The global climate during most of 1999 was impacted by
moderate-to-strong cold episode (La Niņa) conditions in the tropical Pacific (Fig. 14). The evolution toward the 19982000 cold episode began
in early 1998 as the oceanic thermocline (approximated by the depth of the 20°C isotherm)
became shallower-than-normal across the central and eastern equatorial Pacific (Fig. 14a). Accompanying this evolution, subsurface temperatures
dropped and the volume of anomalously warm water decreased across the central and eastern
equatorial Pacific. Nonetheless, SSTs remained well above-average across the eastern half
of the equatorial Pacific in association with ongoing very strong El Niņo conditions (Fig. 14c). These conditions set the stage for a rapid transition to
below-normal SSTs during the first week in May. This transition was triggered by a
dramatic return to near-normal easterly winds at lower levels (Fig.
14b), which contributed to enhanced oceanic upwelling that brought cold ocean waters
to the surface.
The La Niņa episode then became well established by July 1998, and
continued to strengthen during SeptemberNovember 1998 as SSTs dropped to more than
2°C below normal across large portions of the central and eastern Pacific (Fig. 14c). This combination of below-normal SSTs and an anomalously
shallow thermocline across the central and eastern Pacific reflected increased oceanic
upwelling in association with the establishment of enhanced low-level equatorial easterly
winds across the central and east-central equatorial Pacific (Fig.
Accompanying this evolution, tropical convection became enhanced
throughout Indonesia and the western Pacific, and suppressed [indicated by positive
anomalies of Outgoing Longwave Radiation (OLR)] across the central and eastern equatorial
Pacific (Fig. 14d). The associated reduction in atmospheric
heating between 160°E and the west coast of South America led to a drop in 200-hPa
heights throughout that region (Fig. 14e), and to cyclonic
circulation anomalies at upper levels which flanked the region of suppressed tropical
convection. These cold episode conditions then persisted throughout 1999 and into the year
2000, and significantly impacted global temperature, precipitation and atmospheric
circulation patterns in a manner consistent with past cold episodes (Ropelewski and
Halpert 1989; Halpert and Ropelewski 1992).
b. Precipitation Impacts
Many areas of the world experienced precipitation patterns during 1999
which were consistent with past cold episodes (Fig. 15). Among
the areas most impacted by La Niņa-related precipitation anomalies were Indonesia, the
tropical Indian Ocean, the western tropical Pacific, and the western and central
subtropical South Pacific, all of which experienced well above-average rainfall throughout
the year. The cold episode also featured a nearly complete disappearance of tropical
rainfall from the central and eastern equatorial Pacific.
Continuing eastward, the Caribbean Sea and tropical Atlantic
experienced above-normal rainfall during much of the year, along with an extremely active
Atlantic hurricane season from mid-August through November [see section 4a(2)]. Above-normal rainfall was also observed
across the Sahel region of northwestern Africa during JulySeptember [see section 4b(2)]. In the extratropics, La Niņa conditions
contributed to above-normal wintertime precipitation in southwestern Canada and the
Pacific Northwest U. S. [see section 4a(1)], and to
suppressed cool-season precipitation across the southeastern U. S. and Gulf Coast states.
c. SSTs and sub-surface ocean temperatures
Over the equatorial Pacific the La Niņa-related pattern of SSTs was
characterized by a well-defined cold tongue extending westward from the west coast of
South America to the vicinity of the date line (Figs. 16a, c, e, g).
This cold tongue extended farther west than normal throughout the year, as indicated by a
westward retreat of the 28°C isotherm to near 160°E. Temperatures within this cold
tongue averaged less than 25°C during most of the year, with the exception of MAM (Fig. 16c) when they approached 26°C in association with the normal
peak in the annual cycle. These temperatures were well below the 28°C value generally
considered to be the approximate threshold for deep tropical convection (Gadgil et al.
1984), and were consistent with the disappearance of deep tropical convection from this
region throughout the year (Fig. 17).
