The American Monsoon System in UKESM1 and HadGEM3

The American Monsoon System is the main source of rainfall for the tropical and sub-tropical Americas. CMIP6 climate model simulations from the MetOffice Hadley Centre (MOHC) models: HadGEMGC3.1 and the Earth System model UKESM1 were analyzed to evaluate the representation of this monsoon. Pre-industrial and historical experiments were compared to reanalyses and observations.  Several diagnostics, such as the Inter-tropical Convergence Zone (ITCZ) location, the Walker circulation and temperature and precipitation seasonal cycles in the American Monsoon System were assessed, as well as El Niño-Southern Oscillation teleconnections to the monsoon. 

These simulations reasonably represent the observed seasonal cycle of precipitation in the American Monsoon System. However, significant biases in the spatial distribution of rainfall in South America are evident. 
These biases in the South American Monsoon System are linked to temperature biases in the Amazon and Atlantic ITCZ biases. 
The midsummer drought regime in Central America, the Caribbean and southern Mexico is reproduced by all the simulations, although with a stronger intraseasonal cycle than observed.
The North American monsoon is relatively well represented by all the simulations, which is a noticeable improvement of these models compared to CMIP5. 

The overall performance of HadGEM3 at different horizontal resolutions was compared to that of UKESM1. 
This work evaluates the role of horizontal resolution and Earth System processes for monsoon representation which will be useful for interpreting scenario experiments or using CMIP6 runs for understanding variability and
teleconnections in this monsoon system.