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A Proposed Mechanism for the Asymmetric Duration of El Niño and La Niña
YUKO M. OKUMURA
Climate and Global Dynamics Division, National Center for Atmospheric Research,* Boulder, Colorado

MASAMICHI OHBA
Central Research Institute of Electric Power Industry, Abiko, Japan

CLARA DESER
Climate and Global Dynamics Division, National Center for Atmospheric Research,* Boulder, Colorado

HIROAKI UEDA
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan(Manuscript received 13 August 2010, in final form 13 May 2011)
ABSTRACTEl Niño and La Niña exhibit significant asymmetry not only in their spatial structure but also in theirduration. Most El Niños terminate rapidly after maturing near the end of the calendar year, whereas many La
Niñas persist into the following year and often reintensify in boreal winter. Through atmospheric generalcirculation model experiments, it is shown that the nonlinear response of atmospheric deep convection to thepolarity of equatorial Pacific sea surface temperature anomalies causes an asymmetric evolution of surface
wind anomalies over the far western Pacific around the mature phase of El Niño and La Niña. Because of theeastward displacement of precipitation anomalies in the equatorial Pacific during El Niño compared to La
Niña, surface winds in the western Pacific are more affected by SST forcing outside the equatorial Pacific,
which acts to terminate the Pacific event.

1. Introduction
Dynamic and thermodynamic feedbacks between theatmosphere and ocean give rise to the El Niño–Southern
Oscillation (ENSO) phenomenon in the equatorial Pacific, the dominant mode of climate variability on interannual time scales. While the tremendous advance inour understanding of ENSO builds upon a linear view ofthe ocean–atmosphere system, studies over the past few
decades reveal that warm (El Niño) and cold (La Niña)
ENSO events are not a simple mirror image. For example,* The National Center for Atmospheric Research is sponsoredby the National Science Foundation.

Corresponding author address: Dr. Yuko M. Okumura, Climate
and Global Dynamics Division, NCAR, P.O. Box 3000, Boulder,
CO 80307.
E-mail: yukoo@ucar.edu
DOI: 10.1175/2011JCLI3999.1
Ó 2011 American Meteorological Society

sea surface temperature (SST) anomalies over the eastern equatorial Pacific cold tongue are larger in magnitude during El Niño compared to La Niña, resulting
in positive skewness of interannual SST variations (e.g.,
Burgers and Stephenson 1999). In addition, equatorial
Pacific precipitation anomalies are displaced eastward
during El Niño compared to La Niña. Nonlinear processes in the atmosphere are fundamental for this asymmetry: large positive SST anomalies can induce deep
convection over the cold tongue, while negative anomalies have no further effect on the normally dry conditions (Hoerling et al. 1997). The accompanying zonal
shift in surface wind anomalies, in turn, is suggested tocause the stronger SST anomalies in the eastern Pacificduring El Niño compared to La Niña (Kang and Kug2002). Jin et al. (2003) propose that nonlinear thermal
advection by ocean currents contributes to the generationof larger-amplitude El Niños compared to La Niñas.
McPhaden and Zhang (2009) also note the asymmetry in1 AUGUST 2011

OKUMURA ET AL.

