Spring Transition Dates and Fall Transition Dates

Each year, along the Pacific Coast of North American between San Francisco (38 North Latitude) and the Queen Charlotte Islands (52 North Latitude), the coastal winds switch from the southerly winds of winter to the northerly winds of summer producing a transition in wind called the spring transition. Conversely, the yearly switch back from the northerly winds of summer to the southerly winds of winter produce a fall transition. The summer winds, which occur after the spring transition and prior to the fall transition, are known to be favorable for upwelling -- a process that transports the nutrients to the ocean surface, feeding the near-shore food chain. Estimates of the transition dates were derived from smoothed synthetic winds computed by the ocean surface currents model OSCURS (Ingraham and Miyihara 1988), which used daily sea level atmospheric pressure fields for years 1946 to 1994 as input. The spring and fall transition dates were calculated for the latitude of the Columbia's mouth (46 deg 12' North Latitude).

The dates of the spring and fall transitions contained on the web page should be considered provisional. They are values which are approximations to the true spring and fall transition dates. The estimates depend on the degree and type of smoothing used on the synthetic winds derived from OSCURS. Neither NOAA nor the University of Washington is responsible for any misuse of these data.

Method Reference:

Logerwell, E.A., N. Mantua, P. Lawson, R.C. Francis, V. Agostini. 2003. Tracking environmental processes in the coastal zone for understanding and predicting Oregon coho (Oncorhynchus kisutch) marine survival. Fisheries Oceanography 12:554-568.

E. Logerwell (pers. com. 2007)

The date of spring transition can be indexed in several ways; the Logerwell et al. (2003) method indexes 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.

Transitions dates contained on the web page should be considered provisional. They are values which are approximations to the true spring and fall transition dates. Neither NOAA nor the University of Washington is responsible for any misuse of these data.

1 April 2008. All spring and fall transition estimates for all years updated based on a re-run of the cluster analysis for the complete zooplankton data set.

Method Reference:

Hooff, Rian C. and William T. Peterson. 2006. Recent increases in copepod biodiversity as an indicator of changes in ocean and climate conditions in the northern California current ecosystem. Limnol. Oceanogr. 51:2042-2051.

Keister, J.E. and W.T. Peterson. 2003. Zonal and seasonal variations in zooplankton community structure off the central Oregon coast, 1998-2000. Prog. Oceanogr. 57:341-361.

Peterson, W.T. and J.E.Keister. 2003. Interannual variability in copepod community composition at a coastal station in the northern California Current: a multivariate approach. Deep-Sea Res. 50:2499-2517.

Peterson, W.T. and F.B. Schwing. 2003. A new climate regime in Northeast Pacific ecosystems. Geophysical Research Letters. 30(17): OCE 6 1-4.

The biological spring transition date is the day of a biweekly Newport Research Station research cruise at hydrographic baseline station NH 05 off Newport, Oregon when copepods sampled in plankton nets cluster out as a northern (cold-water) community (Peterson and Keister, 2003). This date is a useful indicator of salmonid feeding conditions because it marks the first appearance of the kind of food chain that coho and Chinook salmon seem to prefer, that is one dominated by large, lipid-rich copepods, euphausiids, and juvenile forage fish. Taken from: Peterson et al. (2006).

Similarly the biological fall transition date is the last time in a particular year when the "cold water copepods" were found in the plankton samples (William Peterson, pers. comm.).

The estimates of biological spring transition were developed and compiled by Dr. William T. Peterson.

Dr. William Peterson requests the data not be used for publication without his consent.

Dr. William T. Peterson

Fish Ecology Division

Northwest Fisheries Science Center

National Marine Fisheries Service

Newport Research Station

2032 S Marine Science Drive

Newport, Oregon 97365-5275

bill.peterson@noaa.gov

Method Reference:

Bilbao, P. 1999. Interannual and Interdecadal Variability in the Timing and Strength of the Spring Transitions along the United States West Coast. M.S. Thesis. Oregon State University, Oceanography.

The method is the same as that used in Logerwell (2003) to estimate spring transition dates.

Two time series were inspected for seasonal transitions: (1) area averaged daily upwelling indices for 42º to 48ºN, 125ºW (http://www.pfeg.noaa.gov), and (2) daily sea level residuals (corrected for the inverse barometer effect) measured at Neah Bay, WA, 48º22.1'N,124º37.0'W (University of Hawaii Sea Level Center, http://uhslc.soest.hawaii.edu/). High frequency variation was filtered out by applying a low pass filter with a stop frequency of 1/(10 days) (S-PLUS, MathSoft, Inc., Seattle, WA, USA). To extract the seasonal pattern, a low pass filter with a stop frequency of 1/(90 days) was constructed. The date of fall transition was chosen as the date when the 1/(10 days) low pass filtered lines crossed zero. The 1/(90 days) low pass filter line confirmed that the selected date marked the beginning of a new seasonal state.

In most years the time series agree and the date is easy to pick. In other years the signals do not point to a single transition and some judgment must be made. Thus, although the model allows selection of the date, it does not form a completely objective and automated system for choosing that date.

13 April 2007. Estimates for 1997, 2000, and 2004 were updated.

Transitions dates contained on the web page should be considered provisional. They are values which are approximations to the true spring and fall transition dates. The University of Washington is not responsible for any misuse of these data.

Method & Data Reference:

Mean Spring and Fall Upwelling Transition Dates off the Oregon and Washington Coasts. 2007. Van Holmes, Chris. white paper.

Pacific Fisheries Environmental Laboratory publishes indices of the intensity of large-scale, wind-induced coastal upwelling and alongshore transport at standard locations on a monthly basis. The CBR Mean method uses data from 1967 to the present for three locations along the Pacific Northwest coast:

42N125W West of OR/CA border,
45N125W West of Siletz Bay Lincoln, OR,
48N125W West of La Push, WA.

For all years, the CBR Mean method takes each day's upwelling deviations from the site-specific mean offshore transport. The upwelling deviation was used to account for long term trends at each site. Then the daily deviations were averaged from the three sites. The average upwelling deviation indices are then smoothed using a 15 day central mean calculation. The use of a central mean avoids the trailing nature of a running mean. The smoothed cumulative upwelling deviation indices are then examined for spring minima and fall maxima through the entire series. The julian day of these extremes are listed as the CBR Mean Spring and Fall Transition Dates.