Detecting Global Hydrological Cycle Intensification

Abstract:

Global warming is expected to intensify the global hydrological cycle, but significant regional differences exist in the predicted response. The predicted zonal mean response is enhanced horizontal moisture transport associated with increased saturation vapor pressure, which in turn drives additional net precipitation in the tropics and additional net evaporation in the subtropics. The observed two-dimensional response suggests that the spatial pattern of sea surface salinity is, in fact, amplifying. High salinity regions have become saltier while low salinity regions are have become fresher. Most noticeable is the widening gap between the relatively fresh Pacific ocean and the relatively salty Atlantic ocean. Changes in evaporation minus precipitation (E-P) over the global ocean in response to warming are difficult to observe directly, but changes in sea surface salinity provide a useful proxy for these changes in that they integrate changes in the highly variable surface freshwater flux. To quantify the relationship between sea surface salinity and temperature, we project sea surface salinity anomalies onto evaporation minus precipitation (E-P) to create an index of salinity variability. Conceptually, this index can be thought of as an indicator of the strength of the hydrological cycle, and is dominated by the rate of freshwater transport in the Hadley Cell. The results reveal a strong relationship between tropical temperature variability and the dominant pattern of sea surface salinity. This relationship is coherent across a broad range of frequencies. There are also strong differences in the physical response to warming in the Atlantic and Pacific ocean basins, with the Atlantic salinity response more closely tied to mean salinity and the Pacific salinity response more closely tied to evaporation minus precipitation. We hypothesize that this is due to large-scale evaporative regions and mixed-layer dynamics dominating the salinity response in the Atlantic ocean and the Pacific ocean exhibiting the highly variable nature of the major climate modes in the region. We extend the results to early the 20^th century by examining archival of sea surface salinity measurements from 1900-2014.