Rossby wave-coastal Kelvin wave interaction in the extratropics. Part I: Low-frequency adjustment in a closed basin

The interaction of open and coastal oceans in a midlatitude ocean basin is investigated in light of Rossby and coastal Kelvin waves. The basinwide pre ssure adjustment to an initial Rossby wave packet is studied both analytica lly and numerically, with the focus on the low-frequency modulation of the resulting coastal Kelvin wave. It is shown that the incoming mass is redist ributed by coastal Kelvin waves as well as eastern boundary planetary waves , while the incoming energy is lost mostly to short Rossby waves at the wes tern boundary. The amplitude of the Kelvin wave is determined by two mass r edistribution processes: a fast process due to the coastal Kelvin wave alon g the ocean boundary and a slow process due to the eastern boundary planeta ry wave in the interior ocean. The amplitude of the Kelvin wave is smaller than that of the incident planetary wave because the initial mass of the Ro ssby wave is spread to the entire basin. In a midlatitude ocean basin, the slow eastern boundary planetary wave is the dominant mass sink. The resulti ng coastal Kelvin wave peaks when the peak of the incident planetary wave a rrives at the western boundary. The theory is also extended to an extratropical-tropical basin to shed ligh t on the modulation effect of extratropical oceanic variability on the equa torial thermocline. In contrast to a midlatitude basin, the fast mass redis tribution becomes the dominant process, which is now accomplished mainly by equatorial Rossby and Kelvin waves, rather than the coastal Kelvin wave. T he coastal Kelvin wave and the modulation of the equatorial thermocline pea k close to the time when the wave trail of the incident Rossby wave arrives at the western boundary. Finally, the theory is also applied to the wave i nteraction around an extratropical island.