High-resolution 4-D ocean models allowing for the existence of eddies are used with sea surface height (SSH) measurements from the TOPEX/POSEIDON altimeters to understand the variability seen at the ocean's surface and how this signal is related to the changes in the temperature and salinity in the ocean's interior. These analyses will continue with data from the Jason-1 satellite. Altimeters only "see" the surface, and a realistic ocean simulation helps to understand the physical processes contributing to the SSH changes. We also hope to understand how initial conditions affect the predictions in an ocean-atmosphere coupled model.
Through the combined use of ocean simulations from a twenty-year time series, we are trying to understand the causes of the variability in global ocean circulation patterns, and determine if this variability can be predicted.
Several ocean simulations have been created or will be created by our research group. These simulations currently include a 20-year (1979-1998) simulation as well as a 41-year simulation (1958-1998). The ocean models are 4D simulations, with 20 to 40 vertical levels representing the varying ocean depths. Horizontal spacing has a resolution of about 1/6 to 2/3 degree (for example, each model grid represents an approximate 18 km to 60 km box). The simulations require that the ocean models be forced at the surface by winds, by heat (from the sun and from heat losses by the ocean), and by precipation and evaporation. To produce a simulation that corresponds to the referenced time periods, we use data produced by the European Centre for Medium Weather Forecasts and the U.S. National Centers for Environmental Prediction. These data are a combination of observed measurements and a model, and when applied to the ocean model, result in a realistic simulation.
Evaluation and analysis of the simulations
The simulations are evaluated using both satellite data and in situ data such as measurements from tide gauges and buoys. The in situ data, especially the measurements of sea level made by the tide gauges, add to the confidence in the realism of the simulation during periods when no satellite data are available. The altimeter data sets are Geosat (approximately 1987-1989), TOPEX/POSEIDON (1992-present), ERS-1 and 2 (1990s), and will include Jason-1 data. The model output, its forcing, and the observations are compared using a variety of methods, for example, time series analysis at a given location and spatial statistical methods. From these analyses, we can determine how much of the variability is a direct result of the forcing applied, and how much of the variability comes from other processes. We can also identify regions where the model represents the variability of the ocean particularly well. Figure 1 shows a time series of the average height anomaly along 35°N in the Pacific from a model (black line) and as measured from TOPEX/POSEIDON and Geosat (blue line). The heat content of the top 160 m of the model is in red and the North Pacific Climate Index is the black dashed line. The figure indicates that the simulation is similar to the observations and that the upper ocean, indicated by the heat content curve, is contributing to most of the variability at the low frequency.
Predictability of the ocean's variability
To understand if and where the ocean's circulation can be predicted through the use of satellite-measured SSH, we will examine how the sea level at one location influences the variability downstream. A good example of this type of predictability is detecting the occurrence of an El Niño in the tropical ocean and then knowing that the signal travels northward along the western edge of the Americas at a later time. We will examine various locations around the globe to further explore this type of cause-and-effect variability.
Coupled ocean-atmosphere models are also useful to understand the predictability of the ocean's circulation. Depending on the initial conditions from which a simulation starts, the coupled models will produce different results. We will run a prototype coupled ocean-atmosphere model using, as part of our initial conditions, the state of the current ocean (temperature, salinity, and sea level) from the output of an assimilated model, run along with simulations using just the observed SSH and temperatures. In this type of simulation, the coupled ocean model can be initialized with observed data (including altimeter data) and then allowed to run into the future using the winds and other forcings from the atmospheric model. The resulting predicted field of sea level can then be compared with future altimeter observations.
The anticipated results are to show:
Significance of results
Ocean models are becoming more common as tools in trying to understand the ocean and its variability. More realistic simulations, result in higher confidence in the models, which in turn, helps in understanding scientific problems such as the role of the ocean in our global climate system. In addition, knowing how satellite altimeter data can be used to infer subsurface changes can aid in monitoring of our global oceans.
Interannual to decadal ocean variability and predictability