GNSS radio occultation in-filling of the vast radiosonde data gap of the
UTLS over Africa reveals global, regional and coupled climate drivers of
tropopause variability
Abstract
Over much of Africa, radiosonde data are lacking; consequently, the
African UTLS is understudied, and potential proxies such as climate
models and reanalyses fail to capture the behaviour of the UTLS fully.
This study pioneers the use of Global Navigation Satellite System Radio
Occultation (GNSS-RO) data from 2001 to 2020 to address the radiosonde
data gaps over Africa and contributes to a better understanding of the
tropopause (TP) characteristics under the influence of multiple climate
drivers. The analyses show that GNSS-RO data from CHAMP, GRACE, MetOp,
COSMIC, and COSMIC-2 agree with radiosonde measurements with differences
being smaller than 1 K in the UTLS; thereby enabling in-filling of 80%
of the missing radiosonde data in Africa during 2001-2020. By contrast,
the smoothed vertical temperature profiles of reanalysis products lead
to a warm bias of +0.8K in ERA5 and +1.2K in MERRA-2, and these biases
alter some vertical and temporal structure details, with possible
implications on climate change detection and attribution. Furthermore,
the analysis of GNSS-RO data over Africa revealed: 1) influences of
global climate drivers on TP temperature, with QBO > ENSO
> IOD > NAO > SAM >
MJO, and on TP height with ENSO > QBO > NAO
> MJO > IOD > SAM; 2) multiple
coupled global climate drivers such as ENSO-MJO, ENSO-NAO etc.; 3)
coupled global and regional climate drivers that influence the TP
variability, e.g., ENSO-ITCZ; and 4), the deep convection associated
with the Asian Summer Monsoon and Tropical/African Easterly Jet locally
influence TP height.