the zonal propagation characteristics of El Niño and La
Niña.
In addition to the spatial structure, recent studies also
point out the asymmetry in the duration of El Niño and
La Niña (Kessler 2002; Larkin and Harrison 2002;
McPhaden and Zhang 2009; Ohba and Ueda 2009; Wu
et al. 2010; Okumura and Deser 2010; Ohba et al. 2010).
Figure 1 shows the composite evolution of SST, surface wind, and precipitation anomalies along the equator for El Niño and La Niña based on observations
during 1982–2008. In the composite El Niño, positive
SST anomalies decay rapidly after the mature phase in
December and are replaced by negative values in June–
July of the following year. Conversely, in the composite
La Niña, negative SST anomalies persist throughout
the following year and reintensify in boreal winter. The
prolongation of La Niña conditions into the second year
is associated with persistent easterly wind anomalies
over the western Pacific. In contrast, the anomalous
westerly winds during El Niño begin to weaken over the
far western Pacific 1–2 months before the mature phase.
Compared to El Niño, the larger-amplitude zonal wind
anomalies over the western Pacific during La Niña areassociated with a westward displacement of the anomalous precipitation. Okumura and Deser (2010) showthat this asymmetry in the duration of El Niño and La
Niña is a robust feature of ENSO throughout the past
century.
Ohba and Ueda (2009) show that an atmosphericgeneral circulation model (AGCM) reproduces theasymmetric evolution of surface wind anomalies aroundthe mature phase when forced with Indo-Pacific SST
anomalies that are symmetrical between El Niño and La
Niña. Their result suggests the importance of nonlinear
atmospheric processes for the asymmetric evolution of
El Niño and La Niña. It remains unclear what is therelative importance of SST forcing in the different partsof the Indo-Pacific region for the asymmetric windevolution.
Based on observational analysis, Okumura and Deser(2010) synthesized previous findings and proposed amore detailed mechanism for the asymmetric wind response, as summarized in Fig. 2. During El Niño, delayed basin warming of the Indian Ocean increases
tropospheric temperatures and forces a baroclinic atmospheric Kelvin wave into the western Pacific, inducing anomalous surface easterly winds in the westernequatorial Pacific. The associated Ekman divergencesuppresses atmospheric deep convection in the northwest tropical Pacific, forcing an anomalous anticyclonic circulation and enhancing the anomalous easterly
winds north of the equator (Watanabe and Jin 2002;
Annamalai et al. 2005; Xie et al. 2009). The negative SST

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FIG. 1. Longitude–time sections of SST (8C, color shading),
surface wind (m s21, vectors), and precipitation [mm day21; positive (negative) contours in green (orange) at 61, 3, 5, . . .] anomalies along the equator (38S–38N) for (top) El Niño and (bottom)
La Niña based on the Optimum Interpolation Sea Surface Temperature (OISST) (Reynolds et al. 2002), National Centers forEnvironmental Prediction–Department of Energy (NCEP–DOE)
Reanalysis II (Kanamitsu et al. 2002), and Climate Prediction
Center Merged Analysis of Precipitation (CMAP) (Xie and Arkin
1996) data for 1982–2008. The anomalies are composited for strong
El Niño (1982, 1986, 1987, 1991, 1994, 1997, 2002, and 2006) and La
Niña (1984, 1988, 1998, 1999, and 2007) years defined as in
Okumura and Deser (2010). The time axis runs from January of the
El Niño/La Niña year (Jan0) to May of the year after the following
year (May12).

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anomalies and local air–sea interactions within thenorthwest tropical Pacific may also contribute to theformation and maintenance of the anomalous anticyclonic circulation (Wang et al. 2000; Lau and Nath 2003).
In the far western equatorial Pacific, the anomalous
surface easterly wind forced by the Indian Ocean warming offsets or even reverses the anomalous westerly windforced by the positive precipitation anomalies in theequatorial Pacific and rapidly terminates the event after
the mature phase (e.g., Kug and Kang 2006; Ohba andUeda 2007). During La Niña, however, the anomalous
easterly wind forced by the negative precipitation anomalies in the equatorial Pacific is much stronger than thesurface westerly wind induced by the Indian Ocean
cooling because of the westward shift of the Pacific precipitation anomalies. The net easterly surface wind
anomaly over the western Pacific is hypothesized toprolong the La Niña condition. Slow oceanic adjustment
processes are likely to build up the equatorial heat content and eventually terminate the La Niña event (e.g., Jin
1997; Meinen and McPhaden 2000).
The purpose of this study is to test the hypothesis
formulated in Okumura and Deser (2010) by expanding
upon the AGCM experiments presented in Ohba andUeda (2009). We conduct additional experiments toaddress the following questions: 1) What are the relative
contributions of SST anomalies inside and outside theequatorial Pacific to surface wind anomalies over thewestern Pacific associated with El Niño and La Niña?
and 2) Does the zonal displacement of Pacific precipitation anomalies change the balance of wind forcing
over the western Pacific between El Niño and La Niña?
The paper is organized as follows. The AGCM experiments are described in section 2. Section 3 describes theresults, and section 4 provides a summary.

