Revised and Resubmitted to Journal of Climate November 2005 Abstract




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(Kessler and Kleeman 2000; Moore and Kleeman 1999b). There is observational evidence that the seasonal march of atmospheric convection (Vecchi et al. 2004) and intraseasonal disturbances (Takayabu et al. 1999) play an important role. The development of an ENSO event is not governed by the temporal characteristic of a single coupled mode, but is rather due to the interference of many coupled modes. Non-modal growth within the coupled system is responsible for SST anomaly growth during an ENSO event, which can be influenced both by initial conditions and by stochastic forcing. Therefore, initial condition error as well as weather noise play an important role in limiting the predictability of ENSO. This view permits consideration of more variants on the role of the ocean. In particular, it permits the view that ENSO arises essentially through equatorial processes (including both wave propagation and ocean-atmosphere feedbacks) while the off-equatorial ocean behavior can be relegated to lower importance.


In between these two viewpoints is the view that ENSO is self-sustained (due to weak nonlinearity) and is periodic (Battisti 1988; Suarez and Schopf 1988; Jin 1997; Kirtman 1997). Its behavior is governed by the temporal characteristics of the single, most dominant coupled mode plus the influence of weather noise. In this scenario of ENSO, predictability comes from the oscillatory nature of the dominant mode (Chen et al. 2004), while the loss of predictability is primarily due to noise influence. Different from the stochastic ENSO theory where the noise influences the non-modal growth of the coupled system, the role of the noise in this case is to disrupt the regular oscillation of the dominant mode (Fedorov et al. 2003). In this regime, the ocean wave dynamics and reflection properties must also be sufficient to sustain the oscillation.


Pinpointing exactly where in the parameter regime ENSO resides in reality is difficult, if not impossible, given the available observations. Many of the recent studies on this issue are based on relative simple coupled model simulations and prediction experiments. Some of the evidence supporting stochastic ENSO theory are based on the finding that in the damped regime the coupled model forced by stochastic processes produces the best fit to observed ENSO statistics (e.g., Thompson and Battisti 2001; Penland et al. 2000). Other evidence comes from the finding that there is a lack of support for a continuous ENSO cycle, as depicted by the delayed oscillator theory, in the observations (Li and Clarke 1994; Kessler and McPhaden 1995; Weisberg and Wang 1997; Harrison and Vecchi 1999; Zhang and Rothstein 2000; Larkin and Harrison 2002, Kessler 2002a, Lengaigne et al. 2004). In particular, there is little observational evidence that the initiation of an ENSO event relies on the memory of previous event, though the termination of an event is generally consistent with the delayed oscillator mechanism. The break in the cycle suggests that the system is in a damped regime and the onset of ENSO relies on external influences (Kessler 2002a). Other studies dispute the stochastic hypothesis by providing evidence that seems to be more consistent with the self-sustained ENSO theory. As demonstrated in Schopf and Suarez (1988) and discussed in Jin (1997), a system with a stable, periodic oscillation in the absence of noise can become irregular with the addition of stochastic forcing, and will present statistics that appear to be more stable. Chen et al. (2004) provide retrospective forecasts of ENSO over a 148-year period and show that all prominent ENSO events can be hindcasted at lead times up to two years (Fig. 1). Such a long predictability is in better agreement with the self-sustained ENSO theory than the stochastic theory, however it remains to be tested in the crucible of an actual forecast.


