bi: BI - North Pacific Current Bifurcation Index

In pbs-assess/PACea: An R package of Pacific ecosystem information to help facilitate an ecosystem approach to fisheries management

BI – North Pacific Current Bifurcation Index

Description

The North Pacific Current Bifurcation Index is an annual index indicating the north-south variation of where the North Pacific Current bifurcates into the northward-flowing Alaska Current and the southward-flowing California Current.

Usage

bi

Format

A tibble also of class 'pacea_index' with columns:

year:

year of value

value:

absolute value of the proportion of the 215 simulated drifters that ended up south of their starting latitude in that year

anomaly:

standarised index of the 'value' time series calculated by subtracting the mean and dividing by the standard deviation of the 'value' time series. Note that numbers willchange in subsequent updates – see Details

anomaly_2024:

standarised index of the 'value' time series calculated by subtracting the mean and dividing by the standard deviation of the 'value' time series, with the mean and standard deviation restricted to years up to and including 2024. The anomalies for years up to 2024 will not change in subsequent updates since the tandardisation is fixed; later years will be added (but will not affect the standardisation). See Details.

Details

The North Pacific Current Bifurcation Index (BI) was developed by Malick et al. (2017), who found that a northward-shifted bifurcation was associated with increased salmon productivity in British Columbia and Washington State waters. The BI is calculated by generating annual trajectories for 215 simulated drifters. Simulated drifters were released annually on 1st February and locations were tracked until 30th June, to reflect ocean conditions relevant to seaward-migrating juvenile salmon.

The BI for each year is calculated by first calculating the proportion of the 215 simulated drifters that ended up south of their starting latitude for that year. A high proportion of drifters ending south of their starting location indicates a northward-shifted bifurcation (the southward-flowing California Current is starting further north). A low proportion of drifters ending south of their starting location indicates a southward-shifted bifurcation (the southward-flowing California Current is starting further south). The index is standardised by subtracting by the time-series mean and dividing by the standard deviation. We provide absolute numbers (the proportions, given by column 'value') in the pacea data object 'bi' , as well as the standardised index (column 'anomaly').

Since the standardisation of the index will change as new years are added (because the mean and standard deviation will get recalculated with the longer time series), we also include a column 'anomaly_2024' for the anomalies calculated based on the proportions up to 2024, and will add 'anomaly_2025' etc. in future years. Therefore users can specify a definitive anomaly index so that their analyses will not change when pacea is updated in the future (or you can just refer to 'anomaly' so that it does change – you just need to be aware of this). Users can also use 'pacea::standardise()' to standardise to any specified range; see [?standardise].

Values are updated annually by Michael Malick, and the values used here are taken from his .csv file on GitHub: https://github.com/michaelmalick/bifurcation-index/blob/main/share/bifurcation_index.csv. Note that restricting the standardisation to only use the years up to and including 2010 gives slight changes to the original index shown Malick et al. (2017) because when first updating the index (in 2020) after publication, a new landmask file was required (used to truncate drifter tracks when they hit land) that had slightly different spatial resolution.

Some of the above was adapted from Malick et al. (2017). See that for full details, plus the code at https://github.com/michaelmalick/bifurcation-index/. Thanks to Mike for helpful discussions.

Malick, M.J., et al. 2017. Effects of the North Pacific Current on the productivity of 163 Pacific salmon stocks. Fisheries Oceanography 26:268–281. https://doi.org/10.1111/fog.12190

Author(s)

Andrew Edwards

Source

Generated from running 'data-raw/coastwide-indices/coastwide-indices.R'.

Examples

## Not run: 
bi
plot(bi)

## End(Not run)

pbs-assess/PACea documentation built on April 17, 2025, 11:36 p.m.