6.1. Factors Controlling the Cold Tongue-ITCZ Complex in the Modern Climate
 Our results indicate that the cross-equatorial front of the eastern tropical Pacific marking the transition from the cold tongue in the south to the ITCZ in the north was less intense in glacial times. In seeking to explain this result we first consider the mechanisms that maintain the cold tongue-ITCZ complex and associated front in the modern climate. This front is a prominent oceanographic feature and an integral part of the asymmetry in SST distribution about the equator, which manifests itself as a year-round presence of the warmest waters and the overlying ITCZ in the Northern Hemisphere. The origin of this asymmetry is thought to be ultimately related to the hemispheric land ratio and the continental geometry, in particular the slope of the western coast of the Americas with respect to the meridians, which causes offshore cooling by upwelling-favorable winds to be a more efficient process in the southern compared to the northern tropics [Philander et al., 1996]. However, ocean-atmosphere interactions are instrumental in further amplifying this asymmetry. Philander et al.  discuss four positive feedbacks that help accomplish this: (1) latent heat release causing enhancement of the rising motion in the convective region, which in turn helps intensify surface wind flow into the ITCZ; (2) a northward pressure-gradient force caused by the SST gradient which further reinforces the winds [Lindzen and Nigam, 1987]; (3) evaporative cooling in the cold tongue due to the strong wind flow over it; and (4), perhaps most critically, enhancement of surface cooling by stratus clouds forming over the cold waters south of the equator.
 The efficiency of these feedbacks varies seasonally as the insolation maximum crosses the equator. The seasonal insolation cycle attempts to restore symmetry about the equator by “pulling” the ITCZ southward during austral summer. In the absence of land asymmetries the solar forcing should in theory cause a Southern Hemisphere summer ITCZ to develop, producing an annual mean climatology characterized by a double or split ITCZ [Xie, 1996]. This, however, is inhibited by the symmetry-breaking land forcing, which maintains a permanent Northern Hemisphere ITCZ, even as its latitudinal position migrates seasonally. Mitchell and Wallace  examined in detail the seasonal evolution of the cold tongue-ITCZ in relation to convection strength, zonal and meridional wind stress, SST, and cloudiness. Their analysis links the annual appearance and intensification of the cold tongue with the onset of the northern summer monsoon, which strengthens the northward cross-equatorial winds during boreal spring and summer. Increased northward flow induces upwelling and surface cooling south of the equator. In turn, this cooling enhances the meridional and zonal pressure-gradient forces, further intensifying meridional and zonal wind flow, both of which promote upwelling and sustain the cold tongue. In the process the ITCZ attains its northernmost position. Stratus clouds forming south of the equator further enhance surface cooling by increasing the albedo, and appear to be instrumental in maintaining the cold tongue during its mature phase. In the presence of these positive feedbacks the cold tongue can be self-sustaining. According to Mitchell and Wallace  its demise in boreal fall is linked to a perturbation of the wind field caused by the onset of the southern summer monsoon (October–November) when convection over land migrates to the Southern Hemisphere. The northward cross-equatorial flow in the EEP weakens causing the cold tongue to contract and permitting the oceanic front to relax and the ITCZ to move southward. Nevertheless, as noted, formation of an oceanic ITCZ in the Southern Hemisphere is inhibited. Contributing factors may include orographic blocking by the Andes and preconditioning of the ocean surface with cool SSTs, both of which are unfavorable to westward expansion of convection over the ocean, as occurs in the northern tropics.
