Assessment JJA 2000
Near-normal oceanic and atmospheric conditions
prevailed in the tropical Pacific during June-August 2000. Seasonal mean sea surface temperature (SST) anomalies were roughly 0.5 °C
below normal across the central and eastern tropical Pacific, but they decreased in areal
extent and magnitude during the season. The Niño 3.4 index in the central equatorial
Pacific stayed at or below 0.5 throughout the season.
The evolution of the subsurface thermal structure was
consistent with a return toward near normal conditions. The oceanic thermocline remained
deeper-than-normal in the equatorial west-central and western Pacific, but anomalies at
thermocline depth decreased from an average of 4-5°C during March-May 2000 to an average
of 2-3°C during June-August 2000. The negative temperature anomalies that have
characterized the subsurface thermal structure in the east-central and eastern Pacific
since late 1998 continued to weaken during the season.
The pattern of tropical convection [as inferred from
anomalous outgoing longwave radiation (OLR)] remained generally consistent with weak cold
episode conditions, especially during June, with suppressed convection over the western
and central tropical Pacific and enhanced convection over Indonesia. During June
convection was enhanced in the major monsoon regions (southeast Asia, India,
Mexico/Central America and the southwestern United States). During July and August this
pattern reversed, with suppressed convection in these regions. Tropical intraseasonal
activity (Madden-Julian Oscillation) activity (with a period of near 45 days )
also increased during the season. During August the MJO was associated with an increase in
Atlantic tropical cyclone activity during the beginning and middle of the month and a
suppression of activity late in the month.
Seasonal precipitation totals ranged from 50-400
mm above normal over a large region encompassing portions of Southeast Asia and Indonesia.
Unusually strong Meiyu frontal activity in the Yangtzee River Valley, and tropical
cyclone activity in the South China Sea contributed to seasonal rainfall totals that were
excessive over portions of central and southeastern China.
Seasonal rainfall was well below-normal over Burma and portions of northern India. Western
portions of the African Sahel experienced
below-normal precipitation, while eastern portions received above-normal precipitation.
The seasonal mean lower-level (850-hPa) wind
anomalies across the tropical Pacific were generally quite weak. Easterly zonal wind
anomalies in excess of 3 m s-1 over the west_central and western equatorial
Pacific increased the low-level convergence into the area of enhanced tropical convection over Indonesia. The seasonal
mean upper-level (200-hPa) westerly zonal wind
anomalies exceeded 5 m s-1 across much of the central tropical Pacific. These
westerly anomalies were associated with well-developed mid-Pacific
troughs over the low latitudes of both hemispheres.
The tropical sea level pressure (SLP) anomaly
pattern during the season featured weak positive anomalies across the tropical eastern
Pacific and weak negative anomalies throughout the remainder of the global tropics. The
Tahiti-Darwin Southern Oscillation Index (SOI) remained near zero during the season, while
the equatorial SOI decreased from 1.2 in June to 0.4 in August. The August value of the
equatorial SOI was the lowest value since August 1998.
The lower mid-latitudes and subtropics of the Northern Hemisphere exhibited
above-normal 200-hPa heights, while below_normal
heights prevailed throughout the Tropics, especially over the Pacific basin. This pattern
(often referred to as the global monsoon mode) was most prominent during June,
consistent with enhanced precipitation in the major monsoon regions during the month.
Below-average heights tended to dominate the middle latitudes of the Southern Hemisphere,
especially late in the season when an anomalous wave-3 pattern was evident at higher
latitudes. An animation of 200-hPa heights shows
that this pattern was accompanied by enhanced blocking activity over the high latitudes of
the eastern South Pacific and near the Antarctic Peninsula, which contributed to an
unusually strong series of cold air outbreaks into southern and southeastern Brazil during
Much of southern Europe and eastern Asia experienced above-average surface temperatures during the season, with anomalies
exceeding the 90th percentile from southern France eastward to the Black Sea
and across eastern Russia, Mongolia and Japan. This warmth was associated with an
anomalous upper-level anticyclonic circulation and an overall reduced strength of the
storm track across the region. In Australia below-average precipitation and above-average
surface temperatures were observed over much of the continent during the season.
Over the United States an amplified summertime ridge, clearly evident in the seasonal 500-hPa height pattern, contributed to
warmer-than-average surface temperatures over much
of the West and South. Enhanced northwesterly flow over the northeastern third of the
nation contributed to cool conditions extending from the United States-Canadian border to
the mid-Atlantic region. Accumulated seasonal precipitation
was well below-normal and conditions were particularly dry (in the 10-20th percentile)
along the southern tier of the United States extending from the Gulf Coast region of Texas
to Florida. No precipitation was observed at Dallas,Tx
during July and August. The intermountain west also recorded below-average seasonal
precipitation, which contributed to numerous and vast wildfires across the region.