VOLUME 24

FIG. 2. Schematic diagram illustrating the hypothesized cause of
the asymmetric wind response in the far western equatorial Pacificbetween (top) El Niño and (bottom) La Niña during the maturephase. Pink (blue)-shaded ellipses depict positive (negative) SST
anomalies over the Pacific and Indian Oceans. Red (blue) arrows
indicate surface wind anomalies forced by equatorial Pacific SST
anomalies during El Niño (La Niña); black arrows indicate surface
wind anomalies forced by Indian Ocean and northwest tropical
Pacific SST anomalies. During El Niño (La Niña), the atmospheric
Kelvin wave forced by tropospheric warming (cooling) over the
Indian Ocean induces an easterly (westerly) surface wind anomaly
in the far western equatorial Pacific. In the MRI AGCM, thenorthwest tropical Pacific cooling (warming) and associated precipitation decrease (increase) also force an easterly (westerly)
surface wind anomaly to their south. These easterly (westerly)
surface wind anomalies forced by the Indian Ocean and northwest
tropical Pacific SST anomalies balance (fail to balance) the westerly (easterly) surface wind anomaly forced by equatorial Pacificprecipitation anomalies over the far western equatorial Pacificduring El Niño (La Niña). The lack of balance of the surface wind
anomalies over the far western Pacific during La Niña results fromthe westward shift of the Pacific precipitation anomaly because of
the nonlinear dependence of atmospheric deep convection on theabsolute SST. The net easterly surface wind anomaly over the far
western Pacific during La Niña is hypothesized to prolong theevent. Modified after Okumura and Deser (2010).

2. Model experiments
The numerical experiments are conducted with the
AGCM developed at the Meteorological Research Institute (MRI) in Japan (Shibata et al. 1999; Yukimoto
et al. 2001, 2006). We use a version with T42 horizontal
resolution (;2.88 latitude 3 2.88 longitude) and 30 hybrid sigma-pressure levels in the vertical. In the control
simulation, the model is forced with monthly climatological SSTs based on Levitus and Boyer (1994) andintegrated for 30 yr. As in Ohba and Ueda (2009), twosets of 15-member ensemble simulations are conductedby adding monthly SST anomalies for El Niño and
La Niña over the Indian Ocean and Pacific (408S–408N,
308E–608W) from January of the developing year throughNovember of the following year (‘‘IO 1 PAC’’ runs).
The SST forcing for the El Niño and La Niñaexperiments is identical except for the sign of theanomalies and is derived from a composite El Niñominus La Niña using the extended reconstructed SST
version 2 (Smith and Reynolds 2004). The five strongest
El Niño (1965, 1972, 1982, 1987, and 1997) and La Niña(1955, 1973, 1988, 1998, and 1999) years based on the
December–February (DJF) SST anomalies averaged in
the Niño-3.4 region (58S–58N, 1708–1208W) are used forthe composite (see the second row of Fig. 3 for the SST
anomaly pattern in DJF). The 15 different initial conditions for the ensemble runs are taken from 1 January
states of the control simulation.
In addition to the IO 1 PAC runs, two more sets of
15-member ensemble simulations are conducted bylimiting the SST forcing to the equatorial Pacific. In

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OKUMURA ET AL.

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FIG. 3. DJF anomalies of SST (8C, color shading), surface wind (m s21, vectors), and precipitation [mm day21; positive (negative)
contours in green (orange) at 6 1, 4, 7, . . .] for (left) El Niño and (right) La Niña from (top row) observations and (bottom three rows) MRI
AGCM experiments (IO 1 PAC, EPAC, and their difference). The observational composite maps are based on the same methods anddatasets as in Fig. 1.

these ‘‘EPAC’’ runs, we prescribe only the positive andnegative SST anomalies east of 1608E for El Niño and
La Niña, respectively. Note that these SST anomalies
extend into the subtropical Pacific off the west coastof the Americas (Fig. 3, third row). By comparing the
IO 1 PAC and EPAC runs, we can assess how SST
anomalies outside the equatorial Pacific modify thesurface wind response to the equatorial Pacific SSTs.
Additional experiments suggest that the impacts of
SST anomalies inside and outside the equatorial Pacificare nearly linear (not shown), in agreement withAnnamalai et al. (2005). The pattern, amplitude, andseasonal evolution of SST forcing are perfectly symmetrical between El Niño and La Niña in both the IO 1
PAC and EPAC runs, and therefore any asymmetry in
the AGCM’s response can be attributed to nonlinear
atmospheric processes. In the next section, we will
present ensemble-mean anomalies in the IO 1 PAC
and EPAC runs relative to the climatology of thecontrol simulation.

