Hot Spot in Climate System:Coupled Ocean-Atmosphere Variability over Monsoonal Asia due to Contiguousness between the Tropical Warmness and Arctic Coolness - Grant-in-Aid for Science Research in Innovative Areas (MEXT) 2010-2014

Academic Background


The planet Earth, where life has been sustained and civilization has been developed, is characterized by water that circulates around the ocean and atmosphere. In addition, the ocean and atmosphere are important in our climate system, as they transport excessive heat poleward from the Tropics to release it toward the space. The heat transported from the Tropics mainly by ocean currents is released into the atmosphere in midlatitudes. The heat release is concentrated in a narrow area along a major warm current, including the Kuroshio and Gulf Stream, strengthened as a western boundary current under the effect of the earth’s rotation. Previous numerical studies with atmospheric models with relatively low resolution suggested that the heat release from warm ocean currents and its long-term variability may be important for the formation and variability of the climate system (Latif and Barnett 1994; Hoskins and Valdes 1989; Kushnir et al. 2002). However, it was the recent development of supercomputers such as the Earth Simulator (ES) of JAMSTEC and the availability of high-resolution satellite data that activated the study of specific processes involved in the oceanic influence on the atmosphere. Since then, we have been leading the study on the interactions between the midlatitude ocean currents and the atmosphere, taking advantage of the latest technological advancement mentioned above.

Academic Background

A study on the Kuroshio off Japan by M. Nonaka, one of the PIs of this project, was the first to identify the influence of an extratropical ocean current on surface winds (Nonaka and Xie 2003). Since the corresponding study on the Gulf Stream published in the following year in Science (Chelton et al. 2004), a series of studies have been published on how warm ocean currents influence surface winds locally.

Recently, S. Minobe, A. Kuwano-Yoshida and N. Komori, a PI and members, respectively, of this project, have revealed that thermal influence of a warm ocean current is not confined to the atmospheric boundary layer but can penetrate deep into the troposphere, where weather phenomena mostly occur (Minobe et al. 2008, Fig. 1). Their Nature paper demonstrated that enormous heat release from the Gulf Stream causes intensive precipitation and updraft, based on their analysis of satellite data and their simulation with a high-resolution atmospheric model on ES. The paper immediately motivated German scientists to pursue the atmospheric response to the Gulf Stream. Heat and moisture supplies from warm ocean currents can also have social impact. N. Hirose, M. Yamamoto and Y. Tachibana, members and a PI of this project, respectively, found that anomalous evaporation from the Tsushima Warm Current may yield interannual variability in snowfall in the Japanese coastal areas along the Sea of Japan and that meso-scale ocean eddies can modify distribution of surface evaporation to yield geographical dependence of snowfall (Hirose and Fukudome 2006; Takano et al. 2008; Yamamoto and Hirose 2007). It was also suggested that a rainband that tends to form along the Kuroshio axis in the East China Sea found by S. Minobe and others (Small et al. 2008) may influence precipitation in Okinawa.

A sharp meridional contrast in sea-surface temperature (SST) is maintained across an oceanic front where a warm western boundary current is confluent with a cool current. H. Nakamura, the chief PI of this project, and others pointed out that frontal SST gradient between warm and cool ocean currents leads to the recurrent development of extratropical cyclones that, as ensemble, act to maintain a midlatitude westerly jet (polar-front jet), whose fluctuations, in essence, manifest the annular mode, including the "Arctic Oscillation (AO)", the dominant natural variability exerting substantial impact on weather and climate over the extratropical Northern Hemisphere, including Japan. Their idealized model experiment indicates that the annular mode cannot maintain its strength as observed without sharp meridional SST gradient in midlatitudes, postulating a new concept about the atmospheric general circulation (Nakamura et al. 2004, 2008). Meanwhile, Kawamura, one of the PIs of this project, suggested that a typhoon approaching the Kuroshio south of Japan or an explosive cyclone off the coast of Japan could force large-scale atmospheric waves aloft with possible downstream influence reaching as far as North America (Yamada and Kawamura 2007; Yoshiike and Kawamura 2009).

It is also of scientific and social importance to study how the variability of midlatitude ocean currents can influence the atmosphere and how predictable the variability is. H. Nakamura and Y. Tanimoto, a member of this project, found that the meridional displacement of the frontal zone east of Japan, where the Kuroshio and Oyashio are confluent, yields marked long-term variability in SST (Nakamura et al. 1997, 2003) and thereby changes heat release into the atmosphere (Tanimoto et al. 2003). The first realistic reproduction of the long-term variability of the frontal zone was realized with a high-resolution ocean model experiment on ES by M. Nonaka, H. Nakamura and Y. Sasaki, a member of this project, (Nonaka et al. 2006, 2008). Analyzing the output data of this experiment, B. Taguchi, a member of this project, and M. Nonaka have shown that the intensity and axial latitude of the Kuroshio Extension (KE) fluctuate through interactions between meso-scale ocean eddies with several dozens of kilometers in size and large-scale wind-forced ocean waves, the latter of which potentially yields the predictability to the variability with lead time of several years (Taguchi et al. 2007).


Fig. 1: The cover of Nature (Mar. 2008), based on the paper by Minobe, Kuwano-Yoshida and Komori. The white band denotes the Gulf Stream from the Florida Peninsula, with released heat and associated updraft in red.


As mentioned above, many of the core members of this project have been claiming the importance of "hot spot" in the climate system to the international community. We emphasize that such a "hot spot" is a midlatitude region where thermodynamic effects of the ocean on the atmosphere are concentrated and thus intense along a strong warm current such as the Kuroshio and its long-term variability yields pronounced SST anomalies that can enhance low-frequency atmospheric variability. The primary in the study of climatic impact of the ocean variability on the atmosphere, however, has been placed on the Tropical variability, including the El Niño/Southern Oscillation (ENSO) in the Pacific and the Indian Ocean Dipole (IOD). Our studies as described above are among those that first highlighted the importance of midlatitude air-sea interactions in the climate system. There are many aspects still left unsolved, but paradigm shift is about to begin in climate research. It is the mission of our "hot spot" project to develop and substantiate the new paradigm "active roles of the midlatitude ocean in the formation of the climate system and its variability", as we have been pursuing. While we are retaining a global perspective, our main target area is set over the Far East and western North Pacific, where the most pronounced "hot spot" over the globe is residing. In our effort to develop the new paradigm in climate dynamics through challenging unexplored, overlooked and/or unsolved issues on thermodynamic forcing of midlatitude ocean onto the atmosphere, we try to contribute to the improvement of prediction skill of extreme weather conditions and interannual/decadal variability in the extratropical climate system and surface environment under the influence of global warming.

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