Weather systems associated with synoptic variability in the moist margin

Abstract. The moist margin is a sharp gradient of humidity that separates the moist deep tropics from the drier subtropics, and its movement is known to have an important effect on rainfall variability. In this work, we investigate how weather systems are related to synoptic variability in the moist margin. The weather systems considered include convectively coupled equatorial waves and the Madden–Julian Oscillation (MJO), monsoon low-pressure systems (LPSs), and extratropical interactions with the moist margin characterised by upper-level potential vorticity (PV) anomalies. We use an object-based approach in which, first, objects are defined to describe the variability of the moist margin and are then related to weather objects representing the above weather systems. Overall, the results indicate that these weather systems are associated with a large proportion of variability in the moist margin. The MJO and equatorial Rossby waves have a significant modulating effect on the moist margin. In comparison, monsoon LPSs are infrequent but strongly influence the moist margin when they occur. Interactions with the extratropics occur for around one-quarter of moist margin perturbations and display a clear extratropical wave-like signal, often with anticyclonic PV anomalies near the perturbed margin and cyclonic PV anomalies upstream. Overall, moist margin objects associated with weather systems are larger, longer-lived, and precipitate more, highlighting the important role of weather systems.