Mechanisms controlling warm water volume interannual variations in the equatorial Pacific: diabatic versus adiabatic processes
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- Lengaigne, M., Hausmann, U., Madec, G. et al. Clim Dyn (2012) 38: 1031. doi:10.1007/s00382-011-1051-z
Variations of the volume of warm water above the thermocline in the equatorial Pacific are a good predictor of ENSO (El Niño/Southern Oscillation) and are thought to be critical for its preconditioning and development. In this study, the Warm Water Volume (WWV) interannual variability is analysed using forced general circulation model experiments and an original method for diagnosing processes responsible for WWV variations. The meridional recharge/discharge to higher latitudes drives 60% of the ENSO-related equatorial WWV variations, while diabatic processes in the eastern equatorial Pacific account for the remaining 40%. Interior meridional transport is partially compensated by western boundary transports, especially in the southern hemisphere. Diabatic equatorial WWV formation (depletions) during La Niña (El Niño) are explained by enhanced (reduced) diathermal transport through enhanced (reduced) vertical mixing and penetrating solar forcing at the 20°C isotherm depth. The respective contribution of diabatic and adiabatic processes during build-ups/depletions strongly varies from event-to-event. The WWV build-up during neutral ENSO phases (e.g. 1980–1982) is almost entirely controlled by meridional recharge, providing a text-book example for the recharge/discharge oscillator’s theory. On the other hand, diabatic processes are particularly active during the strongest La Niña events (1984, 1988, 1999), contributing to more than 70% of the WWV build-up, with heating by penetrative solar fluxes explaining as much as 30% of the total build-up due to a very shallow thermocline in the eastern Pacific. This study does not invalidate the recharge/discharge oscillator theory but rather emphasizes the importance of equatorial diabatic processes and western boundary transports in controlling WWV changes.
KeywordsEl Niño/Southern OscillationWarm water volumeEquatorial PacificWestern boundary currentsSolar penetrationVertical mixing
The El Nino/Southern Oscillation (ENSO) is the dominant mode of climate variability at interannual time scales, and impacts weather patterns across the globe (McPhaden et al. 2006a). The physical processes behind ENSO have been studied intensively over the past decades (e.g. Wang and Picaut (2004) and Collins et al. (2010) for reviews). The equatorial Warm Water Volume (WWV), generally defined as the volume of water above 20°C in the equatorial Pacific, has been suggested for long as a key ingredient in ENSO dynamics (Cane and Zebiak 1985; Wyrtki 1985; Springer et al. 1990). These studies show that an El Niño/La Niña is associated with a zonal redistribution of water along the equator, and preceded by a build-up/depletion of WWV. Jin (1997a, b) built on these findings to propose one of the leading ENSO theories: the “recharge-discharge oscillator”. In this paradigm, WWV leads the SST evolution by a quarter of a period and the cyclic nature of ENSO arises from the disequilibrium between zonal winds and WWV anomalies. Using observed measurements of WWV, Sea Surface Temperature (SST) and zonal wind, Meinen and McPhaden (2000) further showed that the recharge discharge theory was consistent with their observational analysis, with WWV preceding SST variations by 6 months to one year.
The WWV has been shown to be a useful predictor of ENSO (e.g. McPhaden et al. 2006b; Izumo et al. 2010). It is therefore of interest to understand the physical mechanisms responsible for these warm water volume changes in the equatorial strip. The recharge-discharge theory originally assigns the recharge and discharge of the equatorial Pacific to interior meridional transports between the equatorial strip and higher latitude on ENSO time-scale. WWV variations can however be driven by both anomalous horizontal transports (in the interior of the domain or along the western boundaries) and anomalous diathermal transports (through changes in vertical turbulent mixing and penetrative solar forcing along 20°C isotherm). Several studies have attempted to quantify the main contributors (i.e. horizontal and diathermal transports) to WWV changes from observations (Meinen and McPhaden 2001; Alory and Delcroix 2002; Clarke et al. 2007; Bosc and Delcroix 2008) and model outputs (Brown and Fedorov 2010), with contrasted results. Using data from the Tropical Atmosphere Ocean moorings, Meinen and McPhaden (2001) found that the build-up of WWV between the weak 1994–1995 El Niño and the onset of the strong 1997–1998 El Niño resulted primarily from an anomalous transport in the western-central Pacific. Their results however suggest that downward diathermal mass flux contributed to about half of the warm water discharge during the 1997–1998 El Niño. The importance of diathermal processes in controlling WWV changes have been further highlighted by Clarke et al. (2007) who suggest that diathermal transports cannot be neglected but are largely balanced by boundary transports in the western Pacific. Besides, although they did not explicitly quantify the role of diathermal transports, Alory and Delcroix (2002) and Bosc and Delcroix (2008) concluded that interior meridional transport alone can account for WWV changes.
