The 1998–2000 Pacific Cold Episode (La Niņa)

a. Overview

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 1998–2000 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 September–November 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. 14b).

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 July–September [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 June–September 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 episodes.

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)].

       2) Zonally 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 August–November 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 1997–98 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 hurricane season.