Oceanography Section Narrative

The mission of the Oceanography Section (OS) is to understand 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 and Sea-Ice Modeling

Every member of the OS has spent a large fraction of their time on work towards the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). This includes the final parameter choices that went into defining the IPCC version of the Community Climate System Model, Version 3 (CCSM3) and executing the many integrations at various computing centers. Most important, however, is the analysis of these integrations and writing of papers for a special issue of the Journal of Climate dedicated to CCSM3. The papers in preparation are:

Attribution and implications of upper ocean biases in CCSM3 by William Large (OS), Gokhan Danabasoglu (OS), and Ilana Wainer (Universidade de Sao Paulo, Brazil). The specific purpose of this work is to expose some of the largest and potentially most important upper ocean biases in the CCSM3 fully coupled simulation of the present day climate. These biases include not only sea surface temperature (SST) and sea surface salinity (SSS), but also the mean and seasonal cycle of equatorial temperature, salinity, and velocity. Of particular interest are attribution and impact determinations. Biases that are merely the end product of cumulative forcing and model errors and that do not feedback into the large scale, multi-component solution are perhaps tolerable, and less of a priority than those that significantly impact extensive elements of the solution.
Diurnal coupling in CCSM3 by Danabasoglu, Large, Joseph Tribbia (CDP), Peter Gent (OS), and James McWilliams, University of California, Los Angeles (UCLA). The primary purpose of this work is to evaluate the standard, once-a-day air-sea coupling implemented in the CCSM3 relative to possible alternatives. These alternatives include one-hourly and three-hourly coupling, as well as an idealized diurnal cycle parameterization, which distributes the daily net solar radiation received from the coupler over a day. With this scheme the ocean model is still coupled once a day. The short wave heat flux is zero between 6 p.m. and 6 a.m., and in between it follows a cosine function attaining its peak value at noon. The paper documents the effects of these alternatives on the coupled model drift, deep cycle turbulence, rectification of SST, the atmospheric mean state, and El Niño Southern Oscillation (ENSO) variability. It is demonstrated that the idealized diurnal cycle significantly reduces the cold bias in the equatorial Pacific and improves the ENSO variability.
Thermocline and deep ocean ventilation during the 20^th Century in the CCSM3 by Gent, Frank Bryan (OS), Danabasoglu, Keith Lindsay (OS), Daisuke Tsumune (OS), a visitor from the Central Research Institute of Electric Power Industry (CRIEPI) in Japan, Matthew Hecht, Los Alamos National Laboratory (LANL), and Scott Doney, Woods Hole Oceanographic Institution (WHOI). An assessment of the ocean component is performed by evaluating the ventilation rate of the thermocline and deep-ocean in simulations of the 20^th Century. An ensemble of these simulations from 1870–2000 has been made with the CCSM3 using the best estimates of solar, volcanic, and greenhouse gas forcing variations over the 20^th Century. Results from the ocean component are compared to observations from the second half of the 20^th Century. These comparisons include the uptake of chlorofluorocarbon (CFC) and heat, changes in ocean salinity, and sea level.
Transient response of the thermohaline circulation and ocean ventilation by Bryan, Danabasoglu, Tsumune, and Nori Nakashiki (CRIEPI). This work concentrates on the response of the thermohaline circulation (THC) to the transient climate forcing, primarily focusing on the dominant physical processes affecting the THC. The effects of the atmospheric resolution on the strength and variability of THC are also considered. The ideal age tracer is used to analyze how the changes in ventilation relate to the THC changes. Implications of the ventilation rate changes on CO₂ uptake are also discussed.
The low resolution version of CCSM3 by Stephen Yeager (OS), Christine Shields (CCR), James Hack (CMS), and Large. A description of the new developments and the accompanying (coupled and uncoupled) control simulations of the low resolution ocean component of CCSM3 is given. An assessment of these results is presented in comparison with those from higher resolutions versions. The primary focus is to determine if this low resolution version can address the needs of the paleoclimate, biogeochemistry, and climate change communities. The metrics include the overturning circulation of the North Atlantic, SST biases, equatorial circulation, and ENSO variability.