 The relative role of the meridional and zonal wind components in driving upwelling and maintaining the cold tongue-ITCZ front is not entirely understood. Mitchell and Wallace  found that cold-tongue SST is more highly correlated with meridional than zonal wind stress. A uniform northward meridional wind flow in the easternmost tropical Pacific has been shown in modeling studies to induce upwelling south of the equator, partly explaining why the cold tongue is centered south of the equator [Cane, 1979; Philander and Pacanowski, 1981]. Yet, satellite observations show that the wind field is far from uniform and is locally modified through the influence of SST on the vertical stability of the atmospheric boundary layer [Chelton et al., 2001]. Over the coldest waters of the equatorial cold tongue the winds are up to four-fold weaker than to the south and north, apparently due to stabilization of the boundary layer by the cold SSTs. Increased stability inhibits downward mixing of northward momentum from the stronger winds aloft, decelerating the surface circulation [Wallace et al., 1989; Chelton et al., 2001]. Observed interactions between SST and the meridional and zonal wind component suggest that both play important roles for coupled air-sea dynamics in this region. Inadequate representations of the wind field in coupled GCMs in combination with poorly parameterized clouds invariably lead to serious deficiencies in reproducing the modern climatologic structure, producing SSTs that are insufficiently cool south of the equator, a cold tongue that is too narrow and extends too far west, and a spurious Southern Hemisphere ITCZ with unrealistically strong convection south of the equator [Mechoso et al., 1995]. These inadequacies are likely to be important and must be considered when evaluating the ability of coupled GCMs to simulate the dynamical response of the equatorial Pacific in LGM experiments.
6.2. Factors Responsible for a Weaker Cold Tongue-ITCZ Complex During the LGM
 Given that cold tongue-ITCZ dynamics are instrumental in maintaining the equatorial SST asymmetry and possibly its westward propagation over the entire span of the tropical Pacific [Mitchell and Wallace, 1992], it is of fundamental interest to examine how the mean climatologic state had changed under LGM boundary conditions. Modeling studies bearing on this issue are divergent [e.g., Bush and Philander, 1999; Shin et al., 2003], and may be biased by the endemic model limitations outlined above. Our data show that the oceanic front separating the cold tongue and ITCZ was relaxed in glacial times compared to the Holocene. This indicates that the asymmetry about the equator, though still present, was reduced. What could have caused this change? An obvious candidate is orbital forcing through its effect on the relative strength of the northern versus the southern summer monsoon. Northern summer (July) insolation was at a minimum during the LGM, whereas southern summer (January) insolation was at a maximum. A weak northern summer monsoon would not favor the annual development and intensification of the cold tongue, while the onset of a strong southern summer monsoon would promote its seasonal demise. Insofar as monsoon circulation is key in initiating convection alternately in each hemisphere, LGM insolation ought to have a symmetry-restoring influence. An overall weakening of the southeasterly wind flow over the cold tongue and across the equator, with consequent southward shift of the ITCZ and attenuation of the front is therefore a plausible consequence of LGM insolation forcing. Northern (southern) summer insolation increased (decreased) over the deglaciation, attaining a maximum (minimum) between 12-6 kyr BP. The resulting strengthening of the northern and weakening of the southern summer monsoon [e.g., Kutzbach and Guetter, 1986] would tend to reinforce the equatorial asymmetry in the early to-middle Holocene causing a pronounced northern ITCZ bias, well-developed cold tongue, and sharp front, consistent with our observations.
 We note, however, that orbitally induced monsoon dynamics cannot explain a weaker cold tongue-ITCZ front in the LGM compared to present because LGM and LH orbital forcing were similar. Other factors must have been important. One candidate is the ENSO system. El Niño suppresses upwelling in the cold tongue and shifts convection toward the equator. El Niño-like SST distribution has been proposed as the prevailing pattern in the LGM [Koutavas et al., 2002] although this may have been as much a consequence as a cause of a more symmetric SST field about the equator. Further, it is unclear as to how such a change in the time-mean background state was related to the frequency, amplitude or duration of individual warm or cold ENSO episodes.
 The presence of extensive land ice cover during the LGM deserves serious consideration as a key player in forcing an equatorward shift of the ITCZ. Such a shift could be accomplished through increased northern trades in the presence of permanent Northern Hemisphere ice sheets, and has been persuasively modeled in the Atlantic basin [Chiang et al., 2003]. A parallel response in the Pacific, while plausible, remains to be demonstrated unequivocally, and is likely to require model advances in simulating the present-day climatology of that basin more accurately.