b. Decadal changes in ENSO and in mean state


When the entire observational record is considered, many studies have pointed out that there is a noted change in ENSO statistics over the past 100 years, including a decadal-scale modulation in ENSO amplitude (Gu and Philander 1997) and ENSO’s phase-locking to the annual cycle (Balmaseda et al. 1994). The change that has caught the most attention took place in the mid 70s. Many have argued that the characteristics of ENSO have changed after 1977, including its predictability (e.g., Balmaseda et al. 1994; Chen et al. 1995). Model studies (Kirtman and Schopf 1998) have seemed to show that there is the possibility of a relationship between the amplitude of ENSO cycles and the limit of its predictability -- some decades may be much more predictable than others. In addition to the low-frequency secular changes of ENSO, there is also evidence for a broad-scale inter-decadal climate fluctuation over the Pacific sector with its phase transitions in 1925, 1947 and 1977 (Graham 1994; Trenberth and Hurrell 1994; Mantua et al. 1997; Minobe 1997; Zhang et al. 1997; Dettinger et al. 2000; Chao et al. 2000; and Mantua and Hare 2002; Deser et al. 2004). Some studies suggest that the low-frequency modulation of ENSO and the inter-decadal fluctuation may be linked, as there appears to be notable changes in ENSO statistics before and after the 1977 climate transition (e.g.Fedorov and Philander 2000). More recent studies have cast some doubts on the linkage between the two. Deser et al. (2004) show that the 1925 and 1947 phase transitions are not evident in ENSO indices, even though the 1977 transition is (Fig.2). Yeh and Kirtman (2004a,b) further argue, based on observational and modeling studies, that the Pacific inter-decadal fluctuation is not related to the modulation of ENSO, suggesting that the two phenomena are governed by different physical processes. We defer the discussion on the Pacific inter-decadal fluctuation to the next section. Here we review some of the ideas that have been put forward about the cause of ENSO irregularity and the dynamics of ENSO predictability.


  1. STOCHASTIC MECHANISM


Some studies argue that the stochastically driven, damped ENSO system can exhibit “decadal regime shifts” that resemble the observed ENSO amplitude modulation (Flügel and Chang 1999; Thompson and Battisti 2001; Kleeman et al. 2003; Yeh et al. 2004; Flügel et al. 2004), and thus propose that the stochastic ENSO mechanism should be regarded as a null hypothesis for decadal modulation of ENSO.


Kirtman et al. (2004) used a new coupled general circulation model (CGCM) coupling strategy, called an interactive ensemble procedure (Kirtman and Shukla 2002), to reduce the impact of internal atmospheric variability on coupled ENSO dynamics. This approach allows a test of the stochastic ENSO hypothesis within a comprehensive coupled system framework. Kirtman et al. (2004) reported that in their CGCM there are well-defined areas in the western tropical Pacific where the coupled variability cannot be explained by the stochastic ENSO theory. It implies that in western tropical Pacific the ocean-atmosphere feedback may be so strong that nonlinearity must be taken into consideration.


2) MEAN STATE CHANGES


A number of recent studies (Timmermann et al. 1999; An and Jin 2000; Fedorov and Philander 2000; Fedorov et al. 2003; Urban et al. 2000; Wang and An 2001, 2002) propose that the low-frequency changes in ENSO can be attributed to changes in the mean state of the coupled system. An implicit assumption built into this proposal is that ENSO behavior is, to a large extent, controlled by the most unstable coupled mode whose characteristics depend sensitively on the mean state of the coupled system.


Using a simple coupled ocean-atmosphere model, Fedorov and Philander (2000) explored the properteries of the most unstable coupled mode in the parameter space spanned by the mean depth of thermocline depth H and the mean strength of the trade wind stresses . Their finding suggests that with a moderate change in H and/or , the dominant coupled mode can migrate from an SST-mode regime where the entrainment of cold water across a shallow thermocline is a controlling factor in regulating SST to a delayed-oscillator-mode regime where thermocline fluctuations induced by equatorially trapped waves are a major factor in controlling SST changes (Fig. 3). The SST-type modes reside in a state where the mean thermocline depth is shallow, so that entrainment is effective in changing SST, whereas the delayed-oscillator-type modes reside in a state where the mean thermocline is deep, so that SST changes require vertical movements of the thermocline. Fedorov and Philander suggest that the present-day mean state of the tropical Pacific will put ENSO in an area close to neutral stability. They went on to further argue that the change in ENSO statistics during the 1980s and 1990s can be attributed to the relatively warm conditions in the eastern tropical Pacific which is caused by a weakening in the trade winds (a small decrease in ) and deepening in the thermocline depth (a small increase in H). According to their stability analysis, such a small change in the mean state is sufficient to alter the structure of the most unstable mode, causing it to move closer to the delayed-oscillator type mode which tends to have longer period (5 years) than that (3 years) of the SST-type mode. Kirtman and Schopf (1998) argued that the decadal modulation in ENSO predictability can be attributed to the fact that ENSO resides near the neutral stability boundary and external stochastic processes can move it above or below the stability boundary, causing changes in its predictability. The predictability is shorter in the decades when ENSO is below the neutral stability boundary than in the other decades when ENSO is above the neutral stability boundary, because the cycle is damped in the former and self-sustained in the latter.


