Climate dynamics

Abstract:

The strongest and most persistent upward propagation of zonal wavenumber 1 (WN1) Rossby waves from the troposphere on record led to the rare Antarctic sudden stratospheric warming (SSW) in September 2019. The dynamical contribution from instantaneous anomalous WN1 and its linear interference with the climatological WN1 contributed equally to the event. The unprecedented WN1 planetary wave behavior is further attributed to a long‐lived midlatitude circumpolar Rossby wave train in the troposphere that was sustained by anomalous convection, first over the subtropical Pacific Ocean east of Australia and then over the eastern South Pacific. Besides the tropospheric wave forcing, the phase of the quasi‐biennial oscillation in the upper stratosphere also facilitated the weakening of polar vortex. Moreover, this SSW strongly influenced the tropospheric circulation via the Southern annular mode, favoring conditions linked to the 2019 bushfires in eastern Australia.

Abstract:

<p><strong>Abstract.</strong> The simulated climate in the American Monsoon System (AMS) in the CMIP6 submissions of HadGEM3.0 GC3.1 and the UKESM1 is assessed and compared to observations and reanalysis. Pre-industrial control and historical experiments are analysed to evaluate the model representation of this monsoon under different configurations, resolutions and with and without Earth System processes. The simulations exhibit a good representation of the temperature and precipitation seasonal cycles, although the historical experiments overestimate summer temperature in the Amazon, Mexico and Central America by more than 1.5 K. The seasonal cycle of rainfall and general characteristics of the North American Monsoon are well represented by all the simulations. The models simulate the bimodal regime of precipitation in southern Mexico, Central America and the Caribbean known as the midsummer drought, although with a stronger intraseasonal variation than observed. Austral summer biases in the modelled Atlantic Intertropical Convergence Zone (ITCZ), Walker Circulation, cloud cover and regional temperature distributions are significant and influenced the simulated spatial distribution of rainfall in the South American Monsoon. These biases lead to an overestimation of precipitation in southeastern Brazil and an underestimation of precipitation in the Amazon. El Niño Southern Oscillation (ENSO) characteristics and teleconnections to the AMS are well represented by the simulations. The precipitation responses to the positive and negative phase of ENSO in subtropical America are linear in both pre-industrial and historical experiments. Overall, the UKESM has the same performance as the lower resolution simulation of HadGEM3.0 GC3.1 and no significant difference for the AMS was found between the two model configurations. In contrast, the medium resolution HadGEM3.0 GC3.1 N216 simulation outperforms the low-resolution simulations in temperature, rainfall, ITCZ and Walker circulation biases.</p>