Mathew Maltrud

Numerical Simulation of the North Atlantic Ocean at 1/10°

Abstract In this paper an initial analysis of an 0.1° simulation of the North Atlantic Ocean using a level-coordinate ocean general circulation model forced with realistic winds covering the period 1985–96 is presented. Results are compared to the North Atlantic sector of a global 0.28° simulation with similar surface forcing and to a variety of satellite and in situ observations. The simulation shows substantial improvements in both the eddy variability and the time-mean circulation compared to previous eddy-permitting simulations with resolutions in the range of 1/2°–1/6°. The resolution is finer than the zonal-mean first baroclinic mode Rossby radius at all latitudes, and the model appears to be capturing the bulk of the spectrum of mesoscale energy. The eddy kinetic energy constitutes 70% of the total basin-averaged kinetic energy. Model results agree well with observations of the magnitude and geographical distribution of eddy kinetic energy and sea-surface height variability, with the wavenumber–fre...

Global eddy‐resolving ocean simulations driven by 1985–1995 atmospheric winds

Results are presented from a high-resolution global ocean model that is driven through three decadal cycles of increasingly realistic prescribed atmospheric forcing from the period 1985–1995. The model used (the Parallel Ocean Program) is a z level primitive equation model with active thermohaline dynamics based on the formulation of Bryan [1969] rewritten for massively parallel computers. Improvements to the model include an implicit free-surface formulation of the barotropic mode [Dukowicz and Smith, 1994] and the use of pressure averaging for increasing the numerical time step. This study extends earlier 0.5° simulations of Semtner and Chervin [1992] to higher horizontal resolution with improved treatments of ocean geometry and surface forcing. The computational grid is a Mercator projection covering the global ocean from 77°N to 77°S and has 20 vertical levels. Three successive simulations have been performed on the CM-5 Connection Machine system at Los Alamos using forcing fields from the European Centre for Medium-Range Weather Forecasts (ECMWF). The first run uses monthly wind stresses for 1985–1995 and restoring of surface temperature and salinity to the Levitus [1982] seasonal climatology. The second run is the same but with 3 day-averaged rather than monthly averaged wind stress fields, and the third is the same as the second but uses the monthly climatological ECMWF heat fluxes of Barnier et al. [1995] instead of restoring to climatological sea surface temperatures. Many features of the wind-driven circulation are well represented in the model solutions, such as the overall current patterns, the numerous regions of hydrodynamic instability which correspond to those observed by satellite altimetry, and the filamented structure of the Antarctic Circumpolar Current. However, some features such as the separation points of the Gulf Stream and Kuroshio and the transport through narrow passages such as the Florida Straits are clearly inaccurate and indicate that still higher resolution may be required to correct these deficiencies. Water mass properties and some aspects of the thermohaline circulation are also not always well reproduced, which is partly due to the relatively short length of the integrations. The use of the ECMWF heat fluxes, rather than restoring to climatological surface temperatures, leads to stronger and more realistic surface and deep western boundary currents (primarily in the Atlantic) as well as more realistic meridional heat transport; this is primarily because the equilibrium meridional heat transport implied by the ECMWF surface fluxes is quite large. The ECMWF heat fluxes also produce improved seasonal cycles of sea surface temperature and height in both the northern and southern hemispheres. The 3-day wind forcing gives rise to modes of model variability that are clearly seen in synoptic observations, such as the large-scale 20–100-day oscillations seen in the TOPEX/POSEIDON data, which are barotropic oscillations induced by the high-frequency wind forcing. Additional studies on other aspects of the simulations described here are being conducted by a variety of investigators, and some of these are briefly described.

