Near-Annual SST Variability in the Equatorial Pacific in a Coupled
General Circulation Model
RENGUANG WU
BEN P. KIRTMAN
School for Computational Sciences, George Mason University,
Fairfax, Virginia, and Center for Ocean–Land–Atmosphere Studies,
Calverton, Maryland
(Manuscript received 20 December 2004, in final form 4 May
2005)
ABSTRACT
Equatorial Pacific sea surface temperature (SST) anomalies in the
Center for Ocean–Land–Atmosphere Studies (COLA) interactive
ensemble coupled general circulation model show near-annual
variability as well as biennial El Niño–Southern Oscillation (ENSO)
variability. There are two types of near-annual modes: a westward
propagating mode and a stationary mode. For the westward
propagating near-annual mode, warm SST anomalies are generated in
the eastern equatorial Pacific in boreal spring and propagate
westward in boreal summer. Consistent westward propagation is seen
in precipitation, surface wind, and ocean current. For the
stationary near-annual mode, warm SST anomalies develop near the
date line in boreal winter and decay locally in boreal spring.
Westward propagation of warm SST anomalies also appears in the
developing year of the biennial ENSO mode. However, warm SST
anomalies for the westward propagating near-annual mode occur about
two months earlier than those for the biennial ENSO mode and are
quickly replaced by cold SST anomalies, whereas warm SST anomalies
for the biennial ENSO mode only experience moderate
weakening.
Anomalous zonal advection contributes to the generation and
westward propagation of warm SST anomalies for both the westward
propagating near-annual mode and the biennial ENSO mode. However,
the role of mean upwelling is markedly different. The mean
upwelling term contributes to the generation of warm SST anomalies
for the biennial ENSO mode, but is mainly a damping term for the
westward propagating near-annual mode. The development of warm SST
anomalies for the stationary near-annual mode is partially due to
anomalous zonal advection and upwelling, similar to the
amplification of warm SST anomalies in the equatorial central
Pacific for the biennial ENSO mode. The mean upwelling term is
negative in the eastern equatorial Pacific for the stationary
near-annual mode, which is opposite to the ENSO mode.
The development of cold SST anomalies in the aftermath of warm SST
anomalies for the westward propagating near-annual mode is coupled
to large easterly wind anomalies, which occur between the warm and
cold SST anomalies. The easterly anomalies contribute to the cold
SST anomalies through anomalous zonal, meridional, and vertical
advection and surface evaporation. The cold SST anomalies, in turn,
enhance the easterly anomalies through a Rossby-wave-type response.
The above processes are most effective during boreal spring when
the mean near-surface-layer ocean temperature gradient is the
largest. It is suggested that the westward propagating near-annual
mode is related to air–sea interaction processes that are limited
to the near-surface layers.
1. Introduction
Tropical Pacific sea surface temperature (SST) anomalies display a
variety of time scales. In addition to the dominant El
Niño–Southern Oscillation (ENSO), there is variability on
quasi-biennial (Ropelewski et al.
Corresponding author address: Dr. Renguang Wu, Center for
Ocean–Land–Atmosphere Studies, 4041 Powder Mill Rd., Suite 302,
Calverton, MD 20705. E-mail:
[email protected]
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1992), decadal (Tourre et al. 1999), and near-annual (Jin et al.
2003) time scales. In particular, the near- annual time scale
variability contributes to some minor El Niño and La Niña events
(Jin et al. 2003; Kang et al. 2004). Documenting the evolution and
physics of the near-annual variability has implications for
understand- ing the tropical Pacific SST anomalies.
Previous empirical and modeling investigations have suggested
near-annual variability of SST in the equato- rial Pacific. Based
on analyses using the National Cen- ters for Environmental
Prediction (NCEP) ocean as- similation dataset (Ji et al. 1995),
Jin et al. (2003) pointed out that there is significant variability
on 12– 18-month time scales for SST in the region of 2°S–2°N,
170°–120°W. Some fairly regular and nearly annual variability
appears in the equatorial Pacific after the major 1997/98 El Niño
event and fluctuations with simi- lar time scales also occurred
between all the major El Niño events (Jin et al. 2003; Kang et al.
2004). SST and wind stress exhibit westward propagation in the
equa- torial Pacific at this time scale. The equatorial zonal
current at 140°W and 10 m derived from current meter moorings
displays a strong annual peak during 1985–89 (Perigaud and Dewitte
1996). Near-annual and suban- nual variability has been identified
in the Zebiak–Cane coupled model (Zebiak and Cane 1987). Jin et al.
(2003) showed evidence for the near-annual variability of SST,
zonal wind, zonal current, and thermocline depth in the equatorial
Pacific with a strong westward propagating tendency in a long-term
simulation of the Zebiak–Cane coupled model. Westward propagation
of equatorial Pacific SST and surface zonal wind stress anomalies
is also found at subannual time scales with a period of about 9
months (the so-called mobile mode: Zebiak 1985; Mantua and Battisti
1995). In a simulation using the Zebiak–Cane coupled model,
Perigaud and Dewitte (1996) found that the SST changes in the equa-
torial central Pacific are dominated by a 9-month oscil- lation,
which corresponds to the oscillation of the simu- lated anomalous
zonal currents that show obvious west- ward propagation. In
examining the temporal evolution along the equatorial Pacific in
the Center for Ocean– Land–Atmosphere Studies (COLA) interactive
en- semble coupled general circulation model (CGCM) (Kirtman and
Shukla 2002), we identified obvious west- ward propagating SST,
rainfall, and surface wind stress anomalies on near-annual time
scale. Understanding the near-annual variability in the CGCM in
comparison with that in observations and even the Zebiak–Cane model
may ultimately lead to improved climate predic- tion.
Previous studies indicate that the physical processes important to
the near-annual variability or the mobile
mode are fundamentally different from those control- ling ENSO. For
ENSO, the thermocline feedback is primary for both the growth and
transition phase, and the zonal advective feedback plays a
secondary role (Jin and An 1999; An and Jin 2001). For the near-
annual or mobile mode, anomalous zonal advection plays a dominant
role for SST anomalies (Mantua and Battisti 1995; Jin et al. 2003;
Kang et al. 2004). The anomalous zonal advection serves not only as
a growth term but also as a phase transition mechanism (Jin et al.
