FY02 Annual Scientific Report - Oceanography Section 

The mission of the Oceanography Section (OS) is understanding the large-scale ocean circulation and the dynamics of climate through studies of the important processes in the ocean and sea-ice, in air-sea-ice interactions, and in coupled systems. 

Global Ocean Modeling 

A 1000-year control simulation of the present day climate has been completed without flux adjustments using the second version of the CCSM.  Minor modifications were made at year 350, which included all five components using the same physical constants.  There are very small trends in the upper ocean, sea-ice, atmosphere and land fields after year 150 of the control simulation.  The deep ocean has small but significant trends, but these are not large enough so that the control simulation couldn't be continued much further.   Jeffrey Kiehl (CMS) and Peter Gent (OS) are writing an overall assessment of this control run. In particular, several aspects of the simulation, especially in the Arctic region, are much improved over those obtained in the first version of the Climate System Model.  Other aspects, such as the tropical Pacific region, have not been improved much compared to those in the Climate System Model, version one.  Priorities for further model development are discussed in the conclusions section of a manuscript that will be submitted to the Journal of Climate. 

The fully coupled CCSM-2 quickly develops a warm SST bias along the coast of South America that exceeds 5C by year 500.  Both the causes and the large-scale impacts of this bias were explored in a series of coupled experiments by Large (OS) and others.  First, the zeroing of the runoff from the western Andes, which is excessive in CCSM-2, had virtually no effect.   However, artificially imposing upwelling favorable winds along the coast did significantly reduce the bias, suggesting that the weak and misdirected winds off the coasts of Peru and Chile are at least part of the problem.  These winds are interpolations that include atmospheric grid points where the orography is 1000m or more above sea level, and where the mountain surface roughness is much larger than found over the ocean.  In other coupled experiments the observed SST was specified along the Pacific coast of South America north to 5°S.  The major effects were a large contraction to the west of the double Inter-tropical Convergence Zone (ITCZ) structure in the South Pacific, and increased rainfall in the North Pacific ITCZ.  This result has motivated efforts to reduce the bias. 

Frank Bryan (OS) and Matthew Hecht, now Los Alamos National Laboratory (LANL), have implemented partial bottom cells, which permit the depth to take on any value, into the  Parallel Ocean Program (POP) CCSM-2 x1 configuration.  In a full ocean general circulation test, the partial cell cases did exhibit less grid scale noise in the vicinity of sloping topography.  On the other hand, the strong smoothing used in preparing the topography was shown to result in first order changes in the general circulation, particularly in the subpolar region of the North Atlantic.  A configuration with partial bottom cells and less smoothing was recommended as providing the most realistic representation of topographic effects, while controlling grid scale noise to acceptable levels.  Other CCSM-2 ocean component developments recommended by the CCSM Ocean Working Group are the implementation of variable diapycnal mixing depending on the roughness of the bottom topography, and the spatial dependence of the parameterized effects of mesoscale eddies. 

Gokhan Danabasoglu (OS) and William Large (OS) have conducted an extensive investigation into the nature of a decadal oscillation in the Southern Hemisphere of global model solutions.   The oscillation could be seen as modulations in the transports of the Antarctic Circumpolar Current and Indonesian Throughflow by as much as 10% of the mean.  It was robustly detected in sea surface temperature (SST) and deep ocean density gradients, as eastward propagating anomalies advected by the mean flow from the Indian Ocean sector and terminating at Drake Passage.  The maxima were found in the Pacific sector, where SST could change by as much as 3°C.  The oscillation was found to be damped by convective vertical mixing, but amplified by coupling to a sea-ice model and/or applying deep tracer time step acceleration, with accompanying changes in the period.  They found no evidence of any such oscillation in observations of either ACC transport, in situ and satellite SST, or deep hydrography.  Despite the limitations of these data, they showed that the modeled oscillation was unlike any naturally occurring variability, at least in the magnitude of the SST signal and in the frequency of occurrence of static instability at depth.  Therefore, in the absence of evidence to the contrary, they regarded the oscillation as spurious. The most effective control was found to be allowing the eddy-induced (bolus) velocity to operate at large isopycnal slopes.  The conclusion was that without this mechanism to flatten steep isopycnals and with vertical mixing tending to steepen slopes even more, the positive feedback leads to an unphysical decadal variability in the Southern Ocean. The results highlight the need for a physical parameterization of mixing for steep and near-convective isopycnal slopes.  A paper was submitted to Ocean Modelling. 

