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Differences in Atmospheric Circulation between the Development of Weak and Strong Warm Events in the Southern Oscillation

DAVID J. STEPHENS AND MICHAEL J. MEULENERS Department of Agriculture and Food WA, South Perth, Western Australia, Australia

HARRY VAN LOON Colorado Research Associates/Northwest Research Associates, Boulder, Colorado

MALCOLM H. LAMOND Lamond Weather Services, Karrinyup, Western Australia, Australia

NICOLA P. TELCIK Yarra Valley Water, Mitcham, Victoria, Australia

(Manuscript received 5 October 2005, in final form 7 September 2006)

ABSTRACT

In this study temporal and spatial aspects of El Niño (warm event) development are explored by comparing composite sequences of sea level pressure (SLP), surface wind, and sea surface temperature (SST) anomalies leading into strong and weak events. El Niño strength is found to be related to the magnitude and spatial extent of large-scale SLP anomalies that move in a low-frequency mode. In association with this, it is also intricately linked to the amplitude and wavelength of the Rossby waves in the southern midlatitudes. The primary signature of the Southern Oscillation is a more pronounced standing wave of pressure anomalies between southeastern Australia and the central South Pacific leading into stronger events. A strong reversal in the strength of the annual cycle between these two regions causes a stronger (weaker) SLP gradient that drives southwesterly (northwesterly) wind stress forcing toward (away from) the western equatorial Pacific in austral winter–spring of year 0 (Ϫ1). Thus, pressure variations in the southwest Pacific preconditions the equatorial environment to a particular phase of ENSO and establishes the setting for greater tropical–extratropical interactions to occur in stronger events. Maximum warming in the Niño-3 region occurs between April and July (0) when a strong South Pacific trough most influences the trade winds at both ends of the Pacific. Cool SST anomalies that form to the east of high pressure anomalies over Indo–Australia assist an eastward propogation of high pressure into the Pacific midlatitudes and the demise of El Niño. Strong events have a more pronounced eastward propogation of SST and SLP anomalies and a much more noticeable enhancement of winter hemisphere Rossby waves from May–July (Ϫ1) to November–January (ϩ1). Weak events require an enhanced South Pacific trough to develop but have much less support from the North Pacific. They also appear more variable in their development and more difficult to predict with lead time.

1. Introduction ability (Latif et al. 1998; Fedorov and Philander 2000). However, the failure of operational forecasting models Recent theories and models of ENSO development to predict the rapid development, intensity, and abrupt have centered on equatorial ocean–atmosphere cou- termination of the intense 1997/98 El Niño (Ander- pling as sole explanators of Southern Oscillation vari- son and Davey 1998; Trenberth 1998; Barnston et al. 1999; Landsea and Knaff 2000) challenged this assumption. Subsequent analysis of the event provided Corresponding author address: Dr. David Stephens, Climate Risks and Opportunities Project, Dept. of Agriculture and Food support for the two main theories of the Southern Os- WA, Locked Bag No. 4, Bentley DC WA 6983, Australia. cillation. E-mail: [email protected] First, the Southern Osillation can be explained as a

DOI: 10.1175/JCLI4131.1

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JCLI4131 2192 JOURNAL OF CLIMATE VOLUME 20 weakly damped oscillator that needs to be triggered by Of critical importance to ENSO dynamics is the in- a random disturbance. Westerly wind bursts in the ference by van Loon (1984) that the quasi-permanent western equatorial Pacific appear necessary at the onset South Pacific trough in the surface westerlies weakens of El Niño as they are associated with 1) an eastward both the South Pacific high and associated trades dur- shift of warm water and convection, and 2) the genera- ing the important months between May, June, and July tion of oceanic Kelvin waves that warm the central and (MJJ, hereafter 3-month periods are denoted by the eastern Pacific through zonal advection and displace- first letter of each respective month). At this time, the ment of the thermocline. The timing and amplitude of trough and subtropical jet stream reach their farthest 1997 warming was attributed to exceptionally strong northward extent as part of the semiannual oscillation westerly wind events/Madden–Julian oscillation (MJO) (SAO) in trough positioning and intensity (van Loon activity, strong downwelling kelvin waves (McPhaden 1967; van Loon 1972; Hurrell et al. 1998). Over the year and Yu 1999; Picaut et al. 2002; Lengaigne et al. 2003), leading into El Niño, the South Pacific trough changes and a buildup of upper-ocean heat early in the year from a weakened to an enhanced state, that is, takes on (Meinen and McPhaden 2000; Sun 2003). MJO activity a more permanent feature (van Loon 1984). Associated was also proposed as a “triggering mechanism” that with this is a west-to-east pressure anomaly reversal or accelerated the ending of the event (Takayabu et al. “seesaw” in the southern midlatitudes (Berlage 1966; 1999; Straub et al. 2006). van Loon and Shea 1985, 1987) and other important However, the influence of the MJO on ENSO re- wind and SST anomalies (Trenberth 1976; Rasmusson mains a controversial topic (McPhaden 2004) as periods and Carpenter 1982; Harrison 1984; Harrison and Lar- of enhanced westerlies and MJO activity do not always kin 1996, 1998). Figure 1 sumarizes this process, which lead to El Niño (Chen and Wu 2000; Bergman et al. we shall call the “van Loon hypothesis.” 2001; Zhang and Gottschalck 2002; Jones et al. 2004; In austral winter/spring in the year before El Niño Levinson 2005). Of the 13 strongest MJO events since (year Ϫ1), the annual cycle is weak in the southern 1979, only the two events late in 1996 and early 1997 midlatitudes, with a strong trough pattern over the re- actually preceded an El Niño (Jones et al. 2004). Slingo gion of the Australian subtropical high, and a weak et al. (1999) found that the statistical relationship be- trough in the South Pacific (Fig. 1a). Northerly wind tween ENSO and MJO indices do not show that they anomalies between these two regions warm the Coral are causally related, and Bergman et al. (2001) specu- Sea and the southwest edge of the South Pacific con- lated that the MJO might be relevant to the timing and vergence zone (SPCZ) as it expands south in austral initial growth of El Niño rather than be responsible for spring (Ϫ1). Increased convection and lower pressures the event. over this warm water then coincide with anomalous Second, the Southern Oscillation can be viewed as a westerlies on the equatorward side of the northern lower-frequency self-sustaining mode of oscillation in SPCZ (Rasmusson and Carpenter 1982; Von Storch et the the tropical Pacific (Chen et al. 2004). Wang and al. 1988). At the southern end of the SPCZ, negative Weisberg (2000) found that off-equatorial SST and as- pressure anomalies continue to form over warm water sociated pressure anomalies in the subtropics played an and this is associated with an equatorward reposition- important role in the development and decay of the ing of the surface trough into the South Pacific (van 1997/98 El Niño. However, what went unnoticed was a Loon and Shea 1985; Fig. 1b). This whole process is major seesaw in pressure in the southern midlatitudes linked to the convective maximum in the annual cycle (van Loon and Shea 1985, 1987), and a large-scale gradually moving from southeast Asia into the Austra- propagating SLP signal noted in previous El Niño lian monsoon region (austral summer), and onto the events (Barnett 1985; Krishnamurti et al. 1986). southern end of the SPCZ (austral autumn; Meehl When Bjerknes (1966, 1969) linked maximum oce- 1987). anic warming in the eastern equatorial Pacific (El Niño) By austral winter in the El Niño year (year 0) the to the “Southern Oscillation” in pressure, the primary annual cycle is enhanced in the southern midlatitudes. cause for these events was attributed to an anomalous Stronger highs occur over Australia, and negative SLP weakening of the trade winds of the Southern Hemi- and upper-level height anomalies become established sphere and associated oceanic upwelling (Bjerknes in the central South Pacific along with an equatorward 1969). Since the South Pacific high dominates the equa- movement in the subtropical jet (Karoly 1989; Kiladis torial flow in the central and eastern Pacific to 10°N, and Mo 1998). The first result of this pattern is strong Bjerknes (1966) speculated that reasons for this warm- southerly wind anomalies to the east of an enhanced ing must be sought south of the equator. Australian high pressure. These southerlies converge

