Moisture transport axes: a unifying definition for monsoon air streams,
atmospheric rivers, and warm moist intrusions
Abstract
The water vapor transport in the extratropics is mainly organized in
narrow elongated filaments. These filaments are referred to with a
variety of names depending on the contexts. When making landfall on a
coastline, they are generally referred to as atmospheric rivers; when
occurring at high latitudes, many authors regard them as warm moist
intrusions; when occurring ahead of a cold front towards the core on an
extratropical cyclone, the most commonly used term is warm conveyor
belt. Here, we propose an algorithm that detects these various lines of
moisture transport in instantaneous maps of the vertically integrated
water vapor transport. The detection algorithm extracts well-defined
maxima in the water vapor transport and connects them to lines that we
refer to as moisture transport axes. By only requiring a well-defined
maximum in the vapor transport, we avoid imposing a threshold in the
absolute magnitude of this transport (or the total column water vapor).
Consequently, the algorithm is able to pick up moisture transport axes
at all latitudes without requiring region-specific tuning or
normalization. We demonstrate that the algorithm can detect both
atmospheric rivers and warm moist intrusions, but also prominent monsoon
air streams. Atmospheric rivers sometimes consist of several distinct
moisture transport axes, indicating the merging of several moisture
filaments into one atmospheric river. We showcase the synoptic
situations and precipitation patterns associated with the occurrence of
the identified moisture transport axes in example regions in the low,
mid, and high latitudes.