Seasonality in long-term trends of tropical intraseasonal wave activity

Long-term trends in the intensity of tropical intraseasonal oscillations, like Madden and Julian oscillations (MJO), tropical depressions and convectively coupled equatorial waves namely Kelvin, Rossby and mixed Rossby–gravity (MRG) waves along with tropical depressions (TD) are presented for the period of 40 years from 1979 to 2018. Daily data of satellite-observed outgoing longwave radiation is used to quantify these waves. From the spatial distribution of waves during different seasons, it is seen that there is large heterogeneity of trends in space and time. Most of the waves are observed to have over all increasing trends in boreal winter (December, January and February, DJF) season, in particular over the Northern Hemisphere. Kelvin wave shows the most homogeneous behavior with significant increasing trends throughout the tropics in this season. In case of MJO, over the period in consideration, spatio-temporal distribution of intensity tends to become more homogeneous with regions of climatological peak having decreasing trends and vice versa. Spatially, MJO and Rossby waves have decreasing intensities, whereas Kelvin and MRG-TD have increasing intensities in almost all the seasons over the Central Pacific. Comparison with long-term trends in sea surface temperature (SST), vertical shear of zonal wind and total column water vapor (TCWV) show that SST and TCWV explain observed trends in waves only in limited cases, since these decrease over a longer period of time over Central Pacific. However vertical shear shows more significant influence on wave tendencies and seems to be a greater contributing factor in long-term variations of waves.