The 30 day Southern Oscillation Index is at +12 – will El Niño fade again next month?

The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.

A strongly and consistently positive SOI pattern (e.g. consistently above about +6 over a two month period) is related to a high probability of above the long-term average (median) rainfall for many areas of Australia, especially areas of eastern Australia (including northern Tasmania) – La Niña.

Conversely, a ‘deep’ and consistently negative SOI pattern (less than about minus 6 over a two month period, with little change over that period) is related to a high probability of below median rainfall for many areas of Australia at certain times of the year – El Niño.

However, it is important to remember that the pattern of relationship between SOI and rainfall (and temperature) can vary depending on the particular season and region. Additionally, the change in SOI over a specified period can be as important in understanding relationships between SOI and rainfall as is the absolute value in SOI.

Here is what BoM has to say about it. While they note that we “remain at El Niño levels” they also note that “The latest 30-day value (+12) is unusual for a developing El Niño”.

Summary: Ocean indicators remain at El Niño levels

Ocean conditions in the Pacific Basin remain at El Niño levels. Should they persist at such levels through the remainder of the southern winter and into spring, as predicted by the world’s leading climate models, 2009 will be considered an El Niño year.

While El Niño indicators have fluctuated over the past few weeks, the overall picture remains one of a developing El Niño event. Pacific Ocean surface temperatures, which drive El Niño events, currently exceed El Niño thresholds and are around 1°C above average. Cloud patterns and rainfall along the equator are becoming consistent with a developing El Niño event.

However, some indicators run contrary to a normal El Niño development. The Southern Oscillation Index (SOI) is currently strongly positive: persistent negative values are a feature of El Niño events. Similarly, ocean temperatures in the western Pacific, Coral Sea and off northern Australia have been much warmer than during previous El Niño events.

El Niño periods are usually (but not always) associated with below normal rainfall in the second half of the year across large parts of southern and inland eastern Australia.

The value of the Indian Ocean Dipole (IOD), as measured by the Dipole Mode Index (DMI), has fallen steadily since the middle of June. The Bureau’s POAMA model suggests the IOD may increase over the coming months.

See IOD forecast, DMI values.

From the NOAA/PMEL/TAO website.

In Brief

• The sea surface is significantly warmer than the long-term average across most of the tropical Pacific Ocean, with El Niño thresholds met in central to eastern areas.
• A large amount of the sub-surface water of the tropical Pacific is also warmer than the long-term average, particularly in the east.
• The latest 30-day SOI value is +12, while the monthly value for June was −2. The latest 30-day value is unusual for a developing El Niño; persistent negative values are a feature of El Niño events.
• Trade winds have been consistently weaker than normal in the far western equatorial Pacific in recent months. Trade winds have fluctuated over the central to eastern tropical Pacific, with winds generally slightly stronger in July than in June.
• Consistent with an emerging El Niño, there is increasing cloudiness near the date-line.
• All international climate models predict the tropical Pacific to continue to warm and to be above El Niño thresholds throughout most of the second half of 2009.

Details

The central and eastern equatorial Pacific sea surface warmed through June and has remained warm through the first part of July. This warming is a continuation of a steady warming trend that has been observed since February 2009. Positive SST anomalies are now well established across most of the equatorial Pacific, with the monthly indices for June being +0.8°C, +0.7°C and +0.6°C for NINO3, NINO3.4 and NINO4 respectively.

In terms of weekly data, the most recent NINO indices are +1.1°C, +0.9°C and +0.6°C for NINO3, NINO3.4 and NINO4 respectively. Although the most recent NINO3 and NINO3.4 values are significantly warmer than their June values, over the past two weeks there has been little change in the weekly NINO index values. Consistent with the establishment of an El Niño, the 7-day SST anomaly map shows a persistence of positive anomalies of greater than +1.0°C across most of the central and eastern equatorial Pacific. Should these warm anomalies persist through the remainder of the southern winter and into spring, 2009 will be classed as an El Niño year. However, contrary to normal SST conditions during El Niño events, ocean temperatures in the western Pacific, Coral Sea and off northern Australia have been warmer than average. An animation of recent SST changes is available.