 Another important factor may have been a change in atmospheric water vapor content. The role of water vapor in fueling convection is two-fold: (1) through the release of latent heat as it condenses to form clouds, and (2) through greenhouse trapping of longwave radiation emitted from earth's surface [Seager et al., 2000]. A postulated decrease in atmospheric humidity during the LGM [Broecker, 1997] may have been a potent way to inhibit rising motions in convective regions, thereby necessitating a decrease in low-level convergence compensating convective updraft. Reduced cross-equatorial low level winds in the EEP are consistent with a reduction in LGM Hadley circulation, particularly of the Southern Hemisphere cell, which is consistent with evidence that the glacial tropics were drier while the subtropics were wetter [Thompson et al., 1998; Rind, 1998]. Reduced meridional winds due to weaker Hadley cell would have inhibited positive feedbacks due to upwelling and evaporative cooling in the cold tongue.
 Given the key role of low-level stratus clouds in the establishment and maintenance of the cold tongue in the modern climate [Mitchell and Wallace, 1992; Philander et al., 1996; Ma et al., 1996], the most effective mechanism for weakening the cold tongue-ITCZ complex in glacial times may have been by a reduction in low clouds over the upwelling region. The radiative forcing of low clouds is estimated at 1 W m−2 per percent change in cloud amount [Klein and Hartmann, 1993]. This compares, for example, with a forcing of 4 W m−2 for doubling of atmospheric CO2. It follows that even a small change in stratus cloud cover can have a strong radiative impact at the surface and hence on SST over regions where stratus clouds form. Indeed, stratus clouds show strong negative correlation with underlying SST in modern observations [Klein and Hartmann, 1993; Oreopoulos and Davies, 1993]. Modeling studies suggest that low-level stratus clouds are in fact a negative climate feedback [Miller, 1997; Seager et al., 2000], tending to increase in a warmer climate. Low clouds increase with increased static stability of the lower troposphere (defined as the difference in potential temperature between a level above the temperature inversion that caps the boundary layer, and the surface) [Klein and Hartmann, 1993]. While the surface temperature is coupled to the underlying SST, the temperature above the inversion is horizontally uniform and depends on surface conditions in convecting regions [Miller, 1997]. Using a simple model Miller  showed that doubled CO2 forcing increases the lower tropospheric static stability and promotes increased low clouds, due to a disproportionate increase of the temperature above the inversion compared to SST. The latter results in part from an increase in SST and moist convection in the model warm pool. In glacial times SSTs in convecting regions were reduced by ∼3°C [Lea et al., 2000; Stott et al., 2002; Visser et al., 2003] and therefore moist convection is likely to have been weaker. We infer that a decrease in low-level static stability of the glacial tropical atmosphere with attendant decrease in low cloud cover over the equatorial cold tongue and subtropical stratus regions is plausible, if not likely. We suggest that this may have been an effective cold-tongue limiting mechanism in the glacial EEP.
 The overall climatic response implied by our data for the Pacific, namely a southward shift of the ITCZ, weakening of the meridional front and contraction of the cold tongue is generally consistent with analogous climatic responses in the Atlantic basin where compelling evidence exists for a more southerly mean ITCZ latitude during glacial and stadial intervals [Peterson et al., 2000; Vink et al., 2001, Arz et al., 1998]. Moreover, a recent GCM simulation using the NCAR CCSM coupled model [Shin et al., 2003] is in general agreement with our results, showing mean LGM cooling of the EEP by 1°C and precipitation anomalies consistent with a southward shift of the tropical rainfall belt (although the model's tendency to produce a double ITCZ in the control run requires caution in interpreting its precipitation response). In this context we suggest that our proposed climatic response in the EEP is not only a plausible physical adjustment of the regional circulation to glacial boundary conditions, but also quite possibly part of a larger-scale fundamental tropical process involving coherent circum-global ITCZ dynamics across ocean basins.