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2021 Atlantic Hurricane Season

After a quiet few weeks in the Atlantic, following a rather busy June and early July, we are quickly approaching the climatological peak of the Atlantic hurricane season. So, what to expect as we climb the climo ramp to the peak? Well, intuition says that we will start to see activity ramp up but there are some indicators occurring on the other side of the globe that suggest we may be in for an active next month to 6 weeks.


Source: https://www.nhc.noaa.gov/climo/images/peakofseason_sm.gif


Arguably the biggest trigger for tropical cyclone formation is called the Madden-Julian Oscillation (MJO). It is a pulse of convection that moves west to east along the equator that typically circumvents the equator every 3 to 6 weeks. As this pulse passes through whichever ocean basin around the world, it is accompanied by rising air indicating increased instability and enhanced tropical cyclone formation. Ahead and behind the pulse, in ocean basins it is not located in, there is a general decrease in convection and an overall sinking motion. The MJO varies by amplitude as well with the convective pulse strengthening or weakening at certain points on its journey around the equator. Currently, it is over the Pacific portion of the equator at a fairly strong state and is expected to cross into the Atlantic basin over the next week. This is why the Pacific has seen a burst of activity and the Atlantic has been quiet.



Source: https://www.climate.gov/sites/default/files/MJO_610_0.png


Note in the image below that phases 1 and 2 traverse the Atlantic returning to Western Hemisphere.


Source: https://www.researchgate.net/profile/Brendan-Power-3/publication/228744365/figure/fig1/AS:301923119976469@1448995489921/Approximate-locations-of-the-MJO-centre-of-convection-RMM-Index-phases-1-8-Phase1.png


Note in the image below that the green line (the average of the ensemble forecast) progresses the MJO from phase 8 into 1 and 2.


Source: https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/ensplume_full.gif



Source: https://www.nhc.noaa.gov/xgtwo/two_pac_5d0.png



Source: https://www.nhc.noaa.gov/xgtwo/two_atl_5d0.png


As indicated by the above images, forecasts currently call for the MJO, currently located in phase 8 (Eastern Pacific) to progress into phases 1 and 2 (Atlantic and Western hemisphere). The burst of activity occurring in the Eastern Pacific will begin to translate into the Atlantic most likely by the second or third week in August. So, while things are quiet now…and will likely remain quiet for the next week or so, it is only a matter of time before we see the Atlantic ramp up.


Another indicator of an active burst coming up is the above average sea surface temperatures in the Atlantic basin. The entire Northern Atlantic (sans a patch near North Africa and Western Europe) are at or above average. This not only provides fuel for developing tropical waves and cyclones, it promotes an overall rising motion indicative of greater than normal instability.

[Insert SST anomaly map] Note in the image below the blue in the Pacific indicative of a developing La Nina (colder than normal sea surface temperatures) and in the Atlantic the yellow and orange (indicative of warmer than normal sea surface temperatures).



Source: https://www.ospo.noaa.gov/data/cb/ssta/ssta.daily.current.png


Note in the image below, the brighter colors (yellows, oranges, and reds) in the Caribbean and Gulf which show high oceanic heat content. This is needed for tropical cyclones to develop and strengthen.



Source: http://isotherm.rsmas.miami.edu/heat/images/ohc_aQG3_latest_natl.gif


The final indicator for an active upcoming several weeks will be the developing colder sea surface temperature anomalies in the equatorial Pacific also called La Nina. La Nina has not been officially declared by the CPC, but will likely be in the upcoming several months. All indications are that we will continue to see cooling in the Pacific which contributes to reduced shear in the Atlantic and an overall sinking and stable motion in the Pacific reducing tropical activity in that basin and enhancing the rising motion aiding in cyclone development in the Atlantic.


Note in the image below the ensemble average (dashed line) is showing an increase in colder sea surface temperatures in the equatorial Pacific throughout hurricane season.


Source: https://www.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/images1/nino34Monadj.gif


So what does this all mean? We are only 5 weeks away from the climatological peak of the Atlantic hurricane season. With that, an increasing likelihood of cyclone development is expected in the Atlantic with perhaps more than one storm ongoing at one time. How impactful these storms will be when they form will be determined by the mid and upper level patterns which are difficult to determine beyond one week. Stay tuned as the Atlantic hurricane season is really only getting started.

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