1. Introduction

The interdecadal sea surface temperature (SST) variability of the Pacific Ocean has been characterized by the third empirical orthogonal function (EOF) of the 13-yr lowpass-filtered near-global SST (Folland et al. 1999). This mode of interdecadal SST variability has been titled the interdecadal Pacific oscillation (IPO) due to its time scale of oscillation and symmetry about the Pacific Ocean equator (Folland et al. 1999, 2002; Power et al. 1999a). Folland et al. (2002) propose that the IPO is the Pacific-wide manifestation of the North Pacific principal mode of decadal variability named the Pacific decadal oscillation (PDO) by Mantua et al. (1997). The basinwide IPO SST pattern is very similar to the SST pattern associated with El Niño–Southern Oscillation (ENSO) events, although there are several distinct differences, in particular that the IPO warm SST anomalies reach the western tropical Pacific and extend farther away from the equator than is typical of ENSO events. Further, the IPO equatorial Pacific warming is accompanied by extensive cooling of the Pacific Ocean’s subtropical gyre circulations, and the time scale of fluctuations between the IPO’s warm and cold phases is approximately 15–30 yr (Folland et al. 1999).

ENSO research has identified an interdecadal signal in event amplitude and frequency that appears to be linked to the decadal variability of the Pacific Ocean (Trenberth and Hurrell 1994; Trenberth and Hoar 1997; Power et al. 1999a, b). Power et al. (1999a), for example, have shown that the IPO is statistically linked with changes in ENSO’s influence on Australian rainfall, surface temperature, river flow, and crop yield. When the IPO is in its negative (cool tropical Pacific) phase, the magnitudes of correlation coefficients between the Southern Oscillation index (SOI) and Australian climate variables are large. In contrast, when the IPO is in its positive (warm tropical Pacific) phase, the correlation coefficients between the SOI and Australian climate variables are weak. While this might suggest that the IPO plays a role in modulating ENSO impacts (Power et al. 1999a), this behavior could be due to the nonlinear relationship between ENSO and Australian rainfall; a larger La Niña SST anomaly tends to produce a larger increase in rainfall, whereas the magnitude of an El Niño SST anomaly is a poorer guide to how dry Australia will become (Power et al. 2006). Thus, the association between ENSO and Australian climate appears to be strong in decades dominated by La Niñas but weak in decades dominated by El Niños.

Our understanding of the physical mechanisms underpinning observed Pacific Ocean interdecadal SST variability is growing, but the question still remains as to whether the IPO operates independently of ENSO, and as such, many different hypotheses have been proposed to explain the variability [see Miller and Schneider (2000) and Power et al. (2006) for recent reviews]. Of the hypotheses proposed, two main types are most relevant to this paper. These are (i) that low-frequency extra-equatorial ocean exchanges with the equatorial Pacific generate SST variability in the Tropics on interdecadal time scales and (ii) that interdecadal SST variations in the tropical Pacific are primarily a consequence of higher frequency atmosphere–ocean dynamics in the equatorial region, in particular ENSO.

