The annual cycle of temperature in the middle troposphere shows a well-developed kernlose (or coreless) pattern. This refers to the fact that temperatures at 500 hPa, for example, seldom fall below -45 °C in winter, despite a continued net radiative loss. This characteristic was originally noted in Antarctica. There, the mechanism
seems to be a balance between outgoing longwave radiation, downwelling longwave radiation, and sensible heat flux from the top of the surface inversion layer, although horizontal thermal advection by sub-polar air masses has also been invoked (van Loon, 1967). For the Arctic, Chase et al. (2002) propose that winter temperatures in the mid-troposphere are regulated by moist adiabatic ascent from airmasses in regular contact with high-latitude open ocean regions, such as the Norwegian Sea. The idea is that airmasses move over such open water areas, where they are convectively heated. They then move over cold snow/ice surfaces. They cool from the bottom upwards, albeit slowly. Before the cooling can significantly affect the mid-troposphere, the airmasses move over open ocean again, where they are re-warmed by convective heating. The proposed mechanism requires further investigation, but finds support in the subsequent study of Tsukernik et al. (2004), who identify the Norwegian Sea as a key region for convective warming (see also Chapter 3).
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