1999

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Oceanography Section

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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, in air-sea interaction, and in coupled systems.

Global Ocean Modeling

OS continues to maintain and upgrade the ocean component of the NCAR Climate System Model (CSM).  A new responsibility taken on this year is to maintain and upgrade the sea-ice component of the NCAR CSM.  Bruce Briegleb (OS) has had most of this new responsibility, since transferring into the OS this year.  A large percentage of the section members' (Julianna Chow (OS), Matthew Hecht (OS), Brian Kauffman (OS), and Nancy Norton (OS)) time and effort this year has gone into changing the base codes used for the CSM ocean and sea-ice components.  We are moving to use the Parallel Ocean Program (POP) and Sea-Ice (CICE) codes that have been developed at Los Alamos National Laboratory (LANL).  This work is being shared with the LANL group, which includes Richard Smith, John Dukowicz, Philip Jones, Matt Maltrud, Elizabeth Hunke, and Bill Lipscomb.  The work is progressing quite slowly because, in addition to changing codes, the models are being implemented on the new IBM computer, as well as on the existing SGI computer.   All the physics packages from the old ocean component have not yet been coded into POP, so this transition is not expected to be completed for several months.

The development of a global, upper ocean model by James McWilliams (OS/University of California, Los Angeles) and Gokhan Danabasoglu (OS) has been completed, based on the old ocean component code.  This model is designed for work on seasonal-to-interannual timescales, and the solutions have been shown to equilibrate in about 30 years, as opposed to the several thousand year equilibration time of a full-depth ocean model.  A manuscript documenting the code, its performance, and some initial integrations has been submitted to the Journal of Climate.

A new formulation for horizontal viscosity has been implemented in both the full depth and upper ocean components.  It is based on having different coefficients in the zonal and meridional directions.  By far the largest benefit of this new viscosity is that the equatorial current system in the tropical Pacific Ocean now compares very well with observations, whereas before, this current system was very weak with the resolutions used for the long climate integrations.  (This figure (25K) shows zonal sections across the Pacific of zonal velocity at the equator: (a) annual average of model case B2, (b) Thermal Array in the Ocean (TAO) current meter climatological annual mean, and (c) TAO current meter 1 year mean, August, 1996 through July, 1997.  The contour interval is 10 cm/s and westward flow regions are shaded.)  This improvement in the ocean component was one of several upgrades to the CSM thought necessary to improve the El Niņo-Southern Oscillation variability.   A manuscript documenting this improvement by William Large (OS), Danabasoglu, McWilliams, Peter Gent (OS), and Frank Bryan (OS) has been submitted to the Journal of Physical Oceanography.

A manuscript (and figures) entitled "What sets the mean transport through Drake Passage" by Gent, Large, and Bryan has been submitted to the Journal of Geophysical Research.  The manuscript is an analysis of twelve experiments using the CSM ocean component alone, coupled to a sea-ice model, and in fully coupled CSM mode.  (This figure (54K) shows the plot of the Cape Horn Sverdrup (Sv) transport in Sv against Drake Passage transport in Sv from ten of the model experiments.)   These experiments have a very wide range of strengths of the Antarctic Circumpolar Current (ACC) and the transport through Drake Passage.  The conclusion is that the transport is set mostly by the zonal wind stress or meridional Ekman transport and by the strength of the thermohaline circulation off the Antarctic shelf.  This conclusion is not in agreement with previous hypotheses for what governs Drake Passage transport.   It is shown that the transport is definitely not set by the curl of the wind stress at the latitude of Cape Horn or by the square root of the average zonal wind stress.   Both these previous theories ignore the effect of the global thermohaline circulation altogether.

