Charles A. Lin

A Reexamination of the polar Halocline Catastrophe and Implications for Coupled Ocean-Atmosphere Modeling

Abstract In this paper, the physical mechanism of the polar halocline catastrophe (PHC) is reexamined with emphasis on the role played by the surface heat flux. It is argued that, in a coupled ocean–atmosphere system, thermal changes in the atmospheric state in response to changes in heat flux from the ocean weaken the feedback responsible for the PHC. So far, the PHC has been observed in models that use mixed boundary conditions; that is, the freshwater flux is specified, but the surface temperature is relaxed to a specified value. Previous explanations of the PHC have focused on the role of the freshwater flux in establishing a freshwater cap and shutting off the deep convection. However, the establishment of a freshwater cap reduces the depth of the water column that is cooled by surface heat loss. As a consequence, the surface temperature is reduced. Since the difference between this and atmospheric restoring temperature is now less, there is a corresponding reduction in the surface heat loss to the a...

Finite elements for shallow-water equation ocean models

The finite-element spatial discretization of the linear shallow-water equations on unstructured triangular meshes is examined in the context of a semi-implicit temporal discretization. Triangular finite elements are attractive for ocean modeling because of their flexibility for representing irregular boundaries and for local mesh refinement. The semi-implicit scheme is beneficial because it slows the propagation of the high-frequency small-amplitude surface gravity waves, thereby circumventing a severe time step restriction. High-order computationally expensive finite elements are, however, of little benefit for the discretization of the terms responsible for rapidly propagating gravity waves in a semi-implicit formulation. Low-order velocity/surface-elevation finite-element combinations are therefore examined here. Ideally, the finite-element basis-function pair should adequately represent approximate geostrophic balance, avoid generating spurious computational modes, and give a consistent discretization of the governing equations. Existing finite-element combinations fail to simultaneously satisfy all of these requirements and consequently suffer to a greater or lesser extent from noise problems. An unconventional and largely unknown finite-element pair, based on a modified combination of linear and constant basis functions, is shown to be a good compromise and to give good results for gravity-wave propagation.

Mesoscale Circulations Forced by Melting Snow. Part II: Application to Meteorological Features

Abstract Various authors have proposed that the cooling associated with melting precipitation contributes significantly to the dynamics of mesoscale precipitation systems. In this study, we use the numerical model described in Part I of this paper to investigate the effects of the cooling-by-melting mechanism in three specific situations: rain/snow boundaries, the production of deep 0°C isothermal layers, and the trailing stratiform region associated with mesoscale convective systems. It is found that melting in the vicinity of a rain/snow boundary produces a thermally indirect mesoscale vertical circulation that may be responsible for enhanced precipitation near a rain/snow boundary. Melting in the presence of warm air advection above the melting layer and cold advection at and below it are necessary for producing deep 0°C layers within realistic times. The dynamic effects of cooling associated with melting and evaporation in the stratiform region of a mature squall line system produce a mesoscale circul...

A Semi-implicit Semi-Lagrangian Finite-Element Shallow-Water Ocean Model

Abstract The finite-element, semi-implicit, and semi-Lagrangian methods are combined together to solve the shallow-water equations using unstructured triangular meshes. Triangular finite elements are attractive for ocean modeling because of their flexibility for representing irregular boundaries and for local mesh refinement. A kriging interpolator is used for the semi-Lagrangian advection, leading to an accurate representation of the slow Rossby modes. The terms that govern fast gravitational oscillations are discretized using the semi-implicit scheme, thereby circumventing a severe time step restriction. A low-order velocity–surface-elevation finite-element basis-function pair is used for the spatial discretization. Results of test problems to simulate slowly propagating Rossby modes illustrate the promise of the proposed approach for ocean modeling.

A decadal oscillation due to the coupling between an ocean circulation model and a thermodynamic sea-ice model

A 3-dimensional, planetary-geostrophic, ocean general circulation model is coupled to a thermodynamic sea-ice model. The thermal coupling takes account of the insulating effect of the ice. A simple approach is taken in the case of the freshwater flux by allowing this to pass through the ice, except that some is used for snow accumulation. It is then modified by salinity rejection/dilution due to freezing/melting. The model has idealized box geometry extending 60° in both latitude and longitude, with a horizontal resolution of 2° and 14 vertical levels. Annual mean surface forcings are used. The coupled system is first spun up using restoring conditions on both surface temperature and surface salinity to reach a steady state which includes ice in the high latitudes. A switch of the surface forcing to mixed boundary conditions (restoring on temperature and flux on salinity) leads to an oscillation of period 17 years in the magnitude of the thermohaline circulation and the ice extent. The oscillation is due to a feedback between ice cover and ocean temperature. Since ice forms only in regions where the ocean loses heat to the atmosphere, the thermal insulation of an increased ice cover makes the ocean warmer. The thermohaline circulation plays a role in transporting this heat polewards, which in turn melts the ice. The heat loss over open water at high latitudes then leads to ice formation and the process repeats itself. Salinity rejection/dilution associated with ice formation/melting is shown to be of secondary importance in this oscillation. Rather, changes in surface salinity are dominated by changes in deep convection and the associated vertical mixing, which are themselves associated with the reduction in surface heat loss due to the insulating effect of the ice. As a consequence the model exhibits the negative correlation between surface salinity and ice extent that is observed in the high latitude North Atlantic

