Northwest Fisheries Science Center

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 of 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.  Intense, late–season storms 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. 

Anomalies in the date of the physical spring transition from 1969 to 2005. Figure PST-01.  Anomalies in the date of the physical spring transition from 1969 to present.  Anomaly is based on an average date of 13 April using the minimum cumulative upwelling index (CUI) value. 

The date of spring transition can be indexed in several ways. Here, we use the date of the minimum value of the Cumulative Upwelling Index (CUI). Further details can be found in Bakun (1973) and Bograd et al. (2009). The average date of upwelling is 13 April (Day 103), but can range from early March to early June. Note from Figure PST-01 the following points: