Upper-Tropospheric Water-Vapor Profiles Steepen With Sea Surface Temperature and Turn Near 303 K Over Tropical Oceans
Abstract
Upper-tropospheric water vapor strongly influences tropical longwave cooling, but many observational benchmarks do not test how moisture is partitioned vertically. This study evaluates whether the contrast between two upper-tropospheric layers changes systematically with local sea-surface temperature (SST). Aura Microwave Limb Sounder (MLS) version 5 water-vapor (H₂O) profiles from August 2004 through December 2023 were collocated with ERA5 SST and 500-hPa vertical velocity; National Oceanic and Atmospheric Administration outgoing longwave radiation (OLR) provided an independent cold-highcloud proxy. The upper-upper troposphere (UUT; 146.78–215.44 hPa) and lower-upper troposphere (LUT; 261.02–316.23 hPa) were summarized by the logarithmic moisture-ratio index dlnq = ln(q_UUT/q_LUT), for which more-negative values indicate a larger vertical contrast. Analyses over the western Pacific warm pool were replicated across tropical oceans using cell-based weighting, 10° spatial-block bootstrapping, dynamical stratification, alternative weighting, OLR subsets, and a layer-boundary sensitivity test. In the warm pool, dlnq became more negative from 300 K to approximately 303 K and then relaxed. The 303–305 K minus 300–303 K contrast was −0.116 (10° spatial-block 95% interval: −0.219 to −0.039). Both layers moistened, but the LUT increased more in fractional terms than the UUT (14.6% versus 9.2%). The qualitative pattern persisted across vertical-motion categories, weighting choices, a high- OLR subset, boundary reassignment, leave-one-year-out resampling, seasonal and ENSO stratification, and the tropical-ocean replication. The approximately 303 K feature is therefore a conditional turning point in vertical moisture partitioning, not a universal convective threshold or a direct estimate of climate feedback. The index provides a process-sensitive target for climate-model evaluation, while retrievalversion sensitivity remains to be established.