Heaving modes in the world oceans

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Climate Dynamics 45 (2015): 3563-3591, doi:10.1007/s00382-015-2557-6.Part of climate changes on decadal time scales can be interpreted as the result of adiabatic motions associated with the adjustment of wind-driven circulation, i.e., the heaving of the isopycnal surfaces. Heat content changes in the ocean, including hiatus of global surface temperature and other phenomena, can be interpreted in terms of heaving associated with adjustment of wind-driven circulation induced by decadal variability of wind. A simple reduced gravity model is used to examine the consequence of adiabatic adjustment of the wind-driven circulation. Decadal changes in wind stress forcing can induce three-dimensional redistribution of warm water in the upper ocean. In particular, wind stress change can generate baroclinic modes of heat content anomaly in the vertical direction; in fact, changes in stratification observed in the ocean may be induced by wind stress change at local or in the remote parts of the world oceans. Intensification of the equatorial easterly can induce cooling in the upper layer and warming in the subsurface layer. The combination of this kind of heat content anomaly with the general trend of warming of the whole water column under the increasing greenhouse effect may offer an explanation for the hiatus of global surface temperature and the accelerating subsurface warming over the past 10–15 years. Furthermore, the meridional transport of warm water in the upper ocean can lead to sizeable transient meridional overturning circulation, poleward heat flux and vertical heat flux. Thus, heaving plays a key role in the oceanic circulation and climate

Defining the spicity

Author Posting. © Sears Foundation for Marine Research, 2011. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 69 (2011): 545-559, doi:10.1357/002224011799849390.Spicity is defined as a thermodynamic variable whose isopleths are 'perpendicular“ to those of density in the (T, S) diagram. A new set of Matlab codes are defined based on the following criteria: First, the isopleths of spicity should be orthogonal to those of density. Second, the ocean is vertically separated into many pressure levels, and the zero point of spicity is set for the water mass with the mean salinity and temperature at each pressure level. This new definition of spicity can provide more accurate information of the water mass properties for the study of thermohaline perturbations

Available potential energy in the world's oceans

Author Posting. © Sears Foundation for Marine Research, 2005. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 63 (2005): 141-158, doi:10.1357/0022240053693770.It is shown that in the case with bottom topography, the available gravitational potential energy cannot be represented by the available pressure potential energy. Thus, a suitable quantity for the study of large-scale circulation is the total available potential energy which is defined as the sum of available gravitational potential energy and available internal energy. A simple computational algorithm for calculating the available potential energy in the world's oceans is proposed and tested. This program includes the compressibility of seawater and realistic topography. It is estimated that the world's oceans available gravitational potential energy density is about 1474 J/m3 and the available internal energy density is -850 J/m3; thus, the net available potential energy density is 624 J/m3, and the total amount of available potential energy is 805 × 1018 J

On the boundary conditions applied to the sea-ice coupled model

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C04S12, doi:10.1029/2006JC003735.The formulation of suitable boundary conditions at the water-ice interface during ice formation (melting) is an important aspect of the sea-ice coupled model. The transfer of water and salt through the water-ice can be posed as different boundary conditions. Behavior of the model under these boundary conditions is illustrated through simple analytical models and a numerical model based on pressure-η coordinate. It is emphasized that the correct handling of the boundary conditions associated with sea ice formation requires an accurate treatment of the equivalent pressure on the top of water column and the total volume (mass) of the water column in the ice formation regime. Improper treatment of these boundary conditions may lead to an artificial loop current near the edge of ice in numerical simulations of oceanic circulation in the Arctic Ocean or near the Antarctica.This study was supported by the Institute of Ocean and Climate Change Institute of Woods Hole Oceanographic Institution

Using the Helmholtz decomposition to define the Indian Ocean meridional overturning streamfunction

Author Posting. © American Meteorological Society, 2020. 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 50(3), (2020): 679-694, doi:10.1175/JPO-D-19-0218.1.The zonally integrated flow in a basin can be separated into the divergent/nondivergent parts, and a uniquely defined meridional overturning circulation (MOC) can be calculated. For a basin with significant volume exchange at zonal open boundaries, this method is competent in removing the components associated with the nonzero source terms due to zonal transports at open boundaries. This method was applied to the zonally integrated flow in the Indian Ocean basin extended all the way to the Antarctic by virtue of the ECCO dataset. The contributions due to two major zonal flow systems at open boundaries, the Indonesian Throughflow (ITF) and the Antarctic Circumpolar Current (ACC), were well separated from the rotational flow component, and a nondivergent overturning circulation pattern was identified. Comparisons with previous studies on the MOC of the Indian Ocean in different seasons showed overall consistency but with refinements in details to the south of the entry of the ITF, reflecting the influence of ITF on the MOC pattern in the domain. Other options of decomposition are also examined.LH was supported by the National Basic Research Program of China through Grant 2019YFA0606703 and “The Fundamental Research Funds of Shandong University” (2019GN051). The authors thank the anonymous reviewers and the editor for their constructive comments. Code availability: The Matlab code that performs the decomposition and produces some figures in this paper is available at https://github.com/lei-han-SDU/IMOC/.2020-09-0

Decadal variability of pycnocline flows from the subtropical to the equatorial Pacific

