Decadal variability: processes, predictability and prediction

TitleDecadal variability: processes, predictability and prediction
Publication TypeTechnical memorandum
Date Published05/2009
Secondary TitleECMWF Technical Memoranda
AuthorsAnderson, DLT, Doblas-Reyes, F, Alonso-Balmaseda, M, Weisheimer, A

The climate system exhibits variability on a variety of timescales. Over much of the ocean the spectra of sea surface temperature and salinity are red, broadly consistent with the ocean acting to integrate atmospheric white noise forcing. However, there are substantial areas where this is not an appropriate approximation and other processes are operative. Various modes of climate variability such as the Atlantic multidecadal oscillation (AMO) and the Pacific Decadal Oscillation (PDO) have been suggested. The thermohaline circulation in the Atlantic is identified as a key component of the AMO through the Atlantic Meridional Overturning Circulation (AMOC) though the details of how they are related are still unclear. Various processes are identified and their predictability assessed. Predictability horizons have been estimated using numerical models and preliminary predictions made and skill assessed. In general there is some hope for predictability in the Atlantic sector on time scales up to about ten years arising from knowledge of the ocean initial state. In the last year or two a beginning has been made to develop decadal prediction systems. The approach is somewhat similar to that developed over the last decade for seasonal forecasting. In addition to initializing the ocean, greenhouse and other radiatively active gas concentrations are used. There is a large scatter in the forecasts, partly arising from errors in the models and uncertainties in the ocean initial conditions. Although decadal prediction is in its infancy, predictability studies indicate some encouraging potential. Large-scale low frequency variability such as temperature over Europe or rainfall over the Sahel offer the potential for huge benefits to society. Delivering accurate reliable prediction is a major challenge for the next decade.

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