Several major hypotheses have been proposed concerning the role of extra-equatorial exchanges with the equatorial Pacific Ocean. (i) The first hypothesis, proposed by Gu and Philander (1997), suggests that the isopycnal (constant density) surfaces linking the midlatitude Pacific Ocean to the tropical Pacific allows the existence of a shallow meridional circulation. The mean meridional overturning circulation is proposed to transport temperature anomalies from the midlatitude Pacific to the Tropics where they alter SST on interdecadal time scales and create unstable atmosphere–ocean interactions to further amplify the signal. (ii) The second hypothesis, proposed by Kleeman et al. (1999), suggests that it is the anomalous advection of the mean temperature in this shallow meridional overturning circulation that is responsible for the tropical Pacific interdecadal variability. Support for this theory has been provided by the observational study of McPhaden and Zhang (2002). The modeling analysis of Capotondi et al. (2005) has further shown that the connection between meridional thermocline transport and equatorial SST variations holds at both interannual and decadal time scales. Nonaka et al. (2002) examined the relative importance of extra-equatorial winds on changes in meridional thermocline transport using an ocean GCM forced with observed wind stresses. They found that extra-equatorial winds (20°–8°S and 8°–25°N) are equally as important as equatorial winds (± 8° latitude) for decadal variations of equatorial SST anomalies, in agreement with the Kleeman et al. (1999) theory, while anomalous winds poleward of 20°S and 25°N do not play any significant role. (iii) The third tropical Pacific interdecadal variability hypothesis concerning oceanic processes linking extra-equatorial regions with the equatorial zone involves Rossby waves impinging on the western boundary, thereby creating coastal Kelvin waves that propagate equatorward and modulate the equatorial thermocline depth via eastward equatorial Kelvin wave propagation. This mechanism of decadal variability has been suggested in theoretical and modeling studies (Lysne et al. 1997; Liu et al. 1999; Hazeleger et al. 2001; Capotondi and Alexander 2001; Capotondi et al. 2003; Galanti and Tziperman, 2003). However, different authors have emphasized the importance of Rossby waves at different latitudes. For example, Capotondi et al. (2003) highlight the importance of Rossby waves at 13°N and 10°S, while the experimentation of Hazeleger et al. (2001) highlights the importance of Rossby waves poleward of 30° latitude. This mechanism has also been explained in a similar context to the delayed oscillator mechanism for ENSO by White et al. (2003) and Tourre et al. (2005), and the recharge/discharge paradigm by Wang et al. (2003).

High-frequency equatorial Pacific ocean–atmosphere (namely, ENSO) interactions are also identified within several different theories. For example, (i) Power and Colman (2006) have shown that ENSO-like interdecadal patterns can arise through random changes in ENSO activity, for example, in the relative number of El Nino and La Niña events in a given decade. They also showed that reddening by thermodynamic and dynamic ocean processes can help to explain the broader structure of decadal ENSO-like modes. (ii) Newman et al. (2003) proposed that ENSO forced variability could be reddened by ocean processes, thus leading to enhanced interdecadal variability in the extratropics. (iii) In the third theory, it is proposed that nonlinear mechanisms of ENSO may generate an apparent decadal signal in the Pacific Ocean (e.g., Jin et al. 1994; Tziperman et al. 1994; Timmermann et al. 2003; Rodgers et al. 2004).

Here, we focus on effectively testing the third hypotheses linking the extra-equatorial regions with the equatorial region via the propagation of planetary waves by seeking to (i) clarify the extent to which ocean dynamic equatorial variability is locally and remotely driven and (ii) determine the importance of Rossby waves incident on the western boundary in the remotely forced response. We address these questions by investigating the relative importance of equatorial wind stress forcing, off-equatorial wind stress forcing and oceanic Rossby wave reflections at the western Pacific boundary with a series of 1½ layer shallow-water model (SWM) experiments. In this study we define the equatorial region as the area within ±12.5° latitude and the off-equatorial region as those SWM latitudes outside of the equatorial region. However, first we seek to identify the importance of upper-ocean dynamics on the tropical Pacific Ocean decadal variability to determine the feasibility of using the SWM as an investigative tool. To this end, we compare the interannual and interdecadal SST variability evident in the Australian Bureau of Meteorology Research Centre (BMRC) CGCM with observations. The role of upper-ocean dynamics in this variability is then investigated using a linear first baroclinic-mode SWM forced with wind stresses from a century-time-scale CGCM simulation. The SWM includes only the important large-scale upper-ocean dynamics, specifically long Rossby waves and Kelvin waves.

