The Southern Oscillation Index (SOI)

Weather Eye
with John Maunder

The Southern Oscillation Index (SOI) is a standardized index based on the observed sea level pressure differences between Tahiti and Darwin, Australia.

The SOI is a leading measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes.

In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific.

The negative phase of the SOI (El Nino) (represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. The positive phase of the SOI (La Nina) represents above-normal air pressure at Tahiti and below-normal air pressure at Darwin.

Prolonged periods of negative SOI values coincide with abnormally warm ocean waters across the eastern tropical Pacific typical of El Niño episodes.

In contrast, prolonged periods of positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of La Niña episodes.

Sustained negative values of the SOI below −8 often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds.

Sustained positive values of the SOI above +8 are typical of a La Niña episode. They are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time.

The graph below ( from the Australian Bureau of Meteorology, BOM) shows monthly values of the SOI from January 2021 to 7 November 2023.

image.png

Monthly ENSO Outlook values 1980 - 2023

Legend

ENW

El Niño Watch

LNA

El Niño Alert

EN

El Niño

N

Inactive

LNW

La Niña Watch

ENA

La Niña Alert

LN

La Niña

El Niño and La Niña years

Shading of text in the year column refers to years in which El Niño (red) or La Niña (blue) events began.

Past ENSO events

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1980 N N ENW ENW ENW ENW ENW N N N N N
1981 N N N N N N N N N N N N
1982 N N N N ENW ENA EN EN EN EN EN EN
1983 EN EN LNW LNW LNW LNW LNW LNW LNA LNA LN LN
1984 LN LNW LNW LNW LNW LN LN N N N N N
1985 N N LNA LNA N LN N N N N N N
1986 N N N N N N ENA ENA ENA ENA EN EN
1987 EN EN EN EN EN EN EN EN EN EN EN EN
1988 EN EN LNW LNA LNA LN LN LN LN LN LN LN
1989 LN LN LN ENW ENW N N N N N N N
1990 ENW ENW ENA ENA N N N N N N N N
1991 N N ENW N ENA EN EN EN EN EN EN EN
1992 EN EN EN EN EN EN N N N N N N
1993 N N ENA ENA EN EN N N N N N N
1994 N N N N ENW ENA N ENA N EN EN EN
1995 EN EN N LNW LNW LNW N LNA LN LN LN LN
1996 N N ENW ENW ENW ENW N N N N N N
1997 ENW ENW ENW ENW ENA EN EN EN EN EN EN EN
1998 EN EN LNW LNW LNW LNA LNA LN LN LN LN LN
1999 LN LN LN LN LN LN LN LN LN LN LN LN
2000 LN LN ENW ENW ENW ENW N N N LNA LNA LNA
2001 LN N ENW ENW ENW N ENW N N N N N
2002 N N ENW ENA ENA ENA EN EN EN EN EN EN
2003 EN N N N LNW LNW N N N N N N
2004 N N N N N N ENW ENA ENA ENA ENA ENA
2005 N N N ENW N N N N N N N N
2006 N N N N N N ENW ENA EN EN EN EN
2007 EN N LNW LNW LNA LNA LNA LN LN LN LN LN
2008 LN LN LNA LNA N N N N N N LNA LN
2009 LN LN ENW ENW ENW ENW ENW ENA ENA ENA EN EN
2010 EN EN EN LNW LNA LNA LN LN LN LN LN LN
2011 LN LN LN N N N N N LNA LN LN LN
2012 LN LN N N ENW ENW N N N N N N
2013 N N N N N N N N N N N N
2014 N ENW ENW ENA ENA ENA ENA ENW ENW ENW ENA ENA
2015 ENA N ENW ENA EN EN EN EN EN EN EN EN
2016 EN EN EN LNW LNW LNW LNW LNW LNW LNW LNW N
2017 N N ENW ENW ENW ENW N N N N LNW LN
2018 LN LN N N N ENW ENW ENW ENW ENA ENA ENA
2019 ENA ENW ENW ENA ENW ENW N N N N N N
2020 N N N N N N LNW LNW LNA LN LN LN
2021 LN LN LN N N N N N LNW LNA LNA LN
2022 LN LN LN LN LN LN LNW LNA LN LN LN LN
2023 LN LN ENW ENW ENW ENA ENA ENA EN EN EN na
                         

Historical values of the ENSO Outlook status prior to 2014 are based on the set criteria alone. Values from the beginning of 2014 include expert analysis by climatologists at the Bureau of Meteorology to make the final assessment on a status level having been reached.

El Niño and positive Indian Ocean Dipole continue

ENSO Outlook

El Niño continues in the tropical Pacific. Warmer than average sea surface temperatures (SSTs) in the tropical Pacific persist above El Niño thresholds, with warmer water beneath the surface to support that at the surface. In the atmosphere, cloud, wind and pressure patterns are consistent with El Niño conditions. Climate model forecasts indicate some further warming of the central to eastern Pacific is likely, with SSTs remaining above El Niño thresholds into the early southern hemisphere autumn 2024.

The positive Indian Ocean Dipole (IOD) event continues. All models indicate that this positive IOD will likely persist into early December. A positive IOD typically leads to reduced spring rainfall for central and south-east Australia.

The Southern Annular Mode (SAM) index is currently positive with forecasts indicating it will return to neutral in the coming days.

The Madden–Julian Oscillation (MJO) is currently weak. Approximately half of international climate models suggest the MJO will strengthen and move eastwards across the western Pacific later this week. When the MJO is in the western Pacific, there is an increased chance of showers and rain over northern parts of the NT and Queensland.

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"Climate Change: A Realistic Perspective"

The title of my climate book "Fifteen Shades of Climate" has been changed to "Climate Change: A Realistic Perspective" to better reflect its contents.

Note: I was born in Nelson (New Zealand) and lived inTakaka in 1938-45 when I watched the Takaka River flood, and as a 7 year old I asked two questions ..Why did it rain,  and What were the consequences of the rain. I still have questions.

The new book is identical to the old book, with the exception that a new Author's Foreword  has been included.

Please  Google  " Climate Change: A Realistic  Perspective"

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