Understanding the contrasting North Atlantic Oscillation anomalies of the winters of 2010 and 2014

Abstract

The present study investigates the two contrasting winters of 2010 and 2014 during which the North Atlantic Oscillation (NAO) was mainly negative and positive, respectively. In the North Pacific, contrasting anomalies were also present, with a straight zonal Pacific jet in 2010 and a strong poleward deviation of the Pacific jet in its exit region in 2014. Using reanalysis data sets and adopting a nonlinear initial‐value approach with a quasi‐geostrophic model, we show that the Pacific‐North American anomalies are responsible for shaping synoptic wave trains propagating across North America. This in turn largely determines the nature of wave breaking and the synoptic eddy feedback onto the mean flow in the North Atlantic and finally the NAO phase. In such a proposed mechanism, synoptic wave activity forms the cornerstone of the dynamical relationship between the North Pacific and North Atlantic large‐scale anomalies during the contrasting winters of 2010 and 2014.

Suppressed eddy driving during southward excursions of the North Atlantic jet on synoptic to seasonal time scales

“…In contrast, composites of C‐ and N‐jet days (Figure b, c, shading) show two distinct jets as in the climatology (blue contours), with the North Atlantic jet positioned poleward of 45 ° N and cleanly separated from the African subtropical jet at approximately 18 ° N. The M‐jet composite (Figure d) gives the impression of a relatively broad, weak jet, but this is in fact an artifact of zonally averaging over a complex jet structure. The S‐jet composite resembles the 2010 winter‐mean jet (Figure a, orange contours), which was also anomalously zonal and southward‐shifted (e.g., Harnik et al ., ; Rivière and Drouard, ).…”

“…Repeating the analyses performed on the synoptic S‐jet for the 2010 winter, we find a consistent picture of a merged jet that is more thermally driven and less eddy‐driven. This allows us to link previous studies that suggested key roles for tropical heating (Harnik et al ., ) and cyclonic wave breaking (Rivière and Drouard, ) for this particular winter season. Enhanced thermal driving from the tropical Pacific during the 2010 winter is similar to, though stronger than, the anomalies in the S‐jet composite (Figure c,a).…”

“…A weak vortex is also associated with increased frequency of Greenland blocking (Papritz and Grams, ), which corresponds to the southern jet (Woollings et al ., ; Madonna et al ., ). Furthermore, the 2010 winter featured a weak ridge over the eastern North Pacific (Figure d, also visible in S‐jet composites Figure c), which has been shown to lead to increased cyclonic wave breaking and a southward‐shifted jet in the North Atlantic (Rivière and Drouard, ). Finally, Eurasian snow cover interacting with the stratosphere can affect the North Atlantic circulation (Cohen et al ., ).…”

“…In terms of North Atlantic jet variability, the 2013/2014 winter also stands out as exceptional (e.g., Davies, ; Rivière and Drouard, ; Watson et al ., ). This winter exhibits the most sustained central jet (C‐jet) in our record, persisting for 72 consecutive days (Figure , black bars), with a seasonal mean comparable to the C‐jet composite (Figure b).…”

An Interdisciplinary Approach to the Study of Extreme Weather Events: Large-Scale Atmospheric Controls and Insights from Dynamical Systems Theory and Statistical Mechanics

“…Finally, according to dynamical systems metrics, the same winter was very low dimensional (and hence predictable). None of these approaches is incorrect, and indeed they are only apparently inconsistent: a more intense eddydriven jet means more eddy momentum deposit and hence more wave-breaking activity (Rivière and Drouard 2015). The different viewpoints discussed above therefore provide complementary (and sometimes overlapping) information.…”