Observational evidences of Walker circulation change over the last 30 years contrasting with GCM results

Abstract

In order to examine the changes in Walker circulation over the recent decades, we analyzed the sea surface temperature (SST), deep convective activities, upper tropospheric moistening, sea level pressure (SLP), and effective wind in the boundary layer over the 30-year period of . The analysis showed that the eastern tropical Pacific has undergone cooling while the western Pacific has undergone warming over the past three decades, causing an increase in the east-west SST gradient. It is indicated that the tropical atmosphere should have responded to these SST changes; increased deep convective activities and associated upper tropospheric moistening over the western Pacific ascending region, increased SLP over the eastern Pacific descending region in contrast to decreased SLP over the western Pacific ascending region, and enhanced easterly wind in the boundary layer in response to the SLP change. These variations, recognized from different data sets, occur in tandem with each other, strongly supporting the intensified Walker circulation over the tropical Pacific Ocean. Since the SST trend was attributed to more frequent occurrences of central Pacific-type El Niño in recent decades, it is suggested that the decadal variation of El Niño caused the intensified Walker circulation over the past 30 years. An analysis of current climate models shows that model results deviate greatly from the observed intensified Walker circulation. The uncertainties in the current climate models may be due to the natural variability dominating the forced signal over the tropical Pacific during the last three decades in the twentieth century climate scenario runs by CMIP3 CGCMs.

Simple dynamical models capturing the key features of the Central Pacific El Niño

“…Corresponding to a nonzero constant easterly trade windâ p < 0, the direct response of the surface wind associated with the Walker circulation at the equatorial Pacific band is shown in Fig. 2C, which is similar to the observed intensification of the Walker circulation in recent decades (13,14).…”

“…Composite analysis of reanalysis data shows that zonal advection has a significant contribution to the SST tendency (5) and is particularly important during the initiation phase of CP El Niño (12). On the other hand, accompanying with the increasing occurrence of the CP El Niño since 1990s, a recent multidecadal acceleration of easterly trade winds is observed (13)(14)(15), which is linked with the strengthening of the Walker circulation (16)(17)(18). Due to the atmosphere-ocean coupling, the enhanced atmospheric circulation then induces the intensification of the westward ocean current with an increased upwelling of cold water in the eastern Pacific ocean, which tends to prevent the full eastward extension of the anomalous warm SST during El Niño phases and leads to the occurrence of El Niño in the central Pacific ocean.…”

“…Here, ϕ 0 ðyÞ has a Gaussian profile centered as the equator and is equal to the first parabolic cylinder function of the atmosphere (SI Appendix). Both the wind bursts perturbations (31) and the strengthening of the trade wind (13,14) are localized over the western equatorial Pacific according to the observations and for simplicity they share the same zonal extent. The evolution of wind burst amplitude a p reads:…”

Recent slowdown of tropical upper tropospheric warming associated with Pacific climate variability

“…These basin-scale SST variabilities exert considerable influence on the observed climate trends found in the tropics and extratropics [e.g., Trenberth et al, 2014], including the Walker circulation and widening of the tropical belt [Sohn et al, 2013;Allen et al, 2014;Bayr et al, 2014;Kociuba and Power, 2015]. The importance of the forced response and internal climate variability on recent global and regional climate variations should be examined further in future studies.…”

A comparative study of large-scale atmospheric circulation in the context of a future scenario (RCP4.5) and past warmth (mid-Pliocene)

“…Changes in Walker circulation have a large impact on global and regional climate (Ropelewski and Halpert, 1989;Power et al, 1999;Holmgren et al, 2001). Thus, it has been a major research focus for many years (Trenberth and Hoar, 1997;Collins et al, 2010;Sohn et al, 2013). Figure 3a-c shows the Walker circulation defined by the divergent component of the horizontal vector, corresponding potential velocity (contours) at 200 hPa and the vertical weighted average of vertical wind ω over each grid cell at each level (from 100 hPa to 1000 hPa), weighted by the level's pressure depth (shaded dots), derived from the control run, mid-Pliocene and RCP4.5 scenario, respectively.…”