mei: ENSO MEI - Multivariate ENSO Index

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

ENSO MEI – Multivariate ENSO Index

Description

The Multivariate ENSO (El Niño-Southern Oscillation) Index combines both oceanic and atmospheric variables, to an assessment of ENSO in in a single index.

Usage

mei

Format

A tibble also of class 'pacea_index' with columns:

year:

year of value

month:

month (1 to 12) of value, representing the two-month calculation covering the given month and the preceding month (i.e. month 2 in year 2023 represents the January-February 2023 value)

anomaly:

anomalies based on a reference period of 1980-2018

Details

NOTE (22th April 2024): on NOAA's PSL website given below it states: "The input OLR dataset is delayed at the source. We will update the index when it's available. The source dataset (JRA-55) for the MEI V2 has been discontinued by the Japanese Meteorological Agency as of January 2024. We have switched to using the JRA3Q reanalysis which yields very similar results." The whole time series looks to be affected, but we have not looked into whether it has reverted to the original 'mei' that was in earlier version of 'pacea', because of the following note. Andy can dig into this if you need.

NOTE (13th Dec 2023): on NOAA's PSL website given below it was noted that there was an issue with one of the data sources for calculating the MEI and the values were therefore recalculated on 13th Dec 2023. These are now used in 'pacea', and means that 'mei' in versions of 'pacea' installed before 21st Dec 2023 will have the old incorrect values. The whole time series looks to have been affected.

The bi-monthly Multivariate ENSO index (MEI.v2) is the time series of the leading combined Empirical Orthogonal Function (EOF) of five different variables:

• sea level pressure (SLP)

• sea surface temperature (SST)

• zonal components of the surface wind

• meridional components of the surface wind

• outgoing longwave radiation (OLR)

These are calculated over the tropical Pacific basin (30 deg S to 30 deg N and 100 deg E to 70 deg W). The EOFs are calculated for 12 overlapping bi-monthly "seasons" (Dec-Jan, Jan-Feb, Feb-Mar, ..., Nov-Dec) in order to take into account ENSO's seasonality, and reduce effects of higher frequency intraseasonal variability.

Key features of composite positive MEI events (warm, El Niño) include:

• anomalously warm SSTs across the east-central equatorial Pacific

• anomalously high SLP over Indonesia and the western tropical Pacific

• anomalously low SLP over the eastern tropical Pacific

• reduction or reversal of tropical Pacific easterly winds (trade winds)

• suppressed tropical convection(positive OLR) over Indonesia and the Western Pacific

• enhanced convection (negative OLR) over the central Pacific

Key features of composite negative MEI events (cold, La Niña) are of mostly opposite phase. For any single El Niño or La Niña situation, the atmospheric articulations may depart from this canonical view.

The above description is adapted from NOAA's Physical Sciences Laboratory website https://www.psl.noaa.gov/enso/mei/, which has more details and references(and is where the values come from). The data on that website are updated on the 10th of each month (if source data are available). Warm and cold thresholds are considered as being +0.5 (warm) and -0.5 (cold) away from 0. Values are calculated for overlapping two-month ‘seasons’, with, for example, February 2023 in 'pacea' representing the January-February calculation.

Associated code adapted from code generously shared by Chris Rooper.

Author(s)

Andrew Edwards

Source

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

Examples

## Not run: 
mei
plot(mei)

## End(Not run)

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