The hurricane season will be upon us soon enough, time to make a seasonal forecast
for it. Right now, the most interesting factor coming into play is the developing
La-Nina. This La-Nina has developed rapidly, and looks to be quite
significant during the hurricane season. Below is the graph of Nino 3.4 and the
SST anomaly maps for the past few weeks, showing this rapid development of La-Nina.
La-Nina is well known to spread more favorable conditions for hurricane development
across the North Atlantic. This is mostly due to a slackening of the
200 mb westerly winds, which results in less shear. Below is a graphic showing
hurricane activity during the ENSO cycles. La-Nina is on the right and left, with
El-Nino in the middle. It can be clearly seen how more tropical cyclones occur
during the La-Nina years.
The next factor, that I have hypothesized has a major impact on tropical cyclones, is
rainfall variability in the African Sahel. I believe this due to the major affect
the Saharan Air Layer has on hurricane season. If the Sahel of Africa is drier
than normal, it is much more likely that dust will be kicked up and a SAL
outbreak will occur.
Currently, things are looking quite dry in the Sahel of Africa, due to a drier than
normal rainy season last year. Using this data I have created the 365 day
rainfall deficit map below. It just outlines where its drier than normal and wetter
than normal. http://www.cpc.ncep.noaa.gov/products/glob...a_1yrprec.shtm
Some stats that I made a few years ago show when the Sahel is wetter than normal
there are on average 5.17 Cape Verde storms, and when it is drier than normal
there are 2.09. The average is 4. Something to look for this cane season
will be cold water anomalies off of Africa, which signify stronger winds
and an imminent SAL outbreak due to the dry surface conditions.
Conclusions
Due to the La-Nina, I expect the Gulf and Caribbean to maintain quite favorable
atmospheric conditions for cyclogenesis throughout the season. The Gulf and
Caribbean will thus need to be watched closely. The Cape Verde region will also
benefit from La-Nina but, SAL intrusions from the drier than normal Sahel
will inhibit Cape Verde activity. This all leads me to believe this season
will have above normal activity.
I have updated my forecast based on a study I did on Sahelian correlations
to tropical cyclone activity, this study can be found here: Sahel Study
I have upped the seasonal forecast to above average activity instead of near
normal based on the study.
Updated Forecast as of late April:
Forecast
11-14 named tropical cyclones
6-9 hurricanes
3-5 major hurricanes
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