Article: Li, X., Xie, S., Gille, S.T., Yoo, C., (2016), Atlantic-induced pan-tropical climate change over the past three decades, Nature Climate Change 6, 275-279, DOI:10.1038/nclimate2840.
Since 1979, satellites have revolutionized our ability to observe the ocean as a whole, with extremely precise measurements. An interesting observation of tropical sea surface temperatures (Fig 1) over the past three decades (aka the “Satellite Era”) is a pronounced warming of the tropical Atlantic, Indian and western Pacific Oceans, despite a cooling central to eastern Pacific Ocean. With the well-established understanding of the interconnectedness of the tropical ocean basins, it is somewhat puzzling that such a temperature gradient exists. Researchers set out to investigate what is causing the cooling of the eastern Pacific, despite warming in the rest of the tropical oceans.
Chapter 1: The Suspects
A classic climate whodunit! Which ocean is to blame for the cooling in the Pacific Ocean? The researchers had their tropical ocean suspects , but how could they get them to talk? Without the luxury of a face-to-face interrogation, as most criminal investigations likely have, the researchers used a series of experiments, enlisting cutting edge climate models. Climate models allow scientists to simulate how each ocean individually responds to changes occurring in a different ocean. Using these climate models, the scientists were able to systematically warm each tropical ocean individually, and observe the response of the other oceans. Warming the Indian Ocean and Pacific Ocean failed to produce changes in the other oceans that were consistent with the trends observed by satellites. Conversely, warming the Atlantic Ocean produced warming in the Indian and western Pacific Oceans, and crucially, cooling in the eastern Pacific Ocean. The Atlantic was caught red handed!
Chapter 2: Probable Cause
So what was the probable cause in our climate mystery? Warming in the Atlantic Ocean producing warming and cooling trends in the other tropical ocean basins consistent with satellite observations is merely correlation; strong science must also provide causation. In this case, what mechanisms can explain this global climate trend?
Carefully designed experiments, again recruiting climate models to help, were performed. The results were conclusive: Atlantic warming causes atmospheric convection in the form of Kelvin and Rossby atmospheric waves. The effects of Kelvin and Rossby waves are changes in wind patterns. Equatorial Kelvin waves are responsible for increased easterly winds over the Indian and western Pacific Oceans, whereas Rossby waves increase westerly winds over the eastern Pacific Ocean. Importantly, these changes in wind patterns drive changes in sea surface temperature, causing warming in the Indian Ocean and cooling in the Eastern Pacific Ocean.
A warming Indian Ocean produces a secondary atmospheric change. The warming of the Indian Ocean causes increasing westerly winds over the Indian and western Pacific Oceans, while simultaneously cooling the eastern Pacific Ocean due to increasing easterly winds. This may sound circular – and that’s because it is (Fig 2)! This is an example of a climate feedback: A change in the atmosphere produces a response in the ocean, the change in the ocean produces a new change in the atmosphere, creating more change in the ocean!
In summary, the warming Atlantic Ocean was found guilty in cooling the eastern Pacific Ocean, resulting from changes in wind patterns that warm the Indian Ocean. This produces additional atmospheric changes that further cool the eastern Pacific Ocean.
The investigation by Li et al. finds that the interconnection of the world’s tropical oceans is stronger than previously thought, and that changes in one ocean basin lead to very rapid (less than one year) changes in other tropical ocean basins. As a result, future climate investigations with time spans of decades should treat tropical ocean basins as one continuous feature.
The findings from this study also are intriguing in light of the warming hiatus – a period of slowed global warming. Much of the warming hiatus has been attributed to the cooling eastern Pacific Ocean. This study goes further, describing the slowdown in global cooling resulting almost paradoxically from enhanced warming in the Atlantic Ocean!