Friday, September 28, 2012

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.


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