This paper is organized as follows. Brief descriptions of the CGCM and SWM used in this study are provided in section 2. Section 3 details the CGCM simulation and the series of SWM experiments designed to investigate the role of equatorial wind stress forcing, off-equatorial wind stress forcing, and western boundary reflections generating the equatorial Pacific Ocean interdecadal SST and thermocline depth variability. Section 4 outlines the statistical techniques and filtering methods used to analyze the model outputs. Experiment results are presented in section 5, and the implications these results have for predictability is investigated and discussed in section 6. Discussion of the results and conclusions from the study are provided in section 7.

2. Models

b. Shallow-water model

The shallow-water model (SWM), developed at Macquarie University for the MATLAB environment, is a 1½-layer model of the stratified ocean (e.g., Philander 1990, 106–108) resolved on a 1° × 1° spatial grid [the Arakawa C grid of Mesinger and Arakawa (1976)]. The upper and lower ocean density layers are separated by an interface (the pycnocline) that approximates the thermocline. The reduced gravity, g′, reflects the density difference between the upper and lower layers. We use the typical value of g′ = 0.03 m s⁻² (Tomczak and Godfrey 1994, p. 37). The lower layer is assumed to be motionless and infinitely deep. We prescribe the mean depth of the upper well-mixed active layer as H = 300 m (Tomczak and Godfrey 1994, p. 37), so the first baroclinic-mode Kelvin wave speed c is 3 m s⁻¹. Motion in the upper layer is driven by the applied wind stresses (per unit density), τ (m² s⁻²), which are anomalies from the long-term monthly means (i.e., seasonal cycle removed). The associated response of the ocean is displayed by the vertical displacement of the pycnocline, η (m), and the horizontal components of the flow velocity. Considering that the pycnocline approximates the thermocline and that the thermocline depth is climatically well understood due to its role in helping to quantify the upper-ocean heat content (or warm-water volume), we use the term thermocline hereafter when discussing results from the SWM experiments. The ocean dynamics are described by the linear reduced-gravity form of the shallow-water equations detailed below [Eqs. (1)–(3)]:

View Expanded

View Expanded

View Expanded

where u and υ are the eastward and northward components of velocity respectively (m s⁻¹) and t is time (s). The long Rossby wave speed C_R(m s⁻¹) is given by the equation, C_R = β(c₁²/f ²), where f (s⁻¹) is the Coriolis parameter and β(m⁻¹ s⁻¹) is the derivative of f northward. The model time step is 2 h and Fischer’s (1965) numerical scheme is utilized for model time stepping. This model formulation permits Ekman pumping and Rossby and Kelvin wave propagation along the thermocline to be generated with appropriate large-scale wind stress forcing.

3. Experimental design

b. SWM experiments

1) SWM control simulation with CGCM wind stress forcing

Anomlous monthly averaged wind stresses (from the average annual cycle) generated by BCM2.2 were used to force the SWM configured for the low to midlatitude Pacific Ocean (41°S–41°N, 120°E–68°W). The western (eastern) Pacific boundaries approximately follow the eastern coastlines of Asia, Australia, and the western Pacific islands (the western coastline of the Americas) while the north (south) model boundaries are at 41°N (41°S). The anomalous monthly wind stresses were linearly interpolated in time and updated at each time step throughout the 100-yr SWM experiments. The three SWM output fields, the thermocline displacement and two horizontal flow velocity components, were archived as 30-day snapshots.

The thermocline depth variability generated in the SWM control simulation is analyzed and compared with the corresponding BCM2.2 VAT variability. Here, we take advantage of the robust correlation between thermocline depth and upper ocean heat content in the tropical Pacific Ocean (to 15° latitude) (Rebert et al. 1985) to usefully compare results from both models. Comparison of the results from the SWM control simulation with the BCM2.2 simulation allows us to investigate whether large-scale wind-stress-forced upper-ocean dynamics play an important and significant role in generating the SST and VAT variations simulated in the much more sophisticated CGCM, and hence whether the SWM is an appropriate tool to investigate forcing mechanisms of the Pacific Ocean’s decadal variability.

