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The two warm ocean currents, the Gulf Stream and the Kuroshio, are located in the western boundaries of the Atlantic and the Pacific, respectively, so they are referred to as the western boundary currents. Meanderings of western boundary currents and the associated sea surface temperature (SST) variations have long been known to affect local weather and climate in the coastal metropolitan areas, mainly because western boundary currents transport heat from the tropics to the extratropics and modulate cyclogenesis and low cloud formation.
More recently, high-resolution satellite observations helped reveal that heat released from western boundary currents have profound impacts on the entire tropospheric layer. The Gulf Stream and the Kuroshio are known as centers of action in the midlatitude intrinsic variability, and also serve as surface fingerprints of low-frequency natural climate variability (e.g., Atlantic Meridional Overturning Circulation, Pacific Decadal Variability). Thus, understanding western boundary currents have major implications for paleoclimatology, climate modeling, and disentangling natural variability from the anthropogenic climate change.
The Gulf Stream and the Kuroshio are separated by the North American continent, and thus, they cannot exchange heat by oceanic processes within a few years. Based on data analyses of satellite observations and high-resolution global climate models (GCMs), we recently showed that sea surface temperatures of the Gulf Stream and the Kuroshio are synchronized at the interannual to decadal time scales. This synchronization, which we refer to as the Boundary Current Synchronization (BCS), is reproduced only in global climate models with high spatial resolution, including GFDL-CM4C192 and MIROC6subhires, and not in lower resolution counterparts.
Both in observations and model simulations, BCS is associated with meridional migrations of the atmospheric jet stream. Changes in the strength and path of the ocean currents associated with the jet shifts lead to the synchronicity of sea surface temperatures. These mesoscale air-sea interactions in both the atmosphere and the ocean have to be resolved to reproduce BCS realistically.
Numerical simulations using a conceptual model and an atmospheric general circulation model are consistent with a notion that BCS is an interbasin air-sea coupled mode. The conceptual model predicts that the warm phase of BCS are associated with a northward shift of jet stream and vice versa. Atmospheric GCM experiments show that SST variability of the MIROC6subhires model actively modulates the position of the westerly jet stream. On the contrary, as suggested by the conventional view, SST variability of the low-resolution version of MIROC6 has less influence on the midlatitude atmosphere.
Understanding BCS have immediate implications for human lives, because the Gulf Stream and the Kuroshio transport heat from the tropics to the extratropics, and their temperature variations affect extreme weather of densely-populated areas in the northern hemisphere. The hot summer experienced in 2018 is a good example of extreme weather associated with BCS. In particular, because other prominent climate modes (e.g., the El Niño Southern Oscillation) were relatively inactive in 2018, the BCS signature may have clearly emerged in the observed air temperature over the entire northern hemispheric extratropics.
BCS also have implications for fisheries productions because the variability of western boundary currents modulates marine ecosystems. Warm SST associated with a northward shift of the Gulf Stream increases the mortality of Atlantic cod (Gadus morhua), whereas migrations of pelagic fish, such as Japanese sardine (Sardinops melanostictus) and Pacific saury (Cololabis sairai), are influenced by the Kuroshio variability because they use the Kuroshio region as spawning and nursery grounds. Continuous monitoring of the two western boundary currents with a finer observational network, as well as development of high resolution GCMs, are necessary for accurate understanding and prediction of BCS in a changing climate.
Dr. Tsubasa Kohyama started his career as a graduate student at the University of Tokyo, under the supervision of Dr. Tomoki Tozuka, and continued his study under the supervision of Dr. Dennis L. Hartmann at the University of Washington. After earning his doctorate, he moved back to the University of Tokyo and joined Dr. Hiroaki Miura's group as a postdoctoral researcher. Since April 2019, he has been serving as an assistant professor at the Department of Information Sciences, Ochanomizu University, Tokyo. His research interests include large scale climate variability and change. In particular, during his graduate study, he investigated sea surface temperature warming patterns of the tropical Pacific in response to global warming (Kohyama et al., 2017, JClim; Kohyama and Hartmann, 2017, JClim; Kohyama et al, 2018, GRL), which is expected to have profound influences on global weather and climate (Kohyama and Hartmann, 2016, JClim). He was also sub-advised by Dr. John M. Wallace for investigating the lunar gravitational atmospheric tide (Kohyama and Wallace, 2014, 2016, GRL). His recent study identified synchronization of the Gulf Stream and Kuroshio Current at interannual to decadal time scales, which is referred to as the boundary current synchronization (BCS) (Kohyama et al. 2021a, Science; Kohyama et al. 2021b, SOLA). He also investigated the sharp downward branch of the Walker Circulation above the western Indian Ocean, or the “Wall” (Kohyama et al. 2021c, JGR), which potentially plays a fundamental role for the Madden-Julian Oscillation convective initiation (Takasuka et al. 2021, GRL).
Department of Information Sciences, Ochanomizu University, Tokyo