## June-August 2004: Seasonal Climate Summary

**By**

**Drs. Gerald Bell and Vernon Kousky**

**NOAA/ NWS/ Climate Prediction Center**

#### TROPICS

## *Highlights* 

1.  A weak Pacific warm episode (El Niño) develops

2.  Ongoing intraseasonal activity linked to Madden-Julian Oscillation
    (MJO)

3.  Record Atlantic hurricane activity for August, suppressed East
    Pacific activity

4.  Near- to below-average rains in the West African monsoon region

[TROPICAL PACIFIC]{.underline}

Positive sea surface temperature (SST) anomalies strengthened across the
central and east-central equatorial Pacific during June-August 2004,
with August departures averaging 1°-1.5°C from the date line eastward to
130°W. This eastward expansion in the area of anomalous warmth indicates
the early stages of a Pacific warm (El Niño) episode. Despite the
anomalously warm SSTs, there was little or no reflection in the average
pattern of deep convection (precipitation) over the central and
east-central equatorial Pacific. This indicates that the anomalously
warm SSTs were not yet having a discernible influence on the atmospheric
circulation. However, this anomalous warmth did contribute to farther
eastward shifts in enhanced convection associated with the convectively
active phase of the Madden Julian Oscillation (MJO).

The MJO activity during June-August resulted in considerable
week-to-week and month-to-month variability in the Tropics. For example,
during mid-June through early July the easterlies weakened in many areas
of the equatorial Pacific, as enhanced convection shifted eastward from
the Indian Ocean to the western tropical Pacific. By mid-July the
low-level winds and equatorial convection returned to near average in
many areas of the equatorial Pacific. However, a strong oceanic Kelvin
wave, initiated by the weaker-than-average easterly winds in June,
propagated eastward resulting in a substantial deepening of the oceanic
thermocline and an increase in the subsurface temperature anomalies in
the central and eastern equatorial Pacific. This Kelvin wave reached the
South American coast during late August, resulting in an increase in
SSTs along the coasts of Ecuador and northern Peru.

[ATLANTIC and EAST PACIFIC HURRICANES]{.underline}

The Atlantic hurricane season runs from 1 June through 30 November, with
the normal peak in activity occurring from mid-August through early
October. No tropical storms formed during June and July 2004, followed
by record activity during August. Eight tropical storms (TS) formed
during August, with five becoming hurricanes (H) and three becoming
major hurricanes (MH) (Alex, Charlie, and Frances). This sets a record
for the most major hurricanes during in that month. It is noted that the
increased hurricane activity also continued through September, with a
total of twelve tropical storms, seven hurricanes, and five major
hurricanes, recorded by the end of the month. This makes 2004 one of the
most active seasons in the record dating back to 1945.

For August-September 2004 combined, six named storms made landfall in
the United States. Tropical storm Bonnie hit the Florida Panhandle on
August 12^th^, MH Charlie hit the west coast of Florida as a Category-3
storm on August 13^th^, TS Gaston hit South Carolina on Aug. 30^th^,
Hurricane Frances hit the east coast of Florida as a Category-2 storm on
Sep. 5^th^, MH Ivan hit Alabama as a Category-4 storm on Sep. 16^th^,
and MH Jeanne hit the east coast of Florida as a Category-3 storm on
Sep. 26^th^.

During the last three Atlantic hurricane seasons (2002-2004) seventeen
named storms have made landfall in the United States (6 in 2002, 5 in
2003, and 6 in 2004). Thirteen of these systems made landfall along the
Gulf coast, defined as the region from the southeastern tip of Florida
to the southern tip of Texas. Five of these systems hit the Gulf Coast
in both 2002 and 2003 each, and three hit during August-September 2004
(TS Bonnie, MH Charley, and MH Ivan).

The above-average activity in 2004 partly reflects the overall increase
in activity that began in 1995. Eight of ten hurricane seasons were
above normal during 1995-2004, compared to only three above-normal
seasons during the previous 25-yr period 1970-1994. Because Atlantic
hurricane seasons exhibit such strong multi-decadal variability, with
extremes in activity typically lasting 20-25 years (Goldenberg et al.
*Science*, 2001), it is likely that the above-normal activity observed
since 1995 will continue for at least the next several years. On
average, the United States experiences 2-3 landfalling hurricanes during
active seasons, and could therefore be at an increased risk of hurricane
landfalls in upcoming years.

During 2004 several conditions known to favor above-normal Atlantic
hurricane seasons were present across the tropical Atlantic. These
include 1) an amplified subtropical ridge at upper levels, 2) reduced
vertical wind shear, which results from an expanded area of easterly
winds in the upper atmosphere and weaker easterly trade winds in the
lower atmosphere, 3) a configuration of the African easterly jet that
favors hurricane development from tropical disturbances moving westward
from the African coast, and 4) above-average sea-surface temperatures
(0.5°C-1.0°C above average). Over the central and eastern tropical
Atlantic these conditions have prevailed since 1995, and are associated
with the ongoing active Atlantic phase of the multi-decadal signal (Bell
et al., *Bull. Amer. Meteor. Soc*., May 2004).

In contrast by the end of September 2004 the East Pacific hurricane
season had produced only 11 TS, 6H and 3 MH (Darby, Howard, and Javier).
These numbers are below the1971-2000 means of 15 TS, 9H, 4.4 MH. This
represents a continuation of generally suppressed East pacific hurricane
seasons that began in 1995.

### West African monsoon season

The rainy season in the Sahel and Sudan regions of western Africa
normally lasts from June-October, with peak rains typically observed
during August and September. During June-August, rainfall totals in
these areas were generally near- to below- average. Farther south
rainfall in the Equatorial Guinea region was considerably below average,
with seasonal totals in some areas below the 10^th^ percentile of
occurrences.

