Line P ocean temperature and salinity, 1956–2005

Abstract

Vertical profiles of temperature and salinity have been measured for 50 years along Line P between the North American west coast and mid Gulf of Alaska. These measurements extend 1425 km into the gulf at 13 or more sampling stations. The 10–50-m deep layer of Line P increased in temperature by 0.9 °C from 1958 to 2005, but is significant only at the 90% level due to large interannual variability. Most of this increase in temperature accompanies the 1977 shift in wind patterns. Temperature changes at 100–150 m and salinity changes in both layers are not statistically significant. Much of the variance in temperature is in the upper 50 m of Line P, and temperature changes tend to be uniform along Line P except for waters on the continental margin. Salinity changes are dominated by variability in the halocline between 100 and 150 m depth and are less uniform along Line P. Largest oscillations in temperature and salinity are between 1993 and 2003. These events can be understood by considering changes in eastward wind speed and wind patterns that are revealed in the first two modes of the Pacific Decadal Oscillation. Changes in these patterns are indicators for both Ekman surface forcing (Surface ocean currents flow to the right of the wind direction) and Ekman pumping (Surface waters diverge away from regions of positive wind stress curl, leading to upwelling of colder saltier water). Changes in temperature along the nearshore part of Line P suggest Ekman surface forcing is the stronger of the two processes in the upper layer. The change in salinity anomalies in the halocline along the seaward end of Line P, following the wind shift in 1977, is in agreement with enhanced upwelling caused by stronger Ekman pumping in this region.