Eulerian and Lagrangian Statistics from Surface Drifters and a High-Resolution POP Simulation in the North Atlantic

Abstract Eulerian and Lagrangian statistics were calculated from the North Atlantic surface drifter dataset for the years 1993–97 and a high-resolution eddy-resolving configuration of the Los Alamos National Laboratory (LANL) Parallel Ocean Program (POP) model. The main purpose of the study was to statistically quantify the state of the surface circulation in the North Atlantic Ocean for this period and compare it with the equivalent modeled state. Diffusivities and time and length scales are anisotropic over most of the ocean basin, except in most of the subpolar regions. Typical time and length scales are 2–4 days and 20–50 km. Longest timescales are found in the energetically quiescent regions in the south and southeast sectors of the basin. The longest length scales are found in the energetic western boundary current system, the most dispersive region of the domain. In many respects the eddy-resolving model reproduced a surface circulation in good statistical agreement with that depicted by the drifte...

Comparison of global climatological maps of sea surface dimethyl sulfide

We have examined differences in regional and seasonal variability among sevenglobal climatologies of sea-surface dimethyl sulfide (DMS) concentrations. We foundlarge differences between recent climatologies and that typically used by mostatmospheric sulfur models. The relative uncertainty (1s/mean) in the latitudinaldistribution of the annual mean DMS concentration increases from about 50% in tropicaland temperate regions to nearly 100% in the high latitudes. We also compared theseclimatologies to new measurements in the North Atlantic Ocean taken during the 2001Programme Oce´an Multidisciplinaire Me´so Echelle (POMME) expeditions.

Changes in dimethyl sulfide oceanic distribution due to climate change

Dimethyl sulfide (DMS) is one of the major precursors for aerosols and cloud condensation nuclei in the marine boundary layer over much of the remote ocean. Here they report on coupled climate simulations with a state-of-the-art global ocean biogeochemical model for DMS distribution and fluxes using present-day and future atmospheric CO{sub 2} concentrations. They find changes in zonal averaged DMS flux to the atmosphere of over 150% in the Southern Ocean. This is due to concurrent sea ice changes and ocean ecosystem composition shifts caused by changes in temperature, mixing, nutrient, and light regimes. The largest changes occur in a region already sensitive to climate change, so any resultant local CLAW/Gaia feedback of DMS on clouds, and thus radiative forcing, will be particularly important. A comparison of these results to prior studies shows that increasing model complexity is associted with reduced DMS emissions at the equator and increased emissions at high latitudes.

Transit-Time Distributions in a Global Ocean Model

Abstract Results from a simulation of the ocean “transit-time distribution” (“TTD”) for global and regional ocean surface boundary conditions are presented based on a 5000-yr integration using the Parallel Ocean Program ocean general circulation model. The TTD describes the probability that water at a given interior point in the ocean was at some point on the ocean surface a given amount of time ago. It is shown that the spatial distribution of ocean TTDs can be understood in terms of conventional wisdom regarding time scales and pathways of the ventilated thermocline and the thermohaline circulation–driven deep-ocean circulation. The true mean age from the model (the first moment of the TTD) is demonstrated to be very large everywhere, because of very long-tailed distributions. Regional TTD distributions are presented for distinct surface boundary subregions, and it is shown how these can help in the interpretation of the global TTD. The spatial structure of each regional TTD is shown to become essential...

Sea level variability in the Arctic Ocean from AOMIP models

[1] Monthly sea levels from five Arctic Ocean Model Intercomparison Project (AOMIP) models are analyzed and validated against observations in the Arctic Ocean. The AOMIP models are able to simulate variability of sea level reasonably well, but several improvements are needed to reduce model errors. It is suggested that the models will improve if their domains have a minimum depth less than 10 m. It is also recommended to take into account forcing associated with atmospheric loading, fast ice, and volume water fluxes representing Bering Strait inflow and river runoff. Several aspects of sea level variability in the Arctic Ocean are investigated based on updated observed sea level time series. The observed rate of sea level rise corrected for the glacial isostatic adjustment at 9 stations in the Kara, Laptev, and East Siberian seas for 1954–2006 is estimated as 0.250 cm/yr. There is a well pronounced decadal variability in the observed sea level time series. The 5-year running mean sea level signal correlates well with the annual Arctic Oscillation (AO) index and the sea level atmospheric pressure (SLP) at coastal stations and the North Pole. For 1954–2000 all model results reflect this correlation very well, indicating that the long-term model forcing and model reaction to the forcing are correct. Consistent with the influences of AO-driven processes, the sea level in the Arctic Ocean dropped significantly after 1990 and increased after the circulation regime changed from cyclonic to anticyclonic in 1997. In contrast, from 2000 to 2006 the sea level rose despite the stabilization of the AO index at its lowest values after 2000.

