Understanding ENSO related tropical teleconnections using Quasi-equilibrium tropical circulation model

Abstract

Atmospheric teleconnections are remote impacts associated with atmospheric processes transmitted through planetary-scale waves like the Rossby wave. Tropical heat sources like El Nino Southern Oscillation (ENSO) could force such planetary-scale wave responses. The El Nino events are classified into Non-OLR El Nino events and OLR El Nino events based on its convective signal over the central-eastern equatorial Pacific using an OLR based El Nino Index. The key purpose of this study is to analyse the difference in teleconnection patterns during these OLR based El Nino events and understand its baroclinic-to-barotropic mode responses using an intermediate complexity atmospheric circulation model called QTCM. Chapter 1 analyses the difference in the distribution of atmospheric variables and Rossby wave source anomalies during Non OLR El Nino events and OLR El Nino using QTCM experiments. It is seen that the OLR El Nino events have a larger barotropic contribution to the positive anomaly of SLP over the western Pacific and a larger baroclinic contribution to the negative anomaly of SLP over the eastern Pacific compared to Non-OLR El Nino events. This is due to stronger baroclinic Rossby waves from the eastern and central tropical Pacific that propagates towards western Pacific and force barotropic wave trains due to barotropic-baroclinic interactions. Also, on analysing the effective RWS forcing and its components over certain regions during OLR and Non OLR El Nino, we see a difference in their distribution due to contributions from the absolute vorticity advection by divergent wind flow and subtropics vortex stretching. Chapter 2 investigates the baroclinic-to-barotropic interaction over the midlatitude and tropical teleconnection through baroclinic-barotropic interaction terms in barotropic Rossby wave during Non OLR El Nino and OLR El Nino. It was seen that among the barotropic Rossby wave source interaction terms, the shear advection term has the largest contribution and the mean baroclinic zonal wind that advects the baroclinic zonal wind anomaly due to tropospheric heating is the most relevant component. The effective RWS over the tropics and the subtropics arise from the mean state baroclinic flow that acts on the baroclinic wind structure arising due to the ENSO tropospheric heating that spreads over a scale of equatorial radius of deformation from the deep tropics to the subtropics. This baroclinic wind structure is stronger for OLR El Nino compared to Non OLR El Nino. The experiment is also extended to preindustrial and mid-Holocene periods using data from CESM. The mid-Holocene OLR El Nino has a weaker RWS response than the preindustrial OLR El Nino due to the relatively weaker tropospheric heating and temperature structure, resulting in a weaker baroclinic wind structure.