viddaloo,
EDIT: Just wanted to say I concur with ASLR's response in reply 1715.
I don't believe that El Nino had ever started. Negative values for the 30 day avg have not persisted long enough or strong enough to declare out and out El Nino conditions. However, I believe that the current run of negative(below ZERO) 30 day avg SOI values, is at least indicative of El Nino “like” conditions. That is, conditions that are NEUTRAL, bordering on weak El Nino. The daily, 30 day, and 90 day avg SOI values all indicate that for the past few months, SLP(Sea Level Pressure) at Darwin has been slightly higher than normal, while SLP at Tahiti has been slightly lower than normal. This is what causes SOI values to become negative and likely suggests that there has been some level of weak atmospheric response to the slightly above avg SSTA in the tropical Pacific. There have been other contributing factors to negative daily SOI values(negative daily SOI values of course influence the 30 and 90 day avg SOI values) over the past few months, such as the SPCZ(South Pacific Convergence Zone), weak active phases of the MJO, and high and low pressure systems passing over these regions. However, none of those contributing factors would be able to hold daily SOI values in the negative range for longer than a few weeks, except the MJO, which just hasn’t been strong enough since April to do that. In fact, we saw negative daily SOI values continue even when there was a supposed inactive phase of the MJO in the Western Pacific, which should have caused daily SOI values to become positive. So something else must be going on. Notice that in the attached image from Weatherzone, the recent run of negative values for the SOI 30 day avg is longer than anything we've seen since at least 2011(if the plot showed data back to 2010 then one would see that its actually the longest run since 2010). Not the strongest, but the longest. Also, notice how the SOI remained essentially positive through all of 2011 when we were in moderate La Nina conditions. Provided below are definitions that should help bring some understanding to what the SOI is and how it pertains to ENSO conditions.
The first definition of the SOI comes from STORMSURF.COM, by Mark Sponsler:
SOI: First we look at the Southern Oscillation Index (SOI). This index/number compares surface pressure over Darwin Australia with pressure over Tahiti. If this value is negative for an extended period of time, that indicates average surface pressure is lower over Tahiti and higher over Darwin, symptomatic of El Nino (or over short durations, ~ 20 days, the Active Phase of the MJO). Positive values over longer times indicates La Nina (or the Inactive Phase of the MJO for shorter durations). The greater the negative or positive value over time, the stronger the ENSO (El Nino Southern Oscillation) configuration (be it El Nino or La Nina). During El Nino episodes wind flows from generally high pressure over Darwin towards generally lower pressure over Tahiti, which is a reverse of what a normal state is. When it's positive, the reverse it true, with higher pressure over Tahiti and lower pressure over Darwin, typical of La Nina, with wind flowing east to west, typical of the trade wind pattern for this region, only more so. The March-June time frame is called the 'Spring Unpredictability Barrier'. During this time wild swings in the SOI can occur, driven by pulses of the MJO. These swings do not always reflect the start of a long term trend, and often can be 'red-herrings' or 'false starts'.
The Second definition of the SOI comes from the BOM:
The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niño or La Niña events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin. 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, and a reduction in winter and spring rainfall over much of eastern Australia and the Top End. 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. Together these give an increased probability that eastern and northern Australia will be wetter than normal.