3. Results
In this section we focus on the atmospheric response
in boreal winter (DJF) when the asymmetry between
El Niño and La Niña becomes evident in surface wind
anomalies over the far western Pacific (Fig. 1). Figure 3
shows SST, surface wind, and precipitation anomalies in
the AGCM experiments, as well as composite maps
based on observations. In the IO 1 PAC runs, the MRI
AGCM successfully simulates the observed patterns ofprecipitation and surface wind anomalies for El Niñoand La Niña, although the model’s response is toostrong in the Pacific, particularly over the warm pool
region, and too weak in the eastern Indian Ocean. Despite the symmetric SST forcing in the model experiments, the center of the Pacific precipitation anomalies
is displaced westward for La Niña compared to El Niño,consistent with the observations and earlier model studies(Hoerling et al. 1997; Kang and Kug 2002; Ohba andUeda 2009). The longitudinal shift of the precipitation

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FIG. 4. DJF anomalies of surface zonal wind (m s21, color shading) and SLP (hPa, contours at intervals of 0.4) for (left) El Niño and (right)
La Niña in the MRI AGCM experiments: (top) IO 1 PAC and (middle) EPAC runs and (bottom) their difference.

anomalies is more pronounced in the model (208–308)
than in the observations (108–158) because of the moreeastward location of the positive precipitation anomalies
during El Niño in the model compared to the observations. In the northwest tropical Pacific, opposite-signed
precipitation anomalies and associated atmospheric circulation anomalies also show an asymmetry in their zonal
extent between the El Niño and La Niña experiments,
consistent with a recent study by Wu et al. (2010). In association with the zonal shift of the precipitation anomaly
patterns, anomalous easterly winds extend into the far
western Pacific in the La Niña experiment, while anomalous westerly winds are limited to the east of 1608E (withanomalous easterly winds over the far western Pacific) in
the El Niño experiment.
The zonal displacement of precipitation and surface
wind anomalies in the Pacific is reproduced in the EPAC
runs, confirming that the model atmosphere responds
nonlinearly to equatorial Pacific SSTs (Fig. 3, thirdrow). The opposite-signed precipitation anomalies appear over the northwest tropical Pacific in the absenceof local SST anomalies due to adjustments in the Walker
circulation. Compared to the IO 1 PAC runs, precipitation anomalies are reduced not only in the northwest tropical Pacific but also slightly in the equatorial
Pacific.
The impact of Indian Ocean and off-equatorial Pacific
SST anomalies is inferred from the difference between

the IO 1 PAC and EPAC runs (Fig. 3, bottom row).
During El Niño, the Indian Ocean warming and northwest/southwest tropical Pacific cooling induce anomalous easterly winds in the western Pacific, weakening
the anomalous westerly winds forced by the equatorial
Pacific warming. These easterly wind anomalies areenhanced north of the equator by the anticyclonic circulation anomaly that develops over the negative SST
anomalies east of the Philippine Islands. During La
Niña, the surface wind response to SST anomalies outside the equatorial Pacific is similar to those associatedwith El Niño with the sign reversed, although theanomalous westerly winds over the far western Pacificare stronger and extend less to the east compared to theanomalous easterly winds during El Niño. Thus, SST
anomalies outside the equatorial Pacific produce a negative feedback during both El Niño and La Niña.
Figure 4 shows surface zonal wind and sea levelpressure (SLP) anomalies in the IO 1 PAC and EPAC
runs and their difference for El Niño and La Niña. In
both the IO 1 PAC and EPAC runs, SLP anomalies
exhibit a zonal seesaw pattern over the tropical Pacific.
The SLP anomalies have a local extremum on (off) theequator in the eastern (western) Pacific, indicative of
equatorial Kelvin (Rossby) wave forced by changes inprecipitation and associated diabatic heating. Withina few degrees of the equator—the equatorial radius ofdeformation for the ocean—anomalous zonal winds are1 AUGUST 2011

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FIG. 5. DJF surface zonal wind anomalies along the equator(38S–38N) for (top) El Niño and (bottom) La Niña in the MRI
AGCM experiments: IO 1 PAC (thick gray), EPAC (red in the
El Niño panel; blue in the La Niña panel), and their difference(black). The gray shading indicates the far western equatorial Pacific (1408–1658E), where the wind response exhibits strong asymmetry between the El Niño and La Niña experiments.