2) Experiment I: The role of equatorial and off-equatorial forcing

To quantify the role of extra-equatorial interactions with the equatorial Pacific Ocean and anomalous equatorial wind stresses on the equatorial Pacific thermocline depth variability, we perform an SWM experiment consisting of two parts that essentially separate the Pacific Ocean basin into two regions. They are the equatorial zone, which is the area within ±12.5° latitude, and the extra-equatorial region, which includes those SWM latitudes outside of the equatorial zone. Part a, titled the “equatorial wind stress forcing” (EqWF) experiment, uses the same SWM configuration as in the control simulation, but only those anomalous wind stresses in the equatorial Pacific between ±12.5° latitude are retained (wind stress anomalies are tapered to zero between 10° and 15° latitude). Part b, hereafter titled the “off-equatorial wind stress forcing” (OffEqWF) experiment, also uses the same SWM configuration as in the control simulation, but here the anomalous wind stresses are retained in the extra-equatorial zone (those latitudes poleward of ±12.5° latitude) and wind stress anomalies are tapered to zero between 15° and 10° latitude. As in the control simulation, 30-day snapshots of the thermocline depth displacements are archived over the 100-yr simulation for both of these experiments. We note here that we are seeking to identify the role of wind stress forcing and the important regions of this forcing on ocean dynamic processes and we do not attempt to address the origin of the wind stress variability.

3) Experiment II: The role of western boundary interactions

The second SWM experiment has four components (experiments IIa–d) each using the same SWM configuration and forcing wind stresses as per the OffEqWF experiment (experiment Ib). This series of experiments was designed to allow us to investigate the role of oceanic Rossby waves reflected at the Pacific Ocean western boundary on the oceanic interactions between the extra-equatorial region and the equatorial zone. Reflection of these Rossby waves explicitly relies on the conservation of mass. Thus, to prevent wave reflection at the western Pacific boundary and, hence, evaluate the importance of wave reflection on the interdecadal equatorial thermocline depth anomalies, we introduce a sizeable linear damping term, F_d (m), into the calculations of the thermocline depth between the selected latitudes along the western boundary, Eq. (4). This term acts to damp out Rossby waves impinging on the boundary, thereby preventing the coastal Kelvin waves from transporting the incoming Rossby mass equatorward. The introduction of this term into the SWM calculations affects the instantaneous conservation of mass, causing it to oscillate around the original steady state. However, the integrated effect is such that the SWM mass is conserved in time:

View Expanded

The damping term (F_d) is calculated using the method of Burgers et al. (2002) for the prevention of Kelvin wave propagation around their artificial north and south boundaries. Here, however, instead of calculating the linear damping term for all longitudes at a specific latitude, the damping term at the western boundary as a function of latitude was calculated by the equation:

View Expanded

where is the thermocline depth in meters, t is time in seconds, ξ_wb is the western boundary longitude in degrees, and γ is the nondimensional damping constant. The damping coefficient, r_k, is specified by the equation:

View Expanded

where r_k(0) = 2.5 × 10⁻⁶ and ξ_crit = ξ_wb + 3.

The magnitude of the damping term is increased by increasing the value of the damping constant, γ, since Rossby waves tend to impinge effectively normal to the boundary rather than parallel, as occurs with Kelvin waves along the artificial north and south boundaries. The damping constant value was tested through a series of SWM experiments in a rectangular domain (not shown) and the damping constant was found to have an optimal value of ∼1.4. Experiments IIa–d incorporate this damping term between (a) 41° and 30°S, 41° and 30°N only; (b) 41° and 20°S, 41° and 20°N only; (c) 41° and 10°S, 41° and 10°N only; and (d) 41°S and 41°N (the entire western Pacific boundary). The 30-day snapshots of the thermocline depth anomalies from each experiment were analyzed and compared with the interdecadal thermocline depth anomalies produced in the OffEqWF experiment.