Comparison calculations have been run to explore the effects of high-frequency and high-wavenumber wind forcing on basin scale general circulations in (a) the Pacific Ocean, and (b) the Mediterranean Sea.  The high-resolution winds in each comparison are derived from the NASA Scatterometer (NSCAT) record as described in the appendix of a paper by Ralph Milliff (OS) et al. (1999).  These collaborations are continuing between Milliff and Dale Haidvogel (Affiliate Scientist, Rutgers University), and Nadia Pinardi (Istituto per lo studio delle Metodologie Geofisiche Ambientali-Consiglio Nazionale delle Ricerche, Italy, IMGA-CNR), respectively.  In the collaboration with Haidvogel, the comparison calculations with respect to National Centers for Environmental Prediction (NCEP) winds are just now completing in a five-layer spectral-element model with finer than 20 km resolution in the equatorial and coastal waveguides.  The collaboration with Pinardi's group in Italy compares the Mediterranean general circulation response with the response to European Centre for Medium-Range Weather Forecasts (ECMWF) winds in a 1/8 degree ocean model.  The response to high-resolution winds exhibits higher-wavenumber spatial structure that is in better qualitative agreement with observations.  Quantitative analyses are beginning to determine the direct versus indirect forcing of this favorable response.

Satellite altimetry and gravimetry systems with typical orbit repeat periods of 10-30 days will alias oceanic variability at shorter periods into some longer timescale.  Bryan, along with Craig Tierney (University of Colorado, Boulder, CU), John Wahr (CU), and Victor Zlotnicki (Jet Propulsion Laboratory), used a non-eddy resolving version of the POP global ocean model to de-alias ocean Topography Experiment (TOPEX) altimeter measurements of sea surface height for the period 1995-1997.  The model has predictive skill, and was able to reduce the residual variance of the mapped sea surface height by up to 7%.  The magnitude of the correction is comparable to some of the other routine corrections applied to the TOPEX data, such as the sea state bias or ionospheric corrections.  A model forced with both winds and atmospheric pressure loading provided the best results.  The study showed that, for these high frequencies, a barotropic model is adequate, making a quasi-realtime correction practical.  On the other hand, the model did not have adequate skill for periods less than five days, so further model refinement will be required before operational corrections can be provided to the TOPEX user community.  

North Atlantic Basin Studies

Analysis of a series of integrations at 0.4, 0.2, and 0.1 degrees horizontal resolution that were carried out in collaboration with Richard Smith (LANL) and Matthew Maltrud (LANL) continues. An analysis of the structure of the Gulf Stream in stream-following coordinates provided the first quantitative comparison between the Gulf Stream in an eddy-resolving simulation and the dense observations obtained during the Synoptic Ocean Prediction (SYNOP) program. While the separation and maximum flow speeds and general cross-stream structure in the simulation agree quite well with observations, there are a number of notable biases. In particular, the meander envelope is too broad to the west of the New England Seamounts, and the flow becomes more barotropic than observed.  This suggest that the baroclinic eddies are too strong in the model in this region. On the other hand, a detailed comparison of the wavenumber-frequency distribution of sea surface height variability shows the 0.1 degree model to be in excellent agreement with observations, in contrast to the lower resolution versions of the model.

Bryan and Scott Doney (OS) continued a collaborative project with Dennis McGillicuddy (Woods Hole Oceanographic Institution, WHOI) and Smith (LANL) and Maltrud (LANL) to study mesoscale biological-physical interactions in the framework of the POP eddy-resolving North Atlantic Basin simulations. The scientific rationale for this work sprung from a number of recent observational and modeling studies that showed that the biological productivity in the nutrient-poor regions of the subtropical gyre appeared to be significantly elevated over that expected only if the supply of nutrients to the surface ocean was via weak background diffusion.  Mesoscale eddies can generate an asymmetric vertical nutrient flux when an isopycnal surface is displaced upward into the euphotic (light) zone, and this may be a major cause for the discrepancies in previous work.   We are pursuing two experimental tracks: (a) full ecosystem simulations at both eddy-permitting and non-eddy-resolving resolutions, (b) and a simplified nutrient uptake simulation at the highest available spatial resolution.  The high-resolution (1/10 degree) nutrient uptake simulation was designed and completed in Fiscal Year 1999 and the analysis is underway.  An NCAR postdoctoral fellow, Ivan Lima (OS), also began implementation of a more complete nitrogen-based ecosystem model into a North Atlantic version of POP.

