How the North Atlantic storm track builds the moisture supply to warm conveyor belt ascent

Abstract. The moisture entering warm conveyor belt (WCB) ascent constrains the latent heat release that shapes extratropical cyclone development and the downstream circulation, yet the processes establishing this moisture remain poorly studied. Using 10-day backward trajectories initialized from North Atlantic WCB inflows in ERA5 over 43 winters (1980-2022), we identify four main dynamical histories of the inflowing air based on which environments shaped their moisture pick-up: cold air outbreak (CAO), fast descent, slow descent, and subtropical origin. WCB inflow is a superposition of these pathways, whose moisture is predominantly of recent extratropical origin and is acquired on strongly pathway-dependent timescales, ranging from about two days for the CAO and fast descent pathways to four to six days for air of subtropical and slow descent origin. We then ask why some WCBs are anomalously moist, while others remain relatively dry. We find that moist and dry inflows differ markedly in pathway composition, with subtropical origin air replacing CAO air for anomalously moist WCBs, but this shift contributes little to the moisture anomaly because the opposing compositional changes nearly cancel. The anomaly instead arises within the pathways: air arriving through every pathway is itself anomalously moist. Within each pathway, moist and dry inflows undergo similar thermodynamic evolution, but moist inflows progress farther toward warmer and more saturated conditions before ascent, while dry inflows are incorporated into the WCB at an earlier stage of this evolution. This difference is established well before WCB inflow: air feeding moist WCBs takes up moisture earlier, after an extended equatorward excursion on the flank of an antecedent anticyclone, whereas air feeding dry WCBs is moistened within a recent CAO shortly before ascent. The two groups are nonetheless embedded in nearly identical sea level pressure patterns at inflow, so their contrasting moisture is not apparent from the circulation at the onset of WCB ascent. How moist a WCB inflow becomes is therefore not determined by pathway identity alone, but by the preconditioning accumulated along that pathway. The antecedent circulation shapes both the route taken toward WCB ascent and how far air progresses along its associated thermodynamic evolution, thereby preconditioning the moisture available for latent heat release during WCB ascent.