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FIG. 1. Schematic diagram showing major weather systems and anomalies in the Southern Hemisphere in winter–spring in (a) the year before El Niño [year (Ϫ1)] and (b) during El Niño [year (0)]. The Walker Circulation (white) and westerly surface troughs (red curve) sit above surface anomalies measured at key stations represented by crosses [Alice Springs (A), Darwin (D), Easter Island (E), Mildura (M), Noumea (N), Rapa Island (R), Tahiti (T), Willis Island (W)]. The SPCZ is represented by parallel gray lines. Warm (red) and cold (green) SST anomalies are indicated, as are wind anomalies (black arrows). High and low SLP are indicated by “H” and “L” and anomalous pressures in the midlatitudes are dashed. with and intensify westerly wind anomalies in the equa- 2004; Thompson and Lorenz 2004; Anderson and Ma- torial western Pacific and appear to play a determining loney 2006). The implication of these studies is that the factor in the onset time of El Niño (Harrison 1984; Southern Oscillation and southern midlatitudes have a Mitchum 1987; Chen and Wu 2000; Xu and Chan 2001). minimal role in ENSO, but is this the case? Second, transients within the South Pacific storm track We propose that the van Loon hypothesis becomes shift equatorward with the axis of maximum baroclinic- more pronounced in stronger El Niño events and that ity (Solman and Menendez 2002). The resultant west- air–sea interactions between the southern midlatitudes erly wind changes are a vital part of a gradual progres- and the equator are necessary for such events to de- sion of negative SLP anomalies into the wider Pacific velop. If the Southern Oscillation is principally a see- basin, that is, weakening of the South Pacific high (van saw (or standing wave) in atmospheric mass exchange Loon et al. 2003). The enhanced southwesterly airflow between the Western and Eastern Hemispheres (Ber- in the southwest Pacific cools the SST along the SPCZ lage 1966; Trenberth 2000), then strong negative pres- (Fig. 1b), which then preceeds a strengthening of the sure anomalies observed in the Australian region in South Pacific ridge (trades) and the demise of El Niño. year Ϫ1 should be observed in the South Pacific as El More recently, the focus has been on atmospheric Niño approaches. We tested this hypothesis by compar- variability and SST anomalies in the northern midlati- ing the composite sequence of sea level pressure (SLP), tudes as a coupled component of the stochastic forcing vector wind, and SST anomalies leading into stronger of ENSO (Vimont and Battisti 2003; Anderson 2003, and weaker El Niño events.