2003; Kang et al. 2004). The anomalous upwelling is important for
the genesis of SST anomalies in the east- ern Pacific (Mantua and
Battisti 1995; Kang et al. 2004). The mean upwelling acts as strong
damping for SST anomalies in the mobile mode (Mantua and Battisti
1995).
The near-annual variability is especially active during the cold
phase of the ENSO cycle when intensified zonal surface currents and
larger zonal SST gradients allow for more intense zonal SST
advection, although it can exist independently of the ENSO cycle
(Mantua and Battisti 1995; Jin et al. 2003). The fast (i.e., near
annual) and slow (i.e., interannual) modes also coexist in more
complex models (Neelin 1990; Philander et al. 1992). The
interaction between the mobile mode and ENSO is a cause for the
irregularity of ENSO in the Zebiak–Cane coupled model (Mantua and
Battisti 1995). Thus, understanding the occurrence of this mode may
have implications for ENSO prediction.
In analyzing the output of the COLA interactive en- semble CGCM, we
found that the model displays both ENSO and near-annual variability
in the equatorial Pa- cific. There are two types of near-annual
variability. In the first type, the warm phase appears in boreal
spring in the eastern Pacific and propagates westward to the
central and western Pacific in boreal summer and fall. In the
second type, warm SST anomalies develop near the date line in
boreal winter and decay locally. These two types appear to be
similar to the westward propa- gation and intensifying period of
the model’s canonical ENSO. Identifying the mechanisms for the
westward propagation and the local decay of SST anomalies in the
near-annual variability, which is the main focus of this study,
would be helpful for understanding the de- velopment and
propagation of ENSO anomalies in the model. The spatial structure
and temporal evolution of the near-annual westward propagating SST
and wind stress anomalies along the equatorial Pacific in the model
has similarities with observations as documented in previous
studies (e.g., Jin et al. 2003). Thus, under- standing the physical
processes associated with this study may help diagnose and predict
ENSO events in observations.
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The organization of the rest of the paper is as follows. In section
2, we describe the coupled model. The evi- dence for the three
modes (one biennial ENSO and two near-annual modes) in the coupled
model is presented in section 3. Section 4 describes the spatial
structure and temporal evolution of the three modes. A budget
analysis of the ocean mixed layer temperature is per- formed in
section 5 in order to understand the pro- cesses for the generation
and evolution of SST anoma- lies in the near-annual modes and
differences from the biennial ENSO mode. Section 6 discusses the
long-term tendency of the frequency of near-annual modes, im- pacts
of ENSO phase on the near-annual modes, im- pacts of the
near-annual variability on the irregularity of ENSO, and the
trigger for the near-annual modes. Section 7 summarizes the main
results.
2. The coupled model
This study use outputs from a long-term integration of the COLA
interactive ensemble CGCM (Kirtman and Shukla 2002). The
atmospheric component is the COLA global spectral atmospheric
general circulation model (AGCM) with a horizontal resolution of
T42 (about 2.8° 2.8°) and 18 unevenly spaced vertical levels
(Kinter et al. 1997). The ocean component is ver- sion 3 of the
Geophysical Fluid Dynamics Laboratory (GFDL) modular ocean general
circulation model (OGCM MOM3: Pacanowski and Griffies 1998), which
has 25 vertical levels. The longitudinal resolution of the ocean
model is 1.5°. The latitudinal resolution changes from 0.5° within
10°S–10°N to 1.5° in the extratropics. The thickness is 15 m for
the top 9 layers and increases to 900 m for the lowest layer.
The CGCM employs an anomaly coupling strategy (Kirtman et al. 1997,
2002) as follows. The atmosphere and ocean models exchange
anomalies of heat, mo- mentum, and freshwater fluxes, which are
computed with respect to their own model climatology. The
climatology upon which the anomalies are superim- posed is
specified by observations. The ocean model climatology is
determined from an uncoupled extended integration with observed
momentum flux and sur- face relaxation of temperature and salinity
to obser- vations after a 50-yr spinup in the same way using the
observed climatology. The atmosphere model cli- matology is
determined from a multidecadal simula- tion with specified observed
SST. The models are coupled once a day, exchanging daily mean
fluxes and SST.
The CGCM further employs an interactive ensemble coupling strategy
(Kirtman and Shukla 2002) with six realizations of the COLA AGCM
coupled to one real-
ization of the GFDL MOM3 OGCM. The six atmo- spheric realizations
only differ in terms of their initial conditions. Each atmospheric
realization experiences the same SST produced by the ocean model.
The ocean model is subjected to the ensemble average of fluxes of
heat, momentum, and freshwater from the six atmo- spheric
realizations. The interactive ensemble coupling technique reduces
the impacts of atmospheric internal dynamics on the fluxes at the
air–sea interface and thus facilitates the detection of coupled
atmosphere–ocean signals (Wu and Kirtman 2003; Yeh and Kirtman
2004). The coupled model has been integrated over a period of more
than 900 years with no flux adjustments applied other than the
anomaly coupling. In the first 400 years of the model integration,
the SST and heat content (the average temperature for the upper 300
m) in the equa- torial central Pacific has a cooling trend of about
0.1°C and 0.2°C (100 yr)1, respectively. After that, there is no
apparent trend.
The model performance has been evaluated with re- spect to SST
observations (Reynolds and Smith 1994) for the mean, annual cycle,
and ENSO behavior in pre- vious studies (Kirtman et al. 2002;
Kirtman and Shukla 2002; Wu and Kirtman 2003). The mean equatorial
Pa- cific SST is lower than observations by 0.5°–1.0°C and the cold
tongue is too strong and extends too far to the west (Kirtman et
al. 2002; Wu and Kirtman 2003). The SST annual cycle in the
equatorial Pacific agrees gen- erally well with observations
(Kirtman et al. 2002) since this is largely prescribed in the
anomaly coupling strat- egy. The model ENSO is biased to the
biennial time scale (Kirtman and Shukla 2002; Wu and Kirtman 2004).
The ENSO SST anomalies are somewhat weaker and extend too far to
the west compared to observa- tions (Kirtman et al. 2002; Wu and
Kirtman 2003). The global teleconnection associated with ENSO is
cap- tured quite realistically by the model (Kirtman and Shukla
2002).
3. The three anomalous SST modes
Equatorial Pacific SST anomalies display both inter- annual and
near-annual variability during the long-term integration of the
coupled model. To provide evidence for the near-annual SST
variability, we display in Fig. 1 the time series of SST anomalies
averaged over the region of 2°S–2°N, 170°E–170°W for three periods.
