1. Northern Hemisphere

The 500-hPa circulation during January 2004 featured above-average heights at high latitudes and below-average heights in the middle latitudes (Fig. E10). This anomaly pattern is referred to as the negative phase of the Arctic Oscillation (AO) (Fig. A2.1), and reflects a meridional mass exchange between middle and high latitudes. A pronounced stratospheric warming is partly responsible for the exceptionally high stratospheric penetration of the AO signal during January, which extended up past the 30-hPa level (Fig. S1). Over the North Atlantic this stratospheric warming spread southward to 60EN, where mean lower stratospheric temperatures ranged from 8E-11EC above average (Fig. S2, bottom).

Regional aspects of this hemispheric anomaly pattern included blocking flow configurations over Alaska and Greenland, amplified troughs across the central North Pacific and North Atlantic, and a large-amplitude trough over northern and eastern Europe. These regional anomalies reflected the negative phases of several prominent teleconnection patterns, including the West Pacific (WP) Oscillation, the East Pacific pattern, the North Atlantic Oscillation (NAO), the East Atlantic/ West Russia pattern, and the Scandinavia pattern (Table E1, Figs. E7, E8).

Prominent temperature departures during January included significantly colder than average conditions across Alaska, Canada, the northeastern United States, and eastern Europe, and a continuation of warmer than average SSTs over large portions of the North Atlantic (Fig. E1). Overall, Northern Hemisphere land-only temperatures averaged slightly below normal (Fig. E2), which is only the fourth such occurrence since early 1997. Prominent precipitation anomalies during the month included above-average totals in the northwestern U.S., and both northwestern and southeastern Europe, and below-average totals in the southeastern and eastern U.S. (Figs. E3, E4).

a. Pacific/North America

The mean upper-level circulation during January featured an amplified ridge over Alaska and an amplified trough over eastern Canada (Fig. E10). This circulation contributed to anomalously cold air across Canada and the northeastern U.S., with surface temperatures throughout the region averaging 2^o to 4^oC below average (Fig. E1). In southeastern Canada and the New England states monthly mean temperatures were below the 10^th percentile of occurrences.

b. North Atlantic/Eurasia

Over the North Atlantic positive 500-hPa height anomalies at high latitudes reflected a continued absence of the mean Icelandic Low. These anomalies, in combination with below-average heights in the middle latitudes, reflected the negative phase of the NAO (Table E1). This circulation, combined with the exceptionally warm SSTs at high latitudes, contributed to above-average surface temperatures over Great Britain and across large portions of southern Europe (Fig. E1). It also contributed to above-average precipitation in northwestern Europe where the mean jet core entered the continent, and over southeastern Europe in the vicinity of the mean upper-level trough (Fig. E3).

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during January featured below-average heights over western Antarctica, and a zonal wave-3 pattern in the middle latitudes with above-average heights over the three ocean basins. (Fig. E16). Anomalously warm and dry conditions were observed over southern South America during the month, with many locations reporting mean temperatures in the warmest 30^th percentile of occurrences (Fig. E1) and precipitation totals in the driest 30^th percentile of occurrences (Fig. E3).

In southern Africa the rainy season normally lasts from October to March. Rainfall was above-average during January (Fig. E3) with area-average totals exceeding the 80^th percentile (Fig. E4). Overall, the 2003-2004 rainy season has been below-average as area-averaged totals during November and December were in the lowest 10^th percentile of occurrences.