Recent evidence shows that wind‐driven ocean currents, like the western boundary currents, are strongly affected by global warming. However, due to insufficient observations both on temporal and spatial scales, the impact of climate change on large‐scale ocean gyres is still not clear. Here, based on satellite observations of sea surface height and sea surface temperature, we find a consistent poleward shift of the major ocean gyres. Due to strong natural variability, most of the observed ocean gyre shifts are not statistically significant, implying that natural variations may contribute to the observed trends. However, climate model simulations forced with increasing greenhouse gases suggest that the observed shift is most likely to be a response of global warming. The displacement of ocean gyres, which is coupled with the poleward shift of extratropical atmospheric circulation, has broad impacts on ocean heat transport, regional sea level rise, and coastal ocean circulation.
This paper presents a horizontally two-dimensional theory based on a variable-coefficient Kadomtsev–Petviashvili equation, which is developed to investigate oceanic internal solitary waves propagating over variable bathymetry, for general background density stratification and current shear. To illustrate the theory, a typical monthly averaged density stratification is used for the propagation of an internal solitary wave over either a submarine canyon or a submarine plateau. The evolution is essentially determined by two components, nonlinear effects in the main propagation direction and the diffraction modulation effects in the transverse direction. When the initial solitary wave is located in a narrow area, the consequent spreading effects are dominant, resulting in a wave field largely manifested by a significant diminution of the leading waves, together with some trailing shelves of the opposite polarity. On the other hand, if the initial solitary wave is uniform in the transverse direction, then the evolution is more complicated, though it can be explained by an asymptotic theory for a slowly varying solitary wave combined with the generation of trailing shelves needed to satisfy conservation of mass. This theory is used to demonstrate that it is the transverse dependence of the nonlinear coefficient in the Kadomtsev–Petviashvili equation rather than the coefficient of the linear transverse diffraction term that determines how the wave field evolves. The Massachusetts Institute of Technology (MIT) general circulation model is used to provide a comparison with the variable-coefficient Kadomtsev–Petviashvili model, and good qualitative and quantitative agreements are found.
[1] Understanding periods associated with climate variations has been challenging and has attracted scientific study. In the work presented here, we establish a theoretical dynamical model driven by Sun-Moon gravitation (SMG) and present basic SMG wave characteristics and SMG-induced nonlinear motions for geophysical fluids. As compared to observations, waves and motions demonstrate climate variations associated with abundant structures and climatic rhythms, including the 30-60 day oscillation, seasonality, El Niño-Southern Oscillation-like interannual variation, etc. In our work, periods depended upon the obliquity and revolution velocity of the Sun and Moon; the speed of geophysical fluids; and the latitude, radius, and rotation velocity of Earth. The rotation of Earth helps fluids remember and accumulate momentum in geophysical fluids that are provided by the SMG on multiple time scales, which may contribute to multiperiods of climate oscillations. The speed-dependent periods of SMG-induced flow are of a broad spectrum (i.e., faster speeds, shorter periods). SMG-induced flow in an atmosphere of faster flow tends to have shorter (e.g., seasonal and annual) periods, while an ocean of slower flow tends to have longer (e.g., annual and interannual) periods.
Long‐term observations of nonlinear internal waves in the South China Sea reveal seasonal to interannual variability. During two selected segments of inverted echo sounder observations, tidal forcing in Luzon Strait is almost identical, but the observed amplitudes of nonlinear internal waves in the South China Sea are very different. The effects of the Kuroshio and mesoscale eddies, reproduced by HYbrid Cooridnate Ocean Model (HYCOM) reanalysis simulation, are then investigated. The Kuroshio can enhance the zonal tilt of the thermocline and induce intruding flow in Luzon Strait. During the two selected segments, different thermocline slopes did not significantly change the internal tide generation, but the intruding flow may result in a 11% difference in the amplitude of generated M2 internal tides. During the two selected segments, mesoscale eddies appeared on the path of internal wave propagation, a cold eddy in one case and a warm one in the other. The eddies changed local stratification and induced additional background currents, thus affecting the nonlinear evolution of internal tides. In addition, wave front steering due to the mesoscale eddies dramatically affected the observed amplitude changes of the nonlinear internal waves: the edge, rather than the center, of the nonlinear internal wave front passed through the observational stations, resulting in reduced amplitude in the observations.
Tropical eastern boundary upwelling systems are characterized by rich marine ecosystems (Carr & Kearns, 2003). They undergo strong intraseasonal to interannual variability dominantly associated with equatorial forcing and are often subject to intense hypoxia (e.g.,
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