Analysis of diurnal to seasonal variability of Integrated Water Vapour in the South Indian Ocean Basin Using Ground‐Based GNSS and 5th generation ECMWF Reanalysis (ERA5) data

The spatial and temporal distribution of tropospheric water vapour in the South Indian Ocean (SIO) basin is investigated using observations collected from 12 IGS GNSS stations spanning throughout the basin. The comparison of GNSS‐derived integrated water vapour (IWV) contents against radio soundings and satellite borne microwave radiometer data shows good agreement, with global uncertainties ranging from 0.76 kg.m^–2.to 1.17 kg.m^–2., depending on GNSS station locations.

GNSS‐derived IWV contents show a strong seasonal cycle, characterized by higher water vapor content during the austral summer, when the inter tropical convergence zone (ITCZ) is located in the southern hemisphere. At the seasonal time scale, the observed annual IWV amplitude varies from 10‐15 kg.m^–2. near the equator to 20‐30 kg.m^–2. in the subtropics. The GNSS IWV signature of the Madden Julian Oscillation (MJO) is hardly noticeable

during the Austral winter, but varies from a 1‐2 kg.m^–2. to 4 kg.m^–2. between the active and suppressed phases of the MJO during austral summer.

At diurnal time scales, GNSS IWV shows larger diurnal amplitude over land (2‐3 kg.m^–2.) than over open ocean (1‐2 kg.m^–2.), with highest amplitudes (up to 7 kg.m^–2.) observed over large and mountainous islands. The phase analysis of the IWV diurnal cycle indicates that the diurnal maximum (resp. minimum) is reached in the late afternoon/evening (resp. morning) over land, at night (resp. mid‐day) over ocean and in the early morning (resp. late afternoon) at coastal locations.

A comparison of GNSS‐derived IWV contents against 5th generation European Center Medium Weather Forecast (ECMWF) Reanalysis (ERA5) data shows that ERA5 generally correctly reproduces the IWV content at both seasonal, intra‐seasonal and diurnal time scales, although some discrepancies can be noticed over small island characterised by steep orography. The signature of the Madden‐Julian oscillation in ERA5 also shows good agreement with GNSS observations at most studied locations.