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Friday, September 28, 2012

SOI Remains Erratic; El Nino Not Evident

The Southern Oscillation Index, commonly called the SOI, remains too erratic and unsteady to provide clear evidence that an El Nino is present.

This roughly-2 year chart of the SOI shows that the La Nina was very well defined in summer-fall-winter 2010-2011, and even in 2011-2012. However, as of recently, the SOI has been all over the place. It has mainly been in negative territory, but the fact that it is usually above -8.0 (the threshold for an El Nino is -8.0 or below) during the last few months tells me that the SOI is not recognizing the presence of an El Nino.

I believe that this winter will hold a neutral ENSO, possibly warm neutral (above 0.0 but not above 0.5) . I have yet to nail down exactly the analogues appropriate for that situation, but I will find it and give it to you ASAP.

Andrew

Special: Why Was This Hurricane Season Active?

This is a special report by The Weather Centre.

This summer was an El nino summer, and in an El Nino, tropical winds turn to a point that drives down Atlantic activity, and raises Pacific tropical activity. However, as shown in this post, the Atlantic is currently above normal. So why is this happening?

I would like to introduce you to the AMM, or the Atlantic Meridional Mode. The AMM, in a nutshell, is traced in either a positive or negative phase. In the positive phase of the AMM, tropical activity in the Atlantic is above normal. In the negative phase, below normal activity is observed.

Let's take a look at recent statistics of the AMM over the past through years. Screenshot courtesy of the ESRL.

The chart shows months vertically, one column for each month. The circled part is the last 3 months of the AMM. See how it has recently spiked positive after a negative winter. As of the latest August tally, the AMM ended up above 2.00, indicating the AMM is well into the positive phase.

I expect enhanced Atlantic tropical activity to continue, as the AMM takes time to switch phases. However, as we edge into fall, this possibility is slowly dwindling.

Andrew