Mountain Tropical Rainfall: Evidence of Phase-Locking between the Diurnal, Annual and Interannual Cycles in the Andes of Colombia

Abstract:

We study the diurnal cycle of rainfall using 15-min data from 15 raingauges located along two neighboring valleys of the Central Andes of Colombia, quantifying its long-term behavior (1998-2008), and its variability at annual and interannual (ENSO) timescales. The long-term mean annual cycle is characterized by the well-known bimodal regime, with two seasons of high rainfall during April-May and September-November, owing to the meridional oscillation of the Inter-Tropical Convergence Zone, plus other concomitant mechanisms. The long-term mean diurnal cycle of rainfall exhibits a seemingly bimodal behavior, with peak values during the afternoon (15:00 to 17:00 LST), another peak around midnight (23:00 to 02:00 LST), and minimum values from 07:00 to 10:00 LST. But a detailed examination shows that the diurnal cycle is mostly unimodal throughout the year, with a seasonally-dependent shifting phase. From May to September rainfall maximum occurs between 22:00 and 02:00 LST, while from October to April it occurs from 13:00 to 16:00 LST (see Figure 1). During short transition periods, a bimodal diurnal cycle can be observed. Our results evidence the existence of phase-locking between the diurnal and annual cycles of rainfall. At inter-annual (ENSO) timescales, the amplitude of the diurnal cycle is reduced (augmented) during El Niño (La Niña), and the phase of the diurnal cycle remains shifting between the same hours and seasons during normal, El Niño and La Niña years, which in turn evidences phase-locking between the diurnal, annual and interannual cycles of rainfall. We provide a detailed explanation of the physical mechanisms involved in such phase-locking among those three oscillators.

Figure 1. Seasonal march (y-axis) of the diurnal cycle of rainfall at selected raingauges. The diurnal cycle is defined from 07:00 to 07:00 LST (x-axis). Interpolated iso-lines represent percent of total daily rainfall at each hour, with the color scale at the bottom.