Doney, Large, Marika Holland (OS), and Stephen Yeager (OS) have completed two companion simulations of the global NCAR CSM Ocean Model (NCOM) using realistic atmospheric forcing from 1958-1997.  The runs differ in the sea-ice forcing, which is specified in the model simulations, with one allowing for variable sea-ice extents taken from observations.  An analysis of the North Atlantic variability in these model simulations is on-going.  An empirical orthogonal function (EOF) analysis indicates that a substantial fraction of the observed interannual to decadal variability can be constructed from the model solutions.  Work in progress includes:  (a) evaluating the magnitude and phasing of the simulated ocean variability, (b) exploring the dynamical processes which govern the oceanic response to anomalous wind and buoyancy forcing, and (c) determining the effects of variable sea-ice forcing on the simulated variability.  In particular, the formation of surface water mass anomalies and their propagation into the ocean interior is being examined.

Air-Sea Interaction

Milliff, Chris Wikle (visitor, University of Missouri), Mark Berliner (visitor, Ohio State University), Large and Peter Niiler (visitor, University of California, San Diego, Scripps Institution of Oceanography) have begun a three-year NASA-funded project on a Bayesian Hierarchical Model (BHM) system to hindcast upper ocean momentum convergence and pre-conditioning for oceanic deep convection in the Labrador Sea.  The BHM theory blends in-situ observations and remotely-sensed observations with dynamical models of the upper ocean.   In-situ measurements are taken from autonomous drifting buoys (MiniMETs) that measure wind speed and direction as they track the upper ocean currents.  The method will employ TOPEX altimeter data and MiniMET drifter trajectories to inform dynamical prior probability distributions for the upper ocean circulation and surface vector wind data from NSCAT and MiniMET drifters to inform prior probability distributions for the surface windfield. The posterior distributions for these fluid flows will inform boundary-layer prior probability distributions on either side of the air-sea interface to predict posterior distributions of upper ocean momentum convergence.  The distributions of pre-conditioning for deep convection will be compared with oceanographic field observations obtained during the Labrador Sea Experiment (Winter 1996-1997).

In duties associated with the NASA Scatterometer Science Working Team, Milliff supported launch and calibration/validation activities for the Quick Scatterometer (QuikSCAT) launched in July, 1999.  The calibration/validation experiment is in collaboration with Niiler (Scripps) wherein MiniMET drifting buoys are being continuously deployed in the Japan/East Sea by Korean fisheries vessels and as part of an Office of Naval Research (ONR) field campaign.  Preliminary comparisons with the unreleased QuikSCAT winds demonstrate surface wind speeds within satellite design specifications for the range 1 to 8 m/s. Directional variability is slightly larger than pre-launch specifications, but this is expected to be remedied in a re-processing of the early QuikSCAT data before release to the scientific community in January or February, 2000.

Milliff and Jan Morzel (OS) have analyzed the long-term average global wind-stress curl field computed from the entire NSCAT record (October, 1996 through June, 1997).  The average wind-stress curl differs from classical notions in that (a) narrow, large-amplitude, boundary wind-stress curl features appear along eastern ocean boundaries of the sub-tropical oceans in Northern and Southern Hemispheres; and (b) a mesoscale patchiness dominates the long-term average wind-stress curl over the sub-polar and sub-tropical gyres, and over the ACC.  The patchiness is shown to be an artifact of insufficient sampling of large-amplitude, intermittent wind-stress curl signals associated with atmospheric storms over the open ocean.  Milliff has helped NASA make the case that multiple, coordinated, coincident scatterometer missions are required to sample the large-amplitude surface wind events that contribute to a climate signal in the ocean general circulation.

Ocean Biogeochemical Modeling

Marine biogeochemical processes as they relate to the global carbon cycle and climate system are under study by Doney.  Three paths are currently being pursued: (a) ecosystem modeling and remote sensing; (b) global tracer and biogeochemical modeling; and (c) observational data analysis.  A global, mixed-layer marine ecosystem model has been developed, based on extensions of the simple, nitrogen-based ecosystem model for the Sargasso Sea.   The mixed-layer model is being used by Doney and a joint Advanced Study Program/Climate and Global Dynamics postdoctoral fellow, Keith Moore, as a test bed for evaluating and improving biological parameterizations for such things as iron limitation and zooplankton grazing.   Satellite ocean color images, a proxy for surface phytoplankton distributions, play an important role in evaluating model ecosystem solutions, and effort is being devoted to the data analysis of the satellite images in collaboration with David Glover (WHOI) and Montserrat Fuentes (visitor, Geophysical Statistics Project/North Carolina State University).   A preliminary statistical analysis has been carried out on the mesoscale spatial variability and length-scales of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color data for the North Atlantic.

