The added value in CMIP6 models for simulating west African rainfall and
its related extreme indices.
Abstract
West African precipitation is characterized by a strong variability from
intra-seasonal to climatic scales. In this region, it typically rains
from March to October with differences in the annual cycle of
precipitation (e.g. Sow et al., Atmosphere 2020). Studies based on the
Coupled Model Inter-comparison Project (CMIP) phase 3 and 5 show that
the representation of the West African Monsoon (WAM) mean state and its
response to global warming, are both highly model-dependent (Roehrig et
al., J. Atmos. Sci. 2013). CMIP5 simulations display large inter-model
spread in both the mean precipitation climatology and higher-frequency
indices by overestimating the frequency of wet days (Sow et al, 2020).
Likewise, strong uncertainties are found in the simulation of the
consecutive wet and dry days (CWD and CDD respectively) over west
Africa. In this study we analyse the added-value of the new generation
models available from the CMIP6 archive in simulating precipitation and
its extreme indices over West Africa. The same group of CMIP5 and CMIP6
models is evaluated with CRU and REGEN datasets for a long time period
(more than 30 years) as well as more than four other observational
datasets in a shorter time period (15 years). Results show that the
inter-model spread as well as the overestimation of these indices
slightly decreases in most CMIP6 historical simulations compared to
their CMIP5 counterpart. When considering experiments where sea surface
temperature (SST) is prescribed over a long period (1870-2014),
performed in the Global Monsoons Model Intercomparison Project (GMMIP),
we find more improvement in the annual structure of precipitation over
the Guinean region with less inter-model spread compared to
observations. The inter-annual variability of precipitation is then
relatively well captured by all CMIP6 models with significant
correlations. Almazroui et al. (2020) found that precipitation in CMIP6
models will increase in the central part and decrease in the western
part of west Africa during the twenty-first century under Shared
Socio-economic Pathways (SSP). In this study we further analyse how
extreme precipitation indices are represented in SSP5-8.5 over West
Africa.