Estimation of Zonally Resolved Edges of the Tropical Belt Using GPS-RO Measurements

Most of the recent studies on the expansion of the tropics have used several zonally averaged metrics. However, as tropical heating is closely tied to the land-sea distribution, the expansion rate is envisaged to vary from region to region. A zonally averaged metric is thus inadequate to derive a zonally resolved expansion trend, which is a lacuna in the present understanding of the regional features of the tropical belt. In this regard, the present study focuses on obtaining the zonally resolved width of the tropical belt from three objectively defined tropopause-based metrics for the first time, using seven years of COSMIC GPS-RO observations. Definitions of two of the existing tropopause-based metrics, viz. the meridional gradient of tropopause altitude and tropospheric bulk static stability, are extended to determine the zonally resolved widths of the tropical belt. In addition, a third metric is proposed, which is based on the pronounced amplitude of the annual oscillation of the tropopause height over the subtropical region. The annual cycle of width of the tropical belt derived from the three tropopause-based metrics shows a good agreement with reanalysis data both at seasonal and interannual time scales. Regional features of the width of the tropical belt during boreal winter and summer, and their contributions to the zonal mean tropical width are discussed. The significance of the present study lies in bringing out the regional features of width of the tropical belt using three tropopause-based metrics derived from radio occultation measurements for the first time.