Title: Observed Tropical Pacific Temperature Trends Outside Modelled Range When Computed in a Novel Temperature Coordinate.
Abstract:
Tropical Pacific sea surface temperatures (SSTs) exert a substantial influence on regional and global climate, shaping large-scale atmospheric circulation and driving seasonal climate extremes. Atmosphere–ocean interactions in the tropical Pacific are tightly coupled, complex, and undergo large internal variability modulated by a strong background SST gradient across the Pacific basin. From 1982 to 2020, this gradient has intensified, but it remains unclear if this is due to internal variability or anthropogenic warming, and whether the observed trend lies within the envelope of climate model simulations. Here, we show that clarity on these questions can be achieved through judicious choices of gradient metrics that better isolate trends from internal variability. Past literature has examined long-term trends in the SST gradient using fixed east–west boxes—but the temperature within these boxes is strongly influenced by internal variability due to factors such as El Niño–Southern Oscillation. We introduce a semi-Lagrangian metric based on the difference between dynamic areas characterizing the warmest and coldest 50% of the tropical Pacific surface ocean. Previous studies using fixed east–west boxes have shown that the observed and modeled trends are unlikely to be consistent; our study shows that when utilizing a cold–warm semi-Lagrangian metric, the consistency is beyond unlikely, becoming virtually impossible.
