Annual and interannual variability of baroclinic transports across Line P in the northeast Pacific Ocean

Abstract

The more than 25 years (mainly during 1959–1983) of the hydrographiclSTD data along Line P in the northeastern part of the North Pacific Ocean have been examined to depict the annual and interannual variability of baroclinic transports in the upper 1000 m of water. The mean transport across the line, between Station P and the coast, based on well over 100 transects is 5 × 106 m3 s−1 (Sv), poleward. The western and eastern half of the line contribute 3 and 2 Sv, respectively. The fluctuations of transport are greater in the eastern half due presumably to the greater eddy motion on that side. Transports relative to the 1200 and 1500 db surfaces are, respectively, 20 and 40% greater than those relat'lve to the 1000 db surface for each side of the line. There is very little annual cycle of transports. However, the tendency exists for the transports to be slightly greater in winter than in summer. This cycle is more discernible for the transports across the eastern half of the line.The time-series transports are characterized by the presence of a well-defined interannual variability. For the entire and the western half of the line persistent strong or weak transports occur for 3–4 years at a time. Spectral peak at a period of 6–7 years is evident but is not significant, even at the 80% confidence interval. At the annual frequency the observed and computed (based on curl of wind stress) transports are highly correlated for the eastern half of the line, suggesting that the coastal portion of the transports are more responsive to changing wind-stress curl than for the offshore portion. It would appear that at the seasonal scale a significant part of the atmospheric forcing goes to produce transports in the barotropic mode in the offshore waters.There is a small out-of-phase relationship between the Alaska Current and the coastal component of the California Current. While this result is not inconsistent with the bifurcation proposal it is more likely that this is due to the in-phase relationship of the coastal currents along the Pacific coast of Canada-U.S.A.The widespread occurrence of ocean events at the period of 6–7 years in the North Pacific is believed to be associated with atmospheric forcing of similar time scale. Information upon the barotropic component of the circulation is lacking. Such data are needed to understand the coupling between the atmosphere and the ocean in both the annual and interannual scales.