This region is chosen because the near-annual variabil- ity is the
most pronounced in this region. The three episodes are selected as
examples for the three differ- ent SST events. Similar temporal
evolutions as those in Fig. 1 are present in other periods. The
evolution in the three selected periods is relatively regular and
thus is
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convenient as a sample for the composite analysis. The results
based on these periods are representative of general results of the
model simulation.
Near-annual variability is prominent for the period of model years
2053–56 during which warm SST anoma- lies recur every year (Fig.
1a). The warm SST anomalies peak in boreal summer and are followed
by cold SST anomalies, which reach their largest amplitude around
the end of the year. Near-annual variability is also ob- vious for
the period of 2615–20 during which warm SST anomalies appear in the
beginning of each year, though the amplitude of the SST anomalies
changes from year to year (Fig. 1c). These warm SST anomalies are
usu- ally followed by a cooling in the same year. Apparent biennial
variability is seen for the period of 2115–28 during which the
model shows alternating warm and cold events every other year (Fig.
1b). Note that the data for January 2122 were lost during the
postprocess- ing of the model output. The warm SST anomalies peak
in the beginning of the year as in Fig. 1c. There is a secondary
peak in the preceding summer and fall. These secondary peaks are
similar to those in Fig. 1a.
The three types of variability are further demon-
strated using Fig. 2, which shows Hovmöller diagrams along the
equator (2°S–2°N average). During the pe- riod of 2053–56,
near-annual variability dominates (Fig. 2a). Westward propagation
of warm SST anomalies is apparent in the equatorial central Pacific
during the middle of the year, followed by cold SST anomalies that
also propagate westward in the same year. Prominent near-annual
variability is also seen during the period of 2615–20 (Fig. 2c).
Warm SST anomalies appear near the date line every year, and weak
or moderate cold SST anomalies are seen between warm SST anomalies.
During the period of 2115–28, biennial variability domi- nates
(Fig. 2b). Large warm and cold SST anomalies appear in the
equatorial central Pacific every other year. The warm SST anomalies
propagate eastward to coastal South America within a few months.
This dif- fers from the period of 2615–20 during which the warm SST
anomalies decay locally in the equatorial central Pacific. Before
the peak of warm SST anomalies, we see westward propagating warm
SST anomalies similar to those seen in the period of 2053–56. These
warm SST anomalies correspond to the secondary peak seen in Fig.
1b. In contrast, the warm anomalies are followed
FIG. 1. SST anomalies (°C) averaged over the region of 2°S–2°N,
170°E–170°W for the Jan to Jan period of (a) 2051–61, (b) 2111–31,
and (c) 2611–21. Shading highlights the periods during which the
same type of variability recurs and for which the composite is
made. The vertical dashed lines denote the time of Jul in (a) and
Jan in (b), (c).
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by cold anomalies in Fig. 2a, but only show moderate weakening in
Fig. 2b.
The evolution of SST anomalies discussed above in- dicates that the
coupled model simulates both biennial and near-annual time scale
variability. The SST anoma- lies on the biennial time scale consist
of a westward propagating phase, an amplification phase in the
equa- torial central Pacific, and an eastward propagation phase.
The two types of near-annual variability are similar to the
westward propagating phase and the local amplification stage of the
biennial variability, respec- tively. Distinct from the biennial
variability, however, warm SST anomalies associated with the
westward
propagating near-annual variability are followed imme- diately by
large cold SST anomalies that propagate westward as well, and those
in the stationary near- annual variability decay quickly and do not
propagate to the eastern equatorial Pacific.
Near-annual variability as those in Figs. 1 and 2 but with opposite
warm–cold anomalies is also identified in the model simulation. For
distinguishing, the SST anomalies as Figs. 1 and 2 are called warm
events and those opposite to Figs. 1 and 2 are called cold events.
The spatiotemporal evolution for the cold events is ba- sically
opposite to the corresponding warm events and thus the present
study only documents the warm
FIG. 2. As in Fig. 1 but for SST anomalies along the equator
(2°S–2°N average). The contour interval is 0.2°C with dashed
contours for negative values.
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events. These near-annual warm and cold events recur during the
model integration. The power spectrum for SST anomalies averaged
over the region of 2°S–2°N, 170°E–170°W (not shown here) displays a
significant peak around a 12-month period. The near-annual varia-
tion for SST anomalies in the above region, as extracted by a
bandpass filter with the half-amplitude response at 8 and 16
months, accounts for about 10% of the total variance. In view of
its recurrence with specific spa- tiotemporal structure, in the
following we term the near-annual variability in Figs. 1a and 2a as
the west- ward propagating near-annual mode, the near-annual
variability in Figs. 1c and 2c as the stationary near- annual mode,
and the biennial variability in Figs. 1b and 2b as the biennial
ENSO mode.
Near-annual equatorial Pacific SST variability also appears in the
anomaly coupled model with only a single realization of the AGCM
and, thus, is not specific to the interactive ensemble coupling
strategy. The in- teractive ensemble coupling approach is designed
to reduce the noise level and increase the percent variance
explained by the signal. This approach improves the simulation of
the large-scale SST anomaly pattern as- sociated with ENSO in the
tropical Indian Ocean and the extratropical Pacific Ocean and the
Indian summer monsoon–ENSO relationship (Kirtman and Shukla 2002).
Preliminary analyses indicate that the near- annual variability in
the anomaly coupled model has a spatiotemporal structure similar to
that in the interac- tive ensemble coupled model. This study only
present results from the interactive ensemble coupled model that
has a much longer integration, which facilitates the discussion of
the long-term change of the near-annual variability.
The results presented Figs. 1 and 2 raise several ques-
tions:
• What processes lead to the westward propagation of warm SST
anomalies during the periods of 2053–56 and 2115–28?
• What generates the cold SST anomalies in the after- math of warm
SST anomalies during the period of 2053–56?
• What limits the eastward propagation of warm SST anomalies during
the period of 2615–20?
To answer these questions, we compare the spatial structure and
temporal evolution and perform an ocean budget analysis for all
three modes in the following two sections.