The Origin of Deep Zonal Flows in the Brazil Basin

Recent data from a deployment of Lagrangian floats in the Brazil Basin of the South Atlantic reveal a swift western boundary current and predominantly zonal flow in the interior at a depth of about 2500 m. Dynamical mechanisms for the deep interior flow are considered using two high-resolution models, a global and a regional one, together with a suite of sensitivity studies at low resolution. Outside the western boundary region, model energy levels are similar to observations. The models are able to reproduce, at somewhat reduced strength depending on resolution, much of the meridional structure of the observed deep zonal flows. Several candidates for generating such flows are examined, including nonlinear rectification, baroclinic instability, and thermohaline and wind forcing. A primary mechanism for the deep flow in the models is the response to the wind stress, as recently argued to be the case for a model of the Pacific Ocean. However, thermohaline forcing is significant, especially where density contrasts between basins generate strong currents in deep passages. The deep thermohaline flow appears to be linked to the depth of the midocean ridge. Baroclinic instability of the mean meridional flow, which is alone capable of generating nearly zonal currents of the observed scale, is a possible additional forcing but is not essential in the models investigated here. The meridional scale of the zonal flows in the models is extremely dependent on the horizontal resolution and horizontal mixing.

The Southern Ocean and Its Climate in CCSM4

AbstractThe new Community Climate System Model, version 4 (CCSM4), provides a powerful tool to understand and predict the earth’s climate system. Several aspects of the Southern Ocean in the CCSM4 are explored, including the surface climatology and interannual variability, simulation of key climate water masses (Antarctic Bottom Water, Subantarctic Mode Water, and Antarctic Intermediate Water), the transport and structure of the Antarctic Circumpolar Current, and interbasin exchange via the Agulhas and Tasman leakages and at the Brazil–Malvinas Confluence. It is found that the CCSM4 has varying degrees of accuracy in the simulation of the climate of the Southern Ocean when compared with observations. This study has identified aspects of the model that warrant further analysis that will result in a more comprehensive understanding of ocean–atmosphere–ice dynamics and interactions that control the earth’s climate and its variability.

On the possible long-term fate of oil released in the Deepwater Horizon incident, estimated using ensembles of dye release simulations

We have conducted an ensemble of 20 simulations using a high resolution global ocean model in which dye was continuously injected at the site of the Deepwater Horizon drilling rig for two months. We then extended these simulations for another four months to track the dispersal of the dye in the model. We have also performed five simulations in which dye was continuously injected at the site of the spill for four months and then run them out to one year from the initial spill date. The experiments can elucidate the approximate timescales and space scales of dispersal of polluted waters and also give a quantitative estimate of the dilution rate. Given the uncertainty in rates of chemical or biological degradation for oil or an oil–dispersant mixture, we do not include a decay term for the dye. Thus, these results should be considered an absolute upper bound on the possible spatial extent of the dispersal of oil or oil–dispersant mixture. The model results indicate that it is likely that oil-polluted waters from the Deepwater Horizon incident will, at some time over the six months following the initial spill date, be transported at relatively low concentrations over a significant part of the North-West Atlantic Ocean. However, this does not imply that oil will reach the eastern shores of North America, or that it will even be detectable. We present probabilities for the transport timescales and estimates of ensemble mean arrival times, and we briefly discuss the likely dispersion timescales and pathways of dye released in the subsurface ocean.