4. Analysis methods

For the present study, we investigate the variability of (i) SST and VAT in BCM2.2 and (ii) the thermocline depth from the series of SWM experiments. The unfiltered model anomalies of thermocline depth, SST, and VAT are from the long-term monthly means (i.e., seasonal cycle removed). Since the IPO is defined as the third EOF of the 13-yr lowpass filtered near-global SST (Folland et al. 1999; Power et al. 1999a), interdecadal variability in the model output is analyzed using the lowpass Butterworth-filtered anomalies, with a 13-yr cutoff. As we used the reduced variable domain (near-global scale reduced down to the Pacific basin) as input to the EOF analysis, we expect the IPO-like variability in both model outputs to be the first mode variability instead of the globally observed third mode. The correlation matrix was used in the EOF analysis of both unfiltered and filtered model outputs to provide equal weighting of the different variables analyzed and a sensible comparison of results between the model fields. To reduce computational costs, the EOF analysis was performed on 2° averages of the output fields.

Each SWM experiment was perturbed from rest (i.e., zero initial state) and, as the equatorial region thermocline depth variability includes the effects of both local and remote forcing and signals in the off-equatorial region (namely, Rossby waves) of the SWM domain (±40°) can take up to ∼10 yr to cross the Pacific Ocean basin, the initial 10 yr of each SWM experiment output have been discarded. For the purposes of comparison, the first 10 yr of the BCM2.2 SST and VAT outputs have also been removed.

5. Results

6. Predictability

The experiment II results have established that processes at the Pacific Ocean western continental boundary appear to play a critical role in the transfer of mass generated from OffEqWF ocean dynamics into the equatorial Pacific, with Rossby wave reflection at the western Pacific boundary likely to be critical to this transfer. To confirm that Rossby waves generated in the extra-equatorial region transport this mass, we examine Hovmöller (time–longitude) plots of the thermocline depth variations at 16°S (Fig. 7a) and 16°N (not shown) for the last 90 years of the OffEqWF experiment. Thermocline depth variations at the equator are also examined (Fig. 7b). The 16°S section shows the dominance of westward propagating anomalies with estimated speeds of approximately 13 cm s⁻¹, which is consistent with the Rossby wave speeds for this latitude (12.7 cm s⁻¹). Upon reaching the western boundary, the majority of these waves appear to transfer their mass to the equatorial region (Fig. 7b). It must be noted that the equatorial response shown is produced by off-equatorial wind stress forcing in both hemispheres: thus western boundary reflections in the Northern Hemisphere and at different latitudes in the Southern Hemisphere all contribute to the OffEqWF thermocline depth variability of the equatorial region. This mass transfer is supported by the fact that the quantity of mass transferred (calculated at various times throughout the remaining 90 years of the SWM simulation) is consistent with expectations from linear theory (Kessler 1991). Additionally, an equatorial Hovmöller plot corresponding to the experiment with nonreflecting boundary conditions operating on the entire western boundary (experiment IId, figure not shown) demonstrates that thermocline depth variability in the equatorial zone is in effect absent. This reinforces the critical role that Rossby wave reflection plays at the western boundary and implies a theoretical predictability with lead times of months to years, depending on the latitude and longitude of Rossby wave generation.