3) ANTHROPOGENIC FORCING


Timmermann et al. (1999) and Collins et al. (2000) presented modeling evidence, based on comprehensive coupled general circulation model simulations, that anthropogenic greenhouse warming can have an influence on stability of the coupled system through gradual change in the upper ocean stratification. In particular, these studies noted that greenhouse gas induced warming can lead to an increase in mean stratification of the upper equatorial ocean, which in turn leads to an increase in both the amplitude and frequency of ENSO in the models. This finding contradicts earlier modeling studies that suggest little change in ENSO behavior (Tett 1995) or even weakened ENSO variability in response to anthropogenic greenhouse warming (Knutson and Manabe 1994, Knutson et al. 1997). Knutson et al. attributed the weakened variability to CO2-induced changes in the model’s time-mean basic state, including a reduced time-mean zonal SST gradient. They further note that the multidecadal amplitude modulations of ENSO become more pronounced with increased CO2. Insufficient ocean model resolution in the earlier modeling studies was partly blamed for the different model response (Timmermann et al. 1999). The results of the recent model studies are also consistent with a coupled model sensitivity study by Meehl et al. (2001) which shows that a lower vertical diffusivity yields a greater ENSO variability, because of the sharper and less diffused thermocline.


4) NONLINEAR DYNAMICS


The view that ENSO is inherently nonlinear and chaotic has led to investigations that the decadal variation in ENSO may arise from a nonlinear, internal source. A number of investigators (Burgers and Stephenson 1999; Jin et al. 2003; An and Jin 2004) point out the fact that the probability distribution function (PDF) of ENSO related SST anomalies is skewed to the positive value, which can not be explained by the linear theory. In particular, there seems to be more warm ENSO events than cold events in the past two decades, including the two most intense El Niño episodes in 1982/83 and 1997/98 (Fedorov and Philander 2000). Timmermann et al. (2003), based on a theoretical ENSO model, proposed a nonlinear bursting mechanism for the occurrence of extreme El Niño events and suggested that the nonlinearities in the tropical heat budget can contribute to El Niño decadal amplitude changes. An and Jin (2004) analyzed the upper ocean heat budget of the NCEP ocean data assimilation (Ji et al. 1995; Behringer et al. 1998; Vossepoel and Behringer 2000) and the simple ocean data assimilation (SODA) product (Carton et al. 2000) and found that nonlinear vertical and zonal oceanic advection act to enhance the amplitude of warm ENSO episodes and reduce the amplitude of cold ENSO episodes, resulting the warm-cold asymmetry. They further argued that the nonlinear heating induced changes in ENSO amplitude have a rectification effect on the climate mean state (Jin et al. 2003). Rodgers et al. (2004) have examined decadal variability in a coupled GCM, and find that the decadal changes in the mean state may not be discernable from residuals in the statistics. Schopf and Burgman (2005) present a pure kinematic argument to explain ENSO asymmetry and residual effect on the mean state without invoking changes in stability of coupled system. In their mechanism, if El Niño can be viewed as an oscillating front between warm and cold water, an increase in the excursion of the front (the "strengthening" of El Niño) results in a change to the Eulerian time mean. But at any single instant, the state is characterized by the same front separating the same warm and cold water. This study raises an interesting question: Do changes in the amplitude of ENSO necessarily imply changes in the stability of coupled system?