However, those observational studies are based on a limited amount of data and rely on available observations to estimate geostrophic and Ekman transports. The residual from these two quantities is then attributed to diathermal transport. Important error bars on the estimated horizontal convergences make it however difficult to confidently attribute the observed imbalances between WWV changes and transport to the role of oceanic physics. Analysing Ocean General Circulation Model (OGCM) outputs is an alternative approach to quantify the contributors to WWV changes: it allows an accurate calculation of the diathermal transports and avoids potential errors of observational studies. The recent study of Brown and Fedorov (2010) based on an analysis of MOM3 outputs suggested that WWV variations due to diapycnal transport are small in the eastern Pacific and negligible in the western and central Pacific on ENSO timescales. In the present study we propose to thoroughly quantify the mutual roles of exchange with higher latitudes and formation of warm water by thermodynamic processes in the equatorial band using a different ocean model. There are three main incentives to perform this new modelling study after the one of Brown and Fedorov (2010). First, the quantification of the various processes may be dependent on the model parameterizations, resolution, forcing… A similar analysis using NEMO OGCM (Madec 2008) has therefore been performed and the sensitivity of the model results to the resolution is briefly discussed by comparing three configurations only differing in their horizontal resolution (from 2° to ¼°). Second, while Brown and Fedorov (2010) focused on the 1993–1998 period (as in Meinen and McPhaden’s (2001) study), we extend the analysis over a longer period, i.e. 1976–2004, to assess how the respective contribution of the main mechanisms controlling the WWV evolution may vary from event-to-event (El Niño/La Niña, intense/moderate events). Lastly, in all the above studies investigating WWV changes, the influence of penetrative solar radiation was not evaluated. However, several recent studies suggest that the solar penetration formulation can significantly influence ENSO properties (e.g. Strutton and Chavez 2004; Wetzel et al. 2006; Lengaigne et al. 2007). To account for this additional contributor, we apply an original method, following Walin (1982) and improved by Iudicone et al. (2008), which allows diagnosing all the processes that contribute to WWV variations including solar heating.
This paper is organised as follows. Section 2 describes the model experiment and the diagnostic used to quantify the different factors that control the evolution of the equatorial Pacific WWV. Section 3 validates modelled WWV variability against available observations. Section 4 discusses the processes that contribute to equatorial WWV variations on interannual time scales. Discussion of the results and a conclusion are respectively provided in Sects. 5 and 6.
2 Data and methods
2.1 The ocean model and forcing datasets
The numerical simulations analysed in this study are part of the DRAKKAR hierarchy of global configurations (The Drakkar group 2007) and detailed in Brodeau et al. (2010). The Ocean General Circulation Model (OGCM) is the recent version (3.0) of the NEMO (Nucleus for European Modelling of the Ocean) OGCM (Madec 2008), comprising the ocean model formerly known as OPA, coupled to the Louvain-la-Neuve sea-ice model (LIM) (Timmermann et al. 2005). The model is based on the standard primitive equations, uses a free surface formulation (Roullet and Madec 2000) and computes the density from potential temperature, salinity and pressure using the Jackett and McDougall (1995) equation of state. This OGCM has been extensively validated in uncoupled mode (e.g. Vialard et al. 2001; Lengaigne et al. 2003; Cravatte et al. 2008) and coupled mode (e.g. Lengaigne et al. 2006) in the tropics where it succeeds in reproducing the basin wide structures of currents, sea level and temperature and accurately simulates the equatorial dynamics.
In this paper, we analyze the highest resolution experiment from a series of four simulations that only differ by their oceanic horizontal resolution: two versions with coarse 2° and 1° resolution (ORCA-R2 and ORCA-R1), a higher ½° resolution (ORCA-R05) and an eddy permitting ¼° resolution (ORCA-R025). Comparisons with lower resolution experiments (ORCA-R2 and ORCA-R05) will be briefly discussed in Sect. 5. The horizontal grids of these configurations are based on Mercator grids (i.e. the same zonal and meridional grid spacing). Two numerical inland poles have been introduced in order to remove the North Pole singularity from the computational domain, so that the departure from the Mercator grid starts at 20°N. In the tropics, a local transformation is applied to ORCA-R2 in order to refine the meridional resolution, increasing up to 0.5° at the equator. The common vertical grid of the configurations used has 46 levels with a 6-m spacing at the surface increasing to 250-m in the deep ocean. Bathymetry is represented with partial steps and an enstrophy-energy-conserving momentum advection scheme is used (Barnier et al. 2006; Penduff et al. 2007; Le Sommer et al. 2009).