directed down the anomalous zonal pressure gradient.
The largest equatorial zonal wind change is found near
the node of the Pacific SLP seesaw, with the center
displaced westward by 208–308 during La Niña compared to El Niño.
In the IO 1 PAC El Niño experiment, the sign of theequatorial zonal wind and SLP gradient anomalies isreversed west of 1608E and anomalous easterly winds of
up to 3 m s21 extend into the far western Pacific (Fig. 4).
In the EPAC El Niño experiment, a similar feature ispresent to the west of 1308E, but the SLP gradient andzonal wind anomalies are nearly absent between 1408
and 1608E. The difference map shows that the enhanced
eastward SLP gradient over the far western Pacific in the
IO 1 PAC experiment compared to the EPAC experiment is due to anomalous low pressure extending eastward along the equator from the Indian Ocean andanomalous high pressure extending westward from thenorthwest/southwest tropical Pacific. Thus, it is suggested that both the atmospheric Kelvin wave forced by
the Indian Ocean warming and Rossby wave forced by
the off-equatorial Pacific cooling induce anomalous

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easterly winds over the far western Pacific in the IO 1
PAC El Niño experiment. Because of the presence of
New Guinea Island, equatorial winds in the far western
Pacific appear to be more strongly affected by SST
anomalies over the northwest than southwest tropical
Pacific. The SLP difference map for the La Niña experiments shows a very similar pattern with reversed
sign, suggesting that the same mechanism is at work. The
impact of SST anomalies outside the equatorial Pacific,
however, is not strong enough to affect the net surface
wind anomaly in the IO 1 PAC La Niña experiment.
Additional experiments forced with SST anomalies
over the full Pacific suggest that the relative contributionof Indian Ocean and off-equatorial Pacific SST anomalies to surface wind anomalies over the far western Pacific is seasonally dependent (not shown). The Indian
Ocean SSTs begin to play a dominant role a few months
after the mature phase of El Niño and La Niña when the
Indian Ocean SST anomalies reach their delayed peak
and the off-equatorial Pacific SST anomalies start todecay. Before and during the mature phase, the Indian
Ocean and off-equatorial Pacific SSTs make comparable
contributions. Note that the excessive (insufficient)precipitation and wind anomalies over the Pacific warm
pool region (the Indian Ocean) in the MRI AGCM (Fig.
3) may suggest that this model overestimates (underestimates) the impact of off-equatorial Pacific (Indian Ocean) SST anomalies. In the IO 1 PAC El Niñoexperiment, anomalous westerly winds start to decay
over the far western Pacific in boreal summer whennegative SST anomalies develop in the western Pacific(not shown), nearly two seasons earlier than in the observations (Fig. 1). Annamalai et al. (2010) suggest that
SST anomalies in the Indonesian seas also affect theseasonal evolution of El Niño. Further model intercomparison studies are necessary to address the relative importance of these regional SST anomalies.
Figure 5 summarizes the contributions to surface
zonal wind anomalies along the equator in the El Niñoand La Niña experiments. The wind anomalies forced by
the equatorial Pacific (Indian Ocean and off-equatorial
Pacific) SST anomalies in the EPAC (IO 1 PAC minus
EPAC) runs are shown in colored (black) lines following
the color scheme in the schematic plot of Fig. 2. The lightgray shading indicates the far western Pacific (1408–
1658E), where the total wind anomalies in the IO 1 PAC
run (thick gray curves) exhibit strong asymmetry between El Niño and La Niña. In the far western Pacific,
the zonal winds forced by the Indian Ocean and offequatorial Pacific SST anomalies are of opposite signand similar magnitude for El Niño and La Niña. Becauseof the nonlinear atmospheric response, the zonal winds
forced by the equatorial Pacific SST anomalies are3828

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smaller (larger) in magnitude than the zonal winds
forced by the SST anomalies outside the equatorial
Pacific in the El Niño (La Niña) experiment. As a result,the net surface winds in the far western Pacific areeasterly with magnitudes up to 3 m s21 in both the
El Niño and La Niña experiments. When the MRI
AGCM is coupled to an ocean model, these anomalous
easterly winds force an upwelling oceanic Kelvin wave
and rapidly terminate the El Niño event (Ohba andUeda 2007). A similar experiment needs to be conducted for La Niña to examine the influence of persistent easterly wind anomalies on the ocean. Our AGCM
experiments suggest that the impact of SST anomalies
outside (within) the equatorial Pacific dominates thesurface zonal wind response over the far western Pacificin the El Niño (La Niña) experiment. The zonal displacement of precipitation and surface wind anomalies
in the Pacific causes this difference in the balance of
surface wind forcing between the IO 1 PAC El Niñoand La Niña experiments.

