ENSO is a climate phenomenon that is due to coupling ...



Predicting ENSO

1. WEATHER FORECASTS vs. CLIMATE FORECASTS

For practical purposes climate prediction is different from weather prediction in important ways. First, the adjustment time of the atmosphere to a thermal or dynamical perturbation is very fast (days to a few months) compared to that of the ocean, land surface or land ice (seasons to millennia). Hence, in making weather forecasts you can fix the water temperature and ice extent to be that observed. The accuracy of the weather forecast depends on how good your numerical model is and (mostly) the quality of the model initialization; i.e., how closely you are to reality at the time you start the forecast. The limit for skillful weather prediction is a few days to a week.

Climate forecasts take advantage of the fact that there are media (ocean/land/cryosphere) that have larger thermal and dynamical interia than the overlying atmosphere, and that the atmosphere responds in a statistical sense to these slower changes in lower boundary conditions. Of course, climate is the aggregate statistics of weather. Hence, by definition a climate forecast is a forecast of the coupled atmosphere/ocean/land/cryosphere system. These are most commonly made by coupling an Atmosphere General Circulation Model (AGCM) to a numerical model of the slower media (typically a land surface plus ocean model). It appears that the limit of predictability of a climate forecast is a few seasons to a year, and this limit is mainly determined by the efficacy of the slow component of the coupled model and the AGCM, and by how accurately the slow component is initialized to the real world at the time of the forecast is started; the climate forecast is insensitive to the state of the atmosphere at initialization time.

2. ENSO FORECASTS

El Nino/Southern Oscillation (ENSO) is a climate phenomenon that is due to coupling between the atmosphere and ocean in the tropical Pacific. Important for the evolution of the atmosphere is the distribution of the Sea Surface Temperature (SST) that creates near surface pressure gradients in the atmosphere (via atmosphere-ocean heat exchange, hydrostatic balance, and the ideal gas law) and hence drives the winds. Important for the ocean is the surface wind stress, which drives ocean currents and upwelling (especially along the equator) and hence affects strongly the SST.

The time scale of ENSO (interannual) results from a competition of processes with comparable time scales (6-10 months): the dynamical adjustment time of the upper ocean in the tropics to a change in the wind stress; the time for the upper ocean and atmosphere to come to a thermodynamic equilibrium. The result is a interannual climate phenomenon that occurs every 2-7 years, with an average time between peak events (eg., El Nino) of about four years. ENSO creates global scale climate changes via atmospheric teleconnections.

A forecast of the state of ENSO over the next year is an example of a climate forecast. The forecast accuracy will depend on the quality of the AGCM, the ocean model (often an ocean GCM), the representation of the energy and moisture transfer between the two media, and the quality of the initialization of the ocean. (the state of the ocean at initialization time is a result of the integrated effects of the wind forcing over the past few years). This is very different set of criteria from the criteria for a good weather forecast (cf, section 1).

3. A DIADACTIC MODEL OF ENSO

A simple diadactic model of ENSO can be used to illustrate the basic methodology used to make a climate forecast, and the type of products that result from a climate forecast. The essence of this toy model of ENSO is as follows. ENSO is usually described as the departure (anomaly) of the coupled atmosphere/ocean system from the average climatological annual cycle. For our puposes, the state of ENSO is defined using two quantities: the SST anomalies averaged near the equator in the eastern half of the tropical Pacific (Nino3.4, T) and the thermocline depth anomaly, h, in the eastern equatorial Pacific (the latter tells us something about the dynamical state of the ocean and is important for the evolution of ENSO). In this model, we will assume that the SST changes slowly compared to the time scale of the synoptic waves (weather) in the atmosphere, so that a portion of the atmospheric variability is affected by the changing SST (observations indicate about half the variance in the surface wind in the tropics is directly related to variance in SST). Hence, the slow changes in the atmosphere (e.g., the Southern Oscillation) are implicit in T. The evolution of the toy model of ENSO can thus be written as:

dZ/dt = R Z,

where Z = {T, h}, and t is time. The matrix R contains the ocean dynamics and the part of the atmospheric variability that is affected by the changing SST. Of course, the atmosphere will still have weather events (with time and space scales of days and ................
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