Albert J. Semtner

A Model for the Thermodynamic Growth of Sea Ice in Numerical Investigations of Climate

Abstract A model is presented whereby the thickness and extent of sea ice may be predicted in climate simulations. A basic one-dimensional diffusion process is taken to act in the ice, with modifications due to penetration of solar radiation, melting of internal brine pockets, and accumulation of an insulating snow cover. This formulation is similar to that of a previous study by Maykut and Untersteiner, but the introduction of a streamlined numerical method makes the model more suitable for use at each grid point of a coupled atmosphere-ocean model. In spite of its simplicity, the ice model accurately reproduces the results of Maykut and Untersteiner for a wide variety of environmental conditions. In 25 paired experiments, annual average equilibrium thicknesses of ice agree within 24 cm for 75% of the cases; and the average absolute error for all cases is 22 cm. The new model has fewer computational requirements than one layer of ocean in the polar regions, and it can be further simplified if additional ...

Numerical simulation of the Gulf Stream and Mid-Ocean eddies

Abstract The circulation of the western North Atlantic is simulated with a primitive equation model that has 5 levels and a horizontal grid size of 37 km. The idealized model domain is a rectangular basin, 3000 km long, 2000 km wide and 4 km deep, which is oriented so that the long axis of the basin is parallel to the east coast of the United States. The nearshore side of the basin has a simple continental shelf and slope, whereas the other sides are bounded by vertical wills. The model ocean is driven by a 2½ gyre pattern of steady zonal wind stress and by a Newtonian-type surface heating. After initialization from a 15-year spin-up with a coarser grid, two experiments are carried out, each of several years duration: the first uses a Laplacian formulation for the subgrid-scale lateral diffusions of heat and momentum, the second uses a highly scale-selective biharmonic formulation for these diffusions. Bottom friction is present in each case. In both experiments, a western boundary current forms which sep...

A Numerical Study of Sea Ice and Ocean Circulation in the Arctic

Abstract A sea-ice model based bulk-viscous plastic dynamics and 3-layer thermodynamics is coupled to a multilevel primitive equation model of the Arctic Ocean and Greenland Sea. The combined model is forced by inflow through the Faeroe-Shetland Channel and Bering Strait and by observed monthly atmospheric forcing and river runoff. A long-term integration produces a realistic cycle of ice cover, whose extent is strongly influenced by ocean heat transport. The wintertime maximum is controlled by northward heat transport of 0.4 petawatts in the Greenland Sea and by southward transport of ice and water through the Fram Strait. The summertime minimum extent of sea ice is influenced by subsurface flow through the Fram Strait of warm Atlantic water, which rises in winter and thins the ice lying over the Eurasian continental shelf and along the Alaskan and Siberian coasts. The oceanic circulation in the Canadian Basin is anticyclonic at all depths, but changes to cyclonic in the Eurasian Basin below 200 meters. ...

A General Circulation Experiment with a Coupled Atmosphere, Ocean and Sea Ice Model

Abstract This paper describes the construction and results of a comprehensive, three-dimensional general circulation model (GCM) of the earths climate. The model, developed at the National Center for Atmospheric Research (NCAR), links separate existing models of the atmosphere, ocean and sea ice. The atmospheric model is a version of the third-generation NCAR GCM which has a relatively complete treatment of physical processes. It uses a generalized vertical coordinate with eight layers (∼3 km thick) and 5° horizontal grid spacing over the entire globe. The ocean model, using the primitive equations and the hydrostatic and Boussinesq approximations, was changed to the world domain from an earlier model developed by Bryan (1969) and reprogrammed by Semtner (1974). The model has four unequally spaced vertical layers and 5° horizontal grid structure. The sea ice model is a simple thermodynamic model using a simplified calculation of heat flux through sea ice (Semtner, 1976). The method of coupling the atmosp...

Experiments with a Global Ocean Model Driven by Observed Atmospheric Forcing

Abstract A global ocean model with a 5° horizontal grid size, previously used in the coupled climate model of Washington et al. (1980), is used here with observed monthly mean wind stress and atmospheric temperatures for the purposes of varying model parameters to suggest values for future coupled climate model experiments, and examining heat storage and transport in the model ocean. A control run is carried out with the same internal parameters as in the climate simulation of Washington et al. (1980). Observed atmospheric forcing produces an oceanic circulation closer to the observed than in the coupled climate model. Necessarily large horizontal momentum diffusion leads to weak zonal currents in both cases; however, the stronger observed wind stresses in the decoupled model produce more vigorous and more realistic meridional ocean currents. Three additional ocean experiments test parameter variations. Lowering horizontal heat diffusion increases north-south temperature gradients and intensifies ocean ci...

