The PNI is a mean of three environmental parameters: annual total precipitation, snowpack depth on March 15^th, and annual mean air temperature (Ebbesmeyer and Strickland 1995), with each of the three environmental parameters scaled to a mean of 0 and 1 standard deviation.

Slightly different methods of the PNI have been applied on DART through the years because of data gaps and discontinued stations. We describe three different flavors of the methods that have been applied on DART: 1) Original PNI [v1.0 and v1.1]; 2) Alternative PNI [aPNI; Beta v2.0] that uses dynamic factor analysis to help predict missing air temperature at Olga, WA; and 3) Updated PNI [v3.0], which replaces air temperature at Olga, WA with that at Everett, WA.

The current record of PNI that is reported on the PNI webpage is PNI v3.0. The previous records of PNI that have been published on DART are archived below. We include some comparisons and interpretations of the different versions of PNI.

Methods for calculating PNI on DART through the years

Original PNI (v1.0 and v1.1)

The original PNI was calculated following the method in Ebbesmeyer and Strickland (1995) for the annual index in years 1891-2015 (and the 5-year average PNI in years 1893-2013). We applied PNI v1.0 from mid-2000s to 2016 for reporting on DART. An updated version of the PNI with QCed data conducted in 2025 is also available and is referred to as PNI v1.1.

The PNI v1.0 and v1.1 uses three parameters: 1) annual total precipitation at Cedar Lake, WA in the Cascade Mountains [USC00451233, NCEI]; 2) snowpack depth at Paradise, WA on Mount Rainier on March 15 of each year [USC00456898, NCEI]; and 3) annual mean air temperature at Olga, WA in the San Juan Islands [USC00456096, NCEI].

The annual PNI for year y is calculated as follows:

Please note that the sign of the normalized air temperature data is reversed to align with precipitation and snowpack in their positive or negative effects on organisms. We report the equations above as documented on DART since the mid-2000s. For further breakdown of steps to the calculations, see the section on Updated PNI v3.0.

For PNI v1.0 and v1.1, when there were any missing data, the PNI was still calculated using only two out of the three values of precipitation (P), snowpack (S), and air temperature (T), or even just one out of the three environmental parameters. For data quality purposes, we note whether all three environmental parameters are used (i.e., PST), two of the three environmental parameters are used (i.e., PS, PT, or ST), or one out of the three are used (i.e., P, S, or T).

Alternative PNI (aPNI; Beta v2.0)

The annual aPNI was calculated for years 1891-2021 (and 5-year average aPNI for years 1893-2019). We applied this method on DART because air temperature stopped being observed at station Olga 2 SE, WA (USC00456096, NCEI) after 2016-04-30. We applied a dynamic factor analysis (DFA) to predict temperatures at Olga, WA (Van Holmes and Gosselin 2020) starting in 2020. However, additional data gaps at the stations used in the DFA prevented the continuation of this method starting in 2022. Please note that this method is in development and a new selection of stations to replace discontinued stations is pending for the DFA analysis to predict temperatures at Olga, WA.

For PNI Beta v2.0, when there were any missing data, the PNI was still calculated using the data available (e.g., PS, PT, ST, P, S, or T).

Updated PNI (v3.0)

The PNI v3.0 was calculated starting in 2025 with a similar method to that of PNI v1.0, but using air temperature at Everett, WA (USC00452675, NCEI) instead of at Olga, WA (USC00456096, NCEI). The PNI v3.0 record begins in 1915 and is the current method updated annually on DART.

The PNI v3.0 is calculated as follows:
(Please note that these equations are equivalent to the simplified calculations in Eq.1.)

where P_z,y, S_z,y, T_z,y are respectively the normalized data (i.e., scaled to a mean of 0 and 1 standard deviation; also known as z-scored data). The z-scored data are calculated as follows:

where p_y, s_y, t_y are respectively the annual total precipitation at Cedar Lake, the snowpack depth at Paradise, Mount Rainier on March 15, and the annual mean air temperature at Everett, WA in year y. The time series mean (μ) and standard deviation (σ) for each of the three environmental parameters are calculated from the most complete time series possible. Thus, the means ( μ_P, μ_S, and μ_T) and standard deviations ( σ_P, σ_S, and σ_T) are recalculated with new data each year (see Table 3). Please note that the data for total precipitation and snowpack depth are reversed (negative sign in Eq. 3.1 and 3.2) to align with the direction of effects of temperature on organisms. For clarity, the means are calculated as follows:

where N_P, N_S, N_T are respectively the sample sizes (i.e, number of years of data) available for annual precipitation, snowpack depth, and air temperature.

