Northward excursion of the ITCZ across the inter-Americas during boreal summer
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The study describes northward excursions of the ITCZ across the inter-Americas during boreal summer, using observational datasets at multi-year, annual and daily time-scales. The annual cycle is felt across the region via evenly distributed SST warming in the summer hemisphere. Low-level air is propelled northward from June to October by the southern Hadley cell. Discharges from the Amazon and Orinoco Rivers spread into the Atlantic Ocean at that time of year. A N–S dipole pattern in satellite outgoing longwave radiation (OLR) is calculated and its temporal character reveals northward ITCZ excursions at 4–6 years period corresponding with a warm east Pacific and intensified diabatic heating near Panama. Composite analysis reveals evidence of a standing atmospheric Rossby wave pattern embedded in westerly flow anomalies which bifurcate on passing the Andes and slant across the inter-Americas in Jun–Oct season: lowering SLP and energizing Caribbean hurricanes. Variability at intra-seasonal time scale is underpinned by the southern jet stream and a ridge over Brazil that pushes the ITCZ northward, as seen in statistical regression and case study analysis of satellite OLR and NCEP2 reanalysis fields. The southern Hadley cell plays a prominent role, transmitting signals from winter to summer hemisphere at all time scales.
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