Phase changes of the Pacific Decadal Oscillation are associated with alternating changes in wind speed and direction over the North Pacific. Northerly winds result in upwelling (and a negative PDO) and southerly winds, downwelling (and a positive PDO) throughout the Gulf of Alaska and California Current. These winds in turn affect transport of water into the Northern California Current (NCC). Northerly winds transport water from the north whereas southwesterly winds transport water from the west (offshore) and south.
Thus, the phase of the PDO can both express itself and be identified by the presence of different water types in the northern CC. This led us to develop a "water type indicator," the value of which points to the type of water that will upwell at the coast. Again, cold, salty water of subarctic origin is nutrient–rich, whereas the relatively warm and fresh water of the offshore North Pacific Current is nutrient depleted.
Figure DTS-01 shows average salinity and temperature measured at the 50–m depth from station NH 05 (shown in Figure HP-01). These measurements were taken during biweekly sampling cruises that began in 1997 and continue to the present.
From these data, two patterns have become clear: first, the years 1997-1998 and 2013-2014 were warmer than average, and corresponded to a warm-phase PDO and El Niño conditions during 1997-1998 and positive PDO during 2014. During 1997-98 and 2014, the water was also the freshest it has been during the time period.
Figure DTS-02 shows the same data, but as a scatter diagram, illustrating several noteworthy points. First, during the El Niño event of 1997-1998 and during 2014, deep waters on the continental shelf off Newport were warm and relatively fresh throughout the year. Second, during the contrasting negative-phase PDO years of 1999-2002 and 2007-2008, these waters were cold and relatively salty or intermediate, as in 2009-2012.
Chinook and coho salmon survival is generally high when cold, salty water is present in continental shelf waters, and vice versa (Figure DTS-03). That is, during the summer when these fish first enter the ocean, if deep waters are relatively cold and salty, we can expect generally good salmon survival. Conversely, if deep water is relatively warm and fresh, salmon survival is poor, as was observed for fall Chinook and coho salmon in 1997 and 1998.