Variability and trends of the Pacific Walker circulation indices in multiple reanalyses

The Pacific Walker Circulation (PWC) is an important part of the global atmospheric circulation. Its strength is changing constantly due to internal variability, such as ENSO, IPO, and multi-decadal climate variability, as well as the forced response to increasing greenhouse gas concentrations. Despite the observational studies showing strengthening of the PWC, modelling studies predominantly predict its weakening with the reason for this discrepancy still unknown. Generally, results of different studies are difficult to compare, due to different datasets, different time intervals and different indices used. However, any change in PWC strength may have vastly different effects on the global climate, which underlines the importance of proper simulation of its trends in the changing climate. 

The main aim of our study is to assess the consistency of different PWC indices across reanalyses.  We compared nine different PWC indices between six different reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2, NCEP and NOAA 20CRv3), both over their respective time spans and over the common period from 1981 to 2011. The comparison was performed for the trends of different lengths, as well as correlations between indices for each reanalysis and between reanalyses for each index. 

On average, time series of indices in reanalyses verify well compared to pure observation-based indices. They are able to capture the strongest El Niño years and distinguish between El Niño and La Niña states. As expected, the spread of indices is larger in the surface-based reanalyses than in the full atmospheric reanalyses, as the the lack of observations in the free atmosphere in the former make them less constrained by observations. The spread is generally the largest during the strongest El Niño events. Correlations between the indices are stronger for the indices based on physically related processes, such as indices based on surface pressure and surface wind or 500-hPa vertical velocity and upper tropospheric humidity. Indices based on surface variables are more strongly correlated between reanalyses than indices based on data in the upper-atmosphere.  

Trends of different lengths show mixed results, depending on the reanalyses and indices. The 20-year trends ending at about 2020 are mostly negative but not statistically significant. Ultimately, this raises a key question: Are the traditional PWC indices still suitable for capturing changes in today’s rapidly changing global climate?