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2. Data When examining the composite of SST anomalies, it must be noted that these were affected by warming The present study utilizes four types of data: 1) SLP, beginning along the South American coast and spread- SST, surface winds, and 850-hPa winds. Maps showing ing westward prior to 1979, whereas later events had the spatial evolution of SLP and surface wind anoma- warming progressing eastward from the western Pacific lies associated with El Niño development were gener- (Trenberth et al. 2002), that is, 3 (2) of the 6 (6) strong ated from the National Centers for Environmental (weak) events had eastward warming. Since there is a Prediction–National Center for Atmospheric Research small sample size, and differences between individual (NCEP–NCAR) reanalysis data from 1950 to 2003 events, we will only comment on major differences be- (Kalnay et al. 1996). This has seasonal-mean surface tween the composites. fields of sea level pressure and low-level winds represented at a 2.5° resolution in both the meridional and zonal direction. The reanalysis data should be consid- 3. Southeast Australian pressure ered a blend of observational and model-simulated values. Acording to van Loon and Shea (1985), low pressure To compliment the spatial analysis, monthly SLP over southeastern Australia in year Ϫ1 appears to be a data were obtained for individual stations across the useful predictor of El Niño events 12–18 months later. Pacific Ocean and Australia. SST anomalies over the As an independent test of this proposition, we corre- Niño-3 region in the eastern equatorial Pacific (5°S– lated seasonal SLP averaged over the region 25°–35°S, 5°N, 150°–90°W) and area-averaged 850-hPa trade 135°–145°E from JAS with Niño-3 SST the equivalent wind index values for the western Pacific (5°N–5°S, period a year later. Between 1985 and 2002 the corre- 135°E–180°) were extracted from the Climate Predic- lation was –0.71, significant at the 99% level. This con- tion Center Web site (www.cpc.noaa.gov/data/indices). curs with Anderson (2003), who found that a pressure Area-averaged SST data over the central South Pacific index in this region was significantly related (20°–30°S, 160°–120°W) from 1950 to 1998 were ex- (r ϭϪ0.56) with the JFM Niño-3.4 index 18 months tracted from the Hadley Centre (Met Office) Global later. Sea Ice and Sea Surface Temperature dataset (GISST3), In Fig. 2, SLP anomalies from Mildura (34.2°S, which is available on a 1° grid. Subsequently, the Rey- 142.1°E) in southeastern Australia are compared to nolds SST dataset (RSST; 1950–99) was updated with those at Darwin closer to the equator (12.4°S, 130.9°E). the extended reconstructed SST (ERSST; Smith and As expected, the variability in SLP is greater at the Reynolds 2003) to map spatial composites of SST higher-latitude station, but this is still dominated by anomalies at a 2.5° resolution between 1950 and 2003. ENSO. The largest negative values typically occur in The El Niño events chosen in this study followed the the winter–spring in the year prior to El Niño. Over 11 year (0) events defined by Harrison and Larkin the following year, there is a dramatic rise in SLP (high- (1998), though we used a slightly less stringent criteria lighted with straight dashed line) as the normal (Niño-3 seasonal SST anomalies exceeding ϩ0.5 stan- strengthening of the subtropical high over the continent dard deviations for at least five months) to define 14 is gradually enhanced. What is most striking is the pro- events up to 2003. When mapping spatial composites, nounced pressure reversal leading into stronger El we split these events into strong and weak categories on Niño events, especially in 1957, 1965, 1972, 1982, and the basis of the Bjerknes ENSO index (BEI), a com- 1997. posite measure of eastern equatorial SLP, zonal wind, At Darwin, there is not such a close association with and meridional wind anomalies (Harrison and Larkin El Niño strength and preceding pressure anomalies. 1998). We assigned the six events with the highest BEI3 The pressure reversal from year Ϫ1 to year 0 is also less intensity (maximum intensity of event) as strong, that pronounced and less consistent. The correlation be- is, 1997, 1982, 1972, 1991, 1965, and 1957. The six events tween SLP anomalies at Darwin, between July and Sep- in 1951, 1953, 1963, 1976, 1994, and 2002 were classified tember, and El Niño strength in the following year mea- as weak (note: we did not have a BEI3 value for 2002 sured by BEI intensity was only r ϭ 0.18, compared to but noted that other ENSO indices were weak). It was r ϭ 0.69 at Mildura. The question that results is as decided to seperately map the the late-developing 1968/ follows: does this low pressure anomaly centered over 69 and 1986/87 events, as the weak warming that de- southeastern Australia play a role in the formation of veloped late in the first year was associated with a dif- ENSO events, and are larger low pressure anomalies ferent sequence of pressure anomalies compared with associated with more pronounced tropical–extratrop- other events. ical interactions leading into stronger El Niño events?

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FIG. 2. The 3-month running mean of SLP anomalies at (a) Darwin, northern Australia, and (b) Mildura, southeastern Australia. Solid dashed straight lines are superimposed on the curves for the 12 months leading into El Niño development.

4. Spatial aspects of El Niño development tions and bootstrap method gave very similar results to the normal-z statistic in nearly every location. They We first compare the evolution of SST, SLP, and also found the larger and more intense the composite winds associated with the development of strong and signal, the more robust the signal tends to be. weak El Niño events. In Fig. 3 spatial composites of developing SST anomalies were complimented with a. Sea surface temperatures equivalent composites of SLP and vector wind anomalies in Fig. 4. Composites were plotted as a sequence of For both stronger and weaker El Niños we found the seasonal averages from MJJ (–1) through to FMA same underlying sequence and pattern of SST anoma- (ϩ1). We plot actual anomalies because wind tends to lies from MJJ (Ϫ1) to FMA (ϩ1) as those found by flow downgradient along the equator, and roughly Harrison and Larkin (1998). Warm SST anomalies to along isobars farther poleward, that is, the actual im- the north and east of Australia gradually extend east- pact of large-scale pressure anomalies can be high- ward and become most pronounced in the eastern lighted visually with wind stress forcing arrows. Also, equatorial Pacific as El Niño events begin to develop ENSO monitoring bulletins (e.g., CPC 1997) plot data (Fig. 3). As the warming progresses eastward into the in this way as a standard. Pacific, SST anomalies become cooler in the Australian To address the issue of field significance, SST and region, and in the characteristic horseshoe-shaped re- SLP anomalies were standardized and a Student’s two- gion in the western Pacific. SST anomalies along the tailed t test was applied. Regions that were significantly SPCZ noticeably change sign from warm to cold be- different at the 90% level are highlighted with a solid tween FMA (0) and MJJ (0). dashed line. When considering this approach, Harrison When comparing SST anomalies between stronger and Larkin (1998) noted that the Student’s t calcula- and weaker events, the following features are apparent.

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FIG. 3. The evolution of SST anomalies for strong and weak El Niño composites: from (a) MJJ (Ϫ1) through to (p) FMA (ϩ1). Anomaly contours are 0.1°C with positive anomalies shaded as indicated by the legend on the next page. Superimposed on SST anomalies are dashed thick lines showing regions of significant anomalies at the 90% level based on the normal-z statistic, and the Student’s t test.

First, the spatial extent and magnitude of SST anoma- along the equator or SPCZ do not always lead to El lies are larger for strong events, although critically, the Niño, but require ocean–atmosphere interactions, magnitude of Pacific warming is not indicated in year which we will now elucidate. Ϫ1. Second, SST anomalies in strong events bifurcate b. Mean sea level pressure and wind anomalies around the equator in NDJ (0), before intensifying along the equator (160°E–140°W) in FMA (0), while For both strong and weak El Niño events Fig. 4 con- for weaker events warm SST anomalies do not extend firms the large-scale eastward progression of low SLP toward Hawaii and are cooler near the warm pool be- anomalies from the Eastern Hemisphere to the West- tween NDJ (0) and FMA (0). Third, in the important ern Hemisphere from year Ϫ1 to year 0 identified by months of MJJ (0) warming is weaker and centered Barnett (1985). This generally coincides with the east- farther east along the equator for weaker events. ward progression in SST anomalies along the equator It should be pointed out that warm SST anomalies and subtropics (Fig. 3), and the movement in the con-

