Characterization of transport regimes and the polar dome during Arctic spring and summer using in situ aircraft measurements

The springtime composition of the Arctic lower troposphere is to a large extent controlled by the transport of midlatitude air masses into the Arctic. In contrast, pre- cipitation and natural sources play the most important role during summer. Within the Arctic region sloping isentropes create a barrier to horizontal transport, known as the polar dome. The polar dome varies in space and time and exhibits a strong influence on the transport of air masses from mid- latitudes, enhancing transport during winter and inhibiting transport during summer. We analyzed aircraft-based trace gas measurements in the Arctic from two NETCARE airborne field campaigns (July 2014 and April 2015) with the Alfred Wegener Insti- tute Polar 6 aircraft, covering an area from Spitsbergen to Alaska (134 to 17◦ W and 68 to 83◦ N). Using these data we characterized the transport regimes of midlatitude air masses traveling to the high Arctic based on CO and CO2 mea- surements as well as kinematic 10 d back trajectories. We found that dynamical isolation of the high Arctic lower tro- posphere leads to gradients of chemical tracers reflecting dif- ferent local chemical lifetimes, sources, and sinks. In par- ticular, gradients of CO and CO2 allowed for a trace-gas- based definition of the polar dome boundary for the two mea- surement periods, which showed pronounced seasonal differences. Rather than a sharp boundary, we derived a transi- tion zone from both campaigns. In July 2014 the polar dome boundary was at 73.5◦ N latitude and 299–303.5 K potential temperature. During April 2015 the polar dome boundary was on average located at 66–68.5◦ N and 283.5–287.5 K. Tracer–tracer scatter plots confirm different air mass prop- erties inside and outside the polar dome in both spring and summer. Further, we explored the processes controlling the recent transport history of air masses within and outside the polar dome. Air masses within the springtime polar dome mainly experienced diabatic cooli