The cold episode and anomalous westward extension of the equatorial
cold tongue are further highlighted by the pattern of negative SST anomalies across the
central and eastern Pacific. SSTs averaged more than 0.5°C below normal across the entire
eastern half of the tropical Pacific throughout the year (Figs. 16b,
d, f, h), with the most significant negative anomalies occurring during DJF and MAM
when temperatures dropped to 1°-2°C below average (Figs. 16b, d).
These conditions were accompanied by anomalously warm SSTs over the western tropical
Pacific during most of the year, with the largest positive SST anomalies (averaging
0.5°1.0°C) also observed during DJF and MAM.
The SST anomaly fields also indicate that the cold episode weakened
during JJA, and subsequently strengthened during SON. By December 1999, moderately strong
cold episode conditions had reappeared, with SSTs averaging more than 1°C below normal
across the entire central and eastern equatorial Pacific (Climate Diagnostics Bulletin,
December 1999, their Fig. T18).
An examination of the sub-surface thermal structure indicates that the
anomalously cold ocean temperatures extended down to approximately 150 m depth across the
central and eastern equatorial Pacific (Fig. 18), while
anomalously warm ocean temperatures were evident over the western Pacific between
approximately 50 m and 225 m depth. These subsurface temperature anomalies reflected an
increased slope of the oceanic thermocline, and resulted from the characteristic La
Niņa-related pattern of strong oceanic upwelling over the eastern half of the Pacific and
increased downwelling over the western Pacific. The weakening of the cold episode during
JJA, and subsequent strengthening during SON, is also evident in the pattern of subsurface
temperature anomalies in the eastern equatorial Pacific, which indicates only modest
negative anomalies during JJA (Fig. 18c), followed by a
substantial expansion of the region of below-normal temperatures during SON (Fig. 18d).
d. Tropical Convection
The distribution and intensity of tropical convection represents a
primary forcing onto the atmospheric circulation through its direct modulation of the wind
and mass fields in the global Tropics and subtropics. The La Niņa-related pattern of
tropical convection featured enhanced convective activity (indicated by negative OLR
anomalies) across the climatologically convective regions of Indonesia and the western
equatorial Pacific (Fig. 17), and anomalously weak convective
activity across the central Pacific. Over the eastern Pacific there was an absence of
equatorial convection and comparatively small OLR anomalies throughout the year.
This overall pattern of anomalous equatorial convection was most
pronounced during DJF and MAM, but remained prominent even during JJA and SON. These
conditions were accompanied by enhanced convection over large portions of Central America
and the western Caribbean Sea throughout the year, as well as across the African Sahel
during the JuneSeptember rainy season in that region [see section 4b(2)].
e. Atmospheric Circulation
1) Pacific Basin
Several atmospheric circulation features common to past Pacific cold
episodes prevailed during 1999. In the Tropics, these features included 1) upper-level
westerly wind anomalies across the central and eastern Pacific during DJF and MAM (Figs. 17a, b), and over the western half of the tropical Pacific
during JJA and SON (Figs. 17c, d), 2) lower-level easterly wind
anomalies across the central and western Pacific throughout the year (Fig.
19), 3) enhanced ascending motion and convective activity over the western Pacific and
Indonesia, and 4) anomalous descending motion and suppressed convective activity over the
central Pacific. These conditions are consistent with an enhanced equatorial Walker
circulation across the Pacific basin, which is a well-known feature of Pacific cold
The circulation also featured anomalous upper-level equatorward flow
over the central tropical Pacific in all seasons (Fig. 17), along
with a combination of anomalous upper-level convergence and lower-level divergence in that
region [implied by the anomalous OLR pattern in Fig. 17]. These
conditions were accompanied by a suppressed Hadley circulation over the central tropical
Pacific, another well-known feature of Pacific cold episodes.