Lagged correlations were calculated between the OffEqWF Niño-3 region anomalies and extra-equatorial zone anomalies along 150°W in the NH and 120°W in the SH. These longitudes were selected since most of the modeled Rossby waves that reach the western boundary at 16° latitude appear to form close to these meridians, thereby providing the longest lead times. The maximum correlation coefficient of 0.39 (0.23) corresponding to latitudes greater than 12°S (12°N) at the 120°W (150°W) meridian occurs at 14°S (16°N) where the off-equatorial anomalies lead the OffEqWF Niño-3 anomalies by 22(28) months, with the correlation coefficient significant above the 99% (99%) confidence level. These leads are more or less consistent with the time needed for oceanic Rossby waves, at the given latitude, to transfer their mass to the equatorial region via the western boundary, thereby modulating the equatorial region thermocline depth. We note, nevertheless, that the 22–28-month leads are not exclusive of further modulation of the equatorial thermocline by OffEqWF at longer time scales. To explore the maximum deterministic lead times of OffEqWF equatorial anomalies we carried out a further 17 OffEqWF SWM experiments. These experiments utilized the same SWM configuration and wind stresses as experiment Ib (the OffEqWF experiment). Here, however, the wind stresses used to force each experiment were respectively switched off at 10, 15, 20, . . . , 85, 90 yr. The amplitudes of the Niño-3 region anomalies were subsequently monitored to identify how long after the OffEqWF lapsed the upper-ocean dynamics would be able to modulate the equatorial thermocline depth. Figure 8 shows the Niño-3 region anomalies, after the wind stresses were systematically switched off, for each of the 17 experiments. Interestingly, the envelope of all of the predictability experiment Niño-3 region anomalies shows that OffEqWF can modulate equatorial region thermocline depths for up to approximately 3½ years after the wind stress forcing is switched off (Fig. 8).

The apparent OffEqWF predictability of the equatorial thermocline depth anomalies may not be of large importance given that the OffEqWF equatorial anomalies make up only a modest proportion of the total equatorial variability. However, it is quite possible that such relatively small perturbations of the thermocline depth (and SST) could be intensified by a positive atmospheric feedback (i.e., the Bjerknes feedback). Thus, in a coupled setting the importance of the predictable component may be enhanced. This might also help to explain the apparent predictability of decadal SST variations reported by Karspeck et al. (2004) using the intermediate complexity coupled model of Zebiak and Cane (1987).

7. Discussion and conclusions

In this paper the interannual and interdecadal SST variability represented in a 100-yr simulation of the BMRC CGCM (BCM2.2) was first “ground truthed” against observations to ensure consistency. The role of upper-ocean dynamics in this interdecadal variability was further explored using a linear first baroclinic-mode shallow-water model (SWM) forced with wind stresses from the century-time-scale BCM2.2 simulation, thus confirming the feasibility of using the SWM as an investigative tool to investigate the tropical Pacific Ocean’s decadal variability. The ocean dynamic mechanisms underpinning this decadal variability were then explored in response to changes in the regions of applied wind stress forcing. The relative importance of equatorial wind stress forcing (EqWF, within ±12.5° latitude), off-equatorial wind stress forcing (OffEqWF, those SWM latitudes polewards of ±12.5° latitude), and oceanic Rossby wave reflections at the western Pacific boundary were investigated. We note that, although we seek to identify the role of wind stress forcing regions on the fundamental large-scale ocean dynamics, we do not attempt to address the origin of the wind stress variability.

The OffEqWF is found to be responsible for approximately 10% (1.5%) of the overall interdecadal equatorial thermocline depth variability in the Niño-4 (Niño-3) region. While being relatively small contributions to the total equatorial region variability, the OffEqWF unfiltered (filtered) Niño-3 region anomalies are correlated at 0.86 (0.94) with the OffEqWF unfiltered (filtered) Niño-4 region anomalies with the correlation coefficient significant above the 99% (99%) confidence level. Thus, this indicates that OffEqWF equatorial region thermocline depth variability is near zonal, which is consistent with the earlier work of Hazeleger et al. (2001). This is in stark contrast to the EqWF equatorial thermocline depth variability, which produces a correlation coefficient of 0.08 (0.44) between the unfiltered (filtered) Niño-3 region anomalies and the unfiltered (filtered) Niño-4 region anomalies. Of the near-zonal equatorial thermocline depth variability produced by OffEqWF experiment, approximately 80% of the anomaly variance is transferred from Rossby waves incident upon the western boundary. Analysis of the SWM flow velocity fields in the OffEqWF experiment reveals that the remaining 20% of the equatorial anomaly variance is transmitted to the equatorial region by meridional flow in the upper layer. These results are therefore consistent with studies that identify Rossby waves incident on the western Pacific boundary as a mechanism that links the extra-equatorial Pacific Ocean with the equatorial Pacific Ocean (e.g., Capotondi and Alexander 2001; Galanti and Tziperman 2003; Wang et al. 2003; White et al. 2003; McGregor et al. 2004).

