Lower-tropospheric humidity: climatology, trends and the relation to the ITCZ

Abstract

A B S T R A C TThe tropical region is an area of maximum humidity and serves as the major humidity source of the globe. Among other phenomena, it is governed by the so-called Inter-Tropical Convergence Zone (ITCZ) which is commonly defined by converging low-level winds or enhanced precipitation. Given its importance as a humidity source, we investigate the humidity fields in the tropics in different reanalysis data sets, deduce the climatology and variability and assess the relationship to the ITCZ. Therefore, a new analysis method of the specific humidity distribution is introduced which allows detecting the location of the humidity maximum, the strength and the meridional extent. The results show that the humidity maximum in boreal summer is strongly shifted northward over the warm pool/Asia Monsoon area and the Gulf of Mexico. These shifts go along with a peak in the strength in both areas; however, the extent shrinks over the warm pool/Asia Monsoon area, whereas it is wider over the Gulf of Mexico. In winter, such connections between location, strength and extent are not found. Still, a peak in strength is again identified over the Gulf of Mexico in boreal winter. The variability of the three characteristics is dominated by inter-annual signals in both seasons. The results using ERA-interim data suggest a positive trend in the Gulf of Mexico/Atlantic region from 1979 to 2010, showing an increased northward shift in the recent years. Although the trend is only weakly confirmed by the results using MERRA reanalysis data, it is in phase with a trend in hurricane activity Á a possible hint of the importance of the new method on hurricanes. Furthermore, the position of the maximum humidity coincides with one of the ITCZ in most areas. One exception is the western and central Pacific, where the area is dominated by the double ITCZ in boreal winter. Nevertheless, the new method enables us to gain more insight into the humidity distribution, its variability and the relationship to ITCZ characteristics.

Trend analysis of tropospheric specific humidity over Iran during 1979–2016

“…It is only at a regional scale that the increasing trend of the SH for southern coasts of Caspian Sea and northern coast of Persian Gulf of Iran has reached similar results. The increase of the SH in the southern and northern coasts coincides with the results of Talaee et al () for Iran, and Laderach and Christoph () for the Gulf of Mexico‐Atlantic, which significantly linked to atmospheric warming and more evaporation from water sources. Given the fact that steam is a greenhouse gas, increasing the amount of water vapour and humidity in the troposphere will lead to higher levels of heating in the troposphere (IPCC, ; Dessler and Sherwood, ).…”

Continental drift, plateau uplift, and the evolutions of monsoon and arid regions in Asia, Africa, and Australia during the Cenozoic

“…Especially over the land masses, the meridional zero-wind contour and the convergence line were not suitable for defining the position of the ITCZ (Žagar et al, 2011). Therefore, other methods have been designed to determine the ITCZ, such as using the mean divergence field in the lower stratosphere (Berry and Reeder, 2014) or the field of specific humidity (Läderach and Raible, 2013) to represent the position of the ITCZ. Considering the strong correlations between rainfall, vertical motion, and convective activity in the equatorial regions, we used the mid-troposphere 500 hPa vertical velocity (ω) to determine the position of the ITCZ.…”

Vertical and latitudinal variation of the intertropical convergence zone derived using GPS radio occultation measurements

“…For each longitude, the peak position, location, and width (full width halfmaximum) of the Gaussian is estimated from GPSRO refractivity and specific humidity data grids. The procedure is based on the method of Läderach and Raible (2013) for identifying the ITCZ from ERA-Interim specific humidity and is applied to the GPSRO specific humidity and refractivity data simultaneously.…”

“…As an initial study, we applied this method on the monthly means of specific humidity and the refractive index at different pressure levels excluding the boundary layer to reduce orography effects. The humidity distribution parameters are assessed by fitting a Gaussian function (Läderach & Raible, 2013) at each longitude (this sentence is repeated).…”