The La Niņa-related tropical convection also contributed to coherent
anomaly patterns in the atmospheric mass and wind fields in the subtropics and
extratropics of both hemispheres. In particular, the absence of convective activity over
the central and eastern Pacific led to cooler-than-normal mean tropospheric temperatures (Fig. 7) and to decreased upper-level heights in those regions in all
seasons (Figs. 14e, 20a). These
conditions were accompanied by an amplification of the mid-Pacific troughs in both
hemispheres, and by a confinement of the low-latitude ridges to the heavy-convection
region of the western Pacific and Australasia (Fig. 20). These
circulation features were particularly pronounced during DJF (Fig.
20b), when the strong east-west variations in tropical convective activity were also
manifested in enhanced east-west variations in the upper-level heights across the low
latitudes of the Pacific basin.
In the Northern Hemisphere, this anomalous low-latitude height field
during DJF impacted the structure and location of the East Asian jet stream (Fig. 21), as well as the extratropical atmospheric circulation over
the higher latitudes of the North Pacific. In particular, the confinement of the
low-latitude ridge to the western Pacific (Fig. 20b) contributed
to a more northward position of the East Asian jet in the area west of the date line (Fig. 21), with the jet axis located near 32.5°N in the region of
strong north-south height and temperature contrast along the poleward flank of the ridge.
Farther east, the amplified mid-Pacific trough was associated with a decrease in the
magnitude of both the upper-level height gradient (implied by the anomaly pattern in Fig. 20b) and jet stream winds (Fig. 21)
across the eastern Pacific up to 30°N [indicated by easterly wind anomalies (blue
shading) in the jet exit region (Fig. 21)].
This anomalous low-latitude height field also contributed to enhanced
upper-level diffluence over the central Pacific along the equatorward flank of the East
Asian jet exit region. In turn, this enhanced diffluence favored an amplification of the
thermally-indirect transverse ageostrophic circulation normally found throughout the jet
exit region, as indicated by anomalous upper-level ageostrophic flow directed equatorward
toward higher heights (Fig. 22). This enhanced ageostrophic
circulation is consistent with accentuated along-stream speed decreases that air parcels
experienced as they exited the jet core and decelerated to below-normal wind speeds in the
region south of 30°N.
This anomalous ageostrophic flow in the jet exit region has important
dynamical implications for the extratropical atmospheric circulation at high latitudes.
For instance, it results in enhanced upper-level divergence over the central North Pacific
along the poleward flank of the jet exit region (indicated by the sense of the anomalous
ageostrophic wind in Fig. 22), which provides a source for
enhanced anticyclonic vorticity and increased heights in that region. Enhanced westerlies
and increased storminess were observed in the region poleward of these increased heights
(see section 4a(1), Fig. 27) which ultimately contributed to
above-normal precipitation in the Pacific Northwest U. S. and western Canada [see section 4a(1)].
Symmetric Streamfunction Anomaly Pattern
Anticyclonic circulation anomalies, indicated by positive
streamfunction anomalies in the Northern Hemisphere and negative streamfunction anomalies
in the Southern Hemisphere, were evident in the lower- and middle-latitudes of both
hemispheres in all seasons (Fig. 23). This global-scale anomaly
pattern is a leading mode of interannual variability, explaining approximately 43% of the
total interannual variance (Mo and Kousky 1993). This mode is strongly influenced by the
ENSO cycle, with the pattern of tropical convection typical of La Niņa conditions
favoring the anomalous streamfunction pattern observed during 1999. In contrast, an El
Niņo-like pattern of tropical convection favors a reversal in the sign of the
streamfunction anomalies and an opposite phase of the mode.
This global scale mode of atmospheric variability also strongly modulates Atlantic and
eastern Pacific basin hurricane activity during the AugustNovember period (Bell and
Chelliah 1999), as well as rainfall across the Sahel region of western Africa. The phase
of the mode observed during 1999 favors persistent regional circulation features which are
conducive to increased tropical storm activity over the North Atlantic and to a wet Sahel,
along with a suppressed eastern Pacific hurricane season. The opposite phase of the mode,
as was observed during the 199798 El Niņo, favors persistent regional circulation
features which suppress tropical storm activity over the North Atlantic, which contribute
to a drier-than-normal Sahel, and which are conducive to an active eastern Pacific