Pacific Walker circulation indices in ERA5 reanalysis

The Pacific Walker circulation (PWC) is an integral part of the tropical atmospheric circulation that influences weather patterns globally, including the global distribution of mean precipitation. It also affects the rate of global warming, sea-level rise, and the efficiency of ocean carbon absorption. Due to its importance, many indices have been used historically to quantify the PWC strength and its variability, such as indices based on surface pressure, surface zonal winds, mid-tropospheric vertical velocity and stream function, upper-tropospheric velocity potential, etc.

We reviewed and compared ten different PWC indices for the 1951-2020 period using ERA5 reanalysis data, which is verified against observations.

Our results show good agreement between the time series of normalized indices for annual-mean PWC strength. The exceptions are indices based on 200 hPa velocity potential and 500 hPa stream function. Considering their performance compared to other indices, we explored an alternative definition of the stream-function index and proposed a revised, data-adaptive velocity potential index. Generally, the correlations between the ten indices are high, with the highest correlations between indices based on closely linked physical processes. The largest disagreement between the indices occurs during the strongest El Niños. Therefore, for a robust evaluation of the PWC trends, El Niño events have to be filtered from the time series (as in L’Heureux et al., 2014) and should not coincide with the beginning or the end of the time series. Indices of PWC strength show increasing trends for 1981-2010, mostly neutral trends for the 1951-2020 period, and weak and often insignificant negative trends for the latest 2000-2020 period. PWC trends in ERA5 also hint at a possible multidecadal variability with approximately a 35-year period.