Gokhan Danabasoglu (OS) has been synchronously integrating a nominal three-degree horizontal resolution ocean model to equilibrium.  This follows the first accelerated equilibrium solution obtained by Danabasoglu and Large using the same configuration of the POP model.  A couple of acceleration methods are used routinely in the ocean modeling community to obtain equilibrium solutions without a synchronous equivalent.  Thus, the purpose of the present study is to analyze and document the effects of the acceleration techniques by obtaining an identical, but synchronous, equilibrium solution.  This synchronous case will be the first long-time integration of an actual, full physics GCM without any simplifications. 

Stephen Yeager (OS) and Large (OS) completed in FY02 an analysis based on previous 1958–1997 hindcast simulations which shows how interannual anomalies in the Pacific extratropics are communicated to the equatorial Pacific via isopycnal pathways, thereby contributing to subthermocline decadal variability on the equator.  An exploration of the origins of these spiciness anomalies reveals that they cannot be attributed directly to surface forcing changes, but arise in the extratropics through the complex, non-local interplay of atmosphere-ocean exchange and internal upper-ocean mixing processes.  Conditions which promote interannual spiciness variability in the ocean are identified as unstable upper-ocean salinity gradients paired with low density stratification, and all the world ocean basins have extratropical zones where such conditions exist.  An extrapolation of the analysis performed in the Pacific would imply that high variance on a range of isopycnal surfaces throughout the world ocean could be explained as the result of comparable extratropical anomaly generation.  These results were presented in poster form at the 2002 WOCE conference and are being submitted for publication. 

A multi-century ocean alone integration of the x1 CCSM-2 ocean component was completed using computing resources at CRIEPI in Japan.   The forcing is atmospheric reanalysis data from 1958 to 2000, repeated eleven times to give a 473-year integration.  Surface momentum, heat and freshwater fluxes were computed using a bulk flux forcing scheme from National Centers for Environmental Prediction (NCEP) reanalysis data (winds, air temperature, and humidity) and satellite data products (clouds, insolation, and precipitation). Also included in this integration were Ideal Age and Chlorofluorocarbon (CFC) passive tracers. The Ideal Age tracer in these experiments has been used to diagnose computational problems in the integration. 

Daisuke Tsumune (OS visitor from CRIEPI),  Keith Lindsay,  Scott Doney, now at Woods Hole Oceanographic Institution (WHOI),  Bryan, and Hecht have examined how well CFC tracer age approximates Ideal Age tracer in the North Pacific thermocline as a function of time and explored the magnitude of interannual variability in both age measures.  Overall, the modeled CFC-12 penetration is generally shallower than observed over the North Pacific basin except in the area between 5–10N associated with the divergence of North Equatorial Current and Counter current.  The CFC tracer age distribution is computed on isopycnal surfaces through the main thermocline using the partial pressure approach.  The CFC tracer age agrees broadly with the simulated ideal age fields in the subpolar and subtropical upper thermocline but tends to under-predict Ideal Age in the lower thermocline and tropics.  These deviations are expected due to non-linear mixing effects where the mean ideal age values approach the length of the CFC transient.  An analysis of the simulated interannual variability in CFC and ideal age along isopycnals shows substantial rms variability, with implications for geochemical rate estimates and the representativeness of individual observational sections. In addition, simulated physical interannual variability compares well against historical observations. 

North Atlantic Basin Studies 

Bryan (OS) continues the analysis of a series of North Atlantic basin integrations at resolutions between 0.4 and 0.1 degrees that are being carried out in collaboration with Richard Smith, Matthew Maltrud (LANL), and Hecht (NCAR, LANL).  Integrations currently underway include exploration of topographic specification (partial bottom cell) sensitivities and response to high wave number and high frequency scatterometer derived wind stress.  Analysis during the last year has focused on understanding differences in the simulation of the North Atlantic between this series of basin scale models and recent global model experiments at similar resolution undertaken by LANL and Naval Postgraduate School, where the latter have not performed as well. 

Sea Ice Modeling 

Changes to the CCSM-2 sea ice component over the last year primarily have been to improve the software engineering structure, efficiency, and "user-friendliness" of the code.  Additionally, a considerable amount of time has been spent documenting the model physics and source code.   Bruce Briegleb (OS) and Julie Schramm (OS) have taken the lead in this documentation effort with collaboration by members of the CCSM Polar Working Group, including Cecilia Bitz (University of Washington), Marika Holland (OS), Elizabeth Hunke (LANL) and Bill Lipscomb (LANL). 

Potential new parameterizations of the ice/ocean system have been tested using single-column model integrations.  In particular, Holland has analyzed alternative methods for simulating mixing in leads embedded within the pack ice.  This was motivated by observations from the Surface HEat Budget of the Arctic (SHEBA) field project which showed that leads within the vicinity of the SHEBA camp could obtain very warm and fresh surface conditions during the summer.  Single-column ice-ocean mixed layer model simulations of this SHEBA event were performed and it was found that more realistic simulations resulted when the lead surface was embedded within the pack ice, and hence isolated from lateral mixing with the ocean conditions under the ice.  This influenced the lateral melt rates and open water formation in the simulations and modified the albedo feedback.  A manuscript detailing this work has been accepted by JGR-Oceans. 

