Decadal Differences

Around 1976, significant shifts in fish populations and physical quantities occurred (e.g. Ebbesmeyer et al., 1991). We have analyzed the wind-forced circulation of the North Pacific before and after this regime shift. At mid-latitudes in the North Pacific, zonal winter wind stress doubled in the decade following 1976, leading to increases in several derived ocean variables (Schwing 1998):

• southward Ekman transport
• upwelling due to Ekman pumping
• turbulent mixing
• poleward and zonal transport

The transports of the North Pacific Current and the subtropical gyre also increased.

Ocean circulation in the NEP changed in association with the 1976 climate shift as well. Surface Ekman transport out of the Gulf of Alaska decreased by a factor of two during 1977-86. The pattern of Ekman divergence changed as well, with an area of maximum upwelling due to Ekman pumping expanding west through the Gulf after 1976 . Meridional Ekman transport through the California Current remained the same. Seaward of the CCS, the southward Ekman transport doubled as part of the basin-scale intensification in wind stress described above.

Prior to 1976, the meridional Sverdrup transport into the Gulf of Alaska was constrained in a strong, relatively narrow current along the eastern edge of the Gulf. Transport in 1977-86 was weaker in the east, but extended across the entire southern edge of the Gulf. In contrast, equatorward Sverdrup transport in the California Current was only slightly weaker after 1976.

These circulation changes probably resulted in significant changes in the advection and distribution of heat, nutrients, and organic material throughout the basin. Comparisons of the seasonal differences between the decades show that the largest fluctuations in the North Pacific atmosphere and ocean occurred in winter. The decadal changes that occurred around 1976 may reflect the decrease in the frequency of LN events and the increase in the frequency of EN events that occurred around 1976 (cf. Wolter and Timlin 1998).

These results were presented in the 1998 Proceedings of the Hawaiian Winter Workshop, and in a manuscript by R.H. Parrish, F.B. Schwing, and R. Mendelssohn in review by Fisheries Oceanography.