4. The spatial structure and temporal evolution
As seen in Figs. 1 and 2, the SST anomaly evolution is quite
similar within each of the three periods. As
such, a composite with respect to the calendar month for each of
the three periods is calculated in order to diagnose the spatial
and temporal evolution of the three modes. For the westward
propagating near- annual mode, the anomalies in the 4-yr period
(2053– 56) are averaged to obtain the composite. For the sta-
tionary near-annual mode, the anomalies in the 6-yr period (July
2614–June 2620) are averaged. For the bi- ennial ENSO mode, the
anomalies in the period of 2115–28 except for the lost model output
for January 2122 are averaged every other year. In the following,
we first describe the structure and evolution for the biennial ENSO
mode. Then, we document the structure and evolution for the annual
modes and compare these to the biennial ENSO mode.
a. The biennial ENSO mode
The temporal evolution of SST, precipitation, surface wind stress,
heat content, and surface ocean current anomalies along the equator
(2°S–2°N average) for the biennial ENSO mode is shown in Fig. 3.
The model El Niño peaks around January in the equatorial central
Pacific (Fig. 3a). The evolution of warm SST anomalies consists of
three stages: a westward propagation of moderate SST anomalies
during summer and fall, am- plification in the equatorial central
Pacific around De- cember, and an eastward propagation in boreal
winter and the following spring. The evolution of rainfall
anomalies (Fig. 3b) is consistent with that of SST anomalies.
Westerly and easterly wind anomalies de- velop to the west and east
of the warm SST anomalies, respectively. These wind anomalies
propagate with the SST and rainfall anomalies. Positive heat
content anomalies precede warm SST anomalies during the am-
plification and eastward propagation periods, but they lag warm SST
anomalies during the westward propaga- tion period (Fig. 3c).
Eastward propagating positive heat content anomalies are also seen
in the western and central equatorial Pacific prior to the
development of westward propagating warm SST anomalies. Obvious
propagation is also seen in the ocean surface current anomalies
(Fig. 3c), consistent with that of heat content anomalies. Eastward
surface current anomalies lie over the warm SST anomalies.
The spatiotemporal evolution for the biennial ENSO mode is further
demonstrated in Fig. 4, which shows the two-dimensional structure
of SST, precipitation, sur- face wind stress, heat content, and
surface ocean cur- rent anomalies every other month. In March, cold
SST anomalies are centered on the equator. The spatial structure
and temporal evolution indicates that the heat content anomalies
feature a reflected downwelling
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Kelvin wave from the western boundary in association with eastward
surface current anomalies. In May, weak warm SST anomalies appear
in the eastern equatorial Pacific when the downwelling Kelvin wave
arrives. At this time, eastward surface current anomalies cover the
entire equatorial Pacific with the largest in the eastern
equatorial Pacific. It appears that both the thermocline change and
the anomalous zonal advection contribute to the development of warm
SST anomalies in the east- ern equatorial Pacific as will be shown
later. At the same time, low-level wind convergence starts to de-
velop above warm SST anomalies.
From May to July, the warm SST anomalies intensify and move
westward along the equator, accompanied by westward migration of
the low-level convergence and eastward ocean surface current
anomalies. The heat content changes are relatively small during
this period. From July to November, warm SST anomalies mainly
propagate westward with little change in amplitude. The surface
wind and ocean current anomalies propa- gate westward in the
equatorial Pacific and westerly wind and eastward ocean current
anomalies amplify at
the same time. Relatively large heat content anomalies near the
equator also appear to move westward.
Eastward propagation and strong amplification in SST, wind, heat
content, and ocean current anomalies appear from November to
January. This propagation is preceded by a strong intensification
of westerly wind and eastward ocean current anomalies in the
western equatorial Pacific, and a strong deepening of ther- mocline
in the equatorial central Pacific. In the western equatorial
Pacific, the heat content anomalies also change sign.
After January, warm SST anomalies decay and ex- pand eastward, as
do the surface wind anomalies. In the western equatorial Pacific,
ocean current anomalies re- verse corresponding to the change in
the structure of heat content anomalies. At this time, large warm
SST anomalies appear along coastal South America, which corresponds
to the arrival of large positive heat content anomalies and
accompanying eastward ocean current anomalies. In the following
May, SST and wind anoma- lies further weaken, and ocean current
anomalies re- verse in the eastern equatorial Pacific.
FIG. 3. (a) Composite SST (°C), (b) precipitation (mm day1) and
surface wind stress (dyn cm2), and (c) heat content (°C) and
surface ocean current (cm s1) along the equator (2°S–2°N average)
for the period of Jan 2115–Dec 2128. The contour interval is 0.2°C
for SST, 0.6 mm day1 for precipitation, and 0.2°C for heat content.
The scales for the wind stress and ocean current are displayed at
the top right of the respective panels. The heat content refers to
ocean temperature averaged over the upper 300 m. The y axis is the
time from 1 Jan of the first year to 24 Dec of the following
year.
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b. The westward propagating near-annual mode
The temporal evolution along the equator for the westward
propagating near-annual mode is similarly shown in Fig. 5. The
westward propagation of warm SST, above-normal precipitation,
surface wind, and
ocean current anomalies is pronounced during spring and summer. The
warm SST anomalies are generated in spring around 120°W (Fig. 5a).
The amplitude of the SST anomalies increases as the anomalies
propagate from the eastern to central Pacific, while the anomalies
decay after crossing the date line. Above-normal pre-
FIG. 4. (left) Composite SST (°C) and surface wind stress (dyn
cm2), and (right) heat content (°C) and surface ocean current (cm
s1) for the period of Jan 2111–Dec 2128. (top to bottom) Mar, May,
Jul, Sep, and Nov in the first year and Jan, Mar, and May in the
following year. The contour interval is 0.2°C for SST and 0.4°C for
heat content. The scales for the wind stress and ocean current are
displayed at the top of the respective panels. The heat content
refers to ocean temperature averaged over the upper 300 m.
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cipitation and anomalous low-level convergent winds are coupled
with the warm SST anomalies (Fig. 5b). The easterly anomalies
appear to be much stronger than the westerly anomalies. The
positive rainfall anomalies are located to the east of warm SST
anoma- lies. The development of positive heat content anoma- lies
in the equatorial eastern Pacific (Fig. 5c) seems to occur later
than the warm SST anomalies and appears to be loosely connected
with the SST evolution. The surface ocean current displays westward
anomalies to
the east of warm SST anomalies and eastward anoma- lies over and to
the west of warm SST anomalies.
