Deep–Water Temperature and Salinity
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. These winds in turn affect transport of water into the northern California Current (CC). 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 and salty water of subarctic origin is nutrient–rich, whereas the relatively warm and fresh water of the offshore West Wind Drift is nutrient depleted.
| |
 | |
| |
Figure 12. |
Salinity (upper) and temperature (lower) at the 50–m depth at station NH 05 (water depth 62 m) averaged over all cruises from May to September each year. Note that the deeper waters of the shelf in 2008 are similar to values observed during the previous "cold phase" of 1999-2002. | |
Figure 12 shows average salinity and temperature measured at the 50 m depth from station NH 05. 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 and 1998 (and to a lesser extent 2004 and 2005) were warmer than average, and corresponded to a warm phase of the PDO. Second, the years 1999–2002 (and to a lesser extent 2003) were colder than average and corresponded to a cool phase PDO (and to negative SST anomalies at Buoy 46050).
Also from these data, we note that before upwelling was initiated in 2005, the spring/early summer resembled the
summer of 1997, when the coastal ocean was dominated by warm (+0.6°C anomaly) and fresh water. However, once upwelling became established, the water properties resembled the cooler conditions ( 0.3°C anomaly) seen during 1999–2002.
 |
| Figure 12a. |
Scattergram of temperature and salinity from 1998 to 2008. Note that 2008 was the coldest year to date. |
Temperature and salinity at the 50–m depth from station NH 05 were average in 2006 compared to the 11–year time series for these data. Salinity values were intermediate, similar to those of summer 2003-2005. This indicates summer upwelling of average strength, unlike the cold (and productive) summers of 1999-2002 (Figure 12a).
Coho salmon survival is high when cold salty water is present in continental shelf waters, and vice versa (Figure 13, upper panel). That is, during the summer when coho first enter the ocean, if deep waters are relatively cold and salty, we can expect good coho salmon survival.
 |
 |
| Figure 13. |
Coho survival (A) and Chinook adult returns (B) shown proportional to bubble diameter vs. temperature and salinity at the 50–m depth at hydrographic station NH 05. Coho survival (upper panel) is high when a "cold salty" water type is present on the continental shelf. |
Conversely, if deep water is relatively warm and fresh, coho salmon survival is poor. Thus, we can use presence of different water types as a leading indicator of coho survival. Note that when coho entered the ocean in May–June 2005, deep waters were warm and fresh, signaling poor returns in fall 2006.
Such a relationship was not as clear for spring Chinook salmon. Figure 13 (lower panel) shows the smolt to adult return rates of Snake River spring Chinook (data from Scheuerell and Williams 2005).
Although smolt–to–adult returns are high when waters are cold and salty, they are not necessarily low when waters are warm and fresh. This suggests that other factors may be influencing returns of these stocks. The "plus" sign indicates summer averaged temperature and salinity for 2004.
|