For PNI v3.0, if two environmental parameters are available, we still calculate it using the data available (e.g., PS, PT, or ST). If only one of the environmental parameters is available, then we do not calculate the PNI and note which one environmental parameter was available (i.e., P, S, or T).

Updates to the data

As part of developing PNI v3.0, a new quality control process was established and the historical data were re-examined. The updates applied to the historical data sets, as of February 2025, are summarized below.

• Annual Total Precipitation at Cedar Lake station, WA:
  • For years 1900, 1903, 1904, and 1913, there is not enough data from the Cedar Lake site to obtain an accurate estimate of the annual total precipitation. These years are left as null.
  • For years 1929 and 1930, historical data is slightly different than the data used to calculate PNI v1.0 and aPNI (Beta v2.0); after further investigation, the current data reported for station USC00451233 from NCEI appears more accurate and is thus used in the updated data set.
  • For the year 2017, data used to previously calculate the annual total precipitation was incomplete and has been corrected.
  • For the year 2021, data used to calculate the annual total precipitation was from Mount Rainier station. The data for 2021 has been corrected to precipitation data from Cedar Lake, WA.

• Snowpack Depth on March 15^th at Paradise station, Mount Rainier, WA:
  • For years 1916 and 1919, there is not enough data from the Paradise station to obtain an accurate estimate of the snowpack depth on March 15^th. These years are left as null.
  • For years 1925 and 1937, the data for snowpack depth on March 15^th that was previously obtained and used to calculate PNI v1.0 and aPNI (Beta v2.0) does not match the historical data currently reported by NCEI at station USC00456898. After investigation, current data appears to be more accurate and are used in the updated data set.
  • For years 1927, 1953, 1954, 2007, 2012, 2015, and 2023, there is no snowpack depth measurement on the date of March 15^th, however, there are measurements within a week before and after that. We used these data for linear interpolation to estimate the daily snowpack depth and the value on March 15^th. These calculations were checked and match the estimates used for calculating PNI v1.0 in all of these years, except for 1953, which has now been updated.
  • For the year 1948, no data on snowpack depth was recorded from March 1 until June 1. Given the large data gap, a linear interpolation was not applied. Instead, we used the maximum snowpack depth in February as a surrogate estimate for the March 15^th in the calculations of PNI v1.0 and aPNI (Beta v2.0) and we maintain this rule for PNI v3.0.

Comparisons of versions of PNI and yearly recalculations

Yearly recalculations of the PNI time series using updated means and standard deviations change the historical values of the PNI, but are relatively small and likely negligible (Figures 1, 2 and 3).

The means and standard deviations for the three environmental parameters are listed for each archived record of recalculated PNI time series (Table 1 for v1.0, Table 2 for Beta v2.0, and Table 3 for v3.0).

A comparison of the values of the PNI of different versions (v1.0, v1.1, Beta v2.0 and v3.0) can be viewed in Figure 4. The difference between PNI v3.0 and earlier versions of the PNI can be viewed in Figure 5 and Figure 6.

Table 1. PNI v1.0 means and standard deviations (SD) of the three environmental parameters for each yearly recalculated time series record of PNI.

