The global climate during 1997 was affected by one of the strongest Pacific warm episodes on record. These warm episode conditions developed rapidly in March, with strong ENSO conditions persisting from May through the end of the year (and subsequently well into 1998) (Figs. 20, 21 ). During this episode the abnormally warm SSTs which covered the eastern half of the equatorial Pacific (Fig. 21a) were comparable in magnitude and areal extent to the famed 1982/83 El Niño (Fig. 20a). These 1997 warm episode conditions were accompanied by a strong negative phase of the Southern Oscillation, with the equatorial Southern Oscillation Index (SOI) also comparable in magnitude to that observed during 1982/83 (Fig. 20b). Also evident since April was a markedly reduced strength of the low-level (850-hPa) equatorial easterly winds across the eastern tropical Pacific (Fig. 21c). At times these anomalies indicated a complete disappearance of the easterlies across the entire eastern Pacific, along with a complete collapse of the normal equatorial Walker circulation. These anomalies were also comparable to those observed during the 1982/83 El Niño (Fig. 20c).
The above conditions were associated with a dramatic alteration of the global pattern of tropical rainfall and deep tropical convection, as indicated by above-normal rainfall across the eastern half of the tropical Pacific and by significantly below-normal convection across Indonesia and the western equatorial Pacific (Fig. 22). The combined zonal extent of these rainfall anomalies covered a distance more than one-half the circumference of the earth.
Selected impacts associated with these warm episode conditions included 1) excessive rainfall across the eastern half of the tropical Pacific, 2) significantly below-normal rainfall and drought across Indonesia and the western tropical Pacific, 3) below-normal hurricane activity over the North Atlantic during August-October, with a simultaneously expanded region of conditions favorable for tropical cyclone formation over the eastern subtropical North Pacific [see section 4a(2)], 4) excessive rainfall and flooding in equatorial eastern Africa during October-December [see section 4b(1) ], 5) a dramatic eastward extension of the South Pacific jet stream to well east of the date line during June-December [see section 3a(4)], which resulted in enhanced storminess across southeastern South America and central Chile [see section 4e(2)], and 6) abnormally dry conditions across the Amazon Basin, central America, the Caribbean Sea and the subtropical North Atlantic throughout the period.
2) Evolution of the 1997 El Niño
The year began with a continuation of weak cold episode conditions in the tropical Pacific during December 1996-February 1997 (DJF 1996/97). These conditions included a well-defined tongue of abnormally cold SSTs extending across the eastern tropical Pacific (Fig. 23b), with equatorial SSTs greater than 28°C confined to the area west of the date line (Fig. 23a). SSTs were slightly below normal in the Niño 1+2 region (Fig. 24a), the Niño 3 region (Fig. 24b) and the Niño 3.4 region (Fig. 24c), and substantially below the 28°C threshold for convection (Gadgil et al. 1984) in all three regions.
The colder-than-normal conditions were accompanied by a strongly sloping equatorial oceanic thermocline (Fig. 25a), with increased thermocline depths across the western Pacific and reduced depths over the extreme eastern Pacific [The center of the thermocline is approximated by the 20°C isotherm]. These variations in thermocline depth were accompanied by abnormally warm ocean temperatures in the western and central tropical Pacific between 50-200 m depth and by abnormally cold water in the eastern tropical Pacific between the surface and 50-100 m depth. During this period, the atmosphere featured 1) a positive phase of the Southern Oscillation, with below-normal sea level pressure (SLP) across the western tropical Pacific (Fig. 21b), 2) a broad area of slightly enhanced low-level easterlies across the central tropical Pacific (Figs. 21c, 26a ), 3) enhanced tropical convection over Indonesia and the western tropical Pacific [indicated by negative values of anomalous Outgoing Longwave Radiation (OLR)] and suppressed convection in the vicinity of the date line (Fig. 26a), and 4) westerly wind anomalies at upper levels across the eastern tropical Pacific (Fig. 27a). Collectively, these conditions reflected an enhanced equatorial Walker circulation [see section 3a(3)], and a continued coupling between the positive phase of the Southern Oscillation and below-normal SSTs across the eastern tropical Pacific.
