The precipitation and the SSTs are clearly correlated to a certain degree in the east Pacific, with heavier precipitation occurring over warmer waters. However, the correlation is not perfect, as the strongest northern hemisphere precipitation occurs while the underlying ocean cools from its May maximum (Magaña, Amador, and Medina, 1999; see also Figure 7.). This behavior is similar to that observed in the west Pacific warm pool (Waliser and Graham, 1993). Furthermore, ITCZ convection develops south of the warmest summertime SSTs, which tend to occur close to the Mexican coast, forming the east Pacific warm pool. Interestingly, the maximum in precipitation appears to coincide with a broad maximum in the difference between northern and southern hemisphere SSTs. This lends support to the assertion of Lindzen and Nigam (1987) that the gradient in SST plays more of a role than the absolute SST value with regard to convection and precipitation.
A multitude of explanations for the ITCZ have appeared in the literature over the past 30 years, most of which remain untested. Charney (1971) hypothesized that the off-equatorial location of the ITCZ results from a balance between Ekman pumping, the efficiency of which increases away from the equator, and moisture availability, which increases toward the equator in Charney's idealized model. The east Pacific doesn't fit into this picture, as the SST, which governs moisture availability in Charney's view, doesn't reach a maximum on the equator. If Charney's model were correct, the ITCZ would occur near to or north of the Mexican coast, as the warmest waters in the east Pacific are generally found along this coastline.
Holton, Wallace, and Young (1971) dispensed with the moisture availability criterion and examined the efficiency of Ekman pumping in non-steady flows. This efficiency peaks at a latitude which is determined by the characteristic oscillation frequency of the flow. They then hypothesized that the dominant frequency of oscillation was associated with the periodic modulation of the ITCZ by easterly waves. This yielded reasonable values for the latitude of the ITCZ.
Tomas, Holton, and Webster (1999) present an alternate point of view, pointing out that the cross-equatorial flow is actually driven by a latitudinal pressure gradient. The flow they compute has the character of a relaxation to geostrophic balance between the zonal flow and the pressure gradient. Convergence develops near the latitude at which this balance occurs. Tomas, Holton, and Webster hypothesize that this convergence defines the location of the eastern Pacific ITCZ, which is the rising branch of the cross-equatorial Hadley circulation.
The hypothesis of Tomas, Holton, and Webster can be reconciled with Lindzen and Nigam's if the surface pressure gradient is caused solely by temperature perturbations in the marine layer associated with the SST distribution. However, if at least part of the pressure perturbation is associated with free tropospheric temperature perturbations resulting from quasi-balanced dynamics or from the deep convection itself, then the two theories are distinct.
Heating due to latent heat release in deep convection plays a crucial role in ITCZ theory. As Hess, Battisti, and Rasch (1993) and Numaguti (1993) found, the location of the ITCZ in an atmospheric general circulation model depends heavily on the type of cumulus parameterization used. The Kuo scheme yields off-equatorial ITCZs even when there is an equatorial maximum in SST, whereas moist convective adjustment generally places the ITCZ over the SST maximum. (See also Manabe, Hahn, and Holloway, 1974 and Goswami, Shukla, Schneider, and Sud, 1984, for further confirmation of this characteristic of moist convective adjustment.)
Waliser and Somerville (1994) found in an analytical model that a zonal strip of heating of finite meridional width produces the greatest low level convergence some distance off the equator. They invoked this as an explanation for the observed tendency of the ITCZ to occur off the equator even when there is an equatorial maximum in SST. They also found that a numerical model using Emanuel's (1991) cumulus parameterization (a mass flux scheme) predicts an off-equatorial ITCZ as well. However, it is not clear that the same mechanisms are acting in their analytical and numerical models.
Ferreira and Schubert (1997) suggested that the fluctuations in the ITCZ and the associated generation of tropical cyclones is due to barotropic instability in the ITCZ. This instability could occur if ITCZ convection is strong and persistent enough to develop a lower tropospheric reversal in the north-south potential vorticity gradient. Whether this happens in the east Pacific is not known.