Westward propagating cold SST anomalies immedi- ately follow the
warm SST anomalies (Fig. 5a). The largest cold SST anomalies are
located to the south of the equator, as seen in Fig. 6, which is
similar to Fig. 4 but for the westward propagating near-annual
mode. The warm and cold SST anomalies form an SST cou- plet. A
similar couplet is seen in precipitation anoma- lies (not shown).
The strong southeasterly anomalies
FIG. 4. (Continued)
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are coupled to the SST couplet, indicating that the wind anomalies
are strongly related to the SST gradient. On the other hand, the
southeasterly anomalies produce anomalous cold advection and
anomalous upwelling (as will be shown later), which in turn favors
the develop- ment of cold SST anomalies. Thus, air–sea coupling
processes may be important for the westward propa- gating
near-annual mode. We will return to this point later in the
paper.
Eastward current anomalies develop in the eastern equatorial
Pacific in May, consistent with positive heat content anomalies.
The horizontal structure of the heat content anomalies, however,
changes quickly. In July, the heat content anomalies on the equator
are rela- tively small compared to those in the off-equatorial re-
gions east of 160°W. Correspondingly, ocean current anomalies
reverse in the eastern equatorial Pacific. These current anomalies
propagate westward following westward propagating off-equatorial
positive heat con- tent anomalies. The latter are related to
westward- propagating equatorial easterly anomalies and associ-
ated off-equatorial anticyclonic wind stress curl anoma-
lies.
The westward propagation is seen both for the bien- nial ENSO mode
and the westward propagating near- annual mode. However, there are
important differ- ences. First, the warm SST anomalies occur about
two months earlier for the westward propagating near- annual mode
than for the biennial ENSO mode (Fig. 5a versus 3a). Second, the
easterly anomalies to the east of warm SST anomalies are much
stronger for the near- annual mode (Fig. 5b versus 3b). Third, in
association with these easterly anomalies, cold SST anomalies de-
velop immediately following warm SST anomalies for the near-annual
mode (Fig. 5a). These cold SST anomalies terminate the preceding
warm SST anoma- lies quickly. For the biennial ENSO mode,
warm
SST anomalies only experience moderate weakening (Fig. 3a). Another
difference is that the above-normal rainfall anomalies lie to the
east of warm SST anoma- lies for the near-annual mode, whereas
positive rain- fall and SST anomalies are collocated for the ENSO
mode.
The westward propagation of SST and wind stress anomalies along the
equatorial Pacific seen in the present study resembles that
documented in previous studies (Mantua and Battisti 1995; Jin et
al. 2003; Kang et al. 2004). Another consistent feature is that the
warm SST anomalies are followed by cold SST anomalies. In
comparison, the westward propagation of heat content and zonal
current anomalies is obvious in our model and the Zebiak–Cane
model, but not in the ocean as- similation data (Jin et al.
2003).
c. The stationary near-annual mode
The composite for the stationary near-annual mode (Fig. 7) shows
warm SST anomalies near the date line and cold SST anomalies around
130°W in boreal winter (Fig. 7a). These anomalies develop and decay
locally. Cold SST anomalies are also seen in the western equa-
torial Pacific. Overlying the warm SST anomalies are above-normal
precipitation and anomalous low-level wind convergence (Fig. 7b).
Negative precipitation anomalies are seen around 150°W located
between warm and cold SST anomalies. The positive precipi- tation
anomalies propagate eastward from the Mari- time Continent during
boreal fall and winter. Positive heat content anomalies correspond
to the warm SST anomalies (Fig. 7c). These heat content anomalies
dis- play eastward propagation in boreal winter and the fol- lowing
spring. However, they weaken quickly and be- come very weak in the
eastern equatorial Pacific. East- ward propagating negative heat
content anomalies are
FIG. 5. As in Fig. 3 except for the period of Jan 2053–Dec
2056.
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obvious in the preceding season. Eastward surface ocean current
anomalies also display eastward propa- gation in the western and
central equatorial Pacific (Fig. 7c).
The structure of the anomalies in the mature phase of the
stationary near-annual mode is similar to the bien- nial ENSO mode.
However, there are also obvious dif- ferences from the biennial
ENSO mode. First, warm SST anomalies are located farther to the
west for the stationary near-annual mode compared to the
biennial
ENSO mode (Fig. 7a versus 3a). Second, warm SST anomalies for the
near-annual mode are not preceded by a westward propagation phase,
and heat content anomalies are negative across the entire
equatorial Pa- cific basin in advance of the development of the
warm SST anomalies (Fig. 7c). Third, the SST anomalies for the
near-annual mode are weaker and more localized and cannot propagate
to the eastern equatorial Pacific. This is related to the weak heat
content anomalies present in the eastern equatorial Pacific.
FIG. 6. As in Fig. 4 except for the period of Jan 2053–Dec
2056.
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5. Budget analysis
To understand the physical processes associated with the SST
anomaly evolution for the three modes, we performed a budget
analysis for the ocean mixed layer. In the figures, we show the SST
tendency, which is equivalent to the mixed layer temperature
tendency. In the following, we discuss the contribution of
different terms to the SST tendency.
a. The biennial ENSO mode
Previous studies have demonstrated that for the ENSO the
thermocline feedback plays a primary role in both the growth and
phase transition, whereas the zonal advective feedback is secondary
(e.g., Jin and An 1999; An and Jin 2001). Figure 8 shows composite
SST tendency (calculated using centered differencing), sur- face
heat flux (negative indicating cooling of the ocean surface), and
mixed layer mean advection terms for the biennial ENSO mode. The
heat budget is done based on monthly mean model output and then a
composite is made. The mixed layer depth is determined by a local
temperature difference of 0.5°C, following previous studies (e.g.,
Monterey and Levitus 1997). The nonlin- ear advection terms are
usually small and are not shown. For simplicity, in the following
discussions, the anomalous horizontal (vertical) advection of mean
tem- perature gradient is referred to as anomalous advection
(upwelling), and the advection of anomalous tempera- ture gradient
by mean horizontal current (vertical mo- tion) is referred to as
mean advection (upwelling).
