Progress in the 3-D circulation of the eastern equatorial Pacific in a climate ocean model
Introduction
The Equatorial Undercurrent (EUC) is a major feature of the tropical Pacific circulation. This strong subsurface eastward flow is tightly confined to the equator and lies within the equatorial thermocline from north of New Guinea to the coast of South America. It starts at about 200 m depth around 140°E and shoals towards the east, following the trend of the thermocline. Its cold and nutrient-rich waters are directly feeding the equatorial upwelling in the eastern Pacific, giving rise to the cold tongue and influencing the complex heat budget in this region (Bryden and Brady, 1985, Kessler et al., 1998). The EUC waters are entrained into the surface layers through upwelling and mixing and modify the sea surface temperature (SST). Hence, they may have a strong influence on the coupling between the ocean and the atmosphere (Cronin and Kessler, 2002) and on ENSO (El Niño Southern Oscillation), the dominant mode of variability in the tropics.
In the far eastern Equatorial Pacific, the termination of the EUC and its interactions with the three-dimensional circulation are complex and not well understood (Kessler, 2006). The presence of the Galapagos Islands in the equatorial band also complicates the picture (Brentnall, 1999). The cold waters upwelled from the EUC diverge poleward through Ekman transports and feed the SEC. Some of the diverging surface waters downwell at 4–5°N (4°–5°S) because of negative (positive) wind stress curl respectively, and converge back towards the EUC in the pycnocline, forming the Tropical Cells (TC) and partly compensating for the Ekman divergence (McCreary and Lu, 1994, Johnson, 2001, Sloyan et al., 2003). They participate in the mixing and the recirculation of the upper part of the EUC. Entrainment of EUC waters into the upper layers also contributes to its deceleration and its termination (Pedlosky, 1987, Pedlosky, 1988). At deeper levels, the EUC waters diverge northward or southward as they approach the South American coast, notably feeding the Peru–Chili current system (Lukas, 1986). However, the exact pathways of the EUC waters and their variability are poorly known.
Strong seasonal and interannual variations in the transport, the temperature and the position of the EUC do exist (Johnson et al., 2002). During boreal spring, the EUC strengthens and surfaces (Yu and McPhaden, 1999, Yu et al., 1997). In the central Pacific, it has been shown that the TC recirculating waters play an important role in the seasonal variability of the EUC transport (Blanke and Raynaud, 1997). Whether such a link exists in the eastern Pacific has not yet been determined. During El Niño events, the EUC weakens and may even disappear (e.g. Kessler and McPhaden, 1995, Johnson et al., 2002). In addition, it has been suggested that the large variations in transport and temperature of the EUC in 1998 had an influence on the rapid turn from El Niño to La Niña conditions in June (Izumo et al., 2002). The EUC is also part of the subtropical meridional overturning cells (e.g. McCreary and Lu, 1994) by which subducted subtropical waters can influence the equatorial temperatures at decadal timescales (Gu and Philander, 1997). At all of these timescales, the EUC variability certainly modifies the Peru current system, which is one of the most productive upwelling region in the world.
An adequate simulation of the EUC, of its 3-D circulation and of its termination is thus essential in ocean general circulation models (OGCM). It is also crucial for the ability of coupled general circulation models to simulate realistic ENSO oscillations, and decadal variability. In the global configuration ORCA2 of the OPA OGCM (Madec et al., 1998), which is used for a wide range of oceanographic and climatic studies, notably used for IPCC (Intergovernmental Panel on Climate Change) and paleoclimate simulations (Braconnot et al., 1999), the simulated EUC is very realistic (e.g. Vialard et al., 2001, Lengaigne et al., 2003). However, in the Lengaigne et al. (2003) simulations, the magnitude of the mean simulated EUC in the eastern Pacific is weaker than observed by more than 20 cm s−1 at 110°W. At the surface, the mean westward South Equatorial Current (SEC) is too strong and too deep compared to the observations. This bias can be observed in other OGCMs (e.g. Large et al., 2001). In this region where the thermocline is very shallow, an accurate simulation of the mean equatorial currents is of particular importance for an accurate simulation of the SST.