Ocean Biogeochemical Modeling 

Oceanic biogeochemical processes as they relate to the global carbon cycle and climate system are under study by Doney and Lindsay (OS).  The biogeochemical model used in the Ocean Carbon Model Intercomparison Project (OCMIP) experiments has been extended to form a prognostic carbon-cycle model.  The model now computes production prognostically, as opposed to relying on a nutrient climatology to infer production, and incorporates iron limitation and a full iron cycle.  This model will represent the ocean carbon cycle in the CSM-1 framework and will be used in experiments related to the Coupled Carbon Cycle Climate Model Intercomparison Project (C4MIP). 

The most active U.S. Program in ocean carbon cycle science — the Synthesis and Modeling Project (SMP) of the U.S. Joint Global Ocean Flux Survey (JGOFS) — is coordinated by Doney and Joan Kleypas (OS) and Jorge Sarmiento (Princeton University).  The SMP, which is mandated to synthesize data collected during the U.S. JGOFS field studies into a set of models that reflect the current understanding of the oceanic carbon cycle, includes 65 projects funded by NSF, NASA, NOAA, and DOE, and over 100 investigators.  Doney and Kleypas actively coordinate SMP activities through instigation and organization of meetings, collection and preparation of project data, editing of SMP special issues, and overall facilitation of investigator interactions, particularly between observationalists and modelers. 

This year, Doney and Kleypas organized the annual SMP meeting at the Woods Hole Oceanographic Institution and helped organize a series of workshops which focused on issues such as marine calcification, midwater processes, iron dynamics, and synthesis and modeling of the equatorial Pacific.  The first SMP Special Issue in Deep Sea Research was published in January of this year, and Doney is currently editing a second SMP Special Issue, slated for publication in 2003.  Ongoing collection and dissemination of SMP data are coordinated between NCAR (Kleypas), Woods Hole (Glover), U. Washington (Sabine) and NOAA Pacific Marine Environmental Laboratory (Hankin).  These combined efforts have led to implementation of a live access server for disseminating SMP data, as well as development of a server for displaying non-gridded data sets from U.S. JGOFS field programs.  This information about the SMP is maintained on the website: http://usjgofs.whoi.edu/mzweb/syn-mod.htm. 

Joan Kleypas (OS) also continues collaborative research on coral reef ecosystem response to climate change with Bob Buddemeier (University of Kansas), Rich Aronson (Dauphin Island Sea Lab) and Janice Lough (Australian Institute of Marine Science). 

Air-Sea-Ice Interactions 

The fresh water budgets of the Arctic Ocean and the mechanisms that drive variability in the transport of fresh water from the Arctic to the North Atlantic are being investigated within the CCSM control integration. This work is a collaborative effort between Holland, A. Proshutinsky (WHOI) and H. Goosse (Université catholique de Louvain, Belgium).  The analysis of the CCSM-2 simulations is being compared to results from the ECBILT-CLIO model.  This work is in the preliminary stages.  Analysis has considered mechanisms which force variations in Arctic ice volume and how these volume anomalies contribute to variations in the volume of ice exported from the Arctic. Additional analysis has examined the oceanic fresh water transport through Fram Strait, Barents Sea, and the Canadian Arctic Archipelago.

The mechanisms forcing northern hemisphere polar amplification in coupled climate models have been investigated by Holland and Bitz (U. Washington).  The polar amplification varies considerably between the 15 different coupled climate models that were analyzed.   It was found that conditions of the control climate sea ice, including the extent and thickness, have a significant influence on the location and amplitude of polar warming at 2xCO² conditions. Additionally, changes in cloud cover and ocean heat transport into the Arctic are significantly correlated to the polar warming.  A manuscript discussing these results has been submitted to Climate Dynamics. 

A major update to the ocean forcing data sets used in the OS has been initiated.  Most importantly consistent surface radiation, both long and short wave, monthly data sets have been acquired from W. Rossow (NASA/GISS). They cover the period 1983 through 2000, which doubles the record length.  The NCEP atmospheric state data have also been extended through the year 2001, and it is hoped that new insights into biases in humidity and wind speed will produce a globally balanced heat flux without correcting the radiation.  We have also shifted to the Global Precipitation Climatology Project precipitation, which covers much the same time period. With these data it appears that the past practice of spatially blending different data sets can be avoided.