In the Arctic, the arrival of spring is sudden. The abrupt onset of Arctic spring comes with the development of high pressure over Baffin Bay and the Labrador Sea and shifts in the jet stream and storm tracks. These changes are important for the Arctic climate and environment and have critical influences on Arctic peoples and ecosystems. This project will test the hypothesis that the spring Arctic transition is a dynamical phenomenon, driven by the downward influence of the springtime stratospheric final warming, abrupt changes in the flow over and around Greenland and in atmospheric waves over the North Atlantic, and shifts in the frequency and tracks of storms entering the Arctic. The spring transition effects the surface climate and is likely modified by feedbacks from surface processes. The implications of the dynamical onset of Arctic spring for future changes in the Arctic will be explored, through analyses of atmospheric and Arctic surface data, experiments with a global atmospheric model, and analyses of 20th Century and future climate simulations. This project will develop a dynamical understanding of the Arctic spring transition and its interactions with the Arctic environment. It will use this understanding to evaluate climate models based on their ability to simulate the spring transition, and it will examine model projections of future changes in the Arctic environment resulting from changes in the timing, strength, and structure of the spring transition.