In contrast, March and April featured an extremely rapid transition to one of the strongest warm episodes
the century. SSTs increased nearly 1.5°C over the normal annual cycle in the Niño 1+2 region during March (Fig. 24a), and nearly 1.0°-1.5°C over the annual cycle in the Niño 3, Niño 3.4 and Niño 4 regions. In the Niño 4 region this increase occurred during a two-week period and was greater than the entire annual cycle of SST for that region. A second period of very rapid SST increases in the east-central Pacific then occurred during April, as SSTs in both the Niño 3 and Niño 3.4 regions climbed an additional 1°C over that expected from the normal annual cycle. Thus, by mid-April SSTs exceeded 28°C across the central and east-central equatorial Pacific (Figs. 24b-d ), with values averaging 1°-3°C above normal in all four Niño regions. Area-averaged SSTs in the Niño 3, Niño 3.4 and Niño 4 regions then remained nearly constant at values greater than 28°C throughout the remainder of the year. This warming reflected a nearly complete elimination of the annual cycle in SSTs across most of the equatorial Pacific, which is normally characterized by a peak in temperatures during March-April and a minimum during September-October.
For the MAM season as a whole, mean SSTs greater than 29°C extended to east of the date line, and values greater than 28°C extended to approximately 160°W (Fig. 23c). These temperatures averaged 0.5°-2.0°C above normal across the central and east-central tropical Pacific (Fig. 23d). This warming was accompanied by increased depths of the oceanic thermocline everywhere east of the date line (Fig. 25b), and by a flattening of the thermocline across the region. In the eastern tropical Pacific, this suppressed thermocline reflected substantially reduced oceanic upwelling in association with a weakening of the low-level equatorial easterly winds (westerly wind anomalies) (Fig. 26b ). These conditions were accompanied by suppressed convection throughout Indonesia and enhanced convection in the vicinity of the date line, which is opposite to the pattern observed the previous season.
During JJA, SSTs remained very warm throughout the entire eastern half of the tropical Pacific, with the 29°C isotherm expanding eastward to approximately 150°W, and the 28°C isotherm extending eastward to approximately 125°W (Fig. 23e ). These extremely warm waters are highly abnormal for that time of year (Fig. 23f), a period normally characterized by a marked decrease in SSTs across the eastern tropical Pacific. As a result, SST anomalies increased substantially throughout the region, and exceeded 3°-4°C between 130°W and the west coast of South America (Fig. 23f). This increase in anomalies was accompanied by a further flattening of the oceanic thermocline across the eastern Pacific (Fig. 25c), as the 20°C isotherm dropped to more than 100 m depth and ocean temperatures increased to more than 7°C above normal between 50-125 m depth.
The JJA period also featured an increasingly negative phase of the Southern Oscillation, and a further decrease in the strength of the 850-hPa easterly winds (3-6 m s-1 below normal) across most of the central and eastern tropical Pacific (Fig. 26c ). These conditions were accompanied by increased tropical convection (Fig. 26c) and rainfall across the entire eastern half of the Pacific, and by decreased rainfall across the western tropical Pacific and Indonesia (see section 4f). These changes in tropical convection reflected 1) a pronounced eastward extension of the primary area of tropical convection to well east of the date line, and at times an actual shift of the main region of tropical convection to the eastern half of the tropical Pacific (not shown), and 2) a strengthening and equatorward shift of the intertropical convergence zone (ITCZ) in the Northern Hemisphere (not shown).
Also observed during JJA was the development of an anomalous upper-level anticyclonic circulation in the Southern Hemisphere subtropics between the date line and 90°W (Fig. 27c). This feature is a recurring aspect of the winter hemisphere circulation during strong warm episodes (Arkin 1982), and reflects several important changes in the flow occurring in the Tropics, the subtropics and the extratropics. In the Tropics, the equatorward flank of the circulation anomaly reflects anomalous upper-level easterly flow across the eastern Pacific, and thus comprises important structural elements of the much weaker-than-normal equatorial Walker circulation observed during the period [see section 3a(3)]. In the subtropics, above-normal heights (not shown) accompanying the anticyclonic circulation anomaly reflect an eastward extension of the mean subtropical ridge to well east of the date line, in response to the increase in tropical convection and deep tropospheric heating across the eastern tropical Pacific. In the extratropics, the anomalous anticyclonic circulation is an integral component of the coupling process between changes in tropical convection and changes in the wintertime jet stream across the South Pacific [see section 3a(4) ]. The Southern Hemisphere circulation was also characterized by recurring high-latitude blocking over the high latitudes of the eastern South Pacific, a feature typical of strong warm episode conditions (Karoly, 1989).
Strong warm episode conditions continued during SON (Figs. 23g, h), with SSTs greater than 28°C
ing eastward from Indonesia to 125°W and greater than 29°C extending eastward to approximately 140°W (Fig. 23g). The normal cold-tongue that typically occupies the eastern half of the tropical Pacific at this time of the year was notably absent, consistent with the collapse of the normal annual cycle in SSTs throughout the region (Figs. 24b, c). A nearly isothermal temperature structure was also observed from the surface to 150 m depth, with ocean temperatures exceeding 9°C above normal at 50-150 m depth in the eastern Pacific (Fig. 25d).