On a second front, a series of tracer and carbon cycle calculations are underway in the global CSM ocean model.  The tracer work provides valuable information on the ventilation rates of the ocean model on timescales from several years out to centuries. The carbon simulations are used to study the physical and biological factors governing the ocean inorganic carbon system, as well as the links with the atmospheric and terrestrial biosphere.  Doney and Keith Lindsay (OS) are participating in the international Ocean Carbon Model Intercomparison Project (OCMIP) (main collaborators: Ray Najjar (The Pennsylvania State University); and James Orr (Commissariat ā l'Energie Atomique, France)). OCMIP has a standard set of tracer simulations, and a number of new (or updated) tracer simulations have been generated in NCOM for OCMIP over the last year: natural equilibrium radiocarbon and abiotic carbon; equilibrium biotic carbon (diagnostic surface production); anthropogenic perturbation radiocarbon and anthropogenic carbon; and chlorofluorocarbons. Analysis of the NCOM solutions, separately, and in the context of OCMIP, is currently being carried out.

Observational and data analysis studies make up the third activity in marine biogeochemistry and are part of long-term collaborations with scientists at other institutions.  In collaboration with Inez Fung (University of California, Berkeley) a major project was completed in the last year on the supply and phytoplankton demand for the limiting element iron to the ocean surface euphotic zone.  Joan Kleypas (OS) collaborates extensively with Bob Buddemeier (University of Kansas), Jean-Pierre Gattuso (Observatoire Oceanologique, France), and Bradly Opdyke (Australian National University) on the role of coral reefs and other shallow carbonate systems in the global carbon cycle.

Geophysical Fluid Dynamics and Turbulence

McWilliams and Sonya Legg (WHOI) have investigated the computational fluid dynamics inhomogeneous deep convection.   This occurs in subpolar regions of the ocean where pre-existing mesoscale eddies induce spatial variations in the density stratification, such that when large-scale winter cooling occurs the resulting convection penetrates most deeply in the centers of cyclonic eddies.  Thus, the movement of heat out of the oceans occurs as much by secondary circulations around the disturbed eddies as it does by the small-scale plumes directly excited by the surface cooling, and the associated cold anomaly penetrates deeper and faster on average than it would without the eddies present.  In this situation, several other phenomena occur: the cyclonic eddies are made stronger by the preferential cooling in their cores, become baroclinically unstable to the point of fission, and then release and laterally spread their core temperature anomalies; the mesoscale eddy field gains energy through the convection and its circulation reaches to greater depth; and much of the fine-scale variability in temperature and salinity arises (from the pre-existing interior gradients) with a significant positive correlation, hence, relatively weak associated density fluctuations.  These phenomena have observed counterparts in a recent experiment made in the Labrador Sea.

McWilliams, in collaboration with Chin-Hoh Moeng (Mesoscale and Microscale Meteorology Division, MMM) and Peter Sullivan (MMM), has developed a computational fluid-dynamical model with a moving lower boundary.  Its first use is to calculate the winds over a surface gravity wave, where ocean wave theory is used to specify the boundary movement as the surface orbital velocities propagating at a speed C.  The problem is posed as a variant of the classical Couette shear flow, with a uniform fluid density and a flat boundary far above the waves moving with speed U, at a sufficiently high Reynolds number that the winds are fully turbulent.  The wind behavior is strongly dependent upon the ratio A = C/U ("wave age").  For small A, pressure forces develop at the boundary to enhance the drag force on the winds, but with increasing A, this effect decreases and even reverses to diminish the drag as A approaches values typical of equilibrium wind-waves in nature.  The turbulent Reynolds stress above the waves is partly due to wind fluctuations that are phase-correlated with the boundary motion, up to a height on the order of the boundary wavelength above which only boundary-uncorrelated winds carry the stress.  This alters the shape of the mean wind profile, so that Monin-Obukhov similarity profile (the norm for winds above a solid stationary surface) does not occur within this wave-correlated surface layer, though it does occur at greater heights.  For intermediate values of A, the wave-correlated winds exhibit boundary flow separation and have a critical layer near the surface, across which their associated Reynolds stress reverses in sign.

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