Sea Surface Temperature Anomalies
Given that the Pacific Decadal Oscillation is a basin–scale index of North Pacific sea surface temperatures (SST), how closely does the PDO match local sea surface temperatures off the Pacific Northwest? We examined this using data from NOAA Weather Buoy 46050, located 22 miles off Newport, Oregon.
Figure 5 shows monthly PDO values vs. monthly average sea surface temperatures at NOAA Weather Buoy 46050 from 1996 through 2008. This is the period during which we have been measuring local ocean conditions.
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Figure 5. |
The PDO and monthly sea surface temperature anomalies at NOAA Buoy 46050, 22 miles west of Newport OR. |
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Correspondence between the PDO and local temperature anomalies has been very high. For example, the four years of negative PDO values from late 1998 until late 2002 closely match the negative SST anomalies measured off Newport. Timing of the positive PDO values also matches that of the positive SST anomalies.
This suggests that changes in basin–scale forcing results in local SST changes, and that local changes may be due to differences in transport of water out of the North Pacific into the northern California Current. The data also verify that we can often use local SST as a proxy for the PDO. However, there are periods in which local and regional changes in the northern CC may diverge from the PDO pattern for short periods (usually less than a few months).
Buoy temperatures clearly identify warm and cold ocean conditions. During the 1997–1998 El Niño event, summer water temperatures were 1–2°C above normal, whereas during 1999–2002, they were 2°C cooler than normal (Figure 5a, lower left). The summers of 2003–2005 were again warm, and some months showed positive SST anomalies that exceeded even those seen during the 1998 El Niño event. Some marine scientists refer to 2003–2005 as having "El Niño–like" conditions. In contrast, summertime SSTs were cooler than normal during summer 2006 and 2008 and during winters from 2006 to 2008.
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Figure 5a. |
Upper panel shows average sea-surface temperatures at Station NH 05 off Newport, Oregon, from 1996 through 2008. Lower panel shows temperature anomolies during the same years during summer (left) or May to September and winter (right) or October to April. |
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Note also in Figure 5 that there are time lags between a change in sign of the PDO and change in SST off Newport. In 1998, the PDO changed to negative in July, and SSTs changed to negative in December. In 2002, the opposite pattern was seen, with the PDO change in August followed by SSTs in December. Thus, it takes 5–6 months for the signal in the North Pacific to propagate to coastal waters.
These measurements show that basin–scale indicators such as the PDO do manifest themselves locally: local SSTs change in response to physical shifting on a North Pacific basin scale. Other local changes associated with basin–scale indicators include the source waters that feed into the northern California Current, zooplankton and forage fish community types, and abundance of salmon predators such as hake and sea birds. Thus, local variables change in response to change that occurs on a broad spectrum of spatial scales. These range from basin–scale changes, which are indexed chiefly by the PDO, to local and regional changes, such as those related to shifts in the jet stream, atmospheric pressure, and surface wind patterns.
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