A simple model for tropical-extratropical interactions along the Eastern Pacific ITCZ

From synoptic to interannual timescales, precipitation in the Intertropical Convergence Zone (ITCZ) is known to be affected by both tropical and extratropical processes, as well as the interplay between them. Within the Eastern Pacific ITCZ (EP-ITCZ) observations suggest that the poleward transport of moisture and energy is particularly dependent upon the intrusion of large-scale transient Rossby waves from the extratropics, especially during the cool season. Furthermore, as opposed to rainfall in the equatorial Western Pacific, the EP-ITCZ rainfall peaks in a region of a zonal SST minimum, implying that dynamical forcing is more important in the EP-ITCZ.

The coupling between moist convective processes and large-scale circulation is at the core of tropical-extratropical interactions; thus, model parameterizations of atmospheric convection and clouds are of fundamental importance in representing these feedbacks between EP-ITCZ and extratropical waves. Because cloud feedbacks and deep convection represent two of the largest sources of uncertainty in current predictions of weather and climate, models' representation of tropical-extratopical teleconnections remain problematic.

Theories for the relationship between convection in the EP-ITCZ and extratropical Rossby waves have emphasized the role of upper level potential vorticity (PV) intrusions associated with Rossby wave breaking. Because of the limitations of PV theory at low latitudes, here an idealized ITCZ model is used to investigate an alternative mechanism where modulations of precipitation is seen as a direct response to advection of moisture towards the ITCZ due to the barotropic component of the extratropical forcing. The ITCZ idealized model includes an active water vapor equation coupled to the dynamical variables through a simple parametrization for precipitation, allowing for a two-way feedback between precipitation and the tropical flow.

In agreement with observations, the idealized ITCZ develops a zonal maximum when the model is integrated using observational estimates for the northern winter basic state and extratropical Rossby wave scale and amplitude. Also consistent with observations, the meridional moisture gradients are larger at the northern edge of the ITCZ, and the precipitating band shifts northwards. The transient response, as well as the total ITCZ precipitation depends on both the initial ITCZ position and width, and on the amplitude of the barotropic forcing. The sensitivity to precipitation parametrization, the mechanisms controlling these ITCZ asymmetries, as well as their agreement with observations and more complex models are also discussed.