In addition, Gent and Danabasoglu have finished a study of heat uptake in the ocean component of the CCSM2. They used results from the 1000-year control run, the 1% per year increasing CO₂ run, and the doubled CO₂ run. They analyzed heat content changes in the upper 300 m and the upper 3 km and compared them to an analysis of ocean observations published by Levitus et al. in Science in 2000. They conclude that the rate of heat uptake is about right in CCSM2, and conclude that this supports the climate sensitivity value, which is quite low. They also documented a weak decline in the North Atlantic THC in the 1% CO₂ run, in a paper published in Journal of Climate.

Tsumune, Lindsay, Doney, Bryan, and Hecht have completed a study of the variability in the North Pacific using CFC ages and ideal age in a CCSM Parallel Ocean Program (POP) experiment with realistic forcing data from 1958 to 2000. The simulated CFC concentrations are in good agreement with World Ocean Climate Experiment (WOCE) observations in shallow layers. The CFC tracer ages are computed using the partial pressure approach and the ratio approach. The relationships of pCFC-11, pCFC-12 age, ratio age, and ideal age are as expected from previous studies by simple models. Temporal changes and the spatial distribution of the difference between these age measures have been described for the first time in a manuscript that is ready for submission.

Wanli Wu (OS), Large, Danabasoglu, and Gent are the co-investigators of the two funded Climate Process Team (CPT) proposals. These are highly collaborative projects bringing observationalists, process modelers, and modeling centers (NCAR, NOAA's Geophysical Fluid Dynamics Laboratory, GFDL, and NASA's Goddard Space Flight Center, GSFC) together. A goal is to expedite the incorporation of some new discoveries into the climate models. The funded proposals concern the deep gravity currents and entrainment with particular emphasis on their role in water mass formation, and hence, the THC, and the interaction between meso-scale eddies and mixed layers. Among the objectives are to develop new or modified parameterizations that can be reliably used in ocean models and to assess their impacts on the climate model simulations. The gravity current proposal should be particularly challenging for the CCSM3 z-coordinate ocean model. Wu has implemented a version of the Price and Baringer overflow model and obtained improvement in the representation of the Mediterranean Outflow. He has also implemented making the eddy parameterization coefficient depend on the buoyancy frequency, also with encouraging initial results.

High Resolution Ocean Simulations

A major effort in high resolution (0.1 degree) global ocean modeling in collaboration with oceanographers at CRIEPI, LANL, and the Naval Postgraduate School (NPS) continues using computational resources at the Earth Simulator in Yokohama, Japan. To address some of the biases that were observed in an initial control integration, an extensive series of sensitivity experiments surveying a broad range of parameter space has been carried out. A combination of anisotropic viscosity and an anisotropic generalization of the Gent-McWilliams (GM) eddy parameterization were found to provide the best representation of the Kuroshio region, ameliorating an unrealistically steady large meander off of the Japanese island of Shikoku. An unrealistic path of the North Atlantic Current has proven to be more difficult to correct. The adiabatic nature of the anisotropic GM formulation is an important element of preparing the high resolution model for long running climate simulations. Analysis and further experiments are ongoing to prepare an ocean component of a very-high resolution coupled climate system model.

Richard Smith (LANL) and Gent have developed an anisotropic generalization of the GM parameterization for eddy-induced tracer transport and diffusion in ocean models. They focus on its application in high-resolution eddy-permitting and eddy-resolving models, where it provides an adiabatic alternative to the more commonly used biharmonic horizontal diffusion operator. A series of numerical simulations of the North Atlantic Ocean are conducted at 0.2 degree resolution using anisotropic viscosity, anisotropic GM, and biharmonic mixing operators to investigate the effects of the anisotropic forms, and to isolate changes in the solutions specifically associated with anisotropic GM. They then conduct a high resolution 0.1 degree simulation using both anisotropic forms and compare the results with a similar run using biharmonic mixing. Modest improvements are seen in the mean wind-driven circulation with the anisotropic forms, but the largest effects are due to the anisotropic GM parameterization that eliminates spurious diapycnal diffusion and leads to significant improvements in the model THC. These improvements include the meridional heat transport, overturning circulation, and deep water formation and convection in the Labrador Sea. A manuscript on this work is about to be published the Journal of Physical Oceanography.