For the biennial ENSO mode, the most important budget terms are
anomalous zonal advection and mean upwelling. The anomalous zonal
advection term (Fig. 8b) dominates in the central and western
Pacific. In the eastern Pacific, the mean upwelling term (Fig. 8g)
is the
largest. This is broadly consistent with Huang and Schneider (1995)
whose budget analysis showed that the El Niño development in an
OGCM is mainly due to the anomalous zonal advection in the west,
the mean meridional advection in the central, and the displace-
ment of thermocline in the east Pacific. The mean me- ridional
advection term (Fig. 8f) contributes in the east- ern Pacific,
especially for the coastal warming during the spring in the
decaying year. Anomalous vertical advection (Fig. 8d) has a
nontrivial contribution to the amplification of warm SST anomalies
in the equatorial central Pacific and for the coastal warming. The
surface heat flux term (Fig. 8a) tends to be out of phase with SST
anomalies, and thus mainly serves as a damping term.
The development of westward propagating warm SST anomalies has
significant contributions from both mean upwelling and anomalous
zonal advection. The westward propagation and the amplification in
the equatorial central Pacific occur largely due to anoma- lous
zonal advection. The eastward propagation has sig- nificant
contributions from mean upwelling and anoma- lous zonal advection.
The role of anomalous zonal ad- vection for the amplification and
eastward propagation of warm SST anomalies is consistent with
Picaut et al. (1997), who suggested that the anomalous zonal advec-
tion could be responsible for the origin of ENSO.
b. The westward propagating near-annual mode
For the westward propagating near-annual mode, most of the terms
contribute to the SST tendency in different stages. The generation
of warm SST anoma- lies in the eastern equatorial Pacific is due to
anoma- lous zonal advection (Fig. 9b) and anomalous upwelling (Fig.
9d). These two terms also contribute to the west- ward propagation
of the warm SST anomalies. An ad-
FIG. 7. As in Fig. 3 except for the period of Jul 2614–Jun 2620
with the time starting from Jul (5) of the first year to Jun (6) of
the following year.
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ditional contribution comes from the surface heat flux (Fig. 9a).
The generation of cold SST anomalies in the eastern Pacific is due
to anomalous upwelling (Fig. 9d), surface heat flux (Fig. 9a),
anomalous meridional ad- vection (Fig. 9c), and anomalous zonal
advection (Fig. 9b). The westward propagation of cold SST anomalies
is due to anomalous zonal advection, anomalous me- ridional
advection, anomalous upwelling, mean zonal advection (Fig. 9e), and
surface heat flux. The mean upwelling term (Fig. 9g) acts to damp
the SST anomalies.
The results of the heat budget for the westward propagating
near-annual mode are consistent with pre- vious studies (Hirst
1986; Mantua and Battisti 1995; Jin et al. 2003; Kang et al. 2004).
Agreement is found in the important role of anomalous zonal
advection for the growth and westward propagation of SST anomalies,
the role of the anomalous upwelling for the genesis of SST
anomalies in the eastern Pacific, and the role of mean upwelling as
a damping term. Note that the anomalous meridional advection was
neglected in pre- vious studies (Mantua and Battisti 1995; Kang et
al. 2004).
The main processes contributing to the SST tendency for the
westward propagating near-annual mode have similarities and
differences compared with those attrib- uted to the biennial ENSO
mode. The anomalous zonal advection contributes to the generation
and westward propagation of warm SST anomalies for both the ENSO
mode and the near-annual mode. However, the role of mean upwelling
is very different. For the ENSO mode the mean upwelling term has a
positive contribu- tion to the generation of warm SST anomalies
(Fig. 8g), whereas for the near-annual mode the mean upwelling is
mainly a damping term (Fig. 9g).
Westward propagating warm SST anomalies for the biennial ENSO mode
are only subjected to a moderate weakening (Fig. 3a), whereas for
the westward propa- gating near-annual mode they are replaced
quickly by cold anomalies (Fig. 5a). We note that the anomalous
upwelling term is negative and large for the near- annual mode
(Fig. 9d), but not for the ENSO mode (Fig. 8d). The anomalous zonal
and meridional advec- tion terms are also larger for the
near-annual mode than for the ENSO mode (Figs. 8b,c versus 9b,c).
These differences are linked to easterly wind anomalies in the
eastern equatorial Pacific, which are much stronger for the
near-annual mode than for the ENSO mode (Fig. 3b versus 5b). The
strong easterly wind anomalies in- duce large anomalous upwelling
to the east of warm SST anomalies (Fig. 9d). The large easterly
anomalies along the equator are associated with anticyclonic wind
stress curl off the equator (Fig. 6, May–September), which deepens
the thermocline off the equator. This
induces anomalous westward ocean current along the equator (Fig. 6,
July–September). As a result, large anomalous zonal and meridional
advection develops for the near-annual mode (Figs. 9b,c). These
terms overcome the mean upwelling (Fig. 9g) and lead to cold SST
anomalies. The surface heat flux term (Fig. 9a) also contributes,
related to an enhanced surface evaporation induced by large
easterly anomalies between the warm and cold SST anomalies. The
eastward location of above-normal rainfall anomalies relative to
warm SST anomalies for the near-annual mode (Figs. 5a,b) also
contribute to negative surface heat flux anomalies through
cloud–radiation feedback.
The anomalous advection and upwelling terms are most effective in
boreal spring when the mean near- surface–layer ocean temperature
gradient is large. This is demonstrated in Figs. 10a,b, which show
the annual cycle of the mean SST. In boreal spring, the eastern
equatorial Pacific SST is the warmest. The zonal SST gradient is
the largest in the eastern equatorial Pacific (to the east of the
warm SST anomalies, Fig. 10a). The region of negative meridional
SST gradient is closest to the equator in the eastern tropical
Pacific (Fig. 10b). In association with the warmest SST, the mean
zonal wind stress is weakest at this time (Fig. 10c). Correspond-
ingly, the vertical mixing is also the weakest. This in- creases
the vertical temperature gradient in the upper ocean (Fig. 10d).
All of these factors favor the contri- bution of anomalous zonal,
meridional, and vertical ad- vection of the background ocean
temperature gradient to the SST tendency.
Based on our analysis of the westward propagating near-annual mode
and the biennial ENSO mode, we suggest that the critical difference
is most apparent dur- ing May through July. For example, during
July, the two modes have comparable warm SST anomalies near the
equator but the near-annual mode has considerably stronger cold SST
anomalies to the southeast of the warm SST anomalies. Consistent
with these stronger cold SST anomalies are stronger easterly wind
anoma- lies occurring between the warm and cold SST anoma- lies,
presumably through a Rossby-wave-type response to the SST anomalies
(Matsuno 1966; Gill 1980). Fur- thermore, the enhanced low-level
convergence is con- sistent with the enhanced rainfall during this
period. The stronger easterlies associated with the westward
propagating near-annual mode enhance the local SST cooling through
enhanced evaporation (Fig. 9a), stron- ger upwelling of cold water
(Fig. 9d), and cold advec- tion from the southeast (Figs. 9b,c).
