Structure of the Pacific Walker Circulation Depicted by the Reanalysis and CMIP6

Emmanuel Olaoluwa Eresanya,Yuping Guan

doi:10.3390/atmos12091219

Abstract

The Pacific Walker circulation (PWC) is one of the most important components of large-scale tropical atmospheric circulations. The PWC and its influences have been studied extensively by numerical models and reanalysis. The newly released ERA5 and NCEP2 are the most widely used reanalysis datasets and serve as benchmarks for evaluation of model simulations. If the results of these datasets differ significantly, this could lead to a bias in projected long-term climate knowledge. For better understanding of future climate change, it is necessary to evaluate PWC reanalysis productions. As a result, we compared the PWC structures between the ERA5 and NCEP2 datasets from month to seasonal time scales. We used the zonal mass streamfunction (ZMS) over the equatorial Pacific to indicate the strength of the PWC. The PWC’s average monthly or seasonal cycle peaks around July. From February to June, the NCEP2 shows a higher PWC intensity, whereas the ERA5 shows greater intensity from July to December. The circulation center in the NCEP2 is generally stronger and wider than in the ERA5. The ERA5, however, revealed that the PWC’s west edge (zero line of ZMS over the western Pacific) had moved 10 degrees westward in comparison to the NCEP2. In addition, we compared the PWC mean state in the reanalysis and CMIP6 models; the mean state vertical structures of the tropical PWC in the CMIP6 multi-model ensemble (MME) are similar to those of the reanalyses in structure but weaker and wider than in the two reanalysis datasets. The PWC is broader in CMIP6, and the western boundary is 7 and 17 degrees farther west than in the ERA5 and NCEP2, respectively. This study suggests that, when using reanalysis datasets to evaluate PWC structural changes in intensity and western edge, extreme caution should be exercised.

Highlights

The Pacific Walker circulation (PWC) is an important component of the global climate system; it features low-level winds blowing from east to west across the central Pacific, a rising motion over the Maritime Continent and the warm western Pacific, returning flow from west to east in the upper troposphere, and a sinking motion over the cold water of the eastern Pacific [1,2,3,4,5,6]
Pacific) and core of the PWC were farther west by 5◦ and 10◦ than in the NCEP2 (Figure 1a)
The mean state difference between the ERA5 and the NCEP2 shows that the western Pacific is controlled by a positive zonal mass streamfunction (ZMS), meaning that the PWC’s intensity was weaker in the ERA5

Summary

Introduction

The Pacific Walker circulation (PWC) is an important component of the global climate system; it features low-level winds blowing from east to west across the central Pacific, a rising motion over the Maritime Continent and the warm western Pacific, returning flow from west to east in the upper troposphere, and a sinking motion over the cold water of the eastern Pacific [1,2,3,4,5,6]. The PWC regulates the global exchange of heat energy, momentum, and water vapor within the tropics through substantial overturning motions. It performs a major task in the steadiness of atmospheric energy. A consensus from studies on the PWC dynamics and variation is the respective strengthening and weakening in the west and east Pacific in recent decades [8,10,12,13,14,15,16,17,18,19,20]. The PWC intensity and structure have changed over time in response to changes in precipitation and temperature, such as the drying of eastern Africa, intensified Northern Hemisphere summer monsoon precipitation, and a recent global warming hiatus [15,21,22,23,24,25,26,27,28]

Methods

Discussion

Conclusion