## 

## EXTRATROPICS

### [Northern Hemisphere]{.underline}

### Highlights 

1.  Persistent anomalous circulation over North America that began in
    April,

2.  Above-average temperatures in Alaska, the western U.S., and Europe

3.  Below-average temperatures across the eastern U.S. and Canada,

4.  Above-average precipitation in the southern and eastern U.S., and
    central Russia, and below-average precipitation in Alaska, the
    southwestern U.S., and southern Europe,

5.  Continuation of exceptionally warm SSTs in the North Atlantic at
    both high latitudes and the subtropics.

###### North America

The period April-August 2004 featured exceptionally warm and dry
conditions from Alaska to the southwestern U.S., with record
temperatures set at many locations in Alaska. For the state as a whole,
this was the warmest April-August period in the 1950-2004 record. In the
southwestern U.S., below-average precipitation occurred during
June-August in association with a suppressed monsoon season.

The April-August period also featured well below-average temperatures
across the eastern half of Canada and the central/ eastern United
States. Central Canada experienced its third coolest such season in the
record, with mean temperatures averaging 2°-3°C below normal. Across the
central and eastern U.S., the most significantly below-average
temperatures occurred during June-August. Several significant cold-air
outbreaks occurred during this period, in association with an amplified
upper-level ridge over western North America/ Alaska and an amplified
trough extending southward into the central or eastern United States.
These conditions favored the southward penetration of cold fronts well
into the deep South. In Texas, the exceptional southward penetration of
the cold fronts was also related to a strong southward funneling of the
cold air along the eastern slopes of the Rocky Mountains. These frontal
systems also brought below-average temperatures to much of the eastern
United States, and contributed to above-average precipitation from Texas
northeastward to New England.

On average, the anomalous temperature and precipitation patterns over
North America during April-August were related to a persistent blocking
ridge over Alaska and northwestern Canada, a westward shift and
strengthening of the mean trough over central/ eastern Canada, and a
persistent jet stream in the Great Lakes region. During the second half
of June and the latter part of July, these conditions were associated
with a westward shift of the mean summertime ridge that normally extends
from Texas northward to south-central Canada. This mean ridge not only
influenced the summertime temperature and precipitation patterns over
North America, but also helped to regulate the strength of the Southwest
U.S. monsoon system. Overall, this monsoon system was weaker than
average during June-August.

While it is not possible to attribute a specific cause or causes of the
persistent April-August circulation, three factors likely contributed to
its longevity. These include a persistent zonal wave-3 pattern at high
latitudes, and a pronounced stratospheric extension of the high-latitude
height anomalies, with the blocking ridge over Alaska and the downstream
amplified trough extending past the 30-hPa level. They also include a
highly diffluent trough over eastern Siberia, and a westward retraction
of the East Asian jet core and jet exit region, which favored the
persistence of a blocking ridge over western North America.

### North Atlantic and Europe

Over the North Atlantic the mean 500-hpa circulation during June-August
featured above-average heights at high latitudes and below-average
heights from Canada eastward to Great Britain and Scandinavia. This
circulation was associated with enhanced westerlies across northern
Europe. It was also associated with an above average strength of the
subtropical ridge across southern Europe, situated along the equatorward
flank of the enhanced jet stream.

This circulation contributed to a continuation of anomalous warmth
throughout Europe. It also contributed to a dipole pattern of
precipitation anomalies over the continent, with northern Europe
experiencing generally above-average precipitation and southern
Europe/northern Africa experiencing below-average precipitation.

Also over the North Atlantic, the season featured a continuation of
exceptionally warm SSTs at both high latitudes and the subtropics, in
association with the ongoing warm phase of the Atlantic multi-decadal
mode that began approximately in 1995 (Goldenberg et al. *Science*,
2001). This anomalous warmth is consistent with above-average heights
across the high latitudes of the North Atlantic, with above-average
temperatures across Europe, and with the above-normal Atlantic hurricane
season.

### 

### [Southern Hemisphere]{.underline}

### Highlights 

1.  Zonally-symmetric pattern of anticyclonic circulation anomalies in
    middle latitudes

2.  Above-average surface temperatures across the three continents,

3.  Below-average precipitation in Amazon Basin, Central America, and
    eastern Australia.

The mean Southern Hemisphere circulation during June-August featured
below-average heights at upper levels across Antarctica, and generally
above-average heights in the middle latitudes. These conditions were
associated with considerable zonal-symmetry of the anticyclonic
streamfunction anomalies in the middle latitudes, and contributed to
warmer than average surface temperatures across the three continents.

### 

### South/ Central America

Over South America the upper-level circulation featured cyclonic
streamfunction anomalies over the Amazon Basin and anticyclonic
streamfunction anomalies at higher latitudes. In the Tropics this
anomaly pattern reflected a weakening of the upper-level "monsoon"
ridge, and was consistent below-average precipitation in the northern
Amazon Basin and Central America. In both regions June-August
precipitation totals were below the 10^th^ percentile of occurrences.
The Amazon Basin also experienced warmer than normal conditions during
the season, with mean temperatures ranging from 1°-2°C above average.

In the middle latitudes the anomalous upper-level anticyclonic
circulation was associated with reduced jet stream winds, and a
reduction in the number of synoptic-scale troughs and major cold frontal
passages east of the Andes Mountains. These conditions contributed to
warmer than normal conditions across extratropical South America, with
seasonal temperatures in many regions ranging from 1°-2°C above average.

###### Eastern Australia

An amplified upper-level subtropical ridge covered most of Australia
during June-August, resulting in warmer than average temperatures over
much of the continent. Downstream of the mean ridge axis, eastern
Australia experienced below-average precipitation, with seasonal totals
generally below the 30^th^ percentile of occurrences.