PNI Archival Year	Data Year Range	Annual Total Precipitation at Cedar Lake		Snowpack Depth on March 15 at Paradise		Annual Mean Air temperature at Olga
		Mean	SD	Mean	SD	Mean	SD
1996	1891-1995	102.224	17.107	172.156	51.460	49.999	1.239
1997	1891-1996	102.343	17.057	171.410	51.547	49.995	1.234
1998	1891-1997	102.464	17.009	172.506	52.134	50.006	1.234
1999	1891-1998	102.488	16.923	172.425	51.808	50.018	1.235
2000	1891-1999	102.514	16.838	173.642	52.635	50.023	1.230
2001	1891-2000	102.265	16.937	173.805	52.330	50.027	1.225
2002	1891-2001	102.089	16.944	172.940	52.604	50.030	1.220
2003	1891-2002	101.943	16.924	173.548	52.582	50.036	1.216
2004	1891-2003	101.931	16.841	173.071	52.453	50.044	1.214
2005	1891-2004	101.773	16.837	173.012	52.146	50.065	1.228
2006	1891-2005	101.585	16.866	171.414	53.942	50.081	1.235
2007	1891-2006	101.734	16.856	171.750	53.723	50.084	1.230
2008	1891-2007	101.657	16.795	171.416	53.510	50.076	1.228
2009	1891-2008	101.689	16.720	171.856	53.372	50.054	1.245
2010	1891-2009	101.590	16.674	171.659	53.108	50.038	1.253
2011	1891-2010	101.606	16.599	171.326	52.912	50.051	1.256
2012	1891-2011	101.654	16.531	171.656	52.720	50.043	1.254
2013	1891-2012	101.678	16.458	172.000	52.542	50.033	1.254
2014	1891-2013	101.575	16.421	171.895	52.271	50.028	1.250
2015	1891-2014	101.748	16.452	171.677	52.039	50.054	1.278
2016	1891-2015	101.750	16.379	170.505	53.039	50.090	1.337

Table 2. PNI Beta v2.0 means and standard deviations (SD) of the three environmental parameters for each yearly recalculated time series record of PNI. The air temperature at Olga WA was predicted from a dynamic factor analysis using several other nearby stations of air temperature.

PNI Archival Year	Data Year Range	Annual Total Precipitation at Cedar Lake		Snowpack Depth on March 15 at Paradise		Predicted Annual Mean Air Temperature at Olga
		Mean	SD	Mean	SD	Mean	SD
NA	1891-1995	102.224	17.107	172.156	51.460	49.672	1.273
NA	1891-1996	102.343	17.057	171.410	51.547	49.667	1.268
NA	1891-1997	102.464	17.009	172.506	52.134	49.678	1.267
NA	1891-1998	102.488	16.923	172.425	51.808	49.692	1.269
NA	1891-1999	102.514	16.838	173.642	52.635	49.690	1.264
NA	1891-2000	102.265	16.937	173.805	52.330	49.686	1.258
NA	1891-2001	102.089	16.944	172.940	52.604	49.681	1.254
NA	1891-2002	101.943	16.924	173.548	52.582	49.677	1.249
NA	1891-2003	101.931	16.841	173.071	52.453	49.686	1.247
NA	1891-2004	101.773	16.837	173.012	52.146	49.704	1.256
NA	1891-2005	101.585	16.866	171.414	53.942	49.716	1.257
NA	1891-2006	101.734	16.856	171.750	53.723	49.725	1.255
NA	1891-2007	101.657	16.795	171.416	53.510	49.728	1.250
NA	1891-2008	101.689	16.720	171.856	53.372	49.721	1.248
NA	1891-2009	101.590	16.674	171.659	53.108	49.717	1.243
NA	1891-2010	101.606	16.599	171.326	52.912	49.733	1.249
NA	1891-2011	101.654	16.531	171.656	52.720	49.733	1.244
NA	1891-2012	101.678	16.458	172.000	52.542	49.739	1.241
NA	1891-2013	101.575	16.421	171.895	52.271	49.752	1.245
NA	1891-2014	101.748	16.452	171.677	52.039	49.783	1.287
NA	1891-2015	101.750	16.379	170.505	53.039	49.821	1.348
2017	1891-2016	101.790	16.314	170.816	52.854	49.854	1.393
2018	1891-2017	101.703	16.270	170.576	52.639	49.868	1.396
2019	1891-2018	101.751	16.209	170.540	52.373	49.888	1.410
2020	1891-2019	101.584	16.244	170.416	52.126	49.907	1.420
2021	1891-2020	101.767	16.300	170.647	51.920	49.922	1.425
2022	1891-2021	101.744	16.234	171.049	51.825	49.929	1.422

Table 3. PNI v3.0 means and standard deviations (SD) of the three environmental parameters for each yearly recalculated time series record of PNI.

