McGregor, S., Timmermann, A., Stuecker, M.F., England, M.H., Merrifield, M., Jin, F.F., Chikamoto, Y. Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming. Nature Clim. Change Online (2014). DOI: http://dx.doi.org/10.1038/nclimate2330
This article discusses the subject of intensifying Pacific trade winds and resulting wind-driven circulation from 1992 – 2011. Recently I covered an article that connected the increased wind-stress to the global warming hiatus (see England et al., 2014). The trend of increased trade winds is not consistent with changes in Pacific Ocean sea surface temperature anomalies, therefore signaling a potential remote driving mechanism from outside of the Pacific basin. A global see-saw pattern of sea-level pressure gradients, positive in the Pacific and negative in the Indian Ocean to Atlantic Ocean region, occurs simultaneously with increased Pacific trade winds. This suggests that a driving mechanism outside of the Pacific basin must exist. This study analyzes the observed sea surface temperature (SST) and sea-level pressure (SLP) trends, and uses climate modeling to determine what remote mechanism(s) are responsible for the acceleration of Pacific trade winds.
SST Observed Trends 1992 – 2011
General warming trends were observed in the Atlantic, western subtropical Pacific and Indian Oceans (Figure 1a). A cooling trend was observed in the eastern Pacific Ocean. These observed trends differ from general circulation model simulated warming hiatuses, which typically show cooling SSTs in all ocean basins.
Climate Modeling Experiments
This study utilizes the Community Atmospheric Model, version 4 (CAM4) to determine the mechanism responsible for the observed trends in trade wind acceleration, SST and SLP. Unlike the general circulation model, CAM4 captures the SLP see-saw and accelerated Pacific trade winds when forced by observed global SST trends (Figure 1c). Though the general patterns were captured, CAM4 underestimated the 1992-2011 central Pacific trade wind intensification (Figure 1d).
These CAM4 simulations do not apply SST anomalies to all ocean basins, but rather one basin at a time to best determine which SST anomalies are forcing the atmospheric circulation changes in the Pacific Ocean. The particular simulation where the Atlantic SST trend is solely used as the forcing mechanism, while the Indian Ocean is prescribed climatological SST and the Pacific Ocean is allowed to adjust to the forcing, yields the recent Pacific Ocean trade wind intensification and SLP see-saw (Figure 1e). This experiment reproduces observed eastern Pacific cooling and global precipitation patterns (e.g. the current southwest US drought) (Figure 1f). These results strongly suggest that increasing Atlantic SST trends greatly impact Pacific trade wind intensification.
This experiment is similar to Experiment 2 in that the climatic forcing is solely Atlantic SST trends. However, in these simulations, both the Pacific and Indian Oceans are free to adjust to the forcing. The trans-basin SLP see-saw is still evident, however, the gradient is reduced due to a smaller magnitude eastern Pacific pressure lobe (Figure 2a,b). The results of this experiment link Atlantic SST to Pacific Walker circulation, as evidence of rising air in the Atlantic basin and sinking air in the eastern Pacific basin. Walker circulation is the east-west atmospheric circulation caused by rising warm air, in this case the Atlantic basin, and sinking cool air in the eastern Pacific basin.
The final experiment runs CAM4 simulations only forced by Indian Ocean SST trends. There is no resulting SLP see-saw or increase in Pacific trade winds. These findings contrast previous studies that identify Indian Ocean SSTs as the major contributor to changes in the Pacific Ocean trade winds.
The rapid warming of Atlantic SST beginning in the early 1990s is responsible for the rapid intensification of Pacific trade winds. Eastern Pacific SST cooling is a consequence of wind-intensified upwelling. Though the global SST forced CAM4 experiments generally captured observed trends, current climate models consistently underestimate the connectedness of atmospheric basins on decadal timescales. Additionally, CAM4 experiments were able to explain the current severe drought in the southwestern United States. The findings of this study suggest that warming Atlantic SST is directly related to Pacific Ocean Walker circulation. Walker circulation is the zonal (east-west) atmospheric circulation caused by rising warm air, in this case the Atlantic basin, and sinking cool air in the eastern Pacific basin. Rising warm air creates a region of low-pressure, whereas sinking cool air creates a region of high-pressure (Figure 3). This can be conceptualized as a see-saw, where high pressure “sinks” one end of the see-saw, and low pressure causes the other end to rise. An increasing pressure gradient consequently accelerates trade winds from high to low pressure. Walker circulation is closely tied with El Niño Southern Oscillation (ENSO). Typically, easterly trade winds (blowing east to west) push and pile up water in the western Pacific. An El Niño mode occurs when Walker circulation weakens, resulting in relaxation of easterly trade winds and warm water from the western Pacific “sloshing” back towards the eastern Pacific, reversing the see-saw.
It is important to constantly improve our understanding of our modern climate system during an epoch of change. This study provides strong evidence that rapidly warming Atlantic SST is the mechanism for rapidly intensifying Pacific trade winds and cooling Pacific SST. These climatic changes have been in place since the early 1990s and have contributed to the warming hiatus, the most recent decade of negligible global warming. Studies such as the one highlighted here will improve our chances of holding predictive power so that better policy decisions can be made amidst a changing climate.
I am a recent graduate (Dec. 2015) from the University of Rhode Island Graduate School of Oceanography, with a M.S. in Oceanography. My research interests include the use of geophysical mapping techniques in continental shelf, nearshore and coastal environments, paleoceanography, sea-level reconstructions and climate change.