The sub-surface of the equatorial Pacific has steadily warmed since February 2009. A large volume of warmer than normal sub-surface water has been evident across most of the tropical Pacific since May. A four-month sequence of Pacific Ocean equatorial temperature anomaly is available here. The sequence shows a slight cooling of the sub-surface in the western tropical Pacific through July. A recent map for the 5 days ending 20 July shows warmer than normal sub-surface water extending across most of the equatorial Pacific, with anomalies exceeding +2.0°C over much of the eastern Pacific. When compared with two weeks ago, the sub-surface has cooled slightly, particularly in the west, however, most of the sub-surface remains significantly warmer than the long-term mean. An animation of recent sub-surface changes is available.

An archive of past SST and sub-surface temperature charts is available.

Trade Winds have been consistently weaker than average over the far west of the tropical Pacific in recent months. The Trade Winds have fluctuated over the central to eastern equatorial Pacific with Trades being generally weaker than the average in June and slightly stronger, closer to the long term mean in July. The latest weekly wind anomalies are shown in the TAO/TRITON map (small image above) for the five days ending 20 July.

The Southern Oscillation Index (SOI) is currently strongly positive with a 30 day value (30 July) of +12. The SOI is usually negative during normal El Niño development, therefore the current positive SOI trend is unusual. Ocean temperatures in the western Pacific, Coral Sea and off northern Australia have been warmer than average. The warmer waters near Darwin may have impacted on the SOI by keeping mean sea level pressure (MSLP) lower at Darwin, while recent weather patterns have kept Tahiti MSLP high. Some cooling of the warm waters off northern Australia has occurred in the past week. As the equatorial Pacific continues to warm and Trade Winds remain generally weak the SOI is expected to fall to negative values again. (SOI graph, SOI table).

Cloudiness near the date-line over the central to western Pacific is another important indicator of warm/cool ENSO conditions, as it normally increases/decreases (negative OLR/positive OLR anomalies) during these episodes. Cloudiness near the date-line has increased steadily in recent months. Consistent with a developing El Niño, cloudiness near the date-line has generally been greater than normal over the past few weeks. This is supported by satellite measurements of rainfall, which now show above average in rainfall in these areas.

All international dynamic computer models surveyed by the Bureau of Meteorology predict further warming of the Pacific Ocean SST in coming months. All models predict SST to be above El Niño thresholds throughout most of the second half of 2009. One of the surveyed models shows less vigorous warming than other models, however all models predict El Niño conditions to be established by the southern spring at the latest. As all models surveyed agree El Niño conditions will persist, and as historically the southern winter is a time of good model predictability, the probability of El Niño conditions remaining through 2009 is high. If this occurs, 2009 will be declared an El Nino year. Recent forecasts from the POAMA model, run daily at the Bureau of Meteorology, show a steady warming with El Niño conditions developing in July. Pacific conditions and model predictions will continue to be monitored closely.

For the mathematically minded (from Australia’s BoM)

There are a few different methods of how to calculate the SOI. The method used by the Australian Bureau of Meteorology is the Troup SOI which is the standardised anomaly of the Mean Sea Level Pressure difference between Tahiti and Darwin. It is calculated as follows:

         [ Pdiff - Pdiffav ]

SOI = 10 -------------------

             SD(Pdiff)

where

Pdiff   =   (average Tahiti MSLP for the month) – (average Darwin MSLP for the month),

Pdiffav   =   long term average of Pdiff for the month in question, and

SD(Pdiff)   =   long term standard deviation of Pdiff for the month in question.

The multiplication by 10 is a convention. Using this convention, the SOI ranges from about –35 to about +35, and the value of the SOI can be quoted as a whole number. The SOI is usually computed on a monthly basis, with values over longer periods such a year being sometimes used. Daily or weekly values of the SOI do not convey much in the way of useful information about the current state of the climate, and accordingly the Bureau of Meteorology does not issue them. Daily values in particular can fluctuate markedly because of daily weather patterns, and should not be used for climate purposes. A table of monthly SOI values is available here. Approximate 30-day values are often included in the weekly El Niño Wrap-Up.

h/t to WUWT reader rtgr