than that in the other two straits based on frictional balance around islands, that is, frictional stresses exerted around an island integrate to zero. In the Tsugaru Strait, the flows induced by the frictional integrals around the northern (Hokkaido) and southern (Honshu) islands are in opposite directions and tend to cancel out. Frictional balance also suggests that the annual cycle at the Tsugaru Strait is likely in phase with that at the Soya Strait because the length scale of the northern island is much shorter than that of the southern island.S. Kida is supported by KAKENHI (22106002). B. Qiu is supported by NASA (NNX13AE15G). J. Yang is supported by the U.S. National Science Foundation. X. Lin is supported by the Natural Science Foundation of China (41222037 and U1406401), China’s National Basic Research Priorities Programme (2013CB956202), and the Global Air-Sea Interaction Project (GASI-03-01-01-02).2016-07-0

Wrap, tilt and stretch of vorticity lines around a strong thin straight vortex tube in a simple shear flow

the mechanism of wrap, tilt and stretch of vorticity lines around a strong thin straight vortex tube of circulation &#915; starting with a vortex filament in a simple shear flow (U=SX2x^1, S being a shear rate) is investigated analytically. an asymptotic expression for the vorticity field is obtained at a large reynolds number &#915;/&#957; » 1, &#957; being the kinematic viscosity of fluid, and during the initial time St « 1 of evolution as well as St « (&#915;/&#957;)1/2. the vortex tube, which is inclined from the streamwise (X1) direction both in the vertical (X2) and spanwise (X3) directions, is tilted, stretched and diffused under the action of the uniform shear and viscosity. the simple shear vorticity is on the other hand, wrapped and stretched around the vortex tube by a swirling motion, induced by it to form double spiral vortex layers of high azimuthal vorticity of alternating sign. the magnitude of the azimuthal vorticity increases up to O((&#915;/&#957;)1/3S) at distance r=O((&#915;/&#957;)1/3 (&#957;t)1/2) from the vortex tube. the spirals induce axial flows of the same spiral shape with alternate sign in adjacent spirals which in turn tilt the simple shear vorticity toward the axial direction. as a result, the vorticity lines wind helically around the vortex tube accompanied by conversion of vorticity of the simple shear to the axial direction. the axial vorticity increases in time as s2t, the direction of which is opposite to that of the vortex tube at r=O((&#915;/&#957;)1/2 (&#957;t)1/2) where the vorticity magnitude is strongest. in the near region r « (&#915;/&#957;)1/3 (&#957;t)1/2, on the other hand, a viscous cancellation takes place in tightly wrapped vorticity of alternate sign, which leads to the disappearance of the vorticity normal to the vortex tube. only the axial component of the simple shear vorticity is left there, which is stretched by the simple shear flow itself. as a consequence, the vortex tube inclined toward the direction of the simple shear vorticity (a cyclonic vortex) is intensified, while the one oriented in the opposite direction (an anticyclonic vortex) is weakened. the growth rate of vorticity due to this effect attains a maximum (or minimum) value of ±S2/33/2 when the vortex tube is oriented in the direction of X^1+X^2[minus-or-plus sign] X^3. the present asymptotic solutions are expected to be closely related to the flow structures around intense vortex tubes observed in various kinds of turbulence such as helical winding of vorticity lines around a vortex tube, the dominance of cyclonic vortex tubes, the appearance of opposite-signed vorticity around streamwise vortices and a zig-zag arrangement of streamwise vortices in homogeneous isotropic turbulence, homogeneous shear turbulence and near-wall turbulence.</p

Pediatric cardiorespiratory failure successfully managed with venoarterial-venous extracorporeal membrane oxygenation: a case report

Background: Venoarterial-venous extracorporeal membrane oxygenation (VAV ECMO) configuration is a combined procedure of extracorporeal membrane oxygenation (ECMO). The proportion of cardiac and respiratory support can be controlled by adjusting arterial and venous return. Therefore, VAV ECMO can be applicable as a bridging therapy in the transition from venoarterial (VA) to venovenous (VV) ECMO. Case presentation: We present an 11-year-old girl with chemotherapy-induced myocarditis requiring extracorporeal cardiorespiratory support. She showed progressive hypotension, tachycardia, hyperlactemia, and tachypnea under support of catecholamines. Echocardiography showed severe left ventricular hypokinesis with an ejection fraction of 30 %. She was placed on VA ECMO with a drainage catheter from the right femoral vein (19.5 Fr) and a return catheter to the right femoral artery (16.5 Fr). Extracorporeal circulation was initiated at a blood flow of 2.0 L/min (59 mL/kg/min). On day 31, although cardiac function had improved, persistent pulmonary failure made weaning from VA ECMO difficult. We planned transition from VA ECMO to VAV ECMO to ensure gradual tapering of extracorporeal cardiac support while evaluating cardiopulmonary function. An additional return cannula (13.5 Fr) was inserted from the right internal jugular vein, which was connected to the circuit branch from the original returning cannula. We then gradually shifted the blood from the femoral artery to the right internal jugular vein over 24 h. She was successfully switched from VA to VV ECMO via VAV ECMO. Conclusions: VAV ECMO might be an option in ensuring oxygenation to the coronary circulation and allowing time to adequately evaluate cardiac function during transition from VA to VV ECMO. Further investigations using larger cohorts are necessary to validate the efficacy of VAV ECMO as a bridging therapy in the transition from VA to VV ECMO.This work was supported by a JSPS KAKENHI Grant (Number JP 16K09541)

The upper-oceanic response to overflows : a mechanism for the Azores Current

Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 880–895, doi:10.1175/2007JPO3750.1.The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows enter the open ocean as dense gravity currents that flow along and down the continental slope. While descending the slope, overflows typically double their volume transport by entraining upper oceanic water. The upper oceanic layer must balance this loss of mass, and the resulting convergent flow produces significant vortex stretching. Overflows thus represent an intense and localized mass and vorticity forcing for the upper ocean. In this study, simulations show that the upper ocean responds to the overflow-induced forcing by establishing topographic β plumes that are aligned more or less along isobaths and that have a transport that is typically a few times larger than that of the overflows. For the topographic β plume driven by the Mediterranean overflow, the occurrence of eddies near Cape St. Vincent, Portugal, allows the topographic β plume to flow across isobaths. The modeled topographic β-plume circulation forms two transatlantic zonal jets that are analogous to the Azores Current and the Azores Countercurrent. In other cases (e.g., the Denmark Strait overflow), the same kind of circulation remains trapped along the western boundary and hence would not be readily detected.SK’s support during the time of his Ph.D. research in the MIT/WHOI Joint Program was provided by the National Science Foundation through Grant OCE04-24741. JP and JY have also received support from the Climate Process Team on Gravity Current Entrainment, NSF Grant OCE-0611530