These local feed- backs weaken or even reverse the SST anomalies,
ulti- mately inhibiting the development of a warm ENSO event. It
appears that surface air–sea interaction pro-
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cesses dominate the difference between the biennial ENSO mode and
the westward propagating near- annual mode during the May–July time
frame.
Why are surface easterly anomalies and cold SST anomalies to the
southeast of the warm SST anomalies stronger for the westward
propagating near-annual mode than for the biennial ENSO mode? We
are un- able to answer this question at this point; however, we
speculate that random or stochastic atmospheric pro- cesses may
play a key role in the excitation of the west- ward propagating
near-annual mode versus the biennial ENSO mode. Indeed, our search
for some mode select- ing “precursors” in, for example, the heat
content has failed. The differences are largely in surface
processes, and we have not found consistent triggering mecha-
nisms. This, perhaps, suggests the role of stochastic pro- cesses
as a trigger, whereas local air–sea feedbacks lead to the
amplification of the anomalies.
c. The stationary near-annual mode
For the stationary near-annual mode, the develop- ment of warm SST
anomalies near the date line is due to anomalous zonal advection
and upwelling terms (Figs. 11b,d). The mean upwelling term (Fig.
11g) is negative in the eastern Pacific, which limits the east-
ward propagation of warm SST anomalies. The surface heat flux (Fig.
11a) is mainly a damping term.
Compared to the biennial ENSO mode, the SST ten- dency in the
equatorial central Pacific is smaller and lying to the west for the
stationary near-annual mode (Fig. 11a versus 8a). This is related
to the weaker zonal current and upwelling anomalies. Weaker
anomalous zonal advection and upwelling are also linked to smaller
zonal and vertical gradients of mean ocean tem- perature in the
western compared to the central equa- torial Pacific. Another
difference from the biennial
FIG. 8. Composite SST tendency (shading, °C month1): (a) surface
heat flux (heatf, °C month1), (b) anomalous zonal advection
(UadxTm), (c) anomalous meridional advection (VadyTm), and (d)
anomalous vertical advection (WadzTm) of mean temperature gradient,
(e) mean zonal advection (UmdxTa), (f) mean meridional advection
(VmdyTa), and (g) mean vertical advection (WmdzTa) of anomalous
temperature gradient along the equator (2°S–2°N average) for the
period of Jan 2115–Dec 2128. The advection terms (°C month1) are
averages for the mixed layer, defined by a local temperature
difference of 0.5°C. The contour interval for heat flux and
advection terms is 0.2°C month1. The y axis is the time from 1 Jan
of the first year to 24 Dec of the following year.
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ENSO mode is that the mean upwelling term is nega- tive in the
eastern equatorial Pacific (Fig. 11g). This is related to negative
heat content anomalies (Fig. 7c), which is unfavorable for the
eastward propagation of warm SST anomalies.
6. Discussion
The frequency of occurrence for warm and cold events of near-annual
modes shows apparent long-term change based on an examination of
the temporal evo- lution of SST anomalies averaged over the region
of 2°S–2°N, 170°E–170°W. The warm events of the west- ward
propagating near-annual mode and the cold events of the stationary
near-annual mode are more frequent in the earlier part of the
900-yr simulation, whereas the opposite occurs during the later
period. We suspect that these tendencies are related to the
mean-state change in the model. One important long- term change
occurring during the model simulation is
the decrease of heat content across the tropical Pacific. We note
that one unfavorable term for the generation of SST anomalies in
the westward propagating near- annual mode is the mean upwelling.
For the warm (cold) events associated with the westward propagating
near-annual mode, the effect of this term increases (decreases)
when the heat content decreases. This would lead to a less (more)
frequent occurrence of westward propagating warm (cold) events in
the later period of the model. For the stationary near-annual mode,
the mean upwelling term has the role of limiting the eastward
propagation of warm SST anomalies. Lower heat content would enhance
this role, thus in- creasing the occurrence of stationary warm
events in the later period. For the stationary cold events, lower
heat content would increase the likelihood of cold SST anomaly
amplification and eastward propagation, which could turn into La
Niña events. As such, the occurrence for the stationary cold events
shows a de- creasing trend.
FIG. 8. (Continued)
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An apparent long-term trend is also seen in the frequency of
occurrence for El Niño and La Niña events. El Niño events are more
frequent in the early period, whereas La Niña events are frequent
in the later period. We speculate that this trend is related to the
decreasing trend of heat content in the tropical Pacific.
The anomalous zonal advection contributes to the generation and
westward propagation of warm (cold) SST anomalies preceding El Niño
(La Niña). The mag- nitude of the anomalous zonal advection depends
on the zonal gradient of mean temperature. As such, the mean state
change may affect the strength of westward propagating SST
anomalies. If these westward propa- gating SST anomalies are
considered to occur on the El Niño and La Niña background, then the
magnitude of these SST anomalies may differ between El Niño and La
Niña. Examination of filtered time series of equa- torial central
Pacific SST anomalies around the annual frequency indicates that
these SST anomalies are stron-
ger when they occur before La Niña than before El Niño. This is
consistent with previous studies (Mantua and Battisti 1995; Jin et
al. 2003; Kang et al. 2004). These studies suggest that the La Niña
state enhances the zonal SST gradient and, thus, the effect of
anoma- lous zonal advection, which ultimately favors the devel-
opment of stronger SST anomalies.
The presence of near-annual modes enriches the SST variability in
the equatorial Pacific. The stationary near-annual mode is relevant
to an aborted ENSO event and can become a “mini-ENSO” if the SST
anomalies attain sufficient magnitude. Further, the westward
propagating SST anomalies preceding El Niño and La Niña contribute
to the irregularity of ENSO. We speculate that this may make
prediction of ENSO more difficult. The westward propagating SST
anomalies and associated wind anomalies also play an important role
for the amplification of SST anomalies in the equatorial central
Pacific. Note that there is a substantial intensification of
rainfall and westerly
FIG. 9. As in Fig. 8 except for the period of Jan 2053–Dec
2056.
