Trends in Tropical Wave Activity from the 1980s to 2016

A frequency–wavenumber power (P) spectrum was constructed using satellite derived outgoing longwave radiation, and brightness temperature for the tropical latitudes. Since the two datasets overlap for over 34 years with non-intersecting 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 P characterized by high interannual variability for wave activity occurring in the low frequency part of the spectrum, and a steady increase in P 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 P, and other parts of the spectrum representing Kelvin waves, mixed Rossby gravity waves, and tropical disturbance like wave activity gaining P. Linear trends in P 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 (ENSO) variability does not appear to control the overall spatial patterns and trends observed in the P spectrum.

Reference: Raghavendra, A., P. E. Roundy, and L. Zhou, 2018: Trends in Tropical Wave Activity from 1980s–2016. Journal of Climate (in revision)