Mechanisms Linking Global 5-Day Waves to Tropical Convection

Abstract

Reanalysis data and satellite-derived rainfall measurements are examined to determine possible mechanisms linking the “5 day” Rossby–Haurwitz wave to localized variations of tropical convection. The mechanisms in all regions rely on the modulation of zonal winds near the equator by the wave, but the nature of these mechanisms depends strongly on local topography and local climate. In the upper Amazon basin, the wave modulates the strength of prevailing easterlies and thus the upslope flow and associated convection on the eastern edge of the Andes. Similar modulation of upslope flow is involved off the Panamanian and Colombian Pacific coasts, but the deflection and confluence of low-level wind in the presence of the Andes and moisture transports across the Andes from the Amazon basin are also factors. Similar deflection and confluence of winds around and through the Maritime Continent lead to low-level divergence and convection anomalies over the eastern Indian Ocean. Anomalous moisture transports from the Congo basin to the eastern and northeastern Gulf of Guinea due to the wave affect atmospheric moisture over the Gulf of Guinea and thus convection in the region. Over oceanic convergence zones, modulations of the prevailing winds by the wave affect the overall wind magnitude, changing evaporation from the ocean surface and atmospheric moisture. Most of these mechanisms arise from the nonuniform nature of Earth’s surface and suggest that other external Rossby–Haurwitz waves may have similar interactions with convection.

Space–Time Spectral Analysis of the Moist Static Energy Budget Equation

“…Exploring deeper, we found that zonal and meridional winds have barotropic structures at the corresponding wavenumber and frequency and that the horizontal advection of MSE integrated over the troposphere coherently varies with zonal wind in the stratosphere as well as in the troposphere (not shown). Based on previous works (e.g., Hendon and Wheeler 2008;King et al 2015King et al , 2017, this signal apparently reflects the 5-day Rossby-Haurwitz wave. In fact, Hendon and Wheeler (2008) show that zonal wind associated with the 5-day Rossby-Haurwitz wave lags negative outgoing longwave radiation (OLR) by about 458 and the same-signed phase lag from precipitation (about 708) is implied at the corresponding wavenumber and frequency in our analysis (not shown, but estimable from Figs.…”

Hydroclimate of the Andes Part I: Main Climatic Features

“…The meridional migration exhibited by this LLJ, characterized by a further north location during SON, is related to the seasonal migration of the ITCZ (Sierra et al, 2018). It is also associated with the dynamics of Mesoscale Convective Systems (MCS) off the Colombian Pacific (Velasco and Fritsch, 1987;Poveda and Mesa, 2000;Mapes et al, 2003;Zuluaga and Poveda, 2004;Mejia and Poveda, 2005;Zuluaga and Houze, 2015;Jaramillo et al, 2017;King et al, 2017).…”

“…One of the rainiest places on Earth (Lloró; 5 • 30 N, 76 • 32 W) is situated alongside the Pacific coast of Colombia, witnessing mean annual precipitation rates reaching 13,000 mm (Snow, 1976;Meisner and Arkin, 1987;Mesa, 1999, 2000;Adler et al, 2000;Mapes et al, 2003;Sakamoto et al, 2011;Zuluaga and Houze, 2015;Jaramillo et al, 2017;King et al, 2017). Such world record precipitation is explained by the dynamics of the Chocó low-level jet (hereafter, Chocó Jet), enhanced by atmosphere-ocean-land surface interactions Mesa, 1999, 2000).…”

How I Learned to Love Normal-Mode Rossby–Haurwitz Waves

“…Already, there is considerable evidence that, despite its small associated convergence, the 5-day wave affects tropical convection (Burpee 1976;Patel 2001;King et al 2015King et al , 2017. King et al (2015) argue that it is not large-scale convergence associated with the 5-day wave but rather the interaction of its associated wind field with local terrain that is the defining influence. Careful studies of other modes may reveal additional important effects on local weather.…”