Ocean Biogeochemical Modeling

Oceanic biogeochemical processes, as they relate to the global carbon cycle and climate system, are being studied by Lindsay and external collaborators, including Doney, Jefferson Keith Moore, University of California, Irvine (UCI), and Mathew Maltrud (LANL). Two paths currently being pursued are: (a) global ocean biogeochemical and tracer modeling and, (b) the study of the relationship between the ocean carbon cycle and the global carbon cycle.

Work on (a) has primarily consisted of continued studies on the incorporation of alternative tracer advection schemes into the CCSM POP. This research is building on the experience of other ocean modeling centers, notably the Massachusetts Institute of Technology (MIT) and GFDL. The work is focusing on flux limited schemes that are based on the work of Peter K. Sweby. In the coming year alternative advection schemes will be evaluated on their performance on a variety of tracers, including the potential temperature and salinity, and passive tracers.

With respect to (b) above, a suite of multi-century, fully coupled, carbon-climate integrations has been performed using a modified version of the Climate System Model, Version 1.4 (CSM1.4). These experiments include a stable 1000 year pre-industrial control run and multiple 180 year historical and 100 year future scenario fossil fuel emissions experiments. These experiments are being analyzed to understand connections and feedbacks between the carbon-cycle and the physical climate.

Air-sea-ice Interactions

Analysis of the CCSM2 control integration has been performed to examine variations in the southern hemisphere ice cover. This is collaborative work between Marika Holland (OS), Cecilia Bitz (University of Washington) and Elizabeth Hunke (LANL). The dominant mode of ice variability exhibits a dipole pattern with positive anomalies in the Pacific associated with negative anomalies in the Atlantic. This variability compares well to the observations. The mechanisms driving this variability and the atmosphere and ocean conditions associated with this variability have been investigated. The influence of ENSO variability and the southern annular mode on variations in southern ocean sea ice extent and thickness have also been considered. A manuscript documenting this research is under review at Journal of Climate.

In related work, in collaboration with Richard Cullather (ASP, now at Lamont Doherty Earth Observatory, LDEO), an evaluation of the CCSM3 Antarctic sea ice simulation compared to observations has been performed. This includes an analysis of the simulated interannual variability. Additionally, an appraisal is performed by Cullather of the recently released European Center for Medium-Range Weather Forecasts (ECMWF) 40-Years Re-Analysis data set in Antarctic latitudes.

Work has continued to investigate mechanisms forcing the relatively high Arctic amplification simulated by the CCSM3 model. Sea ice model parameterizations which may affect the simulation of high latitude feedbacks are being assessed by Holland, Julie Schramm (OS), Bitz, Bill Lipscomb (LANL) and Hunke. More specifically, an evaluation of the influence of the sub-gridscale ice thickness distribution (ITD) on climate sensitivity is underway. The ITD parameterization has a significant effect on the simulated climate, and there are indications that this parameterization also enhances the albedo feedback by resolving thin ice categories that are easily melted away under a climate change scenario. A manuscript discussing the influence of the ITD on the simulated climate is in preparation for Journal of Climate.

Work is underway to determine the conditions and consequences of a seasonally ice-free Arctic Ocean. This involves collaborative research between Holland, Laurence Hamilton (University of New Hampshire), Gifford Miller (University of Colorado), Don Perovich (Cold Regions Research and Engineering Laboratory, CRREL), Bruce Peterson (Marine Biological Laboratory, MBL), and Gavin Schmidt (NASA Goddard Institute for Space Studies, GISS). Climate model projections suggest that the Arctic may become seasonally ice-free in the not too distant future (in approximately the next 100 years depending on the forcing scenario). However, there are many feedbacks, which may influence how and when an ice free Arctic state is reached. Many of these, such as the potential for changes in Arctic Ocean stability to affect ice formation, are not well understood. For this study, evidence from multiple sources, including numerical model experiments, the instrumental record, process studies, and paleoclimate analogues, is used to develop a conceptual model of a seasonally ice-free Arctic system. Uncertainties in the projected climate state and processes that may lead to considerably different conditions are assessed. Additionally, the consequences that an ice-free Arctic state might have on socio-economic and biological systems are being explored.

