Moisture transport axes: a unifying definition for tropical moisture exports, atmospheric rivers, and warm moist intrusions

Abstract. The horizontal water vapour transport is mainly organised in narrow elongated filaments. These filaments are referred to with a variety of names depending on the context, e.g. tropical moisture export, atmospheric river, warm moist intrusion, warm conveyor belt, and feeder air stream. Despite the various names, these features share essential properties, such as their narrow elongated structure. Here, we propose an algorithm that detects these various lines of moisture transport in instantaneous maps of the vertically integrated water vapour transport. The detection algorithm extracts well-defined maxima in the water vapour transport and connects them to lines that we refer to as moisture transport axes. By only requiring a well-defined maximum in the vapour transport, we avoid imposing a threshold on the absolute magnitude of this transport or the total column water vapour. Consequently, the algorithm is able to trace moisture transport axes at all latitudes without requiring region-specific tuning or normalisation. We demonstrate that the algorithm can detect both atmospheric rivers and warm moist intrusions as well as tropical moisture exports, prominent monsoon air streams, and low-level jets with moisture transport. 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. As our detection algorithm performs seamlessly from the tropics across the mid-latitudes into the polar regions, our approach might turn out to be particularly useful to study moist interactions between the tropics and subtropics, mid-latitudes, and polar regions.