Contrastive Influence of ENSO and PNA on Variability and Predictability of North American Winter Precipitation

In the study, Li et al (2019) examined the role of ENSO in the variability and predictability of the Pacific–North American (PNA) and precipitation anomalies in North America in winter. It is noted that 29% variance (Fig. 1) of PNA is linked to El Niño-Southern Oscillation (ENSO) during winter 1950/51-2017/18, and thus may be largely predictable. The remaining 71% variance of PNA is associated with atmospheric internal variability and sea surface temperature (SST) anomalies in the North Pacific, which may be largely unpredictable. That is a common and basic feature of climate variability over the mid-high latitudes, where the variations are dominated by local atmospheric internal variability and to a minor extent constrained by remote forcings, such as SST in the tropics associated with ENSO.

Specifically, the ENSO impact is mainly meridional from the tropics to the mid-high latitudes, while the connection between the North Pacific and the North American continent is mainly confined in the zonal direction, resembling a Pacific Decadal Oscillation (PDO) pattern. Interferential connection of ENSO and PDO on PNA as well as North American climate variability and predictability may reflect a competition of local internal dynamical processes (unpredictable part) and remote forcing (predictable part).

The signal-to-noise ratio (SNR) of winter precipitation in North America is mostly smaller than 0.4, indicating that the local atmospheric internal dynamics is more important than remote/boundary forcing (SST) in generating the precipitation in North American winter and implying that its predictability is low in nature. The model responses to observed SST and model forecasts confirm that the predictability and prediction skill of winter precipitation in North America is dominated by the influence of ENSO and the skill is low and largely regional dependence.

Li, X.^*, Z. Hu, P. Liang, and J. Zhu, 2019: Contrastive influence of ENSO and PNA on North American winter precipitation. J. Climate., 32(19), 6271-6284.

Fig. 1: (a) Total, (b) ENSO-related part, and (c) ENSO-unrelated part of DJF PNA index during 1950/51-2017/18.