Dynamical and thermodynamical influences of the tropics and midlatitudes on arctic hydroclimate variability
Hegyi, Bradley Michael
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The Arctic is an important component of the Earth’s climate system, and it is a region dynamically coupled to climate phenomena at lower latitudes, through both atmospheric and oceanic paths. The coupling has significant effects on the hydroclimate variability in the Arctic, including effects on sea ice and Arctic precipitation. In this dissertation, we explore the coupling of the lower latitudes and the Arctic hydroclimate through atmospheric mechanisms with dynamical and thermodynamical components, with a focus on the following examples of variability: i) the decadal variability of boreal winter Arctic precipitation, ii) the variability of the strength of the stratospheric polar vortex in boreal winter, and iii) the initial melt of Arctic sea ice in late boreal spring. The goal of the research is to understand what drives the Arctic hydroclimate variability in each of these examples through improved knowledge of the mechanisms linking them to the tropics and Northern Hemisphere midlatitudes. In the first part of the analysis, we explore the mechanisms responsible for the decadal variability of boreal winter Arctic precipitation. We find that the decadal variability of cool-season Arctic precipitation is at least partly connected to decadal modulation of tropical central Pacific sea surface temperatures related to the El Niño-Southern Oscillation (ENSO). The modulation can be described as the oscillation between periods favoring central and eastern Pacific warming events [CPW and EPW, respectively], which are two common types of ENSO variability. By analyzing a collection of CPW and EPW events in reanalysis data, we establish the following connecting mechanism. First, the increase of central Pacific SSTs drive a Rossby wave train that destructively interferes with the zonal wavenumber 1 component of the background extratropical planetary wave in the subpolar region. Next, as a result of this interference, the magnitude of the vertical Rossby wave propagation from the troposphere to the stratosphere decreases and the stratospheric polar vortex strengthens. Finally, the strengthening of the vortex translates into a tendency towards a positive Arctic Oscillation (AO) in the troposphere and a poleward shift of the Northern Hemisphere midlatitude storm tracks, increasing moisture transport from lower latitudes and increasing total Arctic precipitation. In a further investigation of a crucial component of the above mechanism, the initial response of the stratospheric polar vortex to the influence of CPW and EPW is investigated. A 20-member ensemble run of an idealized model experiment in the NCAR Whole Atmosphere Community Climate Model (WACCM) is conducted with prescribed CPW and EPW pattern SST anomalies. Both CPW and EPW events weaken the polar vortex in the ensemble mean. The weakening is mainly tied to changes in the eddy-driven mean meridional circulation, with some contribution from eddy momentum flux convergence. There is a significant spread between ensemble members with identical CPW and EPW forcing, where a few of the ensemble members exhibit a weak strengthening response. The initial conditions of the extratropical atmosphere and subsequent internal variability after the introduction of the CPW and EPW forcing help drive the spread in response between individual members. In the last part of the analysis, using MERRA reanalysis data, the means by which atmospheric eddies affect the trend and variability of the initial melt of Arctic sea ice are explored. We focus specifically on the effects of lower troposphere (i.e. 1000-500 mb average) meridional heat transport by atmospheric eddies, a dynamical component of the atmospheric eddy mechanism, and eddy-generated surface downwelling shortwave and longwave radiation anomalies, a thermodynamical component. Although in a climatological sense, atmospheric eddies in all major frequency bands transport heat poleward into the Arctic, we find that the lower-troposphere eddy meridional heat transport does not contribute to the trend of an earlier initial melt date. However, eddy heat transport still plays an important role in the initialization of individual episodes of initial melt with large areal coverage. In the investigation of two specific episodes, the meridional heat transport term that represents the interaction between the eddy wind and mean temperature fields (i.e. the product of the meridional eddy wind and the mean temperature fields) is most associated with the initial melt in both episodes. Additionally, melt in one of the episodes is also associated with surface downwelling longwave and shortwave radiation anomalies, a result of eddy-generated cloud cover anomalies. Therefore, in individual melt events, the combination of direct eddy meridional heat transport and surface longwave and eddy-driven shortwave radiation anomalies may significantly contribute to the initial melt of Arctic sea ice. This combination may be especially important in episodes where significant initial melt occurs over a large area and over a period of a few days.