What are the physical causes of this deficiency? How can we improve the representation of the equatorial currents in this region? Part of the weakness of the OGCM simulations lies with the inaccurate representation of subgrid-scale oceanic processes, and in the parameterisations of momentum and tracer turbulent mixing. Yet, mixing is of primary importance for the EUC simulation, and it has been shown that both vertical and lateral mixing are important terms of the EUC zonal momentum equation (Bryden and Brady, 1989, Qiao and Weisberg, 1997, Wacongne, 1989, Maes et al., 1997). Maes et al. (1997) conducted sensitivity experiments by drastically decreasing the horizontal mixing in the z-coordinate OPA model. They concluded that in the eastern Pacific, the EUC is mainly decelerated by horizontal advection and vertical diffusion in its upper part, and by the horizontal diffusion in its lower part. Megann and New (2001) also investigated the effect of reducing the viscosity on the equatorial circulation in an isopycnal model. They showed that reducing the viscosity leads to a stronger and narrower EUC simulation. Lateral and vertical momentum mixing thus act as a brake for the equatorial circulation, and the weakness of the simulated EUC may be due to an unrealistically strong vertical and lateral turbulent mixing in this region. Another possible weakness of the modelled currents in the Eastern Pacific could be due to the bathymetry. For example, in many models, the Galapagos Islands do not reach the surface. It is suspected that the presence of the Galapagos Islands in the simulations may influence the equatorial circulation in the Eastern Pacific (Brentnall, 1999), and may reduce the mean surface currents in the eastern Pacific, as suggested by the study of Eden and Timmermann (2004).
The goal of this study is to improve the EUC simulation in the global configuration ORCA2 (based on a 2° Mercator mesh) of the OPA OGCM, and to better understand the sensitivity of the tropical cells and of the EUC termination to different parameterisations. We perform sensitivity experiments whereby we decrease both the vertical and lateral eddy viscosity and diffusivity coefficients used to calculate isopycnal mixing in the tropics. We also perform a simulation that includes the Galapagos Islands, and explore the impact of this on the currents and temperatures in the Equatorial Pacific Ocean. The paper is organized as follows. Section 2 describes the model and the different sensitivity experiments. Section 3 compares the mean currents of the experiments, explores the physical mechanisms involved in their differences and studies the sensitivity of the mean tropical cells and of the EUC termination to the different parameterisations. It also compares the mean currents of the experiments to observations. Section 4 examines the EUC and its link to the associated seasonal cycles of the 3-D circulation. In Section 5, the impact of the different parameterisations on the temperature fields is studied. Section 6 provides a discussion and concludes on our results.
Section snippets
Model description and experiments
The OGCM used in this study is the OPA model (Madec et al., 1998) in its global configuration ORCA2. The horizontal mesh is based on a 2° by 2° Mercator grid, and following Murray (1996), two numerical inland poles have been introduced in order to remove the North Pole singularity from the computational domain. The departure from the Mercator grid starts at 20°N, and is constructed using a series of embedded ellipses using the semi-analytical method of Madec and Imbard (1996). In addition, the
The equatorial undercurrent
The effect of reducing the background vertical turbulent mixing in the upper layers is first evaluated by comparing the zonal currents in the STD and RV experiments. The effect of reducing the lateral turbulent mixing is then evaluated by comparing the RV and RVRI experiments. Fig. 1 shows the longitude/depth diagrams of the 1993–2000 mean equatorial zonal currents for the four experiments, as well as their pairwise differences. Fig. 2 shows the latitude/depth diagrams of the 1993–2000 mean
The EUC seasonal cycle
In the eastern Pacific, the EUC transport is strongly varying at seasonal timescales, resulting from both wind variations and wave propagations. Fig. 7 shows the mean seasonal cycle of the observed and modelled STD and RVRI zonal currents at 110°W, at 35 m depth (the TAO time series at 15 and 25 m depth are more gappy). At this depth and longitude, RV and GAL zonal currents are almost identical to RVRI zonal currents and are not included for the clarity of the figure. At 110°W, mean surface
Impacts on surface temperature
In the four sensitivity experiments, the vertical and lateral mixing coefficients for tracers were also changed along with those for momentum. Therefore, we expect changes in the mean simulated temperature and salinity fields. Moreover, modifying the strength of the EUC will certainly affect the SST in the eastern Pacific and the vertical temperature profiles in the equatorial region.
Fig. 9 shows longitude–latitude maps of 1993–2000 mean equatorial temperatures for the STD experiment, the
Conclusions and discussion
In this paper, sensitivity experiments have been performed in ORCA2, the global 2° configuration of the OPA OGCM, to better understand the 3-D circulation in the eastern equatorial Pacific. They allowed us to determine the structure of the Tropical Cells in the East, defined as west of the Galapagos Islands, and appear to be useful tools to investigate the link between EUC mass transport and its associated meridional and vertical circulation which together govern its termination. They also
Acknowledgements
The authors thank W. Kessler for very helpful discussions, and two anonymous reviewers for comments that greatly improved an earlier version of this manuscript. They also wish to thank R. Morrow for a careful reading of the paper. The TAO array data were made available by the TAO Project Office. The authors finally wish to acknowledge the use of the NOAA/PMEL Ferret program for analysis and graphics in this paper. This work was supported by the CNRS (Centre National de la Recherche
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Present address: Frontier Research Center for Global Change (JAMSTEC), Yokohama, Japan.