In these experiments, the model is forced from 1958 to 2007 with the Drakkar Forcing Set #3 (DFS3) described in detail in Brodeau et al. (2010). The starting point of DFS3 is the CORE dataset developed by Large and Yeager (2004) and used to intercompare various global ocean components of coupled system (Griffies et al. 2009). To calculate latent and sensible heat fluxes, the CORE bulk formulae algorithm is used, with surface atmospheric state variables derived form ERA40 reanalysis and ECMWF analysis after 2002 (air temperature, humidity and winds at 10 m). Corrections are performed on these selected input fields to correct temporal discontinuities and yield better agreement with some recent high quality data. Radiation fluxes are based on the CORE v1 dataset, using a corrected ISCCP-FD radiation product (Zhang et al. 2004) available from 1984. Before 1984, climatology is imposed, which leads to better results than the use of reanalyses data. Precipitation is derived from the GXGXS dataset (Large and Yeager 2004), based on the blending of several existing products from 1979. Before 1979, its climatology is imposed. No surface temperature restoring is performed and a salinity restoring, corresponding to a relaxation time scale of 33 days for 10 m, is used even under sea ice. In the following, analysis of these simulations are restricted to the 1976–2004 period.
2.2 Volume budget diagnostic
The role of mixing and forcing in the transformation of water masses is quantified using the thermodynamical method of Iudicone et al. (2008). It is a generalisation of Walin’s (1982) approach that first derived elegant relations between water mass formation and diffusive and radiative (non-advective) heat fluxes, combining heat and volume budgets for an isothermal layer. Iudicone et al. (2008) included internal buoyancy sources (to account for the solar penetrative irradiance) in the estimation of transformations. Therefore, by using this method, we are able to quantify the contribution of each component to the WWV evolution in the OGCM.
Since we can exactly diagnose volume time variations and formation of warm water by atmospheric forcing and domain boundary fluxes (Ψθ and Eθ), the formation by turbulent diffusion will be estimated in the following as a residual and we will refer to this term as “physics”.
This method has been applied to all isotherms between 16 and 24°C with an interval of 0.5°C. Computations have been made on 5-day averages model outputs. Differences between the computation of volume budgets from 5-day model outputs and from outputs at the model time-step arising from nonlinearities are negligible (Iudicone et al. 2008). Volume time derivations have been calculated using a centred difference on the 5-day data and from these results the physics have been estimated as a residual.
In Sects. 3 and 4, we define the Warm Water Volume (WWV) as the volume of water warmer than 20°C within 5° of latitude either side of the equator as it has been originally done in Meinen and McPhaden (2000). We choose to extend the meridional boundaries from the western coast to the eastern coast so that no zonal transport occurs within our domain. The sensitivity of the results to the choice of the isotherm used to define WWV is discussed in Sect. 5.
2.3 Validation datasets
The equatorial WWV calculated from the model outputs is validated against the Bureau of Meteorology Reasearch Center (BMRC) tropical Pacific subsurface temperature dataset (Smith 1995). This dataset includes XBT measurements and data from moorings of the TAO Array, from which monthly WWV timeseries are computed as by Meinen and McPhaden (2000) and made available for the period of 1980 up to present (http://www.pmel.noaa.gov/tao/elnino/wwv/). Meinen and McPhaden (2001) also used this subsurface temperature dataset along with historical hydrographic data and FSU wind product to quantify the Ekman and geostrophic transports (see Meinen and McPhaden (2001) for further details). We compare our modelled interior meridional WWV transports to this estimate as well as to the estimate provided by Bosc and Delcroix (2008) who computed their geostrophic transports using sea level anomalies derived from the AVISO altimetry product and their Ekman transport using wind stress data derived from ERS and Quickscat satellites (see Bosc and Delcroix (2008) for further details). In the following section, these observational estimates are compared to the model outputs from the higher-resolution configuration (ORCA-R025).
3 Model validation
In this simulation, the Pacific WWV, its relationship with the ENSO cycle and the meridional transport agree very well with observational estimates. In the next section, we therefore use the modelled WWV variability with some confidence to investigate the main processes controlling the WWV evolution on interannual time-scales. In the following, interannual modelled anomalies are calculated by removing the mean seasonal cycle to the raw data and applying a 16 months to 8 years band pass Hanning filter to these anomalies.