While we note that the amplitude of the OffEqWF equatorial anomalies may be underestimated by our relatively simple model experiments, since we neglect atmospheric feedbacks in the equatorial region, the significance of these results is the potential for longer-term prediction of the equatorial region’s background state. In this study we find that oscillation peaks in the Niño-3 region thermocline depth anomalies can occur up 3 yr after the OffEqWF is switched off. This suggests that some of the variability in the equatorial thermocline generated by OffEqWF can be predicted up to 3 yr ahead. We also find that thermocline depth anomalies at 120°W, 14°S (150°W, 16°N) lead thermocline depth anomalies in the Niño-3 region by approximately 22 (28) months with a correlation coefficient of 0.39 (0.23), which is significant above the 99% (99%) confidence level. The correlation between thermocline depth at 120°W, 14°S (150°W, 16°N) and those of the Niño-3 region increases to 0.58 (0.59), significant at the 98% (99%) confidence level after applying 5-yr running mean to the thermocline depth anomalies. Hence, it might therefore be possible to identify a fraction of the observed Pacific Ocean equatorial thermocline depth variability produced by OffEqWF by taking into account the amplitudes and locations of off-equatorial region Rossby waves several years prior.

It is clear from our results and the results of Hazeleger et al. (2001) that Rossby waves forced by OffEqWF do not appear to produce enough variability in the equatorial region to be the sole source of the tropical Pacific Ocean decadal variability. Further to this, our results indicate that near-equatorial wind stresses are the dominant source of variability in the control simulation equatorial thermocline depth (i.e., when wind stresses are applied over the entire basin). Again, however, it must be noted that the EqWF used in this study were generated in the CGCM. Hence, these wind stresses implicitly incorporate atmospheric feedbacks that could be generated through OffEqWF equatorial Pacific SST variations. Thus, our results give an indication of the importance of EqWF for the interdecadal modulation of tropical Pacific Ocean thermocline depth variations, but they are unable to unambiguously quantify the role of the equatorial region wind stresses. Considering that Hazeleger et al. (2001) and the present study have shown that OffEqWF, without coupling in the equatorial region, has the ability to modulate the thermocline depth of the equatorial region zonally, and that Meinen and McPhaden (2000) have shown that the equatorial region’s warm-water volume (defined as the volume of water above the thermocline) plays an important dynamical role in the oscillation of ENSO events, the following questions are raised. Is OffEqWF equatorial region thermocline depth variability, with and without coupling, enough to affect ENSO events in any way? Second, could the decadal modulations of ENSO by OffEqWF ocean dynamics account for a proportion of the apparent dominant role of EqWF? We investigate these questions in Part II of this study.

In summary, our SWM control experiment confirms that the SWM forced with BCM2.2 wind stresses captures the important ocean dynamic processes fundamental to the interdecadal SST variability produced in BCM2.2. Results from the SWM wind stress experiments demonstrate that extra-equatorial Rossby waves generated in response to off-equatorial wind stress forcing are also responsible for a small but significant fraction of the total equatorial thermocline depth variability. We have further shown through experiment II that the reflection of Rossby waves at the western boundary within approximately 20° of the equator is required to transfer virtually all of the energy generated by the OffEqWF to the equatorial region. The implications of this result for the predictability of the decadal variations of thermocline depth were investigated, and we conclude that there is potential predictability of the OffEqWF equatorial thermocline depth anomalies with lead times of up to 3 yr when taking into account the amplitudes and locations of off-equatorial region Rossby waves.