Atmospheric moisture anomalies associated with ENSO and future changes in CMIP6 simulations

Atmospheric moisture is perturbed during ENSO variation, as can be quantified using regression with ENSO indices. Seasonal and annual anomalies of the water column, horizontal moisture flux, surface evaporation, rainfall, and other basic variables, associated with the sea-surface temperature indices NINO34 and Pacific-Indian Dipole are evaluated from ERA5 reanalyses over 1980-2019. The skill in the corresponding regression coefficients (at one standard deviation) from historical climate simulations by the ten (only) CMIP6 models for which the vertically integrated flux was submitted is assessed, subject to the statistical uncertainty in ENSO from 40-year series. The ten-model mean fields are encouragingly realistic, although ENSO anomalies in the equatorial Pacific extend farther westward. The future change for the period 2040-2079 under the SSP585 scenario of rising greenhouse gases is evaluated. There is generally little change in the standard deviation in the two indices or in the SST and wind anomalies. The water column, moisture flux, and rainfall anomalies tend to be amplified in the low latitudes, but with limited change in the teleconnections to higher latitudes. The climatological changes in rainfall and moisture flux resemble those of ENSO in the tropical Indo-Pacific, in part linked to a small positive shift in both the indices. Elsewhere, widespread increases in water column, evaporation, midlatitude surface pressure, and, of course, temperature are not ENSO-like. Implications for the reliability of future projected means and variability will be considered. An obvious recommendation is that the vertically integrated moisture fluxes be routinely output by climate models and be a requested variable in future CMIPs.