Also during SON, El Niño-related enhanced rainfall and heavy tropical convection developed across equatorial eastern Africa [see section 4b(1)]. This rainfall was associated with low-level easterly wind anomalies across the tropical Indian Ocean (Fig. 26d), and with a continuation of extremely suppressed convection throughout Indonesia. Also observed was a continuation of enhanced upper-level westerlies and an extended jet stream across the subtropical South Pacific (Fig. 27d), resulting in continued heavy precipitation across Chile and southeastern South America. Elsewhere, above-normal rainfall and increased storminess developed across the Gulf Coast of the United States (Fig. 22), in association with enhanced upper-level westerlies across the southern tier of the country.
3) Equatorial Walker Circulation
Over the equatorial Pacific the divergent component of the atmospheric circulation is intimately related to the distribution of tropical convection, which in turn is an integral part of the still larger Southern Oscillation (Bjerknes 1969). This divergent circulation is often partitioned into its zonal and meridional components, respectively called the equatorial Walker circulation and the tropical Hadley circulation.
The equatorial Walker circulation is characterized by ascending motion over Indonesia and the western tropical Pacific, and descending motion over the east-central equatorial Pacific, with upper-level westerly (low-level easterly) flow completing the "direct" circulation cell. Following Halpert and Bell (1997), we illustrate the equatorial Walker circulation using pressure-longitude plots of the vector field whose horizontal component is the divergent zonal wind and whose vertical component is the scaled pressure vertical velocity. The pressure vertical velocity was subjectively scaled to give a sense of the relative vertical motion in the equatorial plane. The seasonal mean equatorial Walker circulation and anomalies during 1997, along with the accompanying seasonal relative humidity anomalies, are shown in Fig. 28.
During DJF 1996/97, a well-defined equatorial Walker circulation was present (Fig. 28a), with ascending motion over the western tropical Pacific, descending motion over the eastern Pacific and a circulation center near 170°W. These conditions reflected a slight strengthening and an overall westward shift of the circulation center compared to normal (Fig. 28b), consistent with weak cold episode conditions and a positive phase of the Southern Oscillation. These conditions dissipated rapidly during MAM 1997, as a near-normal strength and location of the Walker circulation prevailed (Figs. 28c, d).
By JJA 1997, ascending motion and deep tropical convection encompassed the tropical Pacific between 140°E and 120°W (Fig. 28e ), while no well-defined pattern of vertical motion was evident over Indonesia. This anomalous vertical motion field (Fig. 28f) reflected a nearly complete disappearance of the equatorial Walker circulation. The pattern was also accompanied by enhanced relative humidity everywhere east of the date line, and by reduced relative humidity across the western tropical Pacific and Indonesia. These conditions strengthened during SON 1997, with the equatorial Walker circulation again nearly absent (Figs. 28g, h).
4) South Pacific jet stream during July-September 1997
In both the Northern and Southern Hemisphere, the extratropical wintertime jet stream over the western and central Pacific is intimately related to the distribution of tropical convection across Indonesia and the tropical Pacific. Thus, the interannual variability of these jet streams is strongly influenced by the ENSO. During strong El Niño conditions the wintertime jet stream extends eastward to well east of the date line, and over the eastern Pacific is shifted well equatorward from normal. These changes in the jet stream reflect a deep baroclinic jet structure often extending across the entire Pacific Basin, along with a pronounced eastward shift of the normal jet exit region to well east of the date line. These conditions then contribute to enhanced storminess and above-normal precipitation at lower latitudes of both North and South America.
During 1997, the South Pacific jet stream was particularly impacted during July-September by the
ongoing strong El Niño conditions, while the primary impacts on the North Pacific jet stream did not occur until
early 1998. Thus, this analysis focuses on the wintertime South Pacific jet stream, which extended across the
South Pacific and brought enhanced storminess and above-normal precipitation throughout Chile and southeastern South America [see section 4e(2)].
The core of the South Pacific jet stream (approximated by wind speeds greater than 50 m s-1) during July-September is typically located between 22.5°-32.5°S and extends eastward from eastern Australia to approximately 150°W (Fig. 29a ). The jet entrance region is normally located over eastern Australia, and is characterized by a local maximum in along-stream increases in geostrophic wind speed. Additional characteristics of the entrance region include confluent geostrophic flow at upper levels and a strong poleward component of the horizontal ageostrophic flow directed toward lower geopotential height. This ageostrophic flow is one component of the thermodynamically direct, transverse ageostrophic circulation typical of any midlatitude jet entrance region (Palmen and Newton 1969, sections 1.5 and 8.3; Hoskins et al. 1978, Keyser and Shapiro 1986), and produces the required westerly momentum and kinetic energy increases that air parcels experience as they approach the jet core.
