Upper-tropospheric troughs and North American monsoon rainfall in a long-term track dataset


The North American monsoon is frequently affected by transient, propagating upper tropospheric vorticity anomalies. Sometimes called Tropical Upper-Tropospheric Troughs (TUTTs), these features have been claimed to episodically enhance monsoon rainfall. Here we track long-lived TUTTs in 40 years of reanalysis data, producing composites and case studies from 340 TUTTs which last, on average, seven days as they move westward across the North American monsoon region. TUTTs are thought to form from midlatitude Rossby wave breaking; case studies from our dataset support this theory. TUTTs move westward within the easterly upper-level flow in which they are embedded. In vortex-centered composites along the full tracks of long-lived TUTTs, we find no detectable increase in rainfall within the main TUTT circulation. Instead, negative precipitation anomalies lie within about 500 km of the TUTT center. Quasi-geostrophic ascent occurs in the southeast quadrant of TUTTs but is confined to the upper troposphere and does not appear to interact with precipitation. Positive anomalies of ascent and rainfall occur south and southeast of TUTTs but lie outside the main TUTT vortex, perhaps indicating concurrent variations in nearby climatological precipitation maxima. In contrast with previous case studies and subjective analyses that showed TUTTs enhance precipitation in parts of northwestern Mexico, our composites along the tracks of long-lived TUTTs portray these systems, to first order, as strong vorticity anomalies trapped in the upper troposphere that interact only weakly and indirectly with precipitation.

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