5) OBSERVATIONAL EVIDENCE


Parallel to the theoretical debate on whether there is a linkage between changes in ENSO statistics and changes in mean state, there is a lack of agreement from observational analyses on this issue. Some studies report findings that support the modeling and theoretical results, while others do not. For example, Urban et al. (2000) presented an analysis based on a 155-year ENSO reconstruction from a central tropical Pacific coral and found that there is a tendency for ENSO frequency to shift from a shorter-period (less than 3 years) to a longer period as the mean state becomes warmer from the mid-late nineteenth century to the present. Solow and Huppert (2003), on the other hand, performed a test on the evolutionary spectral analysis of a century-long sea-level pressure time series at Darwin, as well as the Niño-3 SST time series, and found that local variations in ENSO over the past century are not inconsistent with overall stationarity. Cobb et al. (2003) pieced together a fossil-coral record over the past 1,100 years and found no consistent evidence that variations in ENSO statistics are linked to changes in mean climate conditions in the tropical Pacific. Some of the inconsistencies may reflect the fact that the dominant fluctuation patterns at decadal or longer time scales are different from those associated with the low-frequency modulation of ENSO, as noted recently by Deser et al. (2004) and Yeh and Kirtman (2004a,b). Clearly, there is a need to continue scrutinizing the existing instrumental and paleo-proxy data sets for further evidence of possible relationship between variations in ENSO and changes in the mean state. While the search for this relationship continues, there is considerable interest in developing a thorough understanding of the role of the tropical ocean in interdecadal or longer-term fluctuations of the coupled mean state, as these changes can have an impact on global climate, even if they are not directly linked to ENSO. In the following section, we review some of key aspects concerning the role of the ocean in low-frequency changes of tropical mean state.


c. Role of the ocean in tropical mean climate


1) Subtropical Cells and Equatorial Thermocline Variability


Two of the key features of the tropical oceans that hold primary interest for climate study are the sharp thermocline and the "cold tongue" of surface waters along the equator at eastern side of the ocean basins. This latter feature is prominent in the Pacific, and seasonally apparent in the Atlantic. It is a manifestation of the surfacing of the thermocline in the east. The structure of the thermocline affects the sensitivity of the surface temperature to the subsurface ocean variability, which in turn affects the coupling between the ocean and atmosphere. Simple models for ENSO have shown sensitivity to the sharpness and tilt of the thermocline, and a growing body of literature has developed around the issue of how the mean conditions of the thermocline may be altered on longer time scales.


The equatorial thermocline is now understood to connect to the subtropical thermocline through a circulation system known as the subtropical cells (STCs) or shallow overturning circulation. The "canonical'' zonal-average picture of the STC circulation shows that subducted water from the subtropical gyres flows towards the equator at depths of about 100-400m, feeds into the Equatorial Undercurrent (EUC), upwells in the eastern equatorial region and returns to the subtropics in the surface layer. Sea-surface temperature distributions reflect this circulation pattern: the SST is much colder in the eastern equatorial region where upwelling prevails than in the western basin. This circulation system can respond to changes in atmospheric conditions not only within the tropics, but also in the extratropics.


This overturning flow occurs in the presence of a complex upper ocean circulation system. In the tropical Pacific and Atlantic the circulation is characterized by alternating bands of eastward and westward flowing currents: a narrow eastward surface current — the North Equatorial Countercurrent (NECC) — between 3ºN and 10ºN, the two broad westward surface currents — the North and South Equatorial Currents — to the north of 10ºN and to the south of 3ºN, respectively, and a swift subsurface jet — the EUC — centered at the equator over a depth of approximately 150 m and a width of approximately 300 km.