On the development of a seasonal change sea-ice model

Abstract A three-dimensional thermodynamical model of sea ice has been developed which is capable of simulating seasonal changes over the arctic and antarctic regions. The model is patterned after the earlier work of Maykut and Untersteiner (1971) and Semtner (1976). Instead of specifying the fluxes of energy at the top of the ice, as is usually done in sea ice modeling, the components of surface energy balance are computed from observed climatological atmospheric data. Also, a new parameterization of leads by Semtner (1976) is tested and shown to improve the simulation. The model results agree with observations in the Arctic, but they are less successful in the Antarctic. Possible reasons for deficiencies of the model are that ice transport is not included and oceanic heat flux is not properly accounted for. These aspects will be added and improved in future development of the model.

Numerical Simulation of the Arctic Ocean Circulation

Abstract The circulation of the Arctic Ocean and Greenland Sea is simulated using the 1969 numerical model of Bryan and Cox. The coastline and bottom topography of the region are resolved by a 110 km horizontal grid spacing and by 14 vertical levels. The transfers of mass, heat and momentum at the ocean surface and at open lateral boundaries are specified from observations. In particular, the pattern of wind stress is obtained using a map of mean annual atmospheric pressure; and a scalar multiplier is applied to account for the nonlinear dependence of stress on wind speed. Three experiments with different values of this scalar multiplier are run to simulate the effect of high, medium and low wind stress. The first experiment is carried out for the combined Arctic Ocean and Greenland Sea, while the other two experiments are run for the Arctic Ocean only. Many of the observed features of the Arctic circulation are reproduced by the simulations. The Greenland Sea exhibits cyclonic flow at all levels and deep...

Numerical Simulation of Equatorial Ocean Circulation. Part I: A Basic Case in Turbulent Equilibrium

Abstract An equatorial ocean experiment has been carried out, using the primitive equation model of Semtner and Mintz (1977) with a highly conservative differencing scheme, with high horizontal resolution (Δx = 0.50°, Δy=0.25°) and with 14 levels in the vertical. A turbulent equilibrium state has been reached for a 3300 km × 2200 km equatorial ocean, driven by constant 0.5 dyn cm−2 wind stress, heated at the surface and cooled at the northern and southern walls. The predicted surface temperature field shows an upwelling-induced cold region along the equator. The temperatures at the equator near the eastern wall are as much as 6°C colder than in the subequatorial regions. Westward moving waves occur in the temperature field a few degrees north and south of the equator. These waves have periods of 33 days, wavelengths of 800 km, and are symmetric about the equator. Their structure is similar to that of equatorially trapped Rossby waves with n=1 in the vertical and m=1 in the horizontal. Shorter wavelength d...

Intercomparison of Quasi-Geostrophic Simulations of the Western North Atlantic Circulation with Primitive Equation Results

Abstract The purpose of this paper is to compare two numerical models of vastly different complexity and computational requirements, which have been used recently in a number of midlatitude ocean simulations. Specifically, the two-layer quasi-geostrophic (QG) model of Holland (1978) is compared with the five-level primitive equation (PE) model of Semtner and Mintz (1977) for a wind-driven multi-gyre ocean, with effects of bottom topography and thermal forcing included. The dominant feature of the circulation predicted in the previous PE calculations is a strong free jet, with intense mesoscale transients which are maintained by baroclinic instability. The configuration of the QG experiment is designed to approximate closely that of the PE experiment, while retaining as much of the simplicity of the Holland (1978) model as possible. The QG model spins up to a state of statistical equilibrium, which is characterized by a meandering jet and by mid-ocean mesoscale eddies with periods and wavelengths much like...

Energy Trapping near the Equator in a Numerical Ocean Model

Abstract The trapped equatorial standing modes described theoretically by Gent (1979) are reproduced in a single vertical-mode numerical ocean model. integrations are carried out in domains whose longitudinal extents are characteristic of the widths of the Atlantic and Pacific Oceans, as well as in a narrow ocean in which the simplest possible standing mode can exist. The modes are shown to be very insensitive to small changes in basin width and to the inclusion of friction, and somewhat sensitive to the inclusion of the nonlinear terms and to rotation of the rectangular basin relative to the equator. Moreover, they arise spontaneously from simple atmospheric forcing or from random initial conditions. Typically, 20–40% of the energy input to the equatorial ocean remains trapped in a number of distinct standing modes after about nine years of integration time. The wider the ocean domain, the more energy remains trapped near the equator. These unexpected results have important implications for equatorial oc...