In collaborative research between Hugues Goosse (University Catholique de Louvain) and Holland, the mechanisms forcing simulated interdecadal climate variability in the Arctic have been assessed from the CCSM2 control simulation. In previous studies a number of mechanisms have been proposed to explain natural climate fluctuations in the Arctic. These mechanisms and their interrelation were assessed within the context of the CCSM2. It was found that changes in ocean ice exchange and heat transport in the Barents Sea dominate in forcing the large-scale Arctic surface temperature variability. Changes in atmospheric circulation are consistent with a wind forcing of this variability, while changes in the THC are more weakly related. A manuscript discussing these results has been submitted to Journal of Climate.

The fresh water budgets of the Arctic Ocean, and the mechanisms that drive variability in the budget terms, including the transport of fresh water from the Arctic to the North Atlantic, are being investigated within the CCSM3 control integration. This work is a collaborative effort between Holland, and Joel Finnis and Mark Serreze of the University of Colorado (CU). The primary goal of this project is to explore the progressive integration of the impacts and processes of Arctic freshwater cycling on the local, pan-Arctic, and global scales. This includes the study of processes that modify the Arctic freshwater budgets in the terrestrial, atmospheric, and oceanic environments; the integration of these processes to produce the exchange of freshwater between the Arctic and North Atlantic; and ultimately, the influence of these processes on northern North Atlantic deep water formation.

Forcing of the Ocean Circulation

The development of a complete set of global forcing fields for ocean and sea-ice models is described in Large and Yeager (2000, NCAR/TN-460+STR). The forcing is based primarily on the International Satellite Cloud Climatology Project – Flux Data (ISCCP-FD) surface longwave and solar radiation (Zhang et al., 2004), the National Centers for Environmental Prediction (NCEP) reanalysis (Kalnay et al., 1996), and a blend of satellite precipitation products. Ancillary data sets of higher quality have been used to objectively correct the surface winds, air temperature, and humidity to produce a globally balanced heat and freshwater flux. The interannually varying forcing (IVF) is complete from 1983 through 2000, and the NCEP based fields vary all the way back to 1958. In response to an expressed community need to force ocean and sea-ice models with a repeating annual cycle of forcing, these datasets have been processed into such a form. The criteria were to preserve the 43-year mean fluxes, to have realistic storm events, and to transition smoothly from December 31 to January 1, all without changing the standard forcing infrastructure. Since this forcing is not overly biased to any particular year, it is referred to as "Normal Year Forcing" (NYF).

The two CCSM3 ocean model configurations, with respective nominal horizontal resolutions of 1 and 3 degrees, have both been forced with several cycles of IVF and about 100 years of NYF. This standard forcing has also been applied to coupled ocean and sea-ice models as part of the Coordinated Ocean Reference Experiments (CORE). Several national and international groups have adopted this forcing for their CORE integrations, so that solution differences can be compared with minimal consideration of forcing contributions.

Investigations are still continuing to study the transfer of wind energy to the ocean through resonant inertial forcing. One aspect of this work, in collaboration with Robert Stockwell and Ralph Milliff of Colorado Research Associates (CoRA), has found that everywhere, but especially at 30 degrees latitude, ocean surface winds do have a preferential anticyclonic rotation at inertial frequencies making them predisposed to resonate with inertial ocean currents (Stockwell et al., 2004). The other research path in collaboration with Patrice Klein, French Research Institute for Exploitation of the Sea (IFREMER) and Hecht aims to quantify the role of ocean meso-scale eddies in extracting wind energy by broadening the resonant frequency band. Two simulations of a 0.1 degree North Atlantic model shows that, although the mean energy input is not much affected, there is a much broader distribution of energy input, so that non-linear processes, such as vertical mixing, could be much more vigorous in eddy-resolving models forced by high frequency (e.g., 6 hourly) winds. The hypothesis is that both eddies and near-inertial wind forcing will need to be parameterized in non-eddying climate models to mix as deep as observed in eddy rich regions of the ocean, such as the Atlantic Circumpolar Current.