VOLUME 24

which in turn induce climate anomalies not only in the
Indo-Pacific region but also at mid- and high latitudes.
For example, the long-lasting La Niña of 1998–2001
caused extensive drought across the United States,
southern Europe, and southwest Asia (e.g., Hoerlingand Kumar 2003). In the central North Pacific, ocean
temperature anomalies associated with atmosphericteleconnections show distinct asymmetry in duration
between El Niño and La Niña (Deser et al. 2011, manuscript submitted to J. Climate). The asymmetric termination process of El Niño and La Niña may also affectthe mechanisms of onset of ensuing events. Finally, thestrong asymmetry of El Niño and La Niña calls forcaution in the application of linear analysis methods forthe study of ENSO.
Acknowledgments. We acknowledge Drs. A. Kitoh,
T. Ose, S. Yukimoto, and O. Arakawa for providing themodel code. We thank the anonymous reviewers fortheir useful suggestions.

4. Summary
REFERENCES

Recent studies show that La Niña tends to persist
longer than El Niño. The MRI AGCM experiments
support the hypothesis proposed by Okumura andDeser (2010) that the nonlinear atmospheric response toequatorial Pacific SSTs is a fundamental cause of theasymmetry in the duration of El Niño and La Niña.
During the mature phase of El Niño and La Niña, SST
anomalies outside the equatorial Pacific induce surface
wind anomalies over the far western Pacific that act toweaken the equatorial Pacific SST anomalies. In addition to the Indian Ocean that was discussed in Okumura
and Deser (2010), SST anomalies in the northwest
tropical Pacific also play an important role for the windresponse over the far western Pacific in the MRI AGCM
(Fig. 2). These wind anomalies forced by SST anomalies
outside the equatorial Pacific have a similar amplitude
between El Niño and La Niña but affect the sign of net
surface wind anomalies only during El Niño. During
La Niña, the influence of equatorial Pacific SST anomalies dominates the surface wind response over the far
western Pacific because of the westward displacement of
the precipitation anomalies. Further modeling studies
are necessary to examine the oceanic response to theasymmetric wind anomalies and the role of ocean–
atmosphere interactions in the subsequent seasonalevolution of warm and cold events.
The results presented in this study may have important implications for climate prediction. The differing
duration of El Niño and La Niña is likely to affect thepersistence of their global atmospheric teleconnections,

Annamalai, H., S.-P. Xie, J. P. McCreary, and R. Murtugudde,2005: Impact of Indian Ocean sea surface temperature on
developing El Niño. J. Climate, 18, 302–319.
——, S. Kida, and J. Hafner, 2010: Potential impact of the tropicalIndian Ocean–Indonesian seas on El Niño characteristics.
J. Climate, 23, 3933–3952.
Burgers, G., and D. B. Stephenson, 1999: The ‘‘normality’’ of
El Niño. Geophys. Res. Lett., 26, 1027–1030.
Hoerling, M., and A. Kumar, 2003: The perfect ocean for drought.
Science, 299, 691–694.
——, ——, and M. Zhong, 1997: El Niño, La Niña, and the nonlinearity of their teleconnections. J. Climate, 10, 1769–1786.
Jin, F.-F., 1997: An equatorial ocean recharge paradigm for ENSO.
Part I: Conceptual model. J. Atmos. Sci., 54, 811–829.
——, S.-I. An, A. Timmermann, and J. Zhao, 2003: Strong El Niñoevents and nonlinear dynamical heating. Geophys. Res. Lett.,
30, 1120, doi:10.1029/2002GL016356.
Kanamitsu, M., W. Ebisuzaki, J. Woollen, S.-K. Yang, J. J. Hnilo,
M. Fiorino, and G. L. Potter, 2002: NCEP–DOE AMIP-II
Reanalysis (R-2). Bull. Amer. Meteor. Soc., 83, 1631–1643.
Kang, I.-S., and J.-S. Kug, 2002: El Niño and La Niña sea surface
temperature anomalies: Asymmetry characteristics associatedwith their wind stress anomalies. J. Geophys. Res., 107, 4372,
doi:10.1029/2001JD000393.
Kessler, W. S., 2002: Is ENSO a cycle or a series of events? Geophys. Res. Lett., 29, 2125, doi:10.1029/2002GL015924.
Kug, J.-S., and I.-S. Kang, 2006: Interactive feedback between
ENSO and the Indian Ocean. J. Climate, 19, 1784–1801.
Larkin, N. K., and D. E. Harrison, 2002: ENSO warm (El Niño)and cold (La Niña) event life cycles: Ocean surface anomaly
patterns, their symmetries, asymmetries, and implications.
J. Climate, 15, 1118–1140.
Lau, N.-C., and M. J. Nath, 2003: Atmosphere–ocean variations in
the Indo-Pacific sector during ENSO episodes. J. Climate, 16,
3–20.

