Association of Convection with the 5-Day Rossby–Haurwitz Wave

Abstract

he seasonality, regionality, and nature of the association between tropical convection and the 5-day wavenumber-1 Rossby?Haurwitz wave are examined. Spectral coherences between daily outgoing longwave radiation (OLR), a proxy for convection, and 850-hPa zonal wind over the period January 1979?February 2013 are compared for different seasons and for phases of El Niño?Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO). Increased coherence, indicating a stronger association, occurs in boreal spring and autumn, with slightly reduced coherence in boreal summer and significantly reduced coherence in boreal winter. The regionality of the association is examined using lagged-regression techniques. Significant local signals in tropical convection are found over West Africa, the tropical Andes, the eastern Pacific Ocean, and the Marshall Islands. The relative phasing between the 5-day wave wind and OLR signals is in quadrature in Africa and the Marshall Islands, in phase with easterlies over the Andes, and out of phase with easterlies over the eastern Pacific. Frequency spectra of precipitation averaged over the identified local regions reveal spectral peaks in the 4?6-day range. The phasing between the large-scale wind and local convection signals suggests that the 5-day wave is actively modulating the convection around the Americas.