Farther downstream, the jet exit region is normally found between the date line and approximately 125°W, and is characterized by a local maximum in along-stream decreases in geostrophic wind speed. Characteristic features of this exit region include diffluent geostrophic flow at upper levels and a strong equatorward component of the ageostrophic flow directed toward higher geopotential height. This ageostrophic flow is one component of the required thermodynamically indirect, transverse ageostrophic circulation typical of any midlatitude jet exit region, and produces the required westerly momentum and kinetic energy decreases that air parcels experience as they exit the jet.
The July-September 1997 period featured an eastward extension of the jet stream across the entire South Pacific (Fig. 29b), and an extension of the jet core to 105°W (nearly 45° east of normal). This extension was accompanied by a pronounced eastward shift in the regions of along-stream decreases in geostrophic wind speed and strong diffluent geostrophic flow, and by a nearly complete elimination of these features in the vicinity of the climatological mean jet exit region. Collectively, these conditions reflected an eastward shift in the location of the jet exit region to between 130°-90°W. This dramatic structural change in the jet stream was accompanied by a dynamically consistent eastward shift in the primary region of equatorward-directed ageostrophic flow at upper levels to the observed jet exit region, indicating a corresponding shift in the entire thermodynamically indirect, transverse ageostrophic circulation that characterizes the jet exit region.
This jet extension and eastward shift of the jet exit region were intimately related to an eastward extension of the subtropical ridge to well east of the date line, which is identified by a well-defined anticyclonic circulation anomaly across the entire eastern subtropical South Pacific during JJA and SON (Figs. 27c, d). Additional aspects of this link between the two features are revealed by examining their common attributes. One common feature is the region of enhanced westerlies over the eastern South Pacific, which comprises both the poleward flank of the anticyclonic circulation anomaly and the extended South Pacific jet stream (compare Figs. 27c, and 29b, c). Two additional common features are the poleward flow and equatorward flow along the western and eastern flanks of the anticyclonic anomaly, respectively, which contain important dynamical information regarding links between the anomalous subtropical ridge and changes in the jet entrance and exit regions.
The anomalous poleward flow comprises several important structural changes occurring in the exit region of the climatological mean Pacific jet. First, it contributes to anomalous geostrophic confluence throughout the region (Fig. 29c), which also coincides with the entrance region of the anomalous westerly wind maximum. Second, it comprises a dynamically consistent pattern of anomalous ageostrophic flow at upper-levels, directed toward lower geopotential heights at an angle nearly orthogonal to the jet axis. This ageostrophic flow reflects an anomalous thermodynamically direct, transverse ageostrophic circulation, and results in abnormally strong Lagrangian increases in kinetic energy throughout the region (Fig. 30a). In this particular case, both the rotational (Fig. 30b) and divergent (Fig. 30c) components of the ageostrophic flow contributed strongly to these kinetic energy tendencies. Collectively, these anomalies are consistent with an almost complete elimination of the normal jet exit region in the vicinity of the date line, and with a reduced strength of its attendant transverse ageostrophic circulation.
A similar examination indicates that the equatorward flow along the eastern flank of the anticyclonic
circulation anomaly comprises important structural and dynamical features of the observed jet exit region. For
example, this equatorward flow contributes to geostrophic diffluence in the observed jet exit region (Fig. 30c), an area which also coincides with the exit region of the anomalous westerly wind maximum. The
flow also comprises a coherent pattern of ageostrophic flow directed toward higher geopotential heights at upper levels, at an angle nearly orthogonal to the jet axis. This ageostrophic component of the flow reflects the well-defined thermodynamically indirect, transverse ageostrophic circulation previously noted in the jet exit region, and results in Lagrangian decreases in kinetic energy throughout the area (Fig. 30a). In this case, the rotational component of the ageostrophic flow contributes more to these kinetic energy tendencies (Fig. 30b) than does the divergent component (Fig. 30c).
Thus, in this case the anomalous poleward and equatorward flow found respectively along the western and eastern flanks of the anticyclonic circulation anomaly are strongly linked to jet dynamical processes through El Niño-related changes in the subtropical ridge. These flow features also highlight the jet-like character of the anomalous westerly wind maximum found along the poleward flank of the anticyclonic circulation anomaly.
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