1 AUGUST 2011

OKUMURA ET AL.

Levitus, S., and T. P. Boyer, 1994: Temperature. Vol. 4, World
Ocean Atlas 1994, NOAA Atlas NESDIS 4, 117 pp.
McPhaden, M. J., and X. Zhang, 2009: Asymmetry in zonal phasepropagation of ENSO sea surface temperature anomalies.
Geophys. Res. Lett., 36, L13703, doi:10.1029/2009GL038774.
Meinen, C. S., and M. J. McPhaden, 2000: Observations of warm
water volume changes in the equatorial Pacific and their relationship to El Niño and La Niña. J. Climate, 13, 3551–3559.
Ohba, M., and H. Ueda, 2007: An impact of SST anomalies in the
Indian Ocean in acceleration of the El Niño to La Niña transition. J. Meteor. Soc. Japan, 85, 335–348.
——, and ——, 2009: Role of nonlinear atmospheric response to
SST on the asymmetric transition process of ENSO. J. Climate,22, 177–192.
——, D. Nohara, and H. Ueda, 2010: Simulation of asymmetric
ENSO transition in WCRP CMIP3 multimodel experiments.
J. Climate, 23, 6051–6067.
Okumura, Y. M., and C. Deser, 2010: Asymmetry in the duration of
El Niño and La Niña. J. Climate, 23, 5826–5843.
Reynolds, R. W., N. A. Rayner, T. M. Smith, D. C. Stokes, andW. Q. Wang, 2002: An improved in situ and satellite SST
analysis for climate. J. Climate, 15, 1609–1625.
Shibata, K., H. Yoshimura, M. Ohizumi, M. Hosaka, and
M. Sugi, 1999: A simulation of troposphere, stratosphereand mesosphere with an MRI/JMA98 GCM. Pap. Meteor.
Geophys., 50, 15–53.

3829

Smith, T. M., and R. W. Reynolds, 2004: Improved extended reconstruction of SST (1854–1997). J. Climate, 17, 2466–2477.
Wang, B., R. Wu, and X. Fu, 2000: Pacific–East Asia teleconnection: How does ENSO affect East Asian climate?
J. Climate, 13, 1517–1536.
Watanabe, M., and F.-F. Jin, 2002: Role of Indian Ocean
warming in the development of Philippine Sea anticyclone
during ENSO. Geophys. Res. Lett., 29, 1478, doi:10.1029/
2001GL014318.
Wu, B., T. Li, and T. Zhou, 2010: Asymmetry of atmospheric circulation anomalies over the western North Pacific between
El Niño and La Niña. J. Climate, 23, 4807–4822.
Xie, P., and P. A. Arkin, 1996: Analyses of global monthly precipitation using gauge observations, satellite estimates, andnumerical model predictions. J. Climate, 9, 840–858.
Xie, S.-P., K. Hu, J. Hafner, H. Tokinaga, Y. Du, G. Huang, and
T. Sampe, 2009: Indian Ocean capacitor effect on Indo–western Pacific climate during the summer following El Niño.
J. Climate, 22, 730–747.
Yukimoto, S., and Coauthors, 2001: The new Meteorological Research Institute Coupled GCM (MRI-CGCM2)—Model climate and variability. Pap. Meteor. Geophys., 51, 47–88.
——, and Coauthors, 2006: Present-day climate and climate sensitivity in the Meteorological Research Institute Coupled
GCM version 2.3 (MRI-CGCM2.3). J. Meteor. Soc. Japan, 84,
333–363.