Equatorward shift of ENSO-related subtropical jet anomalies in recent decades

Abstract

Using observations, NCEP-NCAR and ERA5 reanalysis datasets, numerical experiments and CMIP6 model outputs, this study investigates long-term changes in the meridional position of subtropical jet anomalies in association with El Niño-Southern Oscillation (ENSO). It is found that the subtropical jet anomalies associated with ENSO index (i.e., Niño3.4 index in this study) over both Northern Hemisphere (NH) and Southern Hemisphere (SH) significantly (at the 99% confidence level) shift equatorward since the 1960s, at the speeds of 1.28 and 1.00° per decade, respectively. In recent six decades, the subtropical jet anomalies have moved equatorward by 4.34 and 3.19° over NH and SH, respectively. Our analysis indicates that the tropical positive sea surface temperature (SST) anomalies associated with Niño3.4 (corresponding to its warm phase) during 1990–2020 (P2) are weaker than those during 1950–1980 (P1). The weak, positive SST anomalies induce tropical weak, positive surface latent heat flux and precipitation anomalies, which heat tropical troposphere and produce weak, positive geopotential height anomaly with a narrow meridional width over the tropics during P2. According to geostrophic wind relations, anomalous positive zonal winds occur over the subtropics in the upper troposphere, meaning strengthened subtropical westerly jets over NH and SH. Hence, the relatively weak, narrow positive geopotential height anomaly during P2 favors the equatorward shift of Niño3.4-related subtropical jet anomalies via geostrophic wind relations. The results from numerical experiments and CMIP6 model outputs also indicate that changes in tropical SST anomalies associated with Niño3.4 contribute to this equatorward shift, supporting the results from reanalysis datasets. These results suggest that the intensity of tropical SST anomalies linked to ENSO should be well resolved to better simulate and predict subtropical jet variations.