Climate Summary

December 2003-February 2004

TROPICS
Highlights 
1.	A continuation of ENSO-neutral conditions
2.   Strong intra-seasonal activity associated with the MJO

The December 2003 – February 2004 (DJF) season featured a continuation of ENSO-neutral conditions, with near-average sea surface temperatures (SSTs) across the eastern equatorial Pacific and generally above-average (+1C) SSTs west of the date line. The main evolution in SSTs during the season was observed over the eastern equatorial Pacific, where SSTs went from slightly above average in December to slightly below average in February. This evolution reflected a shoaling of the oceanic thermocline across the eastern equatorial Pacific as the season progressed, and is related to a strong westerly wind burst at low-levels of the atmosphere during December in association with the Madden-Julian Oscillation (MJO).
The low-level (850-hPa) and upper-level (200-hPa) equatorial zonal winds, 200-hPa streamfunction, 200-hPa velocity potential, and patterns of tropical convection [indicated by the Outgoing Longwave Radiation (OLR) anomalies], also exhibited considerable intra-seasonal variability in association with the MJO. The MJO impacts were also observed in the middle latitudes. For example, during December and February enhanced convection was observed over the Indian Ocean/ western tropical Pacific and suppressed convection prevailed near the date line. This anomalous convection contributed to an enhanced subtropical ridge and a strong East Asian jet over the extreme western Pacific, and to an overall poleward shift of the East Asian Jet east of the date line. During January an opposite pattern of tropical convective anomalies contributed to a pronounced extension of the East Asian jet stream to well east of the date line.

EXTRATROPICS
Northern Hemisphere
Highlights 
1. Strong intra-seasonal variability over the North Pacific associated with the MJO (see above), 
2.	Above-average surface temperatures across the middle latitudes, with the largest departures (> 3C) seen in Canada, southern Greenland, and Russia
3.	Below-average precipitation in the southwestern, central, and southeastern U.S., and across northern Europe, and above-average precipitation in the western U.S. and southern Europe/ northern Africa,
4.	A continuation of exceptionally warm SSTs in the North Atlantic at both high latitudes and the subtropics, and
5.	A pronounced negative phase of the Arctic Oscillation and polar stratospheric warming event during January.

The exceptionally warm temperatures observed across Canada, the North Atlantic, Europe, and Russia during DJF 2003-04 were associated with a persistent pattern of positive 500-hPa height anomalies over much of the region. The above-average heights over Canada, in combination with below-average heights over the western and central North Atlantic, reflected a pronounced northwest-to-southeast orientation of the mean Hudson Bay tough. This circulation was associated with a reduced flow of polar air into Canada, and contributed to above-average surface temperatures in that region.   
Over the high latitudes of the North Atlantic the above-average heights reflected a disappearance of the mean Icelandic Low. This pattern, combined with the below-average heights over the central North Atlantic, was also associated with a pronounced split flow configuration over the eastern Atlantic. In addition to the anomalous warmth, this circulation contributed to a dipole pattern of precipitation anomalies over Europe, with northern Europe experiencing below-average precipitation downstream of the mean ridge axis, and southern Europe/northern Africa experiencing above-average precipitation within the enhanced southern branch of the jet stream and its associated storm track.
On monthly time scales, an interesting aspect of the circulation occurred during January 2004, when above-average heights covered the high latitudes and below-average heights spanned the middle latitudes. This zonally symmetric pattern of height anomalies reflects the negative phase of the Arctic Oscillation (AO), and is associated with a meridional mass exchange between middle and high latitudes. A pronounced stratospheric warming was partly responsible for the exceptionally high stratospheric penetration of the AO signal during the month, which extended up past 30-hPa. Over the North Atlantic this stratospheric warming spread southward to 60EN, where mean lower stratospheric temperatures ranged from 8E-11EC above average. 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.

Southern Hemisphere
Highlights 
1. Above-average surface temperatures across all three continents
2.	A persistent pattern of above-average 500-hPa heights across eastern Australia and the subtropical South Pacific
3. Above-average precipitation across northern Australia, and across southern Africa during the peak of that region’s rainy season. 

Anomalously warm and dry conditions prevailed across the Amazon Basin during DJF 2003-04, in association with a suppressed upper-level ridge over the region and an overall equatorward shift of the mean mid-latitude jet stream. Anomalously cool and wet conditions were observed along 30S in association with this anomalous jet stream.
The persistence of above-average 500-hPa heights across eastern Australia is consistent with enhanced convection across the western tropical Pacific and tropical Australia observed in the season mean. This enhanced convection was observed during December and February, and occurred primarily in response to the MJO.
In southern Africa the rainy season normally lasts from October to March. Area-averaged rainfall was above average during DJF 2003-04, with totals in many regions exceeding the 70th percentile of occurrences. Rainfall was sporadic, however, with much of the above-average precipitation occurring during January.