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.

The impacts of climate variability and change on streamflow are of increasing concern, particularly as human demands on water supplies compete with the needs of natural ecosystems. The consequences on the hydrological cycle are predicted to be most severe for mid- to high-latitude regions. Of particular concern is reduced mountain snow accumulation and related reductions in the snow- and glacier-derived water supply. In northern British Columbia (BC), recent snowpack declines have caused a unique water management challenge. Diminishing water security in a region considered water-abundant has intensified over the last decade. Characterizing the climate controls on hydrologic variability is a priority for developing baseline information required for water supply forecasting. This research focuses on developing multi-century, annually-resolved records of snow water equivalent (SWE) and streamflow to provide a better understanding of long-term hydroclimate variability for the design and implementation of management strategies that balance riverine ecosystem services, such as recreation and fish habitat, with increasing economic and social demands. Climate sensitive tree-ring chronologies provide the opportunity to extend instrumental records of hydroclimate by capitalizing on the influence of climate on both annual radial growth and seasonal runoff. Traditional dendrohydrology relies on moisture-limited tree species from dry, continental settings. This dissertation presents a new method by focusing on mid- to high-elevation conifers sensitive to snowpack variability. Ring-width and maximum latewood density records from mountain hemlock (Tsuga mertensiana (Bong.) Carriere), white spruce (P. glauca (Moench) Voss), and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) stands were collected at sites in northern BC. Dendrochronological techniques were used to develop a: 1) 223-year record of April 1 SWE for the Stikine River basin; 2) 417-, 716-, and 343-year record of summer streamflow for the Skeena, Nass and Stikine rivers, respectively; and, 3) a 193-year reconstruction of summer-run Skeena River steelhead abundance based on the influence of ocean-atmospheric forcings on both radial tree growth and steelhead escapement. The April 1 SWE record suggests that there has been considerable variability in snowpack levels in the Stikine basin and a distinct in-phase relationship with seasonalized Pacific Decadal Oscillation (PDO) indices, not seen in basins to the south. The summer streamflow records also support a north-south “see-saw” effect, suggesting an association between moisture transport and atmospheric-ocean circulation in the region. In addition to the snow-sensitive tree-ring data, the streamflow models incorporated paleo-hemispheric records to improve predictive skill. Finally, the steelhead model described alternating intervals of persistently above-average and below-average abundance that corresponded to oceanic PDO-like influences and describe links to “warm-warm” ENSO-PDO years associated with in-river low flow periods. The reconstructions suggest that: 1) recent snowpack and streamflow declines are a rare event over a multi-century context; and, 2) existing instrumental records do not adequately represent the historic range of basin-specific hydroclimate variability necessary for new planning horizons. Mid- to high-elevation, snow-sensitive conifers have strong potential as paleohydrological proxies and for expanding the application of dendrohydrology to non-arid settings. Current conditions in northern BC, compounded by land use changes and climate change, are predicted to become more severe in the future. It is important that planning regimes incorporate long-term hydroclimate data to better understand and quantify how water supply and ecosystems will respond to future changes. / Graduate