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FIG.3.(Continued) vective maximum in the annual cycle (Meehl 1987). sphere as the sequence develops. That is, with signifi- Similar to SST anomalies, the magnitude and spatial cant low pressure over Australia between MJJ and extent of pressure and wind anomalies is greater for the ASO (Ϫ1), the central North Pacific between NDJ (0) strong events, and this pattern begins in Austral winter and FMA (0), and the central South Pacific between in year Ϫ1. MJJ and ASO (0). Figure 4 also confirms the sequence of events pro- We now examine this sequence more closely by ap- posed in the van Loon hypothesis and illustrates that proximately following the first five phases of Harrison the seesaw in low pressure between Australia and the and Larkin’s (1998) schematic ENSO composite. south central Pacific is most pronounced for the strong 1) PHASE 1: PRE—MAY (Ϫ1) TO OCTOBER (Ϫ1) events (Figs. 4a,b cf. Figs. 4i,j). A distinct feature of the seesaw in strong El Niño events is the large area of In this buildup stage, the magnitude of Pacific warm- significant low pressure anomalies in the winter hemi- ing in the following year is indicated by the magnitude

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FIG. 4. The evolution of SLP and surface wind anomalies for strong and weak El Niño composites: from (a) MJJ (Ϫ1) through to (p) FMA (ϩ1). SLP anomaly contoured every 0.5 hPa with negative anomalies shaded. Superimposed on the pressure anomalies are dashed thick lines showing regions of significant SLP anomalies at the 90% level based on the normal-z statistic, and the Student’s t test. The vector wind anomaly is measured by the scale arrow indicated on the next page. of the low pressure anomaly in the Australian region. posed by Jin (1997), and the buildup of sea level Concurrent higher-than-normal pressure over the Pa- (Wyrtki 1975, 1985), is seen here as a wider enhance- cific means that the Southern Oscillation is acting like a ment of the zonal Walker Circulation. tilted seesaw before a major change. In terms of wind anomalies, the increased northerly flow to the east of 2) PHASE 2: ANTE—NOVEMBER (Ϫ1) TO APRIL (0) Australia assists warmer SST anomalies along the SPCZ for stronger events. Along the equatorial Pacific Antecedent conditions occur when the low pressure the southeast trades are enhanced, and combined with anomalies progress east of Indo-Australia in a complex stronger westerly wind anomalies at the western end of way. In strong events low pressure over Australia bi- the Maritime Continent, provide a favorable scenario furcates about the equator [NDJ (0)] and gradually ex- for the buildup of sea level and heat in the western tends over a larger area to the northwest and southwest Pacific. Thus, the “heat pump” picture of ENSO pro- Pacific. This bifurcation in pressure appears to be

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FIG.4.(Continued) coupled to warm SST anomalies, which takes on a simi- cific at this time (Clarke and Van Gorder 2003), but the lar spatial pattern with northwesterly flow along the magnitude of these anomalies only partially relates to SPCZ, maintaining warm SST in the south, and south- the strength of the following El Niño. Two important erly flow warming the northwest Pacific (Figs. 4e–h). In lead time features are evident: 1) low pressure changes strong events, significant lower pressures in the north- in the Pacific midlatitudes lead pressure changes along central Pacific in boreal winter appear to be an en- the equator that become established in MJJ (0); and 2) hancement of planetary Rossby waves and the precipi- the southeast/northeast trade winds weaken in the mid- tation maximum in the annual cycle. Likewise in FMA latitudes before equatorial trades weaken and the east– (0), significant low pressure over the southern end of west SST gradient along the equator is reduced. the SPCZ occurs in the humid wet season coincident 3) PHASE 3: ONSET—MAY (0) TO JULY (0) with the precipitation maximum in the annual cycle. We confirm that westerly wind anomalies migrate In the critical months of MJJ (0) there is a wide- from west of Sumatra into the western equatorial Pa- spread fall in pressure across the Pacific and a rise in

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Fig 4i live 4/C 2200 JOURNAL OF CLIMATE VOLUME 20 pressure in the Indo-Australian region for both event 5) PHASE 5: DECAY—FEBRUARY (ϩ1) TO APRIL types. The van Loon hypothesis is most pronounced in (ϩ1) stronger events, while weak events have significant low As we move to FMA (ϩ1), the SLP pattern differs pressure anomalies farther east in the southeastern Pa- more between individual events and this depends on cific that line up longitudinally with equatorial warming whether a transition to La Niña or neutral conditions is centered closer to South America (Figs. 3i,j and 4i,j). occurring. In stronger events the significant low pres- These results confirm two features of Rossby waves sure anomalies weaken along the equator with stronger identified by Colls and Whitaker (2001). First, as the ridges continuing to gradually extend toward the east zonal flow of air becomes more disturbed, the ampli- (Fig. 4o). Low pressures persist near Tahiti, which tude of the upper waves becomes greater, with the cor- causes the Southern Oscillation index (SOI) to remain responding reduction in the wavelength indicating negative and lag many other indicators that are suggest- baroclinic growth. Second, the net result of these ing a breakdown of El Niño conditions. Associated with changes in pressure is a greater meridional movement this persistent low pressure is a southward shift in equa- of air: increased equatorward movement of cooler air torial westerly winds (Fig. 4o), which reduces the direct into the western equatorial and subtropical Pacific wind forcing along the equator and contributes to the 40°S–10°N, 160°E–170°W; and a greater poleward flow decay process (Harrison and Vechi 1999). By MJJ (ϩ1) in the southeastern Pacific in the region of the South (not shown), there is a return to stronger equatorial Pacific high (10°–35°S, 120°–80°W). In the latter case, easterlies and a stronger South Pacific high in both there is an increase of mass transport to the region types of events. south of 40°S, coincident with greater subsidence and The importance of the southern midlatitudes was blocking over the Bellingshausen Sea (Renwick and also found for the late developing El Niño events of Revell 1999). 1968/69 and 1986/87 (not shown). Warming developed The strength of the El Niño events is clearly a func- in NDJ (0), then reamplified in MJJ (0) as a strong tion of westerly wind anomalies in the western equato- South Pacific trough linked up with low pressure rial Pacific and the increased SLP gradient between anomalies along the equatorial Pacific. Australia and the South Pacific. The North Pacific high is weaker in strong events, but for weak events remains near normal strength. Weaker El Niño events therefore 5. Temporal time series aspects of El Niño form from the Southern Oscillation acting over regions development to the south of 10°N, further emphasizing the require- ment for a weaker South Pacific high in El Niño devel- Given the sparse network of weather stations in the opment (Bjerknes 1966). South Pacific, normalized time series composites of pressure and SST anomalies are presented in Fig. 5 for individual locations illustrated in Fig. 1. For the 11 El 4) PHASE 4: PEAK—AUGUST (0) TO JANUARY Niño events defined by Harrison and Larkin (1998) (ϩ1) monthly normalized SST anomalies are averaged and At the peak stage, significant low pressure anomalies plotted, while normalized pressure anomalies were fur- cover most of the eastern Pacific in strong events; how- ther converted into 3-month running means to mini- ever for weaker events significant negative anomalies mize the Southern Oscillation “signal” being domi- cover less of the equator and are more noticeable nated by monthly “noise” (Trenberth 1976). across a band from 30° to 40°S in the southeastern Pa- In the western region around Australia (Fig. 5a), nor- cific. The increased zonal extent and duration of west- malized pressure anomalies generally coincide with erly wind anomalies in strong events (Philander 1981) is normalized Niño-3 SST anomalies during the transition clearly seen in Figs. 4m–p. The dynamics of El Niño into El Niño, emphasizing the broad-scale coupling of decay proposed by Chan and Xu (2000) can be seen to the ocean and atmosphere. Between June and October begin in the midlatitudes in NDJ (ϩ1) as the high pres- (Ϫ1), the largest pressure anomalies occur in winter sure anomaly over Indo-Australia begins to extend to- over southeastern Australia and Darwin. Negative ward Hawaii and the central South Pacific over two pressure anomalies in the northwestern SPCZ become broad bands of cool SST anomalies. Significant high slightly more significant during NDJ (0) as negative pressures close to Hawaii in weak and strong events SLP anomalies move east at this time (Fig. 4). SLP rises would contribute to the relationship between Hawaii and falls in the Australian region as Niño-3 warms and SLP anomalies and subsequent Niño-3.4 SST anomalies cools, respectively, although SLP returns to normal val- found by Anderson (2003). ues more quickly over southeastern Australia.

