The ocean and atmosphere in the midlatitudes of the Northern Hemisphere are coupled through an intricate series of energy exchanges. In the western Atlantic Ocean, the warm waters of the Gulf Stream snake up the East Coast of the US, collide with the frigid Labrador Current, and swerve eastward. The sharp temperature gradient between the currents leads to an exchange of heat that provides energy for the formation of storms. The storms affect the strength and position of winds at the surface and aloft, which, in turn, drive the currents. The western Pacific hosts a similar setup, in which the warm-water Kuroshio Current clashes with the cold Oyashio Current off the east coast of Japan. The component that links the Atlantic and Pacific features is the jet stream, which interacts with both current convergence zones as it circles the globe from west to east, as shown in the figure below.

Tsubasa Kohyama of Ochanomizu University in Japan and colleagues have now demonstrated that the Gulf Stream and Kuroshio currents have an even stronger link- They form a synchronized system in which the departure of sea-surface temperatures from regional averages increases and decreases in step with each other on the scale of years and decades. The researchers examined nearly four decades’ worth of satellite-derived sea-surface temperature anomalies in the zones where the warm- and cold-water currents mix. They found no significant correlation between the Atlantic and Pacific measurements at time scales of days, weeks, and even several months. But evidence of covariability emerged at scales of 20 months or more. The output of high-resolution general circulation models largely matched the observational results and allowed the researchers to simulate the relationship over longer periods of time. Kohyama and his colleagues note that the Gulf Stream and Kuroshio curves have only a small offset between them—changes in the currents’ temperature anomalies tend to have nearly no lag.

The finding offers the potential to predict the likelihood of extreme events such as heat waves. Kohyama and colleagues highlight the summer of 2018, which featured anomalously high sea-surface temperatures in the northern Atlantic and Pacific. In response the jet stream shifted north, leading to hot, dry conditions in parts of North America, Europe, and Asia. The complexity of the synchrony and the need to filter out uncorrelated short-term phenomena, however, could make prediction of coordinated extreme events a difficult task. In the long term, the study should help researchers better understand the energy exchange between ocean and atmosphere in the Northern Hemisphere’s middle latitudes, as well as how it is changing in a warming climate. (T. Kohyama et al., Science 374, 341, 2021.)