The nature and dynamics of rapid spring onset in the Arctic

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He, Jia
Black, Robert X.
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The Arctic atmospheric circulation undergoes a systematic rapid seasonal transition each year from late winter to early spring. At stratospheric and upper-tropospheric levels, this transition is dominated by an abrupt break down of the circumpolar vortex, which is related to upward planetary wave propagation. At the surface, this rapid spring onset transition is manifested by a dramatic increase in air temperature within a short time period. Very limited prior research efforts exist on the nature and physics of the rapid near-surface warming during Arctic spring onset (ASO). The current dissertation provides a thorough investigation of the abrupt transition of ASO events from the perspective of near-surface air temperature increase. In contrast to conventional measures of spring onset timing (often defined in terms of temperature thresholds related to regional phenology), our investigation views the abrupt transition as either an acceleration of air temperature increase or a transition from a steady winter state into a warming spring state. Thus, the first part of this dissertation provides a comprehensive exploration of three novel techniques for identifying ASO via the rapid increase in the 2-meter temperature (T2m). A 2-phase linear regression model is first employed to identify a transition from an approximately steady winter state to a warming spring state. The other two methods, the time derivative (d2T/dT) and radius of curvature (RoC) techniques, isolate periods of large T2m acceleration. Although all three approaches are largely successful in isolating the state transition associated with ASO, the RoC method is most effective in capturing the most rapid temperature increases and is this adopted in the remainder of the dissertation. It is determined that ASO timing exhibits strong interannual variability but with no significant long-term trend. The Arctic-mean composite evolution reveals T2m increases notably faster than the climatological seasonal trends, which indicates the likelihood of a dynamical driving mechanism. Spatial patterns of T2m changes occurring during ASO events are shown via a composite analysis. The rapid warming observed over Polar latitudes during ASO is roughly zonally symmetric while concomitant warming patterns observed further south distinguish North Siberia as the critical region (CR) in which robust warming occurs during most ASO events. Besides such common structures isolated in the composite, individual events demonstrate distinct primary warming structures that may occur outside the CR region. A hierarchical cluster analysis and a synoptic classification technique are applied to the individual ASO T2m change maps leading to the identification of four subsets of events distinguished by their primary regional warming signature. The synoptic behavior of ASO events is studied via a parallel composite analysis of sea level pressure (SLP) anomalies. This analysis reveals that, during ASO events, changes in the regional semi-permanent surface pressure features provide favorable conditions that promote regional temperature advection and ASO warming. Thus, our synoptic analyses also suggest that ASO events are dynamically driven by large-scale atmospheric processes. To provide a more quantitative understanding of the physical sources for the rapid temperature increases observed during Arctic spring onset, we divide the temperature tendency in the thermodynamic equation into separate terms related to different physical processes: horizontal linear advection, nonlinear eddy heat flux convergence, heat transport via vertical motions, adiabatic heating and cooling, and diabatic effects. After evaluating the contributions of each term to the temperature tendency, we find that nonlinear heat transport is essential for initiating the rapid temperature increase prior to and during the early stage of events while linear temperature advection is the leading factor in maintaining rapid warming during the later stages. This notion is further supported by the observed accumulation of eddy heat transport into the primary warming region during the early ASO stages and the formation of a coherent large-scale circulation pattern favorable for warm advection during middle and late ASO stages. Finally, a case study is performed for the CR event in 2000 (CASE2000) using both observational data and numerical model simulation outputs. During CASE2000, robust T2m increases resembling the composite structure occur over the CR region. A heat budget analysis of CASE2000 identifies nonlinear processes as the primary contributor to the early stages of onset and linear advection as a leading source later on. Model experiments using the Weather Research and Forecasting Model (WRF) are performed for CASE2000 to directly access the role of nonlinear processes due to synoptic eddies. A control experiment is first conducted to simulate CASE2000. Then, two parallel sets of model experiments are performed to simulate conditions in which synoptic eddy activity is suppressed by using a) meteorological variables filtered using a 10-day running mean and b) composite meteorological variables as input for initial and boundary conditions. The timing of CASE2000 is effectively delayed by suppressing synoptic eddy activity at the lateral boundary that also leads to a large reduction on magnitude of T2m increase. The case study provides a ancillary support to the composite phenomenological and mechanistic analysis of ASO events.
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