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FIG. 6. Composite time series of averaged SLP anomalies for 11 El Niño events for Raoul (Kermadec Islands) near the date line (open circles); Rapa, west of the date line (solid circles); and southeastern Australia, east of the date line (solid triangles). All SLP anomalies have been smoothed by a 3-month running mean.

FIG. 5. Composite time series of averaged normalized SST consideration. Negative SLP at Rapa that coincides anomalies in the Niño-3 region (solid line, diamonds) for 11 El Niño events compared to equivalent anomalies at (a) western with rapid Niño-3 warming in April implies an earlier region, SLP at Darwin (solid line, solid triangle), northwest SPCZ northward influence of the trough on the trades. (average Noumea and Willis Island; dashed line, open squares), The underlying mechanism for this process appears and southeastern Australia (average of Alice-Springs and Mil- to be a standing wave of pressure anomalies between dura; thin line, open triangles); (b) eastern region, central South southeastern Australia and the South Pacific (Fig. 6). Pacific SST (solid line, open squares), and SLP anomalies at Rapa Island (solid line, solid circles), Easter Island (thin line, open Three evenly spaced locations between Australia and circles), and Tahiti (dashed line, solid triangles). All SLP anoma- the central South Pacific were chosen at 30°S: south- lies have been smoothed by a 3-month running mean. eastern Australia appears to represent a western antinode, the date line (Raoul, Kermadec Islands, 29°S, 178°W) a stationary node (until November 0), and In regions east of the date line (Fig. 5b), the largest Rapa Island the eastern antinode. The points of maxi- anomaly from late in year Ϫ1 through to March (0) is mum amplitude (antinodes) oscillate from over south- warm SST in the south-central Pacific (20°–30°S, 120°– eastern Australia in year Ϫ1, are equally found at both 160°W) surrounding Rapa Island. Northerly wind antinodes in year 0, and finally end in the South Pacific anomalies that flow down the SPCZ at this time (Figs. in year ϩ1 when El Niño decays. Near the date line, 4c,e,g) maintain warm SST anomalies and enhance the pressure anomalies are part of broad-scale negative normal gradual decrease in pressure observed in the anomalies in the Australian region before El Niño, and annual cycle at Rapa Island. Negative pressure anoma- broad-scale positive anomalies in the South Pacific af- lies first appear at Rapa in October (Ϫ1) and become ter El Niño (Figs. 4 and 6). most significant in the critical months of February–June To investigate the strength of the anomalies in rela- (0). Significantly, these negative pressure anomalies tion to the strength of the event, we divided the 11 lead, both a fall in pressure farther north of the SPCZ warm events defined by Harrison and Larkin (1998) at Tahiti and a gradual erosion of strength of the South into weak, moderate, and strong based on the BEI3 Pacific high centered farther east near Easter Island. power: weak—1951, 1953, and 1969 (BEI3 Power 0–3); The fact that SST anomalies in Niño-3 warm the most moderate—1957, 1965, 1976, 1987, and 1991 as moder- between April and July (0) (Fig. 5b), when van Loon ate (BEI3 power 6–12); and, strong—1972, 1982, and (1984) speculated that the South Pacific trough most 1997 (BEI3 Power 20–70). When the pressure anoma- influences Pacific trade winds (MJJ), is an important lies were averaged for these categories, the magnitude

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FIG. 7. Composite time series of averaged normalized anomalies for three weak El Niño events (faint line), five moderate events (dark thin line), and three strong events (bold thick line) for (a) SLP in southeastern Australia (average of Alice Springs and Mildura), (b) SLP on Rapa Island, (c) Niño-3 SST, and (d) SLP in Hawaii. All pressure data have been smoothed by a 3-month running mean. of negative anomalies over southeastern Australia in the El Niño events. Negative SLPs appear earlier at winter (Ϫ1) is reversed by winter (0) (Fig. 7a). These Rapa at the end of year Ϫ1 and early in year 0 for anomalies match 1) the magnitude of negative anoma- weaker and moderate events, but are more significant lies at Rapa Island in winter (0) (Fig. 7b); and 2) the in austral winter (0) when strong events are developing. rate of SST warming in the Niño-3 region through At Hawaii, very negative SLP anomalies are clearly April–July (0) (Fig. 7c). Thus, consistent with a stand- indicated between April and July (0) for strong El Niño ing wave, the magnitudes of pressure anomalies in win- events (Fig. 7d). This supports the assumption by Wang ter (0) over southeastern Australia (Figs. 4a,b) are later and Weisberg (2000) that a dipole of complementary observed in the central South Pacific in year 0 (Figs. negative pressure anomalies are needed in the northern 4i,j) and these coincide with the rate of SST increase and southern midlatitudes to force the strong westerly farther north along the equator for the critical period of wind anomalies needed at the onset of extreme events. trough enhancement between April and July (0). The For weaker events there are weaker SLP anomalies timing of the trough strengthening in the South Pacific between April and July (0) at Hawaii, implying that the also appears important in determining the strength of North Pacific has a more minor role in the formation of