Mesoscale Circulations Forced by Melting Snow. Part I: Basic Simulations and Dynamics

Abstract The melting of snow extracts latent heat of fusion from the environment. The basic response of the atmosphere to this cooling-by-melting mechanism is investigated by using a nonlinear two-dimensional numerical model. It is found that the resultant melting-induced circulations consist of a forced downdraft which spreads out laterally like a gravity current and transients which are gravity waves. The characteristics of these mesoscale thermally driven circulations are studied under idealistic atmospheric conditions. Model results show that the melting associated with realistic precipitation rates (up to 10 mm h−1) can induce horizontal wind perturbations of several meters per second and vertical motions of tens of centimeters per second. Since the gravity waves and the cold outflow current propagate away from the source, they can have significant dynamic effects on the environment remote from the precipitation region. Moreover, the melting-induced, near O°C isothermal layer in the atmosphere alters...

Thirty‐five year (1971–2005) simulation of daily soil moisture using the variable infiltration capacity model over China

Abstract We use the Variable Infiltration Capacity (VIC) land surface macroscale hydrology model driven by observed maximum and minimum air temperatures and precipitation to map daily soil moisture values over China for the period 1 January 1971 to 31 July 2005. The model is applied over a grid of 10 458 points with a resolution of 30 km × 30 km. The model is first calibrated using observed hydrographs from 35 catchments with drainage areas varying from 190 to 351 530 km2. The model is then validated over these 35 catchments at different periods, and over an additional eight catchments with drainage areas ranging from 1230 to 10 010 km2. An estimation procedure to determine model parameters is developed and applied to catchments where hydrographs are not available for the standard calibration process. In situ soil moisture measurements from 28 sites around the country are also used for model validation. VIC performs well over both calibration and validation catchments especially in humid and semi‐humid regions. The 35‐year soil moisture climatology for the top 1 m from VIC is consistent with known soil moisture conditions in China.

A high resolution numerical study of Gulf of Mexico fronts and eddies

SummaryThe Gulf of Mexico (GOM) circulation is simulated using the DieCAST ocean model, with a horizontal resolution of 1/12° and 20 vertical layers. The results compare well with observations of both large and small scale features, including Loop Current frontal occlusions associated with frontal eddies. The simulation is carried out without any data assimilation. The frontal eddies tend to be spaced at about 90° intervals around the Loop Current, leading to a Loop Current head shaped like a square with rounded corners. The pattern rotates as the eddies circle the Loop, and frontal eddies elongate as they squeeze through the Florida Strait. Major warm core eddies separate regularly from the Loop Current and propagate to the western GOM. Old warm core eddies in the western Gulf dissipate through bottom drag effects, which also generate cyclonic parasitic eddies. Newly arrived warm core eddies merge with old ones in the western GOM. Recently separated elongated Loop Current eddies can rotate and reattach temporarily to the Loop Current. The barotropic flow component develops eddies between the main separated warm core eddy and the Loop Current due to eastward dispersion, as the main eddy itself propagates westward into the Gulf.

Numerical studies of eddy shedding in the Gulf of Mexico

We model the eddy shedding phenomenon in the Gulf of Mexico using the Sandia Ocean Modeling System. In the first part of the study a parameter sensitivity study is performed using a rectangular basin two-level model. In particular, the sensitivity of the model eddy shedding to inflow/outflow conditions, horizontal resolution (80, 40, and 20 km), and reduced gravity is examined. The results are interpreted in terms of the quasi-geostrophic vertical motion equation and vorticity conservation. In the second part of the paper we include realistic coastlines and bathymetry in the model. A horizontal resolution of 20 km is used together with 16 vertical levels. Various observed features of the eddy shedding are simulated by the model. A boundary current which follows topographic contours is generated around the Gulf because of the splitting of the flows associated with the loop current and the shed eddy. This current is likely to be important in the dissipative stages of the eddies in the western Gulf. In addition, our results suggest that higher-order baroclinic modes are important in the dissipation of loop current eddies.

Impact of freshwater on a subarctic coastal ecosystem under seasonal sea ice (southeastern Hudson Bay, Canada). I. Interannual variability and predicted global warming influence on river plume dynamics and sea ice

Abstract Analysis of sea ice cover, runoff and air temperature observations in Hudson Bay shows marked interannual variability. This variability is thought to play a major role in determining overall productivity of the coastal ecosystem by changes to river plume extent, under-ice light conditions and nutrient levels during spring. Extensive field work off the Great Whale River in southeastern Hudson Bay has shown the importance of freshwater discharge, sea ice cover and meteorological forcing on the production of under-ice microalgae and the success of first feeding in fish larvae. Recent global climate model (GCM) results for a doubling of present atmospheric carbon dioxide indicate increases of both air temperature and precipitation in the Hudson Bay area. Predictions based on GCM results are used to estimate future changes to the sea ice and runoff regime. Sea ice breakup in the offshore is predicted to occur about one month earlier than presently. Estimates of the spring freshet in the Great Whale River indicate it will also advance by approximately one month. Onset of the spring freshet will occur about one month before Hudson Bay ice breakup, similar to present. A predicted reduction of about 35% in maximum sea ice thickness will lead to an increase in the ice-ocean interface irradiance and a decrease in melt water input to the Hudson Bay surface waters. These results are used in a discussion of potential effects of global climate change on northern coastal marine environments.