4 WWV interannual variability
4.1 Diabatic versus adiabatic processes
Standard deviation correlation with Niño34, correlation and regression at lag 0 with equatorial WWV changes for diabatic processes, meridional transports and equatorial WWV changes
4.2 Details of adiabatic processes
Standard deviation, correlation with Niño34 SST anomalies and correlation and regression at lag 0 with equatorial WWV changes for interannual anomalies of total, boundary and interior transports at 5°N, 5°S and 5°N–5°S
Total transport 5°N–5°S
Interior transport 5°N–5°S
Boundary transport 5°N–5°S
Total transport 5°N
Interior transport 5°N
Boundary transport 5°N
Total transport 5°S
Interior transport 5°S
Boundary transport 5°S
4.3 Details of diabatic processes
Standard deviation, correlation with Niño34 SST anomalies and correlation and regression at lag 0 with equatorial WWV changes for interannual variations of diabatic processes (diathermal mixing and solar penetration)
Diabatic processes East
Diabatic processes West
A similar argument explains the variations of diabatic heating through solar forcing. With the deepening of the thermocline in the eastern Pacific during an El Niño event, an exponentially smaller part of the solar radiation will reach the 20°C isotherm. Since the convergence of an exponential is an exponential, the formation of WWV by penetrating solar forcing will consequently decrease nearly exponentially with isotherm depth. Therefore during El Niño events, the anomalously small formation of WWV by solar forcing in the east will contribute to the depletion of equatorial WWV. A reverse argument allows explaining the build-up of WWV by penetrating solar forcing during La Niña events, during which the thermocline in the eastern Pacific considerably shoals.
Our results suggest that this diabatic WWV formation/depletion is particularly efficient during strong La Niña events. During the strongest La Niña events (1983/85, 1988/89 and 1998/2000), the thermocline is brought up close to the surface in the eastern Pacific allowing these diabatic processes to account for more than 70% to the WWV recharge, 1/3 being related to solar penetration and 2/3 to vertical mixing processes (Fig. 8). In contrast, build-ups occurring during almost neutral conditions in the eastern Pacific (80–82, 92–93) do not show a significant contribution of these diabatic processes.
5.1 Comparison with previous studies
Our analysis underlines the important role of diabatic exchanges through the 20°C isotherm in the modelled WWV budget. This result agrees with the observational studies of Meinen and McPhaden (2001) and Clarke et al. (2007). It however differs from the results of Bosc and Delcroix (2008) and the modelling study of Brown and Fedorov (2010), which both suggest that diathermal transport only marginally contributes to WWV evolution. The discrepancy with Bosc and Delcroix (2008) may arise from the fact that they suggested the ENSO-related western boundary current variations to be marginal contributors to the WWV evolution. This is in contradiction with the results of Clarke et al. (2007) and Ishida et al. (2008), who have shown, from observations and model outputs respectively, that western boundary layer transport cannot be neglected and oppose the interior transports. An inaccurate estimation of the western boundary currents variations could have led Bosc and Delcroix (2008) to overestimate the role of meridional transport, hence underestimating the role of physics.
Several reasons may explain the discrepancy between our modelling results and those of Brown and Fedorov (2010). First, they did not account for the solar penetrative flux in the diathermal transport calculation. Discrepancies between the two studies could also arise from mean state differences. In Brown and Fedorov (2010), the isopycnal defining the lower boundary of the WWV is about 30 m deeper in the eastern Pacific than the 20°C in the observations (see their Fig. 1) while this overestimation is only of about 10–15 m in the experiment analysed in this study. This probably results in an underestimation of the role of vertical mixing, as this process is most active close to the surface.
Regression coefficients at lag 0 for interannual variations of diabatic and adiabiatic processes with equatorial WWV changes for different three experiments differing by their horizontal resolution
5.2 Sensitivity to isotherm choice
WWV is generally defined as the volume of water warmer than 20°C within 5° of latitude on either side of the equator. The choice of the 20°C isotherm relies on the fact that it is a good proxy for the equatorial Pacific thermocline depth in the observations. In a model study, the 20°C isotherm is not necessarily a good proxy for the thermocline depth owing to biases in the simulated vertical structure. In fact, in the eastern Pacific, where diabatic processes are the most important, the 21/22°C isotherms in our experiment are likely to be a better proxy of the model thermocline and better matches the observed depth of the 20°C in the observations (Fig. 1). There is hence a need to test the sensitivity of our results to the choice of the isotherm above which WWV is calculated.
