Long-term variability and trends of precipitable water vapour derived from GPS tropospheric path delays over the Eastern Mediterranean

The precipitable water vapour (PWV) of 21 Global Navigation Satellite System (GNSS) ground receivers from the Survey Of Israel Active Permanent Network (SOI-APN) in the Eastern Mediterranean area are processed and analysed. We focus on the annual cycles, inter-annual variations and long-term trends. The PWV monthly mean values peak during September-October where October has the highest variability for all stations. PWV values reach minimum level during February-March where Apr is the least variable month for most stations. Generally, the PWV variability decreases with station height. Additionally, harmonic analysis indicates that the annual mode's amplitude decreases with station height. In contrast, the semi-annual mode's accounting for the sub-annual variance increases with station height. The PWV monthly mean anomalies are consistent among all the stations suggesting a common regional driver. Furthermore, a comparison between the PWV station average anomalies and the regional mean anomalies from the ERA5 reanalysis shows a strong correlation with r = 0.95. In addition, a correlation coefficient of r = 0.72 was found with the regional mean moisture flux anomalies at 750 hPa taken from ERA5 and the station average PWV anomalies, thus implying that horizontal moisture flow accounts for most of the inter-annual variability. However, the significance of the 750 hPa pressure level requires future investigation. For the long-term analysis, we find an increasing regional mean trend of 0.5 mm . decade(-1) for the entire data period (1997-2019) as compared to 1 mm . decade(-1) for the last decade (2010-2019) suggesting a recent accelerated moistening of the Eastern Mediterranean region.