The fundamental dynamics of the STC circulation was set forth by McCreary and Lu (1994). Their theoretical and modeling studies demonstrated the connection between the ventilated thermocline theory (Luyten et al. 1983), used to describe the sub-tropical thermocline gyres, and the supply of water to the equatorial cold tongue. They provide a description of a shallow circulation driven by wind stress curl and a specification of the surface density field. Since the net Ekman flow across the equatorward side of the subtropical gyres is poleward, the subsurface geostrophic flow must have a net flow toward the equator. Together with the work of Lu et al. (1998), these models point out consequences of the beta effect and the large scale wind curl as causing a net convergence of relatively cold water into the equatorial belt. In these theories, the subducted water is presumed to flow in an essentially geostrophic, adiabatic fashion, with water mass transformation only possible when the flow is in contact with the near surface layers. Therefore, the net convergence of cold subducted water would imply that this water must emerge at the surface within the tropics. With the mean easterly Trade winds, the necessary upwelling conditions apply at the eastern end of the equator, along the American coasts, and in a few isolated regions such as the Peru upwelling and the Costa Rican dome (Umatani and Yamagata 1991; Kessler 2002b). (McCreary and Lu provide some intriguing examples of how the circuit of cold water can be completed off the equator even if no equatorial winds are available to cause upwelling). Some leakage of northern hemisphere thermocline water through the Indonesian throughflow is accounted for in the Lu et al work, and has been quantified in diagnostic models (Blanke et al. 2001).


Further studies with more complex ocean circulation models have investigated the source waters of the equatorial undercurrent or equatorial cold tongue (Rothstein et al.1998, Harper 2000; Huang and Liu1999; Malanotte-Rizzoli et al. 2000; Rodgers et al.2003; Fukumori et al. 2004), and are largely in agreement that the source waters of the equatorial undercurrent and equatorial cold tongue lie well within the subtropical gyres. Observations support the canonical view of the STCs (Johnson and McPhaden 1999).


With the STC circulation connecting the sub-tropics with the equator, it became a question of interest as to whether anomalies could propagate from the subtropics to the equator and thereby influenced ENSO. Small changes in the transport of properties along the STCs could lead to changes in the equatorial thermocline and, as dicsussed previously, alter the stability properties of the ENSO system.


From the climate perspective, the ventilated thermocline theory is limited by its focus on the ocean. To make a description of the STC flow, both the wind stress and the surface density patterns must be specified. Altering either will induce changes to the STC that may be reflected in a changed structure in the equatorial thermocline. Early work examined the subduction of temperature anomalies onto the mean STC circulation (Gu and Philander 1997). This concept drew upon the observational work by Deser et al. (1996), showing extratropical temperature anomalies appearing to propogate equatorward. Further modeling studies elucidated the concept that anomalies that have a signature in potential density will not simply flow along the isopycnals, but will propagate as planetary waves via undulations of isopycnals (Lysne et al. 1997; Huang and Pedlosky 1999; Liu 1999a, b). An exception to this behavior is found if the temperature anomalies are compensated by salinity, so that they remain on isopycnal surfaces, forming “spiciness” anomalies. The “spiciness” anomalies may be advected by the mean STC circulation (Schneider et al. 1999; Schneider 2000; Zhang et al. 2001; Yeager and Large 2004). A second mechanism for STC-induced variability hypothesizes that changes in the wind stress curl patterns alter the strength of the STCs. They may undergo decadal variations which cause changes in the equatorial thermocline by affecting either the upwelling rate or the relative supply of colder or warmer water (Kleeman et al. 1999; Klinger et al. 2002; Nonaka et al. 2002). Figure 4 provides a conceptual sketch of a zonally averaged view of the mechanisms proposed by Gu and Philander (1997) and Kleeman et al. (1999). Both mechanisms have been extensively studied in models of the Pacific, although no resolution has been reached. Some studies suggest that oceanic teleconnections are not efficient enough to cause modulations in the tropics (Schneider et al. 1999; Hazeleger et al. 2001) and others suggest that oceanic teleconnections seem to be working in the Southern Hemisphere (Chang et al. 2001; Giese et al. 2001; Bratcher and Giese 2002).


