alpi: ALPI - Aleutian Low Pressure Index

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

ALPI - Aleutian Low Pressure Index

Description

The Aleutian Low Pressure Index (ALPI) has been used to describe decadal-scale changes in North Pacific climate-ocean conditions and is linked to patterns in marine productivity.

Usage

alpi

Format

A tibble also of class 'pacea_index' with columns:

year:

year of value

anomaly:

the area of the Aleutian Low is calculated as the mean area of the North Pacific with sea level pressure less than or equal to 100.5 kPa, where the mean is over the four monthly values from December to March (the value for 2015 corresponds to December 2014 to March 2015). The 'anomaly' given here is the resulting ALPI calculated as the anomaly of the area of the Aleutian Low from the long-term (1950-1997) mean of 5.524183 \times 10^6 \mbox{km}^2. Values are those in Table 4 of King and Surry (2015), and units of ALPI are 10^6 \mbox{km}^2.

Details

The Aleutian Low atmospheric pressure system is a semi-permanent feature of the North Pacific, generally centered over the Aleutian Islands. The Aleutian Low strengthens in winter, and weakens the following spring; however the relative intensity of low pressures can vary greatly from year to year. The Aleutian Low affects the intensity of winter storms and the direction of atmospheric circulation off the west coast of North America, since the size and position of the Aleutian Low determines the relative waviness of the westerlies. It was mentioned several times at the 2024 State of the Pacific Ocean meeting, prompting us to update it in pacea (it was previously only available to 2015).

A relatively weak and northward positioned Aleutian Low results in a direct westerly flow in the atmosphere over the North Pacific (i.e. westerly winds).

The ALPI was developed at the Pacific Biological Station (Fisheries and Oceans Canada) in 1993, and updated with documented code by Surry and King (2015). It requires the computation of the mean area (in 10^6 \mbox{km}^2) in the North Pacific that has a sea level pressure lower than 100.5 kPa in winter months (December to March). The ALPI is computed as the anomaly from a long-term mean area (1950-1997) from a gridded sea surface pressure data obtained from the National Center of Atmospheric Research (Surry and King 2015). Positive ALPI values indicate an intense Aleutian Low relative to the long-term mean.

Chris Rooper used the documented R code provided by Surry and King (2015) to calculate values from 2016 to 2022, and these are now included in pacea. The code could will be incorporated into pacea at some point (see progress at https://github.com/pbs-assess/pacea/issues/54), but Chris thinks the websites that the data come from have changed, so this might still be a bit of work. Some of the spatial R packages used have since been replaced by newer ones, which may also take a bit of time.

Much of the above is adapted from:

Surry, A.M., and King, J.R (2015). A new method for calculating ALPI: The Aleutian Low Pressure Index. Canadian Technical Report of Fisheries and Aquatic Sciences. 3135: v + 31 p. https://waves-vagues.dfo-mpo.gc.ca/library-bibliotheque/359380.pdf

and ALPI's use in

Haigh, R., P.J. Starr, A.M. Edwards, J.R. King and J. Lecomte (2019). Stock assessment for Pacific Ocean Perch (Sebastes alutus) in Queen Charlotte Sound, British Columbia in 2017. DFO Canadian Science Advisory Secretariat Research Document 2018/038. v + 227 p. http://www.dfo-mpo.gc.ca/csas-sccs/Publications/ResDocs-DocRech/2018/2018_038-eng.pdf

Also see references within Surry and King (2015), plus https://open.canada.ca/data/en/dataset/4bb821ce-bef7-46d3-95d2-064065f1bda4 that houses the original values.

Author(s)

Andrew Edwards

Source

Generated from running 'data-raw/coastwide-indices/coastwide-indices.R' and then (once) 'data-raw/coastwide-indices/alpi-update.R'

Examples

## Not run: 
alpi
plot(alpi)

## End(Not run)

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