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FIG. 8. (a) Plot of normalized Rapa Island SLP anomalies since 1951 [black line, left (y) axis] vs actual Niño-3 SST anomalies [red and blue color fill, right (y) axis]. (b) Plot of normalized Niño-3 SST anomalies (red curve) vs normalized Rapa Island SLP anomalies (blue curve) and normalized 850-hPa west Pacific trade winds (purple curve) (1979–2004). All curves have been smoothed using a 5-month running mean. these events (Figs. 4j,l). Figure 7d also supports the anomalies at Rapa Island between 1976 and 1997, and leading role that Hawaii SLP in NDJ (0) has at indicat- the increased occurrence of El Niño (Fig. 8a). ing a subsequent warming of Niño-3.4 SST (Anderson To look at the influence of the South Pacific trough 2003) but suggests that this is probably dominated by on equatorial trades, Rapa SLP anomalies are plotted the few strong El Niño events. against the anomalies in the area-averaged 850-hPa trade wind index for the western Pacific (Fig. 8b). Pres- sure anomalies at Rapa lead anomalies in the trades, 6. The relationship between pressure and the but more importantly, coincide in anomaly strength at Pacific trade winds the beginning of El Niño (1982, 1986/87, 1991, and SLP anomalies at Rapa Island in the South Pacific 1997) and La Niña (1988, 1995, and 1998). When the appear to play a vital role in equatorial SST changes trade wind and SLP curves have opposite anomalies, farther north in the Niño-3 region (Fig. 8a). The corre- ENSO events do not develop, even if the trade winds lation between the two curves is Ϫ0.42, significant at are very strong (1984) or weak (early 1990). the 99% level. Warm and cold events are clearly iden- The unprecedented rapid rise and fall in SST in the tified with divergence between the two time series, with 1997 El Niño and the 1998 La Niña appear to be related the largest Rapa pressure anomalies leading the largest to the most pronounced weakening and strengthening SST anomalies. All El Niño events are associated with of the South Pacific ridge between April and July since a stronger South Pacific trough, and all La Niña events 1950 (Figs. 8a,b). In the decay of the event, the South are associated with a stronger South Pacific ridge. The Pacific ridge (trades) are clearly strengthening, and critical role of the South Pacific trough is also empha- Niño-3 SST is cooling, long before the “small-scale” sized by the shift to more consistent negative pressure MJO event reaches the eastern Pacific in May 1998

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FIG. 10. Correlations between bimonthly standardized 850-hPa trade wind anomalies in the western equatorial Pacific and bimonthly standardized pressure anomalies at Rapa Island in the South Pacific and Hawaii in the North Pacific (1979–2001). Dashed lines represent the 99% and 95% significance levels.

gradient between these two regions when we compare 1997 to 1998 (Figs. 9b,c). This difference begins in April and continues through to October. To quantify the relative importance of the Pacific subtropical ridges on ENSO, SLP anomalies at Rapa Island and Hawaii were correlated with the western

FIG. 9. Comparison between SLP at Rapa Island and southeast- Pacific 850-hPa trade wind index (Fig. 10). In spite of ern Australia (average of Alice Springs and Mildura: (a) plot of the fact that Rapa Island is approximately 4880 km to 5-month running mean in 1994–98, Rapa (dashed line) and south- the southeast of the western equatorial Pacific, and east Australia (solid line); and (b) monthly values in 1997, Rapa 1450 km farther from this region than Hawaii, the trade (solid triangle) and southeast Australia (solid dot). (c) Same as in winds are significantly correlated to the strength of the (b), but for 1998. South Pacific ridge between March and November and only between May and August for the North Pacific (Takayabu et al. 1999; Straub et al. 2006). This supports ridge. Weaker correlations over austral summer at the conclusion by Wang and Weisberg (2000) and Bou- Rapa would be expected as the subtropical high is to langer et al. (2004) that strong off-equatorial low (high) the south of Rapa Island and the surface trough in the SLP and westerly (easterly) wind anomalies in the west- westerlies is farthest south in the annual cycle. High ern Pacific in early 1997 (1998) were the main drivers of correlations between trade winds and Rapa SLP in Feb- the accelerated development (termination) of this ruary–March and March–April occur in the wettest event. months in the year at Rapa when the SPCZ is active Figure 9a shows that in El Niño (La Niña) years the and extends southward, that is, El Niño is linked to enhanced (weakened) annual cycle at 30°S causes a enhancement in the annual cycle of precipitation. greater (weaker) pressure difference in austral winter Figure 11a shows that Rapa pressure is lower than between Australia and the South Pacific. In 1997, there normal in March–April for all the El Niño years and was an unprecedented eight consecutive months be- higher than normal for most of the La Niña years. Thus, tween March and October when SLP was lower at the pattern of SLP anomalies found in the South Pacific Rapa Island than over southeastern Australia, that is, appears to play a significant role in setting up a favor- southwesterly wind flow directed into the southwest Pa- able pattern for ENSO development or transitions (r ϭ cific (Fig. 9b). There is a fundamentally different SLP 0.62). In contrast, Fig. 11b shows that for the same

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FIG. 11. (a) Simultaneous correlations between average March–April standardized SLP anomalies at Rapa Island in the South Pacific and standardized 850-hPa trade wind anomalies in the western equatorial Pacific; (b) simultaneous correlations between average March–April standardized SLP anomalies at Honolulu in the North Pacific and standardized 850-hPa trade wind anomalies in the western equatorial Pacific; (c) correlations between average March– April standardized SLP anomalies at Rapa Island and standardized 850-hPa trade wind anomalies in the western equatorial Pacific in May–June; and (d) correlations between average March–April standardized 850-hPa trade wind anomalies in the western equatorial Pacific and average standardized SLP anomalies at Rapa Island in May–June.

