Speakers
Abstract: Daniel Skinner
It is known that the Madden-Julian Oscillation (MJO) excites a response in the behaviour of many extratropical weather regimes at lag times of one to two weeks, acting as a key predictor in weather forecasting. Less well understood, however, is the robustness of these responses over long time scales. We begin by taking a statistical approach to assess the boreal winter response of a selection of key extratropical systems (e.g. North Atlantic Oscillation (NAO), Pacific North American (PNA) pattern) to the MJO, over two non-overlapping time periods (1974-1997 and 1997-2019). It is shown that there is significant change in both the magnitude and structure of the extratropical response signal, as a function of lag, between the two periods.
This is followed by a similar analysis applied to the 1100 year pre-industrial control run of the UKESM-1-0 coupled climate model. By breaking this period into separate 20 year segments and comparing the extratropical responses to the MJO in each segment, we show that although there is a predictable mean signal, it is overwhelmed by the internal variability in the system. Repeating this methodology with segments between 10 and 40 years in length allows us to assess sampling errors and identify the key timescales for the variability. A robust mean signal justifies the current use of the MJO as a predictor in the extratropics, although the variability casts doubt on the reliability of these predictors for the future.
Recent process based analysis has shown that El Niño Southern Oscillation (ENSO) can act to modulate the Rossby wave source associated with the MJO. We investigate this using our statistical approach to assess the impact of ENSO on the MJO teleconnection patterns. In addition to this, we consider lower-frequency modes, for example Atlantic Multidecadal Variability (AMV) and the Pacific Decadal Oscillation (PDO)
Abstract: Ned Williams
Seasonal forecasts cover part of the timescale gap between weather forecasts and climate change projections by aiming to predict climate anomalies a few months ahead of time. They aim to answer questions such as ‘Will the upcoming winter be colder or warmer than usual’? Whilst far from perfect, contemporary operational forecasts do demonstrate a useful level of skill in predicting winter climate in the extratropics. Several modes of climate variability exist in the tropics which vary slowly relative to seasonal timescales and are highly predictable. These tropical drivers influence extratropical climate through teleconnections. One such driver is the El Niño-Southern Oscillation (ENSO), which involves the fluctuation of sea surface temperatures in the tropical Pacific between warm ‘El Niño’ and cool ‘La Niña’ phases.
Teleconnections from the El Niño-Southern Oscillation must be represented accurately in seasonal forecasts in order to translate the high predictability in the tropics into predictability in the extratropics. We analysed late winter teleconnections in five seasonal forecast models and found that the El Niño teleconnections to the North Pacific are underestimated by a factor of a half. We found that the La Niña teleconnection is also weak. We consider the chain of processes involved in the teleconnection and find that tropical precursors are not underestimated. Therefore the underestimation of teleconnection amplitude must be due to processes in the extratropics. We then examine the relationship between the North Pacific and North Atlantic atmospheric circulation – the next step in the main teleconnection pathway between the El Niño-Southern Oscillation and Europe – and find that this is accurately modelled in seasonal forecasts. Consequently the overall teleconnection to the North Atlantic is underestimated, affecting European winter climate predictability.
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