Testing robustness of co-existing climate states
Authors: Ragon Charline , Lembo Valerio , Lucarini Valerio , Vérard Christian , Kasparian Jérôme , Brunetti Maura 
Published: March 2021

&lt;p&gt;&lt;span&gt;The climate can be regarded as a stationary non-equilibrium statistical system (Gallavotti 2006): a continuous and spatially inhomogeneous input of solar energy enters at the top-of-atmosphere and compensates the action of non-conservative forces, mainly occurring at small scales, to give rise to a statistically steady state (or attractor) for the whole climate. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Depending on the initial conditions and the range of forcing, all other parameters being the same, some climate models have the property to settle down on different attractors. &lt;/span&gt;&lt;span&gt;Multi-stability reflects how energy, water mass and entropy can be re-distributed in multiple ways among the climate components, such as the atmosphere, the ocean or the ice, through a different balance between nonlinear mechanisms. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Starting from a configuration where competing climate attractors occur under the same forcing, we have explored their robustness performing two kinds of numerical experiment. &lt;/span&gt;&lt;span&gt;First, we have investigated the impact of frictional heating on the overall energy balance and we have shown that such contribution, generally neglected in the atmospheric component of climate models, has crucial &lt;/span&gt;&lt;span&gt;consequences on conservation properties: it improves the energy imbalance at top-of-atmosphere, typically non negligible in coarse simulations (Wild et al. 2020), strengthens the hydrological cycle, &lt;/span&gt;&lt;span&gt;mitigates the mechanical work associated to atmospheric circulation intensity &lt;/span&gt;&lt;span&gt;and reduces the heat transport peaks in the ocean. &lt;/span&gt;&lt;span&gt;Second, we have compared two bulk formulas for the cloud albedo, one where it is constant everywhere and the other where it increases with latitude, as implemented in the new version of the atmospheric module SPEEDY in order to improve comparisons with observational data (Kucharski 2013). We have che&lt;/span&gt;&lt;span&gt;cked that this new parameterization does not affect energy and water-mass imbalances, while reduces global temperature and water-mass transport on the attractor, giving rise to a larger conversion of heat into mechanical work in the atmosphere.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;In order to perform such studies, we have run the climate model MITgcm on coupled aquaplanets at 2.8 horizontal resolution until steady states are reached (Brunetti el al. 2019) and we have applied the Thermodynamic Diagnostic Tool (&lt;em&gt;TheDiaTo&lt;/em&gt;, Lembo et al. 2019). &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;References: &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Brunetti, Kasparian, V&amp;#233;rard, Climate Dynamics 53, 6293 (2019)&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Gallavotti, &lt;/span&gt;Math. Phys. 3, 530&lt;span&gt; (2006)&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Kucharski&lt;span&gt; et al.&lt;/span&gt;, Bulletin of the American Meteorological Society 94, 25&lt;span&gt; (2013)&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Lembo, Lunkeit, Lucarini, Geoscientific Model Development 12, 3805&lt;span&gt; (2019)&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Wild, &lt;/span&gt;Climate Dynamics 55, 553&lt;span&gt; (2020)&lt;/span&gt;&lt;/p&gt;