Impact of extreme CO2 levels on tropical climate: a CGCM study

Abstract

A coupled general circulation model has been used to perform a set of experiments with high CO 2 concentration (2, 4, 16 times the present day mean value). The experiments have been analyzed to study the response of the climate system to strong radiative forcing in terms of the processes involved in the adjustment at the ocean-atmosphere interface. The analysis of the experiments revealed a non-linear response of the mean state of the atmosphere and ocean to the increase in the carbon dioxide concentration. In the 16 9 CO 2 experiment the equilibrium at the oceanatmosphere interface is characterized by an atmosphere with a shut off of the convective precipitation in the tropical Pacific sector, associated with air warmer than the ocean below. A cloud feedback mechanism is found to be involved in the increased stability of the troposphere. In this more stable condition the mean total precipitation is mainly due to large-scale moisture flux even in the tropics. In the equatorial Pacific Ocean the zonal temperature gradient of both surface and sub-surface waters is significantly smaller in the 16 9 CO 2 experiment than in the control experiment. The thermocline slope and the zonal wind stress decrease as well. When the CO 2 concentration increases by about two and four times with respect to the control experiment there is an intensification of El Nin ˜o. On the other hand, in the experiment with 16 times the present-day value of CO 2 , the Tropical Pacific variability weakens, suggesting the possibility of the establishment of permanent warm conditions that look like the peak of El Nin ˜o.

Monotonic decrease of the zonal SST gradient of the equatorial Pacific as a function of CO2 concentration in CCSM3 and CCSM4

“…The p CO 2 in our simulations will be chosen to cover values from to 16 times the preindustrial level of 286 ppmv. In comparison with doubling and quadrupling of p CO 2 experiments that have usually been investigated, these larger perturbations in greenhouse gas forcing are found to be sufficient to allow us to much more clearly identify the trend of the zonal SST gradient and to understand the mechanisms responsible for the trend, as previously suggested by Cherchi et al [].…”

Will There Be a Significant Change to El Niño in the Twenty-First Century?

“…The stabilized runs in this study only cover a small portion of the potential range in CO 2 forcing due to anthropogenic climate change; does the system continue to respond in the same way at higher and higher CO 2 ? There is some evidence that ENSO behavior may radically change at very high (but plausible) CO 2 concentrations (Cherchi et al 2008). Once the extension runs for the other forcing pathways become available, it will become possible to perform a much more detailed diagnosis of the behavior of ENSO in response to CO 2 .…”

Robust ENSO across a Wide Range of Climates

“…For example, we do not address the ultimate causes of changes in the zonal SST gradient-we simply induce those changes from the extratropics. Consequently, we do not explicitly consider changes in tropical climate caused by variations in Earth's precession or obliquity (Clement et al 1999;Timmermann et al 2007;Clement et al 2000), volcanic or solar forcing (Mann et al 2005), or high CO 2 levels (Cherchi et al 2008). A significant reduction of the mean east-west SST gradient can also be achieved by other means, such as by varying cloud properties within a climate model (Burls and Fedorov 2014).…”