Year: 2016
Type: dissertation
Abstract: Paleoclimatic data, 20th century observations and numerical simulations have all suggested the capability of the extratropics to trigger global teleconnection patterns and drive changes in the distant tropical regions mediated through atmospheric and ocean processes. The emerging general picture is that the Inter Tropical Convergence Zone (ITCZ) tends to shift toward the warmer hemisphere at the same time that the atmospheric energy transport is modified to favour the transport of energy to the colder hemisphere. Apart from paleo and 20th century climate implications, the extratropical to tropical teleconnection problem has also been related with the future climate as the projected pattern of temperature change for the 21st century presents an interhemispheric asymmetry with the strongest warming projected to occur over the Northern Hemisphere (NH) high latitudes. This thesis aims to shed further light into the sensitivity of the tropical climate to an extratropical thermal forcing via a series of numerical simulations performed with an Atmospheric General Circulation Model (AGCM) coupled to either a low or a medium complexity ocean model, using realistic surface boundary conditions in all the cases. The behaviour of two convergence zones, the ITCZ and the South Atlantic Convergence Zone (SACZ), is the main focus of the work. The relative roles of the atmosphere, sea surface temperature (SST), land surface temperature (LST), hemispheric component of the forcing and tropical ocean dynamics are investigated and physical mechanisms mediating in the extratropical to tropical teleconnection are proposed and tested. In simulations in which the AGCM is coupled to slab ocean and land models we find that if the tropical SSTs are not allowed to change in response to the extratropical forcing then the ITCZ response strongly weakens although it is not negligible in particular over the Atlantic Ocean and Africa. If, in addition to the tropical SST constraint, the LST over Africa is prevented to change the ITCZ response to the remote signal completely vanishes. Therefore, our results indicate that the ITCZ response to the extratropical forcing is not possible just trough purely atmospheric processes, but needs the involvement of either the tropical SST or the LST. The clear-sky longwave radiation feedback is highlighted as the main physical mechanism operating behind the land-based extratropical to tropical communication. Regarding the SACZ, we present numerical evidence that indicates that it can be affected by an extratropical thermal forcing with a behaviour that does not replicate the ITCZ response of shifting toward the warmer hemisphere. In fact, we find that when the NH (Southern Hemisphere, SH) extratropics warm (cool), the SACZ experiences a significant weakening associated, mostly, to the NH component of the forcing. Our results indicate that 75% of the SACZ signal in response to the forcing is linked to the development of a secondary tropical convergence zone in the Atlantic Ocean around 20°N-30ºN, which depends on the tropical SST response. The remaining 25% of the signal can be explained through the development of a Walker-type of circulation between western tropical Africa and the SACZ, being this mechanism associated with the African LST reaction to the remote forcing. To assess the role of the tropical ocean dynamics we analyse and compare simulations in which the AGCM is coupled either to a slab ocean model or to a combination of a Reduced Gravity Ocean model in the tropics and a slab model elsewhere. The inclusion of tropical ocean dynamics enables changes in the equatorial Pacific SST seasonal cycle, whose response to a NH (SH) high latitude warming (cooling) consists of a significant strengthening in the eastern portion of the basin. The annual mean response, however, is weaker than the signal the extratropical forcing would generate if only thermodynamic coupling was allowed, thus suggesting that tropical ocean dynamics tends to oppose the extratropical forcing in the mean. We further find that under an extratropical forcing the activity of the El Niño-Southern Oscillation weakens, which is consistent with a strengthening of the seasonal cycle through the previously proposed frequency entrainment mechanism
Source: Americanae (AECID Library)
Author: Talento, Stefanie
Language: English
Subfield: Global and Planetary Change
Field: Environmental Science
Domain: Physical Sciences
SDG: Climate action
Open Access status: green
OpenAlex ID: W7005622652