PNI Archival Year	Data Year Range	Annual Total Precipitation at Cedar Lake WA		Snowpack Depth on March 15 at Paradise		Annual Mean Air temperature at Everett WA
		Mean	SD	Mean	SD	Mean	SD
NA	1915-1995	101.475	18.391	171.067	52.103	50.504	1.245
NA	1915-1996	101.623	18.321	170.316	52.167	50.512	1.239
NA	1915-1997	101.773	18.255	171.455	52.777	50.538	1.255
NA	1915-1998	101.809	18.147	171.385	52.437	50.573	1.289
NA	1915-1999	101.847	18.041	172.646	53.291	50.581	1.283
NA	1915-2000	101.566	18.132	172.825	52.977	50.593	1.280
NA	1915-2001	101.370	18.125	171.951	53.230	50.602	1.275
NA	1915-2002	101.209	18.090	172.585	53.212	50.614	1.273
NA	1915-2003	101.203	17.987	172.108	53.065	50.643	1.295
NA	1915-2004	101.028	17.969	172.060	52.746	50.671	1.314
NA	1915-2005	100.820	17.986	170.435	54.528	50.698	1.331
NA	1915-2006	101.000	17.964	170.791	54.306	50.718	1.338
NA	1915-2007	100.919	17.886	170.460	54.077	50.728	1.335
NA	1915-2008	100.964	17.793	170.920	53.939	50.727	1.328
NA	1915-2009	100.857	17.733	170.730	53.662	50.695	1.356
NA	1915-2010	100.883	17.640	170.400	53.451	50.714	1.362
NA	1915-2011	100.946	17.556	170.747	53.257	50.706	1.357
NA	1915-2012	100.980	17.468	171.109	53.077	50.714	1.352
NA	1915-2013	100.870	17.417	171.011	52.796	50.721	1.347
NA	1915-2014	101.074	17.441	170.798	52.552	50.748	1.367
NA	1915-2015	101.084	17.354	169.611	53.537	50.787	1.415
NA	1915-2016	101.135	17.274	169.938	53.351	50.819	1.446
NA	1915-2017	101.249	17.225	169.701	53.124	50.831	1.444
NA	1915-2018	101.309	17.150	169.673	52.850	50.858	1.464
NA	1915-2019	101.123	17.179	169.556	52.592	50.874	1.466
NA	1915-2020	101.336	17.230	169.800	52.383	50.900	1.483
NA	1915-2021	101.207	17.204	170.218	52.290	50.913	1.482
NA	1915-2022	101.092	17.168	170.020	52.068	50.935	1.493
NA	1915-2023	100.843	17.293	169.990	51.813	50.955	1.500
2025	1915-2024	100.777	17.229	170.058	51.566	50.977	1.511

Reference

Ebbesmeyer, C.C. and R.M. Strickland. 1995. Oyster Condition and Climate: Evidence from Willapa Bay. Publication WSG-MR 95-02, Washington Sea Grant Program, University of Washington, Seattle, WA. 11p.

Van Holmes, C. & Gosselin J. (2020). Development of a Mean Annual Temperature Value for the Continued Calculation of the Pacific Northwest Climate Index. Available from: www.cbr.washington.edu/sites/default/files/papers/Olga_Surrogate_DFA_FINAL.pdf

Williams, J.G., S.G. Smith, J.K. Fryer, M. D. Scheuerell, W.D. Muir, T.A. Flagg, R.W. Zabel, J.W. Ferguson, and E. Casillas. 2014. Influence of ocean and freshwater conditions on Columbia River sockeye salmon Oncorhynchus nerka adult return rates. Fish. Oceanogr. 23(3):210-224. doi:10.1111/fog.12056

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