Author Posting. © American Meteorological Society, 2005. 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 35 (2005): 1861–1875, doi:10.1175/JPO2791.1.A method based on isopycnal trajectory analysis is proposed to quantify the pathways from the subtropics to the Tropics. For a continuous stratified ocean a virtual streamfunction is defined, which can be used to characterize these pathways. This method is applied to the climatological dataset produced from a data-assimilated model. Analysis indicates that in each layer contours of the virtual streamfunction are a good approximation for streamlines, even if there is a cross-isopycnal mass flux. The zonal-integrated meridional transport per unit layer thickness through each pathway varies in proportion to 1/sinθ, where θ is latitude. The vertical-integrated total transport through pathways behaves similarly. Transport through pathways has a prominent decadal variability. Results suggest that in decadal time scales the interior pathway transport (IPT) anomaly may be mainly caused by the wind stress anomaly at low latitude. The western boundary pathway transport (WBPT) anomaly often has a sign opposite to the IPT anomaly, reflecting compensation between the IPT and the WBPT. However, more often than not the wind stress anomaly within tropical latitudes can also be used to explain the WBPT anomaly.QW was supported by The National Natural Science Foundation of China through Grants 40176003 and 40136010 and National Key Program for Developing Basic Research of China through Grants 2005CB422301. RXH was supported by the National Oceanic and Atmospheric Administration through CICOR Cooperative Agreement NA17RJ1223 and National Science Foundation through Grant OCE- 0094807 to the Woods Hole Oceanographic Institution

An experimental study on thermal circulation driven by horizontal differential heating

Author Posting. © Cambridge University Press, 2005. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 540 (2005): 49-73, doi:10.1017/S002211200500577X.Circulation driven by horizontal differential heating is studied, using a double-walled Plexiglas tank (20×15×2.5 cm3) filled with salt water. For instances of heating/cooling from above and below, results indicate that there is always quasi-equilibrium circulation. In contrast to most previous results from experimental/ numerical studies, circulation in our experiments appears in the form of a shallow cell adjacent to the boundary of thermal forcing. The non-dimensional stream-function maximum confirms the 1/5-power law of Rossby, Ψ ∼Ra1/5 L . Dissipation rate measured in the experiments appears to be consistent with theory. For cases of heating/cooling from a sloping bottom, circulation is similar to cases with a flat bottom; circulation is strong if heating is below cooling, but it is rather weak if heating is above cooling. Nevertheless, circulation in all cases is visible to the naked eye.W. W. was supported by The National Natural Science Foundation of China through grant 40476010 and the Research Fund for the Doctoral Program of Higher Education through grant 20030423011. R. X. H. was supported by the National Science Foundation through grant OCE-0094807 and the National Aero- Space Administration through Contract 1229833 (NRA-00-OES-05) to the Woods Hole Oceanographic Institution

Stommel’s box model of thermohaline circulation revisited - the role of mechanical energy supporting mixing and the wind-driven gyration

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): 909–917, doi:10.1175/2007JPO3535.1.The classical two-box model of Stommel is extended in two directions: replacing the buoyancy constraint with an energy constraint and including the wind-driven gyre. Stommel postulated a buoyancy constraint for the thermohaline circulation, and his basic idea has evolved into the dominating theory of thermohaline circulation; however, recently, it is argued that the thermohaline circulation is maintained by mechanical energy from wind stress and tides. The major difference between these two types of models is the bifurcation structure: the Stommel-like model has two thermal modes (one stable and another one unstable) and one stable haline mode, whereas the energy-constraint model has one stable thermal mode and two saline modes (one stable and another one unstable). Adding the wind-driven gyre changes the threshold value of thermohaline bifurcation greatly; thus, the inclusion of the wind-driven gyre is a vital step in completely modeling the physical processes related to thermohaline circulation.YPG was supported by the National Science Foundation of China (NSFC, 40676022), the National Basic Research Program of China (2006CB403605), and the Guangdong Natural Science Foundation (5003672). RXH was supported by the National Oceanic and Atmospheric Administration through CICOR Cooperative Agreement NA17RJ1223 to the Woods Hole Oceanographic Institution

Structure of an inertial deep western boundary current

An inertial model of the deep western boundary current (DWBC) is presented where the cross-stream distribution of potential vorticity varies in a realistic fashion. The case of uniform potential vorticity, which has been solved earlier, is included for comparison. The potential vorticity distribution used in the model is obtained from a hydrographic density section across the North Atlantic DWBC. The model solutions using this distribution differ significantly from the uniform potential vorticity case. Most notably the current is wider and weaker with substantially reduced relative vorticity, more indicative of observed DWBCs. The addition of an exponential continental slope leads to a further constraint on the existence of the current. Finally, it is demonstrated how a topographic ridge can partially block the DWBC and give rise to recirculation of the deepest water, reminiscent of the deep flow near the Southeast Newfoundland Rise in the North Atlantic

Dynamical roles of mixed layer in regulating the meridional mass/heat fluxes

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C05036, doi:10.1029/2006JC004046.The mixed layer is an important component of the oceanic circulation system. Recent progress in energetics of the oceanic circulation suggests that the amount of external mechanical energy available for mixing is directly linked to the strength of the meridional overturning circulation. Using an analytical two-dimensional model and a three-dimensional numerical model, it is shown that the meridional distribution of mixed layer depth plays an important role in regulating the meridional overturning circulation and poleward heat flux. In fact, if the mixed layer at low and middle latitudes is deeper because of increase in mechanical energy input to the turbulence in the upper ocean, the meridional overturning circulation and poleward heat flux are enhanced in a steady circulation system, and at the same time, it may take less mechanical energy to support the subsurface diapycnal mixing.RXH was supported by the National Oceanic and Atmospheric Administration through CICOR Cooperative Agreement NA17RJ1223, CJH was supported by the National Key Basic Research Program of China through grant 2006CB403605, andWWwas supported by the National Natural Science Foundation of China through grant 40476010. This study is also supported through the Chinese 111 Project under Contract B07036