HR: 16:45h
AN: OS52L-10    [PDF]
TI: A Theory of Thermohaline Circulation/Adjustment of the Intermediate Layer in the North Pacific
AU: * Nakamura, T
EM: nakamura@kugi.kyoto-u.ac.jp
AF: Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, 606-8502 Japan
AU: Awaji, T
EM: awaji@kugi.kyoto-u.ac.jp
AF: Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, 606-8502 Japan
AU: Toyoda, T
EM: ttoyoda@kugi.kyoto-u.ac.jp
AF: Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, 606-8502 Japan
AU: Ishikawa, Y
EM: ishikawa@kugi.kyoto-u.ac.jp
AF: Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, 606-8502 Japan
AB: The Okhotsk Sea is believed to be the principal site of the ventilation of the intermediate layer in the North Pacific [e.g., Talley, 1991; 1993]. Recently, from a set of impact experiments using a global ocean general circulation model, Nakamura et al. [2003] have indicated that (1) the strong tidal mixing in the Kuril Straits, which connect the North Pacific and the Okhotsk Sea, deepens the ventilation in the Okhotsk Sea through both enhancement of the subduction associated with sea ice formation in the northern shelf and diapycnal mixing at the straits, and that (2) these processes change the fluxes of potential vorticity and other water properties from the Okhotsk Sea to the North Pacific, eventually leading to the modification in the circulation and freshening of the North Pacific intermediate layer, which are good indicators of enhanced ventilation. In this study, based on their results, we have investigated the mechanism through which the diapycnal mass transport given from the western boundary due to the Okhotsk Sea subduction and the mixing in the Kuril Straits affects the intermediate layer circulation in the interior region of the North Pacific. Our model analysis shows that the dynamical adjustment to the tidal mixing at the Kurils is conducted by both Kelvin and Rossby waves, which include the 2nd as well as 1st and higher baroclinic modes. The Kelvin waves propagate along the coast against the Kuroshio current, and to the equator. On the other hand, the 2nd mode Rossby waves, which are affected by the mean flow field, move eastward along the boundary of the subtropic and subpolar gyres. These results suggest that the understanding of the adjustment of the intermediate layer needs both the western boundary region which allows the Kelvin wave propagation and wind driven circulation which enables eastward propagation of Rossby waves. However, the former is absent in the ventilated thermocline theories [e.g., Luyten et al., 1983], while the latter is not taken into account in the thermohaline adjustment theories in the line of Stommel and Arons [1960] and Kawase [1987]. Thus, we have constructed a theoretical model of intermediate layer circulation by including the both effects. The model is a steady quasi-geostrophic 2.5-layer model, which includes both subpolar and subtropic gyres. In addition to Ekman pumping at the sea surface, mass flux is given at the western boundary as the forcing. The latter mimics the diapycnal transport in the Okhotsk Sea and the Kuril Straits. The solution obtained shows the followings. Net mass flux given at the western boundary excites the 1st-mode response, which is confined in the western boundary region. This effect enables the western boundary current to cross the gyre boundary and to reach the latitude where the given net-mass flux is compensated by the Sverdrup transport. The 1st-mode response thus helps the western boundary flow in the lower layer to reach the subtropics. The lower layer response to the given mass flux then spreads into the pool zones in both the subtropic and subpolar gyres and modifies the intermediate layer circulation. Our simplified theoretical model will be applicable to other basins.
DE: 4203 Analytical modeling
DE: 4215 Climate and interannual variability (3309)
DE: 4532 General circulation
SC: Ocean Sciences [OS]
MN: 2004 Ocean Sciences Meeting