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anomalies in the western equatorial Pacific when warm SST anomalies
arrive (Figs. 3, 4). This intensification is related to a higher
mean SST in the western Pacific. These westerlies deepen the
thermocline and enhance eastward warm advection and anomalous
downwelling (Fig. 9). These contribute to the amplification of warm
SST anomalies.
For the stationary near-annual mode, no preced- ing westward
propagating SST anomalies are seen (Fig. 7a). Warm SST anomalies
develop quickly near the date line, similar to the amplification of
warm SST anomalies for the biennial ENSO mode. What is the trigger
for the stationary mode? We speculate that the large precipitation
anomalies seen in November over the Maritime Continent (Fig. 7b)
move eastward to 150°E in December. In association, westerly
anomalies develop and deepen the ther- mocline (Figs. 7b,c), which
drives eastward warm ad- vection and anomalous downwelling (Figs.
11b,d). This ultimately leads to the development of warm SST
anomalies that, in turn, feed back on the atmo- sphere, resulting
in further growth of wind and SST anomalies.
7. Summary
The COLA interactive ensemble coupled model dis- plays multiple
time scale variability in the equatorial Pacific. In addition to
the biennial ENSO mode, there are two near-annual modes: a westward
propagating mode and a stationary mode. In the westward propa-
gating near-annual mode, the SST anomalies are gen- erated in the
eastern equatorial Pacific in boreal spring and propagate westward
during boreal summer and fall. Consistent westward propagation is
found in the wind, precipitation, and ocean surface current anoma-
lies. In the stationary near-annual mode, SST anomalies develop in
boreal winter near the date line and decay locally during the
following spring. These near-annual modes may contribute to the
irregularity of ENSO. Un- derstanding the mechanism for the
near-annual vari- ability may advance our understanding of the
irregular- ity of ENSO and lead to improved ENSO prediction. The
spatiotemporal evolution of the westward propa- gating near-annual
mode has important similarities with observations. The stationary
near-annual mode re- sembles some observed aborted ENSO events that
fea-
FIG. 10. Climatological annual cycle of SST (°C) along (a) the
equator (2°S–2°N average) and (b) along 130°–110°W, (c) zonal wind
stress (dyn cm2) along the equator (2°S–2°N average), and (d)
change of ocean temperature (°C) with depth (m, y axis) averaged
over the region of 2°S–2°N, 130°–110°W.
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ture weak–moderate SST anomalies near the date line developing and
decaying locally. Understanding the physical processes associated
with this study may help diagnose and predict aborted ENSO events
in observa- tions.
The warm SST anomalies in the westward propagat- ing near-annual
mode resemble those occurring in the westward propagation phase of
the biennial ENSO mode. The anomalous zonal advection acts as a
major mechanism for the generation and westward propaga- tion of
warm SST anomalies. By comparison to the biennial ENSO mode, the
warm SST anomalies for the westward propagating near-annual mode
occur two months earlier and are followed immediately by cold SST
anomaliesthat propagate westward as well. In as- sociation, the
easterly anomalies lying to the east of warm SST anomalies are much
stronger. The mean up- welling is a damping term for the westward
propagating near-annual mode, but contributes to the generation of
warm SST anomalies for the biennial ENSO mode.
For the westward propagating near-annual mode, the warm SST
anomalies are accompanied by large easterly anomalies and cold SST
anomalies. The spatial phase relationship and coherent westward
propagation of SST and wind anomalies leads us to hypothesize that
the development of cold SST anomalies in the aftermath of warm SST
anomalies is associated with surface air–sea interaction processes
occurring in a favorable back- ground state. The larger easterly
anomalies to the east of warm SST anomalies contribute to the
development of cold SST anomalies through anomalous zonal advec-
tion, anomalous meridional advection, anomalous ver- tical
advection, and enhanced surface evaporation. The cold SST
anomalies, in turn, enhance the easterly anomalies. The larger
horizontal and vertical gradients of mean near-surface-layer ocean
temperature in boreal spring are favorable for easterly anomalies
to induce large SST anomalies when these wind anomalies are
triggered. The presence of cold SST anomalies in the southeastern
tropical Pacific is also a favorable condi-
FIG. 11. As in Fig. 8 except for the period of Jul 2614–Jun 2620
with the time starting from Jul (5) of the first year to Jun (6) of
the following year.
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tion. However, it is unclear what triggers the above pro- cesses
for the westward propagating near-annual mode.
The development of warm SST anomalies for the stationary
near-annual mode is related to air–sea inter- action processes
resembling the amplification of warm SST anomalies for the biennial
ENSO mode. Anoma- lous zonal advection and anomalous upwelling
contrib- ute to these SST changes for both modes. In compari- son,
the anomalies for the stationary near-annual mode are weaker and do
not propagate into the eastern Pa- cific. This is related to
smaller zonal and vertical gradi- ents of mean ocean temperature in
the western Pacific compared to the central Pacific. The trigger
for the stationary near-annual mode may be related to anoma- lous
heating over the far western tropical Pacific. For the biennial
ENSO mode, the westward propagating SST anomalies can act as a
mechanism for the amplifi- cation of SST anomalies in the
equatorial central Pacific.
The applicability of the model results depends on the model
performance in the mean simulation, especially differences in the
mean temperature gradient and sur- face wind stress between the
model and observations. In the eastern equatorial Pacific the
difference is small for the zonal gradient of mean SST, whereas the
me- ridional gradient of mean SST is larger and the vertical
gradient of mean subsurface temperature is smaller in the model
compared to observations. This indicates that the contribution of
anomalous meridional advec- tion may be larger and that of
anomalous upwelling may be smaller in the model as compared to
observa- tions. The mean zonal wind stress in the tropical Pacific
is weaker in the model compared to observations, indi- cating
weaker mean upwelling. Thus, the model may underestimate the role
of mean upwelling. Because the mean upwelling in the model is a
favorable term for the biennial ENSO mode but an unfavorable term
for the near-annual mode, this implies that the near-annual
variations relative to the ENSO may account for a larger percent of
the variance compared to observations.
Acknowledgments. The authors thank D. Straus for his careful
reading of an earlier draft of this manuscript. The comments of two
anonymous reviewers led to the improvement of this manuscript. This
research was sup- ported by National Science Foundation Grants ATM-
9814295 and ATM-0122859, National Ocean and At- mospheric
Administration Grant NA16-GP2248, and National Aeronautics and
Space Administration Grant NAG5-11656.
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