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The Pacific–North American pattern (PNA), North Atlantic Oscillation (NAO), and Arctic Oscillation (AO) are three dominant teleconnection patterns known to strongly affect December–February surface weather in the Northern Hemisphere. A partial least squares regression (PLSR) method is adopted in this study to generate wintertime two-week statistical forecasts of these three teleconnection pattern indices for lead times of up to five weeks over the 1980–2013 period. The PLSR approach generates forecasts for the teleconnection pattern indices by maximizing the variance explained by predictor indices determined as linear combinations of predictor fields, which include gridded outgoing longwave radiation (OLR), 300-hPa geopotential height (Z300), and 50-hPa geopotential height (Z50). Overall, the PLSR models yield statistically significant skill at all lead times up to five weeks. In particular, cross-validated correlations between the combined weeks 3–4 PLSR forecasts and verification for the PNA, NAO, and AO indices are 0.34, 0.28, and 0.41, respectively. The PLSR approach also allows the authors to isolate a small number of predictor patterns that help shed light on the sources of prediction skill for each teleconnection pattern. As expected, the results reveal the importance of tropical convection (OLR) for forecast skill in weeks 3–4, but the initial atmospheric flow (Z300) accounts for a substantial fraction of the skill as well. Overall, the results of this study provide promise for improving subseasonal-to-seasonal (S2S) forecasts and the physical understanding of predictability on these time scales.

The response to the Madden–Julian oscillation (MJO) over the Pacific–North American (PNA) region is investigated. In addition, the sensitivity of this response to the interaction between Madden–Julian oscillation forcing and the extratropical initial flow is explored. First, a simple dynamical model is run with an ensemble of 100 randomly selected initial conditions from ERA-Interim data, with no heating, MJO phase-1-like heating, and MJO phase-5-like heating. The 300-hPa geopotential height field is separated into a part that would evolve without an active MJO present, and a part that is a consequence of the MJO heating. A negative 300-hPa geopotential height anomaly centered over northeastern China bounded by positive anomalies on its equatorward and poleward flanks is found to be followed by a large amplitude negative PNA-like response for MJO phase 1 and a positive PNA-like response for phase 5. A similar study is carried out using observational data. An analog approach—using projections to determine the analogs—is used to approximate the part of the flow that results from the MJO heating. The composite initial flow that corresponds to a large MJO response in observational data somewhat matches that in the model, although there is more variability between phases. Finally, the analog method is used to examine questions related to predictability and the MJO. It is found that predictability is improved by taking into account the presence of the MJO and by choosing analogs with high projections. The presence of an active MJO also increases predictability.

In this paper, large-scale aspects for the onset of the extreme cold European weather event in January–February 2012 are investigated. It is shown that the outbreak of this extreme cold weather event may be attributed to the transition from a positive North Atlantic Oscillation (NAO+) event to a long-lasting blocking event over the eastern Atlantic and western Europe (hereafter ENAO−). A persistent decline of the surface air temperature (SAT) is seen over all of Europe during the long-lived ENAO− event, while the main region of enhanced precipitation is located over southern Europe and part of central Europe, in association with the presence of a persistent double storm track: one over the Norwegian and Barents Seas and the other over southern Europe. The NAO+ to NAO− transition events are divided into NAO+ to ENAO− and NAO+ to WNAO− transition events [ENAO− (WNAO−) events correspond to eastward- (westward-) displaced NAO− events whose positive center is defined to be located to the east (west) of 10°W], and a statistical analysis of the NAO+ to ENAO− transition events during 1978–2012 is performed. It is found that there has been a marked increase in the frequency of the NAO+ to ENAO− transition events during the period 2005–12. Composites of SAT anomalies indicate that the marked decline of the SAT observed over much of Europe is primarily associated with NAO+ to ENAO− transition events. Thus, NAO+ to ENAO− transition events may be more favorable for the extreme cold events over Europe observed in recent winters than other types of NAO− events.