6 Summary and conclusions
Interannual variations of equatorial Pacific warm water volume (WWV, defined as the volume of water above 20°C between 5°N and 5°S) are known to be critical for ENSO prediction (e.g. Izumo et al. 2010). In this study, we investigated the mechanisms responsible for WWV interannual variations in a ¼° OGCM experiment. By applying a diagnostic first developed by Walin (1982) and extended by Iudicone et al. (2008) to model outputs, the respective roles of adiabatic processes (interior and western boundary transports) and diabatic processes (diathermal transports induced by vertical mixing and penetrative solar flux divergence) have been quantified. The good agreement of the model with observational estimates in terms of mean equatorial temperature, mean zonal currents along the equator, WWV variability and meridional transports allows us to draw conclusions on processes acting in the real world with some confidence.
Our extended period of study (1979–2004) also allows us to highlight large variations in the respective contribution of diabatic and adiabatic processes during build-ups/depletions from event to event. The WWV build-up during neutral ENSO phases (e.g. 1980–1982) is almost entirely controlled by meridional recharge, providing a textbook example for the recharge/discharge oscillator theory. During the strongest La Niña events (1984, 1988, 1999), however, diabatic processes are particularly active and contribute to more than 70% of the WWV buildup, with heating by penetrative solar fluxes explaining as much as 30% of the total build-up due to a very shallow thermocline in the eastern Pacific. These results are consistent with expectations based on simple ENSO physics: during La Niña events, the anomalously shallow thermocline favours both enhanced vertical mixing of heat and mass across the 20°C and penetrative solar flux heating at the 20°C level hence resulting in increased WWV formation. The opposite occurs during El Niño events.
This study therefore suggests that, in addition to interior meridional transport, both diabatic processes and western boundary currents act to modulate WWV variations on ENSO time-scale. The representation of diabatic processes is likely to depend on the mixing parametrization scheme used. Analysis of other OGCMs solutions with different vertical physics is required to assess the robustness of the present results. The penetrative solar flux has been for the first time identified as a significant contributor to WWV evolution and a more careful parameterization of this process is also required to provide a better quantitative estimate of its oceanic influence. Uncertainties existing in the surface shortwave estimations are likely to result in uncertainties in the penetrative solar flux contribution. In addition, light penetration is supposed to be independent of the visible light wavelength and of the particulate load of seawater in our experiments. However, chlorophyll is known to modify vertical distribution of radiant heating and its variability has been suggested to influence the ENSO-related surface layer heat budget (Strutton and Chavez 2004). Accounting for the influence of biology on solar penetrative forcing in our model experiments could hence improve and ascertain our conclusions on the processes influencing the WWV budget in the real world.
This study also advocates for a better understanding of the main processes and mechanisms controlling western boundary currents variations on ENSO time-scales. These currents are shown here to partially compensate interior transport. In the southern hemisphere, interior and boundary transports are out of phase while in the northern hemisphere, interior and boundary transports are almost in quadrature. A careful examination of the mechanisms controlling these phase relationship is beyond the scope of this paper but will be conducted in a further study. The lack of sufficient long-term in situ observations within the western boundary currents does not allow assessing the model ability in reproducing these interannual variations. International programs such as the Southwest Pacific Ocean Circulation and Climate Experiment (SPICE; Ganachaud et al. 2008) and the Northwestern Pacific Ocean Circulation and Climate Experiment (NPOCE; Wang and Dunxin 2010) will allow giving new insights about the role of western boundary currents on ENSO in observations.
The importance of diabatic processes and boundary transports and their different phase relationship with regard to WWV changes could modulate the simple picture brought by the recharge-discharge oscillator theory (Jin 1997a, b), that only account for interior meridional transport to explain WWV evolution. The varying contribution of diabatic processes during the various phases of ENSO (e.g. stronger during strong La Niñas) may also explain some of the asymmetries between El Niño and La Niña. This suggest that, in the attempt to tackle the differences found in the simulated ENSO statistics of coupled general circulation models, the representation of these processes and their respective contributions to WWV changes should be evaluated.
ML would like to thank C. Meinen and C. Bosc for making their observed meridional transport estimates available as well as M. McPhaden and F. F. Jin for valuable comments and discussions on this work. The authors would also like to thank the two anonymous reviewers of this manuscript and acknowledge the DRAKKAR project (http://www-meom.hmg.inpg.fr/Web/Projets/DRAKKAR/) for providing the oceanic simulations and the TOGA TAO Project Office for making the mooring data easily available.