A relevant climate feature of the Intra-Americas Sea (IAS) is the low-level jet (IALLJ)dominating the IAS circulation, both in summer and winter; and yet it is practicallyunknown with regard to its nature, structure, interactions with mid-latitude and tropi-cal phenomena, and its role in regional weather and climate. This paper updates IALLJcurrent knowledge and its contribution to IAS circulation–precipitation patterns andpresents recent findings about the IALLJ based on first in situ observations duringPhase 3 of the Experimento Clim´atico en las Albercas de Agua C´alida (ECAC), an in-ternational field campaign to study IALLJ dynamics during July 2001. Nonhydrostaticfifth-generation Pennsylvania State University National Center for Atmospheric Re-search Mesoscale Model (MM5) simulations were compared with observations andreanalysis. Large-scale circulation patterns of the IALLJ northern hemisphere summerand winter components suggest that trades, and so the IALLJ, are responding to land– ocean thermal contrasts during the summer season of each continent. The IALLJ is anatural component of the American monsoons as a result of the continent’s approxi-mate north–south land distribution. During warm (cold) El Ni ˜ no–Southern Oscillationphases, winds associated with the IALLJ core (IALLJC) are stronger (weaker) than nor-mal, so precipitation anomalies are positive (negative) in the western Caribbean nearCentral America and negative (positive) in the central IAS. During the ECAC Phase 3,strong surface winds associated with the IALLJ induced upwelling, cooling down thesea surface temperature by 1–2 ◦C. The atmospheric mixed layer height reached 1 kmnearthesurfacewindmaximumbelowtheIALLJC.Observationsindicatethatprimarywater vapor advection takes place in a shallow layer between the IALLJC and the oceansurface. Latent heat flux peaked below the IALLJC. Neither the reanalysis nor MM5captured the observed thermodynamic and kinematic IALLJ structure. So far, IALLJknowledge is based on either dynamically initialized data or simulations of global (re-gional) models, which implies that a more systematic and scientific approach is neededto improve it. The Intra-Americas Study of Climate Processes is a great regional oppor-tunity to address trough field work, modeling, and process studies, many of the IALLJunknown features.
