The objective of this work is to assess the downscaling projections of climate change over Central America at 8-km resolution using the Eta Regional Climate Model, driven by the HadGEM2-ES simulations of RCP4.5 emission scenario. The narrow characteristic of continent supports the use of numerical simulations at very high-horizontal resolution. Prior to assessing climate change, the 30-year baseline period 1961–1990 is evaluated against different sources of observations of precipitation and temperature. The mean seasonal precipitation and temperature distribution show reasonable agreement with observations. Spatial correlation of the Eta, 8-km resolution, simulations against observations show clear advantage over the driver coarse global model simulations. Seasonal cycle of precipitation confirms the added value of the Eta at 8-km over coarser resolution simulations. The Eta simulations show a systematic cold bias in the region. Climate features of the Mid-Summer Drought and the Caribbean Low-Level Jet are well simulated by the Eta model at 8-km resolution. The assessment of the future climate change is based on the 30-year period 2021–2050, under RCP4.5 scenario. Precipitation is generally reduced, in particular during the JJA and SON, the rainy season. Warming is expected over the region, but stronger in the northern portion of the continent. The Mid-Summer Drought may develop in regions that do not occur during the baseline period, and where it occurs the strength may increase in the future scenario. The Caribbean Low-Level Jet shows little change in the future. Extreme temperatures have positive trend within the period 2021–2050, whereas extreme precipitation, measured by R50mm and R90p, shows positive trend in the eastern coast, around Costa Rica, and negative trends in the northern part of the continent. Negative trend in the duration of dry spell, which is an estimate based on evapotranspiration, is projected in most part of the continent. Annual mean water excess has negative trends in most part of the continent, which suggests decreasing water availability in the future scenario.

Abstract. Global Eta Framework (GEF) is a global atmospheric model developed in general curvilinear coordinates and capable of running on arbitrary rectangular quasi-uniform spherical grids, using stepwise ("Eta") representation of the terrain. In this study, the model is run on a cubed-sphere grid topology, in a version with uniform Jacobians (UJ), which provides "equal-area" grid cells, and a smooth transition of coordinate lines across the edges of the cubed-sphere. Within a project at the Brazilian Center for Weather Forecasts and Climate Studies (CPTEC), a nonhydrostatic version of this model is under development and will be applied for seasonal prediction studies. This note describes preliminary tests with the GEF on the UJ cubed-sphere in which model performance is evaluated in seasonal simulations at a horizontal resolution of approximately 25 km, running in the hydrostatic mode. Comparison of these simulations with the ERA-Interim reanalyses shows that the 850 hPa temperature is underestimated, while precipitation pattern is mostly underestimated in tropical continental regions and overestimated in tropical oceanic regions. Nevertheless, the model is still able to well capture the main seasonal climate characteristics. These results will be used as a control run in further tests with the nonhydrostatic version of the model.

The seasonal cycle of precipitation in tropical South America is determined by the monsoonal system. The transition from dry to wet season occurs in austral spring (September–November, SON) when intense convection from northwestern South America rapidly shifts southwards to the southern Amazon Basin and western‐central Brazil (WCB) in October and further to the southeast of Brazil in November. This study evaluates ability of the global atmospheric model, Global Eta Framework (GEF), at 25‐km horizontal resolution, to simulate the onset of the rainy season in WCB region. The simulations are based on a five‐member ensemble seasonal integrations for the years 2011 and 2013. Evaluation of mean global simulated fields, such as 200‐hPa wind, 500‐hPa geopotential height, 850‐hPa temperature and wind, and MSLP at the surface, for the SON period indicates high level of agreement with reanalyses and observations, both in spatial distribution and intensity for most of the variables. The variable of the lowest skill is precipitation, which is overestimated over some tropical oceanic regions and underestimated over tropical continental regions, including South America. The onset of the rainy season is determined using methods based on precipitation and outgoing long‐wave radiation (OLR). The threshold based on simulated precipitation is also calculated as an alternative method for defining monsoon onset. Comparison of the 5‐day averaged values (pentads) of precipitation and OLR of all members of the ensemble and the ensemble mean against the observed data shows the ability of GEF to reproduce the typical pattern of transition from dry to wet season in WCB, although most of ensemble members tend to underestimate precipitation and overestimate OLR. The onset date is delayed for few pentads in the model simulations.

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<p>During deglaciation disintegration of large-scale continental ice sheets represents a continuous threat to reduce the strength of the Atlantic meridional overturning circulation (AMOC) via meltwater perturbations to the northern high latitudes. Nevertheless, an abrupt AMOC recovery is detected half-way through the last deglaciation and  a growing number of studies using Earth System Models (ESMs) of varying complexity have shown that atmospheric CO<sub>2</sub> concentrations and ice sheet volume can influence the operational mode of the AMOC, eventually including the coexistence of multiple states and associated threshold behavior for intermediate climate states between full glacial (e.g. Last Glacial Maximum, LGM) and full interglacial (e.g. pre-industrial, PD)  conditions. In this study we present results from coordinated sensitivity experiments conducted as part of the German climate modeling initiative (PalMod), using three complex ESMs (AWI-ESM, CESM and MPI-ESM). Besides differences in the impact of CO<sub>2</sub> and ice volume changes, we also investigate how variations in these boundary conditions control the AMOC sensitivity to deglacial meltwater injections in the North Atlantic. We find that the AMOC strength responds to ice sheet and/or CO<sub>2</sub> changes in all models, with partly opposing effects.  A similar AMOC strength for PD and LGM conditions is detected in AWI-ESM and MPI-ESM, while CESM shows a weaker LGM AMOC. This weaker LGM state is also characterized by a relatively pronounced AMOC sensitivity to freshwater perturbations. Our inter-comparison experiments suggest that this specific behavior in CESM can be detected for atmospheric concentrations between LGM and intermediate levels of ~220 ppm. This further corroborates in particular the impact of CO<sub>2</sub> changes to modulate the trajectory of deglacial climate changes by an alteration of the AMOC susceptibility to meltwater injections as recently suggested (Sun et al., Glob. Planet. Change, 2021; Barker & Knorr, Nat. Commun., 2021).</p><p> </p><p> </p><p> </p><p>References:</p><p>Sun, Y., Knorr, G., Zhang, X., Tarasov, L., Barker, S., Werner, M. and G. Lohmann (2022): Ice sheet decline and rising atmospheric CO<sub>2</sub> control AMOC sensitivity to deglacial meltwater discharge. <em>Global and Planetary Change</em> 210. https://doi.org/10.1016/j.gloplacha.2022.10375</p><p>Barker, S. and G.  Knorr (2021): Millennial scale feedbacks determine the shape and rapidity of glacial termination. <em>Nature Communications </em>12, 2273. https://doi.org/10.1038/s41467-021-22388-6</p>