Large–scale Ocean and Atmospheric Indicators

Local and Regional Physical Indicators

Local Biological Indicators

Indicators Under Development

Introduction to Pacific Northwest Oceanography

Ocean Sampling Methods

Physical Spring Transition

Winter in the Pacific Northwest is characterized by frequent rainfall and southwesterly winds.  Southwest winds push water onshore and cause downwelling (the opposite from upwelling).  Downwelling in turn brings warm, nutrient–depleted, surface water onshore from offshore sources and results in very low levels of primary production.  The most critical time of the seasonal plankton production cycle is when the ocean transitions from a winter downwelling state to a summer upwelling state.  This time is known as the spring transition. 

The spring transition marks the beginning of the upwelling season and can occur at any time between March and June.  Generally, the earlier in the year that upwelling is initiated, the greater ecosystem productivity will be in that year.  In some years the transition is sharp, and the actual day of transition can be identified easily, but in many years transition timing is more obscure.  It is not uncommon for northerly winds (favorable to upwelling) to blow for a few days, only to be followed by southwesterly winds and storms.  If late season storms are intense, they can erase any upwelling signature that may have been initiated, thus re–setting the "seasonal clock" to a winter state.  This is what occurred during summer 2005. 

	Figure 9.	Anomaly of date of spring transition based on an average date of 6 April, from Logerwell et al. (2003).  Data and plot provided by Robert Emmett.

The date of spring transition can be indexed in several ways.  Logerwell et al. (2003) has indexed the spring transition date based on the first day when the value of the 10–day running average for upwelling is positive and the 10–day running average for sea level is negative (Figure 9).  Based on the index of Logerwell and her associates, the mean date of the transition is 6 April, but it can range from early February to early July.  Note from Figure 9 the following four points:  

	Figure 10.	Coho salmon survival vs. day of spring transition, 1969–2005 (Logerwell et al. 2003).

Figure 10 shows that hatchery adult coho salmon returns are correlated with the spring transition (Logerwell et al. 2003).  A similar analysis using spring Chinook counts at Bonneville or Snake River smolt to adult return rates (from Scheuerell and Williams 2005) did not reveal any significant correlations. 

Another measure of the spring transition comes from monitoring of ocean currents on a daily basis.  Dr. Mike Kosro, College of Oceanic and Atmospheric Sciences, Oregon State University, operates an array of coastal radars that are designed to track the speed and direction of currents at the sea surface.  He produces daily charts showing ocean surface current vectors, and from those one can clearly see when surface waters are moving south (due to upwelling) or north (due to downwelling).  By scanning progressive images, the date of transition can be visualized. 

	Figure 11.	Coho survival vs. spring transition as indicated by changes in temperature of deep water.

We developed a measure of the spring transition based on measurements of temperature taken during our bi weekly sampling cruises off Newport, Oregon.  We define the spring transition as the date after which deep water at the mid shelf is cooler than 8°C.  This indicates the presence of cold, nutrient–rich water that will upwell at the coast, signaling the potential for high plankton production rates. 

Figure 11 shows relationships between this index and coho salmon.  Survival is higher in years with an early transition date and vice versa.  The transition to an "upwelling" water type on 28 July 2005 (Julian date 209) was particularly late, suggesting coho returns in 2006 of around 1%.