Trends in Tropical Wave Activity from the 1980s to 2016

Abstract

A frequency–wavenumber power () spectrum was constructed using satellite-derived outgoing longwave radiation (OLR) and brightness temperature for the tropical latitudes. Since the two datasets overlap for over 34 years with nonintersecting sources in error and compare relatively well with each other, it is possible to diagnose trends in the tropical wave activity from the two datasets with confidence. The results suggest a weakening trend in characterized by high interannual variability for wave activity occurring in the low-frequency part of the spectrum and a steady increase in with relatively low interannual variability for wave activity occurring in the high-frequency part of the spectrum. The results show the parts of the spectrum representing the Madden–Julian oscillation and equatorial Rossby wave losing and other parts of the spectrum representing Kelvin waves, mixed Rossby–gravity waves, and tropical disturbance–like wave activity gaining . Similar results were obtained when trends in variance corresponding to the first principal component were produced using spectrally filtered OLR data representative of atmospheric equatorial waves. Spatial trends in the active phase of wave events and the mean duration of events are also shown for the different wave types. Linear trends in for the entire spectrum and regional means in the spectrum corresponding to the abovementioned five wave types with confidence intervals are also presented in the paper. Finally, we demonstrate that El Niño–Southern Oscillation variability does not appear to control the overall spatial patterns and trends observed in the spectrum.

Deep Convection over Africa: Annual Cycle, ENSO, and Trends in the Hotspots

“…S1 and S2 in the online supplemental material). This is in agreement with the trends considered specifically for April-June of the Congo (Raghavendra et al 2018) and the Sahel wet season of June-September (Taylor et al 2017).…”

“…Changing these thresholds modifies the magnitude of values output by the analysis in this study, but has negligible impact on climatology and interannual variability results in terms of when and where the most convection is observed. However, reported trends in convective activity are greater for the colder brightness temperature thresholds, as reported in the literature (Taylor et al 2017;Raghavendra et al 2018). This will be discussed in section 5 and figures reproduced with the 2608C CCD data are included in the online supplemental material.…”

“…This suggests implications for African easterly wave initiation (Mekonnen and Rossow 2011). Indeed, Raghavendra et al (2019) report increases in activity of the higher-frequency tropical wave activity across the tropics, with the equatorial Rossby wave activity notably increasing downstream (westward) of the Ethiopian Highlands. Patterns in outgoing longwave radiation (OLR) over these highlands suggest a northwestward shift in the location of most intense convection (Raghavendra et al 2019).…”

“…East Congo (Zhou et al 2014). Raghavendra et al (2018) has proposed that this paradox should be understood as taller and narrower convective storms producing less rainfall but cooler cloud-top temperatures over the Congo. This is in line with conclusions in Hamada et al (2015) that demonstrate a disconnect between extreme convection and extreme rainfall.…”

Analysis of Synoptic Disturbance in Maritim Continent Using Spherical Harmonics Transformation Method

“…The WK99 method is the most popular spacetime spectral analysis (STSA) method in the field (Raghavendra et al, 2019), but this method has a shortcoming to describe the global convective pattern as its OLR data domain is limited to the tropics (Ayuliana et al, 2019). To tackle this limitation, an alternative STSA method for spectral analysis of wavenumber-frequency to global OLR data has been proposed based on spherical harmonics (SH) transformations (Ayuliana et al, 2019).…”