11.07.2022: Ocean Circulation and Climate Dynamics Colloquium

Dr. Antje Weisheimer, University of Oxford & European Centre for Medium-Range Weather Forecasts (ECMWF): "Seasonal Forecasts of the 20th Century"

When?     Monday, 11 July 2022 at 11 am
Where?    ZOOM meeting room: https://geomar-de.zoom.us/j/84703072812?pwd=ZnhObUJCb3BaVGRubTRNNmx1MGVZZz09

Meeting ID: 847 0307 2812
Passcode: 600337

The large-scale fluctuations of the equatorial atmosphere and ocean over the tropical Pacific known as El Niño - Southern Oscillation (ENSO), play an important role in the climate system. Forecasting ENSO is at the very heart of seasonal predictions because it provides the largest source of predictability on time scales of months and seasons ahead and is of great relevance to society. Forecasts of seasonal climate anomalies using physically based global circulation models are routinely made at operational meteorological centers around the world. A crucial component of any seasonal forecast system is the set of retrospective forecasts, or hindcasts, from past years that are used to estimate future forecast skill and to calibrate the forecasts for biases. Hindcasts of seasonal predictions are usually produced over a period of around 20–30 years. With an average frequency of 4-5 years, there is only a very limited number of ENSO cases available in the operational reforecast records and sampling the wide spectrum of ENSO flavours is not possible. In the presence of considerable variations in the coupled ocean-atmosphere system, good skill in predicting the most recent ENSO events cannot guarantee that future events will have similar predictability.

In this talk, I will introduce new historical retrospective research forecasting datasets created with a reduced-resolution version of the ECMWF forecasting model that covers all of the 20th Century and extends the forecast lead times to 2 years. The reforecasts show substantial decadal modulations of forecast skill. In particular, the skill to predict ENSO is very high during recent decades, but it is markedly reduced during the 1930s–1950s. ENSO skill at the beginning of the century is, however, as high as for recent high-skill periods suggesting that the loss of skill in the mid-century period is not related to the lack of good observational data. Alternative hypotheses related to intrinsic predictability and the role of the observing system will be discussed. Extratropical modes of variability such as the North Atlantic Oscillation also show distinct multi-decadal fluctuations of forecast skill during the 20th Century, with implications for the so-called signal-to-noise paradox. Our results imply that relatively short hindcasts are not adequate for reliably testing seasonal forecasts and that small hindcast sample sizes can potentially lead to skill estimates that are not robust.


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