Emanuel (1995) and Raymond (1995, 1997) took a different approach to the forcing of cumulus convection in their expositions of boundary layer quasi-equilibrium (BLQ). In this theory, the surface flux of moist entropy is thought to govern the net mass export from the subcloud layer into moist convection. The efficiency with which surface fluxes create mass transports depends on the ratio of updraft to downdraft mass flux at the top of the boundary layer, and on the difference between the equivalent potential temperatures of updraft and downdraft air. These factors in turn depend on cloud dynamics and microphysics and are most likely determined by environmental profiles of temperature, humidity, and wind. In particular, higher relative humidities are thought to suppress downdrafts and to decrease the difference between updraft and downdraft equivalent potential temperatures, thus resulting in greater updraft mass flux for a given surface entropy flux. This in turn results in more rainfall.
In seeking an explanation for the ITCZ in the context of BLQ, one needs to look for regions with (1) higher surface entropy fluxes or (2) higher ratios of mass flux to surface entropy flux. The first condition is supported by higher SSTs and stronger boundary layer winds. The second condition is presumably the result of a moister troposphere, though other factors such as variabilities in convective available potential energy, wind shear, microphysical effects of aerosols, etc., cannot be discounted at this point.
Raymond (1999), in a model which follows the principles of BLQ, found that cloud-radiation interactions could create a cross-equatorial Hadley circulation even in the absence of latitudinal gradients of SST or solar input. This circulation, which was predicted to be at least as strong as the zonally averaged Hadley circulation at the solstice, derives from the absorption by mid-level stratiform cloudiness associated with deep convection of longwave radiation emitted from the ocean surface. The resulting heating drives ascent in the cloudy region, which moistens the atmosphere, resulting in more mass flux and rainfall per unit surface entropy flux. Interestingly, surface entropy fluxes are not particularly enhanced in the vicinity of the simulated ITCZ, which means that the enhancement of rainfall in the ITCZ is a consequence of the tropospheric moistening alone. However, since the moistening is also a consequence of the ITCZ convection, the entire phenomenon must be thought of as an instability. The reasons why this instability manifests itself as a cross-equatorial Hadley circulation are still under investigation.
When significant SST gradients are present, the above radiative process plays less of a role in Raymond's (1999) model, as the enhancement of surface entropy fluxes by the increased SST serves to localize convection over higher SSTs. However, even in these conditions the cloud-radiation interactions can cause significant modifications to the location of the ITCZ.
Transient disturbances may also play a critical role in determining the character and strength of the ITCZ. Raymond, López, and López (1998) point out that the ITCZ in the east Pacific undergoes strong fluctuations in intensity on periods of a few days (see Figure 4). These fluctuations are generally associated with the passage of easterly waves, many of which are intensifying into tropical storms. During these periods of intensification, the strongest convection of the ITCZ near sometimes moves northward into the Gulf of Tehuantepec, leaving little convection to the south of . Convection to the west of this region can also become suppressed at this time (Zehnder, Powell, and Ropp, 1999).
BLQ provides several ways in which the deep convection and rainfall in the ITCZ could be modulated by external phenomena such as easterly waves. These all arise from either their influence on surface entropy fluxes, most likely through control of the boundary layer windspeed, or via their effects on mid-tropospheric moisture. Limited observations from the TEXMEX (Tropical Experiment in Mexico) project suggest that increased boundary layer windspeed is indeed correlated with deep convection in the east Pacific (Raymond, López, and López, 1998), as it is in the west Pacific warm pool (Raymond, 1995, 1997; Zhang, 1996). Zehnder (1991), Mozer and Zehnder (1996), and Farfan and Zehnder (1997) describe how this might occur via the impingement of African easterly waves on Central American topography. During such active phases, the ITCZ is observed to move to the north, perhaps as a consequence of the near-coastal forcing of convection resulting from orographically generated low level jets and the associated enhancement of surface fluxes.
In addition, deep convection in the east Pacific ITCZ is subject to a strong diurnal cycle, as is other tropical deep convection (Hendon and Woodberry, 1993). The evolution of convection over tropical oceans has yet to be intensively studied through the full diurnal cycle. Characterizing ITCZ convection by its behavior during only a small part of this cycle could therefore be quite deceptive.