March–April period there is an insignificant relation- case, then the Pacific subtropical highs and ridges ship between Hawaii pressure and western Pacific trade would weaken after the westerly anomalies appeared. winds (r ϭ 0.26). Very low pressures are seen in the To test this, we correlated South Pacific SLP anomalies strong El Niño years 1982 and 1997, but also in neutral with trade wind strength with lead time, and then re- and La Niña years. Given the stronger negative SLP versed the lead/lag relationship (Figs. 11c,d). Rapa Is- anomalies in strong El Niño years, but the low corre- land SLP anomalies in March–April explained half the lation with trade winds for all years (Fig. 11b), it ap- variance of the following west Pacific trade winds in pears that the North Pacific pressures 1) respond to May–June (r ϭ 0.70, significant at the 99% level), while broad-scale pressure changes from year Ϫ1, 2) contrib- the reverse relationship was weaker and insignificant ute to the formation of strong El Niño events (Figs. (r ϭ 0.34). Shorter-term periods of weaker trades asso- 4e,g,i), and 3) adjust to trade wind strength that devel- ciated with the passage of MJO events are obviously ops along the equator between May and August (0). related to oceanic Kelvin wave development and sub- In recent years there has been speculation that west- sequent east Pacific warming, however Figs. 4 and 5 and erly wind anomalies in the western equatorial Pacific Figs. 7–11 suggest that El Niño events will not develop play an important role in triggering El Niño and weak- unless there is support from the broad-scale SLP pat- ening the trade winds across the Pacific. If this was the tern, especially in the South Pacific.

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7. Discussion and conclusions lower than that recorded over southeastern Australia (Fig. 9). Wang and Weisberg (2000) only showed weak By comparing El Niño development on the basis of southerly wind anomalies off the east coast of Australia strength this study has clarified a number of previous in the 1997 El Niño development phase, but in doing so findings. A broad-scale dynamical framework for did not map anomalies between April and July, when ENSO development is possible if the interactions be- equatorial warming was greatest. tween the midlatitudes and equator are properly ac- Second, we show that the standing wave of SLP in the counted for. southern midlatitudes is part of a low-frequency east- First, we find that the development of stronger El ward propogation of 1) SLP at a large scale (Barnett Niño events is dominated by the classical Southern Os- 1985; Krishnamurti et al. 1986; Meehl 1987; Kiladis and cillation pressure signal observed on the Southern van Loon 1988), and 2) surface wind anomalies along Hemisphere by van Loon and Shea (1985, 1987). We the equator (Gutzler and Harrison 1987; Chen and Wu confirm that wave fluctuations in the southern subtrop- 2000; Clarke and Van Gorder 2003). Gill and Rasmus- ics are closely coupled to the Southern Oscillation (Kid- son (1983), Barnett (1985), and Barnett et al. (1991) son 1975; Trenberth 1980; Karoly 1989). The greatest show that low SLP anomalies on the equator propagate Pacific warming (April–July of year 0) occurs when eastward through an ocean–atmosphere feedback such Rossby waves in the southern midlatitudes have a high that larger-than-normal absolute ocean temperatures amplitude and short wavelength. Energy, or momen- are continually created to the east of the central low tum, in the Rossby waves appears to be tranferred from pressure anomaly and the release of latent heat in the Australia (year Ϫ1) to the South Pacific (year 0) via a middle troposphere. This phase shift between the forc- low-frequency standing wave of SLP, and a fundamen- ing and response effectively “pulls” the center of con- tal reversal in the strength of the annual cycle close to vection and low pressure eastward toward the warmer 30°S. These changes must occur in sequence with glo- SST, with westerly wind anomalies and advective cool- bal-scale pressure changes (Barnett 1985; Krishnamurti ing occurring to the west of the convective zone (Bar- et al. 1986) coincident with a transition from an en- nett et al. 1991). We show that this process extends into hanced Walker Circulation (buildup of heat in Indo– higher latitudes as meridional wind anomalies associ- Australia), to a weaker Walker circulation (movement ated with large-scale pressure anomalies flow from op- of heat heat away from Indo–Australia; Meinen and posite directions in each hemisphere and support SST McPhaden 2000; Sun 2003). Such changes are also changes in the subtropics. linked to the annual cycle of clouds and precipitation, Third, a key finding of this study is that the Pacific and the movement of the convective maximum from troughs strengthen long before changes are observed the Indo–Australian region into the southwest Pacific on the equator (Figs. 4, 5, 7, 8, and 11c,d). Thus, we (Meehl 1987). confirm the lead role that SLP changes play in the Since the Southern Oscillation does act like a seesaw, South Pacific (Quinn 1974; Trenberth 1976, 1980; Chen or tilted pendulum, the van Loon hypothesis appears to 1982; Trenberth and Shea 1987; Wright et al. 1988) and be critical for the development of strong events, but the North Pacific (Anderson 2003, 2004). We do not see must also incorporate the complimentary role of the evidence for a reduced east–west SST gradient along North Pacific Rossby waves in strong events (Wang and the equator reducing the trade winds in the central east- Weisberg 2000). The less significant pressure signals ern Pacific (Lengaigne et al. 2003), but rather see the found prior to weak events suggest that they are more trades weakening as a result of broad-scale pressure variable in their development, and that air–sea intera- changes that first appear in the midlatitudes. Peixoto tions along the equator could be more significant in and Oort (1992) found that changes in the SOI lead their formation. changes in zonal wind stress in the central equatorial The wave, or seesaw features, related to the van Pacific by 2 months, and eastern Pacific SST by 4.5 Loon hypothesis occurred in a dramatic way leading months. Our analysis suggests that pressure changes in into the intense 1997 El Niño. In the southern midlati- the midlatitudes could give an even longer lead warning tudes there was an unprecedented 1) low SLP anomaly of Pacific warming. Significant high (low) pressure over southeastern Australia in JAS 1996 (which Fig. 2 anomalies near Tahiti at the beginning (end) of weaker partly illustrates), 2) east–west pressure reversal be- (stronger) events cause the SOI to lag other indicators tween Australia and the central South Pacific (Figs. 2 in some ENSO transitions. and 8), 3) warm SPCZ in March 1997 (CPC 1997), 4) It could be argued that mid-Pacific surface troughs strong South Pacific trough (March–July; Fig. 8), and 5) are enhanced by anomalous convection near the date eight consecutive months when SLP at Rapa Island was line in MJJ (0) (Kiladis and van Loon 1988) and,

