Narrowing of Lagrangian Cold-Point Pathways into the Tropical Stratosphere

Abstract. The tropical tropopause layer (TTL) plays a crucial role for air mass transport from the well-mixed convective troposphere into the radiatively controlled stratosphere. The minimum temperatures found at the cold-point tropopause are particularly important for regulating water vapour entry into the stratosphere. Extensive research has revealed a slow widening of the tropical belt in the troposphere while the Intertropical Convergence Zone (ITCZ) was found to become narrower. It remains unclear whether such structural changes also occur for the stratospheric entry region near the cold-point tropopause and how this would impact long-term cold-point temperature trends.

Here, we use 44 years (January 1981–August 2024) of Lagrangian trajectory simulations driven by MERRA-2 reanalysis to quantify long-term changes in the temperature and meridional width of the Lagrangian cold-point (LCP) region. We introduce a trajectory-based metric, defined as the 90th-percentile latitude range of the LCP distribution, to diagnose the spatial extent of stratospheric entry pathways. We find statistically significant cooling of LCP temperatures (−0.098 ± 0.027 K per decade) and a narrowing of the LCP width (−0.350 ± 0.098° per decade). A decomposition of LCP-weighted temperature fields reveals that both local temperature variability and changes in transport pathways contribute to the observed long-term cooling. Additional analysis of transport width in vertical layers around the LCP (±15 hPa) shows systematic narrowing toward the LCP core. These results provide strong evidence that stratospheric entry pathways have become increasingly confined over recent decades, with implications for TTL dehydration and lower-stratospheric water vapor trends.