With the TAO array and the extensive measurement programs in place for ENSO prediction, McPhaden and co-workers have been able to quantify the changes in the STCs over the past several decades. Zhang et al. (1999) noted that the STC strength had been spinning down over the past 30 years, and that this could cause a warming in the eastern Pacific. More recent measurements (McPhaden and Zhang 2004) indicate that the circulation has rebounded. The conclusions of McPhaden and Zhang are based on the understanding that the interior cross-gyre flow is geostrophic and large scale. What remains to be examined is whether there is compensation for the equatorward flow in the western boundary current systems.


The work on STCs has largely focused on understanding the mechanisms for its variability and the understanding of its dynamics. It has been somewhat taken for granted that the changes induced in the equatorial structure will lead to a modification in ENSO, altering its amplitude, predictability or frequency. Actual demonstration of this connection remains to be made or observed. One counter suggestion has been made using model studies. Yeh and Kirtman (2004b) obtained model results that exhibit (at least) two modes of decadal variability in the Pacific. One has a high pattern correlation with the observed decadal changes seen by McPhaden and Zhang (2004) (Fig. 5 and Fig. 6), but is not correlated with changes in ENSO variance over time. A second mode correlates well with the amplitude of ENSO (Fig. 6), but has a spatial pattern more akin to the residual modes described by Rodgers et al. (2004). One possible view of the situation is that the low frequency variations in the STCs generate a broad tropical response in the upper ocean, but these do not lead to significant changes in ENSO. At the same time decadal variation of ENSO may arise as a stochastic process (as in Flügel et al. 2004, and other works above) that exhibits the residual effect described by Schopf and Burgman (2005).


2) Maintenance of the Equatorial Thermocline


Continuing beyond the consideration of decadal changes, the examination of the role of the tropical ocean in long-term mean climate requires further study of STCs and their details. Investigators have begun to examine the basic structure of the shallow overturning and its relation to the general circulation of the atmosphere. If the concept that the STCs form an isolated cell in the upper ocean is correct, then these circulations connect the two largest heat sources and sinks in the Pacific - the equatorial cold tongue and the region of the Kuroshio off Japan. The heat transport by the shallow circulations has been examined by Held (2001), and Klinger and Marotzke (2000). Water in the mid-latitudes approaches thermal equilibrium with the atmosphere before being subducted and sequestered away from the heat fluxes at the surface. This water is then brought back to the surface at the equator. The circuit of flow from cool mid-latitudes to the equator acts as a strong conductor, linking the equator to the region of subduction. Boccaletti et al. (2004) frame the problem in a slightly different fashion by asking the question how the heat transport by STCs can work as a constraint for the equatorial thermocline depth, but also find that the connection between the heat gain in the ocean in the equatorial cold tongue and its return to the atmosphere in the subtropics is strong. This implies that a large component of the net heat transport by the ocean is accomplished by the STCs.