Unauthenticated | Downloaded 10/07/21 04:00 PM UTC 15 MAY 2007 S T E P HENS ET AL. 2207 through the mass circulation, force the extension of the the two main theories of ENSO dynamics and our re- subtropical jet and associated South Pacific storm track. sults could be provided by Eisenman et al. (2005), who However, the fact that the SLP changes in the midlati- found that the low-frequency component of westerly tudes lead changes along the equator is a strong coun- wind bursts, which force ENSO, are modulated by terargument to this proposition. Nevertheless, positive ENSO through the large-scale equatorial SST field in feedbacks between weaker trades, surface warming, the western Pacific. We find a notable difference in the and eastward shifts in convection (McPhaden 2004) SST field between stronger and weaker events near the would benefit from lower pressure in the subtropics and Pacific warm pool between NDJ (0) and FMA (0) (Figs. vice versa. Once tropical convection becomes ano- 4e–h). molously strong and extends into the central Pacific, it Finally, there are three implications for ENSO moni- would act as a positive feedback between the ocean and toring that follow from this study. First, stronger events atmospheric circulation that would favor the mainte- should be predictable at a longer lead time if SLP nance and continued strengthening of the event. It is changes over southeastern Australia are monitored. interesting to note that the South Pacific troughs are Second, a broad-scale index of the Southern Oscillation strengthened farther east in weaker events, which is needed to properly account for pressure changes means any feedback between the Tropics and subtrop- mapped in Fig. 4. Third, the greater spatial extent of ics would be weaker in the western Pacific. significant pressure and SST anomalies in strong warm Fourth, we confirm that the strength of winter hemi- events provides a plausible explanation to the spatial sphere planetary Rossby waves has an important role in extent of drought in the global scale (Ropelewski and ENSO development and decay (van Loon hypothesis; Halpert 1987; Allan et al. 1996; Lyon 2004). Anderson 2003, 2004; Vimont et al. 2003; Anderson and In summary, equatorial Pacific warm events are Maloney 2006). In particular, an enhanced South Pa- clearly not only a result of reciprocal action between air cific trough causes wind stress forcing that adds to the and sea, but also reciprocal interactions between equa- westerly momentum change occurring farther north torial and midlatitudinal weather systems. We find that along the equator, both in the western and the eastern midlatitudinal and equatorial pressure anomalies com- Pacific. Such changes in momentum should be a major bine to determine the strength and persistence of west- contributor to ENSO being phase locked to the annual erly wind anomalies in the western/central Pacific and cycle, and to the seasonal dependence in MJO strength- the magnitude of Pacific warming. The reversal of ening in MJJ (0) in El Niño years (Teng and Wang trough strength in the southern midlatitudes before 2003). ENSO extremes, combined with a buildup and weak- Harrison and Larkin (1998) found that the wind ening of trades (heat), offers a consistent dynamical anomaly patterns in their composites mainly occurred framework for the connection between the Southern within 10° of the equator and have “little in common Oscillation and the magnitude of eastern Pacific warm- with the structure of the northeast or southeast trades.” ing. This study, along with Wang and Weisberg (2000) and Anderson (2003), contradicts this view. By grouping Acknowledgments. We thank George Kiladis weak, strong, and late-developing events together, Har- (NOAA/Earth Systems Research Laboratory), Neville rison and Larkin (1996, 1998) could have obscured the Nicholls (Australian BMRC), Ian Foster (DAFWA), role of the midlatitudes in their analysis. We also con- and two anonymous referees for many useful comments sider that the role of the South Pacific trough would on this paper. Ian Smith (CSIRO) kindly provided have been diminished in the study of Anderson (2003) GISST3 South Pacific SST data and Jim Renwick because a detrending process would have removed the (NIWA) NCEP–NCAR reanalysis data. Patrick Var- trend to lower pressure in the South Pacific, which co- ney (Meteo-France), Jim Salinger (NIWA), and Denis incided with more eastern Pacific warming (Fig. 8). Shea (NCAR) all provided SLP data for stations in the Our composites are unsuitable for testing the con- Pacific Ocean. The Australian Grains Research and tasting views of ENSO dynamics although they do sup- Development Corporation (GRDC) provided funding port Chen et al. (2004), who found that stronger El for the project. Niño events appear as low-frequency, self-sustaining oscillations that set up in the large-scale state, espe- REFERENCES cially SST. However, we would concur with Guilyardi Allan, R., J. Lindesay, and D. Parker, 1996: El Niño—Southern et al. (2004) that the atmosphere plays the dominant Oscillation and Climatic Variability. CSIRO Publishing, 416 role in setting El Niño amplitude by allowing air–sea pp. interactions on the equator to occur. The link between Anderson, B. T., 2003: Tropical Pacific sea-surface temperatures

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CORRIGENDUM

DAVID J. STEPHENS Department of Agriculture and Food WA, South Perth, Western Australia, Australia

HARRY VAN LOON Colorado Research Associates/Northwest Research Associates, Boulder, Colorado

MALCOLM H. LAMOND Lamond Weather Services, Karrinyup, Western Australia, Australia

NICOLA P. TELCIK Yarra Valley Water, Mitcham, Victoria, Australia

Due to an error, an incorrect version of Fig. 8 in Stephens et al. (2007) was published in which the x-axis labels were not accurately placed. The correct version of Fig. 8 is presented below.

REFERENCES

Stephens, D. J., M. J. Meuleners, H. van Loon, M. H. Lamond, and N. P. Telcik, 2007: Differences in atmospheric circulation between the development of weak and strong warm events in the Southern Oscillation. J. Climate, 20, 2191–2209.

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JCLI2502 15 MAY 2008 CORRIGENDUM 2333

FIG. 8. (a) Plot of normalized Rapa Island SLP anomalies since 1951 (black line, left y axis) vs actual Niño-3 SST anomalies (red and blue shading, right y axis). (b) Plot of normalized Niño-3 SST anomalies (red curve) vs normalized Rapa Island SLP anomalies (blue curve) and normalized 850-hPa west Pacific trade winds (purple curve), 1979–2004. All curves have been smoothed using a 5-month running mean.

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Fig 8 live 4/C 2332 JOURNAL OF CLIMATE VOLUME 21

CORRIGENDUM

DAVID J. STEPHENS Department of Agriculture and Food WA, South Perth, Western Australia, Australia

HARRY VAN LOON Colorado Research Associates/Northwest Research Associates, Boulder, Colorado

MALCOLM H. LAMOND Lamond Weather Services, Karrinyup, Western Australia, Australia

NICOLA P. TELCIK Yarra Valley Water, Mitcham, Victoria, Australia

Due to an error, an incorrect version of Fig. 8 in Stephens et al. (2007) was published in which the x-axis labels were not accurately placed. The correct version of Fig. 8 is presented below.

REFERENCES

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JCLI2502 15 MAY 2008 CORRIGENDUM 2333

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Fig 8 live 4/C