Interdecadal variability of the ENSO teleconnection to the wintertime North Pacific

Abstract

The El Niño/Southern Oscillation (ENSO) strongly influences the large-scale atmospheric circulation over the extratropical North Pacific during boreal winter, which has an important impact on North American winter climate. This study analyses the interdecadal variability of the ENSO teleconnection to the wintertime extratropical North Pacific, over the period 1900-2010, using a range of observationally derived datasets and an ensemble of atmospheric model simulations. The observed teleconnection strength is found to vary substantially over the 20th century. Specifically, 31-year periods in the early-century (1912-1942), mid-century (1946-1976) and the late-century are identified in the observations when the ENSO teleconnection to the North Pacific circulation are found to be particularly strong, weak and strong respectively. The ENSO teleconnection to the North Pacific in the atmospheric model ensemble is weak in the mid-century period and substantially stronger in the late-century, closely following the variability in the observed ENSO-North Pacific teleconnection. In the earlycentury, however, the atmospheric model also exhibits a weak teleconnection to the North Pacific, unlike in observations. In a subset of the model realisations that exhibit similar ENSO-North Pacific teleconnection as in observations during the early-century period there are large differences in extratropical circulation but not in equatorial Pacific precipitation anomalies, in contrast to the late-century period. This suggests that the high correlation in the early century period is largely due to internal extratropical variability. The important implications of these results for seasonal predictability and the assessment of seasonal forecasting systems are discussed.

How confident are predictability estimates of the winter North Atlantic Oscillation?

“…That means that regions which appear more predictable in the real world than in the model (i.e. paradoxical or underconfident) during recent decades, were strongly overconfident during the mid‐century decades (see also O'Reilly, ). In the earlier parts of the twentieth century the situation appears less extreme in either direction with slightly higher values of perfect model skill over Greenland and the northern North Atlantic and moderate observed skill levels over most of the North Atlantic except for the northeastern parts.…”

“…Minobe, ; Hoerling et al, ; Derome et al, ; Fletcher and Saunders, ; Greatbatch and Jung, ; Douville et al, ; Hegerl et al, ; Huang et al, ) suggesting that such differences are indeed physically based. In particular, the temporal skill evolution of the NAO co‐varies with changes in the skill of the Pacific–North America (PNA) pattern that are highly statistically significant and relate to changes in the ENSO–North Pacific teleconnections (O'Reilly et al, ; O'Reilly, ). Further studies to understand the physical mechanisms of these inter‐decadal general circulation changes in the atmosphere and ocean both in observed data as well as in models (initialised and non‐initialised: Kumar and Chen, ) are clearly needed.…”

“…The corresponding geographical structure of the RPC is displayed in the left column of Figure and confirms a substantial difference in the behaviour of the predictable components over the North Atlantic and Greenland between the more recent decades and the mid‐century period. Almost all of the Northern Hemisphere during the mid‐century period has RPC values substantially below one (see also O'Reilly, ). In the earlier period of the century the Arctic sticks out as a region with small RPCs (overconfidence), with large areas of the North Atlantic being in the range of a perfect value around one.…”

The importance of stratospheric initial conditions for winter North Atlantic Oscillation predictability and implications for the signal‐to‐noise paradox

“…The ERA‐20C IC experiment is the hindcast that was performed by Weisheimer et al () over the entire twentieth century. They found that the skill of the NAO (as well as the Pacific/North American pattern) exhibited a minimum in the mid‐twentieth century, when there was relatively weak forcing from tropical Pacific SSTs (O'Reilly et al ; O'Reilly, ). The hindcast period analysed here primarily covers the later period where the ERA‐20C IC experiment exhibits higher levels of skill.…”

When and where do ECMWF seasonal forecast systems exhibit anomalously low signal‐to‐noise ratio?

“…) suggests that there is significant variation of over‐ and underconfidence during the 20th century linked to long‐term variability of the global circulation and teleconnections (e.g. O'Reilly, ). Analysis of a much longer set of seasonal forecasts such as that described in Weisheimer et al .…”