1. Northern Hemisphere

The 500-hPa circulation during January featured a persistent pattern of positive height anomalies over western North America, Greenland, the low latitudes of the eastern North Atlantic, and central Asia, and negative height anomalies over the central North Pacific, eastern North America, southern Europe and Scandinavia (Figs. E10, E12). These anomalies project onto the positive phases of the Pacific/ North American (PNA) and the East Atlantic (EA) teleconnection patterns, each of which has persisted since October (Table E1, Figs. E7, E8). The persistence of these teleconnection patterns is consistent with the ongoing El Ni�o and with an overall negative phase of the Arctic Oscillation (Fig. A2.1).

In the subtropics a marked inter-hemispheric symmetry of the 200-hPa circulation anomalies was also evident during January in association with the ongoing El Ni�o (Fig. T22). This symmetry included anticyclonic streamfunction anomalies in both hemispheres flanking the region of enhanced convection over the central equatorial Pacific, and cyclonic streamfunction anomalies in both hemispheres over the subtropical eastern Pacific and over most of the subtropical Atlantic.

The prominent temperature anomalies during January included much warmer than normal conditions across the western United States, Canada, and central Asia, and cooler than normal conditions across the eastern United States and Scandinavia (Fig. E1). Prominent precipitation departures during the month included above-average precipitation across portions of southern Europe, and below-average precipitation over the southwestern and eastern United States, and eastern Canada (Figs. E3, E4).

a. Pacific/North America

The strong positive PNA (+1.5) pattern during January (Table E1) reflected amplified upper-level ridges over the subtropical North Pacific and western Canada, and upper-level troughs across the central North Pacific and the southeastern United States (Figs. E10, T22). These anomalies were associated with an extension of the East Asian jet stream across the eastern North Pacific, and with a shift of that jet exit region and accompanying split-flow configuration to well east of the date line (Figs. E11, T21).

The amplified upper-level ridge over western North America during January contributed to above-average surface temperatures across the western United States, Alaska, and Canada. In Alaska and Canada temperatures ranged from 2�C above average while over the western U.S. they ranged from 2-4�C above average (Fig. E1). This anomalous warmth is a typical winter feature during El Ni�o episodes.

b. North Atlantic and Europe

The circulation over the North Atlantic again featured above-average heights at high latitudes and in the subtropics, and below-average heights in the middle latitudes (Fig. E10). This circulation was associated with high-latitude blocking and a pronounced split-flow configuration over the eastern North Atlantic, and with a southward shift of the mean wintertime jet axis to southern Europe (Figs. E11, T21). This overall anomaly pattern has persisted since October, and reflects the ongoing positive phase of the East Atlantic teleconnection pattern (Table E1, Figs. E7, E8). During January these conditions were associated with a continuation of warmer and wetter than normal conditions across southern Europe, and cooler than normal conditions in Scandinavia (Figs. E1, E3).

2. Southern Hemisphere

Above-average 500-hPa heights were evident over southeastern Australia and the high latitudes of the South Atlantic during January, and below-average heights covered the mid-latitudes of the South Atlantic and the area south of Africa (Fig. E16). In the subtropics a strong inter-hemispheric symmetry of the 200-hPa circulation anomalies was again evident during the month in association with the ongoing El Ni�o (Fig. T22). This symmetry included anticyclonic streamfunction anomalies in both hemispheres flanking the region of enhanced convection over the central equatorial Pacific, and cyclonic streamfunction anomalies in both hemispheres over the eastern Pacific and most of the Atlantic.

The dominant temperature and precipitation anomalies during January reflected a continuation of warmer and drier than average conditions over eastern Australia and across northeastern South America. These conditions are consistent with El Ni�o (Fig. E4) but were also linked to the local 200-hPa circulation features, with the anomalous warmth coinciding with the upper level ridge axes in both regions, and the anomalous upper-level convergence (Fig. T23) and implied sinking motion situated between the upper-level ridge and downstream trough axes.