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Abstract
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 MaddenâJulian 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.
Highlights
In the tropics, water vapour and rainfall have been shown to be strongly related (e.g. Sherwood, 1999; Bretherton et al, 2004; Neelin et al, 2009)
The deep tropics have a low frequency of wet perturbations due to these regions almost always being above 45 kg mâ2
There are likely some perturbations that are associated with both cyclonic and anticyclonic potential vorticity (PV) anomalies given the 500 km distance condition used here. We note that these results indicate that while wet perturbations are relatively rare in the extratropics, when they do occur there, they are very likely associated with a PV anomaly
Summary
Water vapour and rainfall have been shown to be strongly related (e.g. Sherwood, 1999; Bretherton et al, 2004; Neelin et al, 2009). Mapes et al (2018) showed that tropical rainfall at any given time is contained in the region of very high total column water vapour (TCWV). They identified the value of TCWV to represent the threshold below which rainfall is rare and termed this the moist margin. Robinson et al (2024b) showed that this moist margin is highly variable on a range of timescales They showed strong variability exists at synoptic, or weather, timescales. This begs the question of what the underlying dynamical processes involved in these perturbations are and whether they are of tropical or extratropical origin (or both)
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