In the simplest view of the STCs as ventilated thermocline pathways complemented by surface poleward flow, the subsurface flow was envisioned as adiabatic and geostrophic, leading the concept of "potential vorticity pathways". If the flow were indeed adiabatic, then some part of the equatorial cold tongue should appear significantly colder than it is. Closer examination of model results indicates that substantial diapycnal transformation is occurring along the flow (Rodgers et al. 2003). The model studies of Boccaletti et al. (2004) demonstrate that increasing the background thermal diffusivity within the ocean has a strong impact on the heat transport as well as the overall structure of the cold tongue. The McCreary and Lu (1994) view of the STC envisions water beneath the thermocline that does not surface in the tropics. It must connect relatively cold waters from the sub-polar gyres from one hemisphere to the other. Such a layer need not transport any heat, but can be in simple thermal equilibrium with the atmosphere in the higher latitudes. In the presence of diapycnal mixing, however, this water will cool the lower limb of the STC, having the practical effect of connecting the equatorial surface water to latitudes further poleward than the adiabatic STC theory would indicate. Much of this mixing occurs in the highly-sheared regions above and below the equatorial undercurrent, but significant diapycnal mixing is also indicated along the pathways between subduction and the equatorial region where the flow returns to the surface. The physics of mixing in models is still not reliable enough to quantify the role of along-path mixing, and the wide variety of model representations of the role of the STCs testifies to the sensitivity of models to their vertical diffusivities. A major model development task in CLIVAR is to improve and verify these subtle, but far-reaching, parameterizations.


One of the most important and vigorous regions of diapycnal mixing in the open ocean is in and around the equatorial undercurrent. Upwelling transport into the upper layer of the east-central Pacific appears to balance the Ekman divergence across 5° latitude, about 30-50 Sv. Part of this transport flows eastward along upward-sloping isopycnals, but there is a significant diapycnal conversion of water mass properties, in which thermocline water flowing into the region at temperatures of 18°-24°C is warmed to flow out meridionally at temperatures 5°C or so higher, a heat gain on the order of 50-80 W/m2 (Bryden and Brady (1989); Weisberg and Qiao 2000; Meinen et al. 2001). This entrainment occurs as the surface gains heat through solar shortwave radiation that is spread downward into the upper thermocline by turbulent mixing. Although the turbulent mixing processes have been the subject of many studies (see Gregg 1998 for a review) the details and mechanisms of these, and how they may be represented in ocean circulation models are not well understood.


The picture of the STC circulation is further complicated by an asymmetry between the northern and southern hemispheres and the involvement of low latitude western boundary currents in the circuit. The bias of the atmospheric inter-tropical convergence zone (ITCZ) to lie in the northern hemisphere imprints a narrow band of anomalous potential vorticity across the Pacific around 10°N. This forms a "PV barrier" that causes the equatorward flow of the thermocline water to divert towards the western boundary. In the southern hemisphere, no such barrier exists, and the flow to the equator is potentially much more direct (see, for example, Johnson and McPhaden 1999, Nonaka and Takeuchi 2001).


Detailed surveys of the processes within the low latitude western boundary currents have not been made, particularly an examination of whether and how significant diapycnal mixing might occur. An intriguing feature of the western boundary current is that the bifurcation from southward to northward flow is not barotropic, but tilts with depth, as first noted by Reid and Arthur (1975). As noted by Pedlosky (1996), a complete theory for the western boundary current bifurcation is not yet in hand. McCreary and Lu (1994) offer a discussion of the western boundary bifurcation in a 2.5 layer reduced gravity model, but a more complete treatment is lacking. Geostrophic calculations based on historical data indicate that in the mean the bifurcation shifts from about 13°N near the surface to at least 18°N at depths around 1000 m  in the North Pacific (Qu et al., 1997, 1999) and from about 15°S to at least 20°S in the South Pacific (Qu and Lindstrom 2002). This tilt varies with time.  On seasonal time scales, the NEC has a northernmost bifurcation in winter and a southernmost bifurcation in summer Qu and Lukas (2003).  Although there are no sufficient data for analyzing the interannual variation, results from high-resolution GCM suggest that the NEC has a northernmost bifurcation during El Niño years and a southernmost bifurcation during La Nina years (Kim et al. 2004). The variation in the South Pacific is not known at this point. While this point may seem a detail in our understanding of the boundary current, the theory of McCreary and Lu relies upon separation of equatorward and poleward flow at the western boundary. Since the subsurface limb of the STC is the important component of the flow for this theory, and since this boundary can change by 5° of latitude, a more complete understanding of this behavior seems in order.


d. Summary

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