August 2023 has already seen Hurricanes Dora, Fernanda and Hilary as well as Tropical Storms Eugene and Greg in the Eastern Pacific, and with three new investigation areas being monitored in the East Pacific and Atlantic Oceans, there are no signs of hurricane activity stopping any time soon. 

Typical Hurricane Season in the Americas starts on May 15th and ends November 30th. On average a total of 15 storms are identified per season, with eight of these developing into hurricanes and only four exceeding category 3. Pre-season forecasts of cyclone activity for 2023, by NOAA & SMN, predicted a total of 14-20/16-22 tropical storms this season respectively, with 7-11 of them becoming hurricanes. 

With just over three months to go, the East Pacific is tracking to meet these above average expectations, with a total of eight tropical storms and six hurricanes, four of which were major (exceeding category 3). Five of the eight hurricanes occurred in August! 

So why the sudden increase in cyclone activity these past weeks? Has something changed or is this normal? 

August has had the most eastern Pacific cyclone activity of all the months in the past 30 years, closely followed by July and September. So, it’s not particularly surprising. The reason we are seeing all the severe storms of the 2023 season so far in the span of two weeks is because the conditions are just right for them to form. 

Tropical storms occur due to a build-up of energy over an area, known as an instability.  For Australia, Asia and the east coast of Africa, monsoons generally cause most of the instability for tropical storms. In the Atlantic and Pacific, they are caused by Easterly Waves.   

Fig 1. An image of a blade nicking a piece of wood and forming spiral shaped shavings 

Imagine two blades placed on opposite sides of a plank of wood but facing the same “forward” direction. The piece of wood is slowly being moved forward, but the two blades are faster. Every time the blades nick the piece of wood, a little wooden shaving (which has a spiral shape) falls off. These are pretty much how Easterly waves work to form instabilities. They are east-to-west winds that tend to be slower than their surrounding air, so little spirals of wind fall off the sides... making instabilities. 

Now for hurricanes to form, these instabilities need to be enhanced. To do so we need: 

1. To be located between 5 and 30 degrees of latitude 
2. Have warm sea surface temperatures (about 24.5 degrees)  
3. Moisture availability (prefers ocean over land) 
4. Low differences in the speeds and directions of winds as you move up in the atmosphere (so it can bring air towards it) 

At the moment, conditions 2, 3 and 4 can be met given that the instability forms in the location designated by condition 1.  

Not only is it summer in the Northern Hemisphere but the surface temperature of the East Pacific is warmer than normal as it develops into El Niño. As a consequence, there is also a greater amount of evaporation occurring and wind speeds reducing. There is also slightly more than average moisture over the East Pacific due to another phenomenon, known as the Madden-Julian Oscillation. This globally circulating cloud band is directly over the East Pacific at the moment, increasing chances of hurricane development. 

Fig .2 A map of sea surface temperature anomalies compared to the mean sea surface temperature at this time of the year (17th August) from NOAA. 

Fig.3 A map of the difference in light reaching the planet's surface due to the prescence of the MJO from BOM (17th Aug 2023). 

These conditions are expected to persist well into September. So, this is not the end of the hurricanes this season, more are expected to come...