Previous observational studies have highlighted a delay in Kuroshio transport variability relative to basin-scale atmospheric forcing, and thus have emphasized baroclinic Rossby wave adjustment as the mechanism by which remote forcing reaches the western boundary. However, recent observational studies and preliminary analyses suggest that, in fact, the Kuroshio and its extension each exhibit both delayed and immediate responses to the atmospheric forcing, suggesting that additional mechanisms play important roles in Kuroshio variability. The project will take a comprehensive look at these two response time scales, paying particular attention to the rapid response and to the connectivity of the components comprising the North Pacific subtropical gyre.
The purpose of this study is to develop a better understanding of how the ocean responds to basin-scale wind forcing, especially in the western boundary currents and their extensions, with a focus on interannual to decadal variability in the Kuroshio system. In the North Pacific, such variability manifests itself in the Pacific Decadal Oscillation (PDO), a climate index reflecting the large-scale sea surface temperature variability, which is also well-correlated with the leading mode of wind stress curl variability over the most recent decades.
A combination of existing observational data, idealized numerical model experiments, and output from an eddy-resolving ocean general circulation model (OGCM) will be used to accomplish three goals: (1) characterize the interannual to decadal variability in the Kuroshio in the East China Sea (ECS-Kuroshio), (2) investigate the relative roles of local versus remote atmospheric forcing on the barotropic and baroclinic oceanic responses in the ECS-Kuroshio and identify waveguide(s) by which remote forcing reaches the ECS-Kuroshio, and (3) relate the variability in the ECS-Kuroshio to variability in other parts of the subtropical gyre circulation, especially that in the Kuroshio Extension region.
Intellectual Merit: The combined use of observations, a two-layer model, and ocean general circulation models output will lead to a more complete description of the Kuroshio system and a better understanding of the dynamics which give rise to variability in the western boundary current and its extension. Advancing the current understanding of the relationship between the inter-annual to decadal variability of ocean circulation and atmospheric forcing will be beneficial to the broader climate research community, particularly in the on-going community effort to improve inter-annual to decadal predictability.
Broader Impacts: The first year of this project will partially support the postdoctoral training of the lead investigator, a junior scientist from an under-represented group. In addition, the lead investigator will perform outreach work with middle school students and their science teachers. This volunteer work will support the efforts of a school in Rhode Island to implement a 2009 Science and Math Scholars (SAMS) grant by NASA to improve the quality of science and math education in middle schools.
The Kuroshio is a strong ocean current in the western North Pacific. This current transports warm, salty subtropical waters northward and plays an important role in regulating the earth’s climate and ecosystems. Ultimately, the Kuroshio exists because of the wind field blowing over the vast interior North Pacific Ocean: the winds over the subtropics force weak southward flows in the ocean interior (so-called Sverdrup flow) and those waters are steered westward (i.e., towards the Philippines, China and Japan). The Kuroshio is the return flow in this wind-driven circuit, funneling vast amounts of water along the North Pacific’s western margin towards southeastern Japan in a narrow (~60 miles across), swift current. Near Tokyo, the Kuroshio leaves the margin and flows eastward back into the ocean interior, completing its subtropical circuit. Changes in the wind field over the ocean interior lead to changes in the Kuroshio’s strength and structure and the energy is transmitted from the wind-forced ocean interior to the Kuroshio via large-scale, long period waves called Rossby waves. These are sometimes called planetary waves since they are supported by a restoring force that arises because the earth is a rotating sphere covered with just a very thin ocean veneer (the ocean’s aspect ratio is 1:1000). Much of the ocean is stratified (lighter water is floating on more dense water), so two types of Rossby waves can be supported in the ocean: slow baroclinic Rossby waves that can take 1 to 10 years to cross the entire North Pacific basin and rapid barotropic Rossby waves, which can cross the basin in weeks to months. Both types of Rossby waves have a westward component to their energy propagation, but the paths (i.e., the waveguides along which the energy propagates towards the basin’s western boundary) depend on the ocean’s stratification and on the bathymetry. The purpose of this study was to develop a better understanding of how the North Pacific Ocean responds to changes in the basin-scale wind forcing with a focus on interannual to decadal variability in the Kuroshio system. A combination of existing observational data, idealized numerical model experiments, and output from an eddy-resolving ocean general circulation model (OGCM) were used to accomplish four goals: (1) characterize the interannual to decadal variability in the Kuroshio, (2) investigate the relative roles of local versus remote atmospheric forcing on the barotropic and baroclinic oceanic responses in the Kuroshio, (3) identify waveguide(s) by which remote forcing reaches the Kuroshio via barotropic and baroclinic Rossby waves, and (4) relate the variability in the Kuroshio to variability in other parts of the subtropical gyre circulation. In the North Pacific, observed variability manifests itself in the Pacific Decadal Oscillation (PDO), a climate index reflecting the large-scale sea surface temperature variability, which is also well-correlated with the leading mode of changes in the wind field over the most recent decades. Previous observational studies have highlighted a delay of several years between changes in the North Pacific’s basin-scale atmospheric forcing and subsequent changes in the Kuroshio’s strength and thus have emphasized baroclinic Rossby wave adjustment as the mechanism by which information about changes in the remote forcing reaches the western boundary. However, the research funded through this grant has demonstrated that the Kuroshio exhibits both delayed and rapid responses to interannual changes in the wind field. This research has also highlighted the important role of topography in shaping the ocean’s response to changes in the winds: topography steers the wind-forced barotropic waves southwestward along the North Pacific’s sloped western boundary so that changes in the subpolar wind field manifest themselves as variability in the subtropical Kuroshio current. Furthermore, baroclinic Rossby waves are generated not only directly by winds, but also by the interaction of barotropic Rossby waves with submerged ridges. The combined use of observations, idealized models and models realistic forcing and topography have led to a more complete description of the Kuroshio system and a better understanding of the dynamics which give rise to variability in western boundary currents. Advancing our understanding of the relationship between the interannual to decadal variability of ocean circulation and atmospheric forcing has been beneficial to the broader climate research community, particularly in the on-going community effort to improve interannual to decadal predictability. The first year of this proposal partially supported the postdoctoral training of the PI. In addition, the PI performed outreach work with middle school students and their science teachers in support of the efforts of a school in Rhode Island to implement a 2009 Science and Math Scholars (SAMS) grant by NASA to improve the quality of science and math education in middle schools. The collaborations fostered by this grant are ongoing, with continued outreach by the PI in the middle school.
