# Season Climate Summary

## December 2003-February 2004

#### TROPICS

## *Season 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
(+1°C) 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. These 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]{.underline}

### Season 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 (\> 3°C) 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 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 60N, where mean lower stratospheric
temperatures ranged from 8-11C 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]{.underline}

### Season 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 30°S 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 70^th^ percentile of occurrences.
Rainfall has been sporadic, however, with much o the above-average
precipitation occurring during January.