The Impact Of El Nino On Hurricane Season

How this year’s tropical weather season is shaping up.

For many of us, the arrival of June signals the beginning of the summer season, and a return to warm weather and outdoor activities. But for those living along the Eastern Seaboard and the Gulf Coast, the arrival of June has more ominous undertones as it signals the onset of hurricane season.

Hurricane season in the Atlantic Basin runs from June 1 to November 30, with a peak in late August/early September. Hurricane season in the Central Pacific Basin, which includes the Hawaiian Islands, also runs from June 1 to November 30, while the Eastern Pacific starts May 15 and runs through the end of November. Eastern Pacific hurricanes can occasionally affect the southwestern US (usually as remnant low-pressure systems), but more commonly affect the West Coast of Mexico and Guatemala, or proceed westward and cross into the Central Pacific (west of 140 degrees longitude).

Collecting clues

There are several factors we need to consider to make a forecast for tropical cyclone activity. The El Nino Southern Oscillation (ENSO) can have a significant impact on how active the tropical season in across all three hurricane basins that affect the United States. Once we have diagnosed the expected conditions for ENSO over the summer months, we can look at other years in which similar conditions existed to get an idea of what might happen during this hurricane season. More localized anomalies in the sea surface temperatures can affect which areas of a particular basin are more or less productive. The location and strength of certain atmospheric features, such as the “Bermuda High”, can not only affect the development of tropical systems, but can also influence where they go once they form. In the Atlantic Basin specifically, the presence of a phenomenon called the Saharan Air Layer can suppress the development of tropical systems by increasing subsidence (sinking air) in the atmosphere.

Looking specifically at this year, a weak El Nino has been in place since December 2018, and we are likely to see those conditions stick around through at least the early fall, when hurricane season is peaking. El Nino is defined as warmer-than-normal sea surface temperatures in the Eastern Pacific Ocean near the equator and this affects the Atlantic and Pacific hurricane basins differently. In the central and eastern Pacific, hurricane activity tends to increase in El Nino years, drawing energy from the warmer waters and causing an associated increase in rising motion in the atmosphere. This leads to an increase in thunderstorm activity, and these clusters of thunderstorms can potentially organize into tropical cyclones. This increase in activity is tempered somewhat by an equatorward shift in the Inter-Tropical Convergence Zone (ITCZ), which is a belt of thunderstorms that develops over the tropical oceans. Since there is less spin in the atmosphere near the equator, the thunderstorms have a lower chance of organizing into tropical systems if they develop near the equator. El Nino typically has a dampening effect on hurricane season in the tropical Atlantic Ocean. The increase in thunderstorm activity over the eastern Pacific causes westerly winds aloft to strengthen. These strong upper-level winds then travel eastward over Central America and into the tropical Atlantic, increasing vertical wind shear in the areas tropical cyclones form. The strong vertical wind shear prevents thunderstorm clusters from organizing into tropical cyclones.

Sea surface temperatures (SSTs) are currently anomalously cool in the Eastern Pacific Ocean in the waters immediately off the coast of Mexico between 10N and 20N where many tropical cyclones originate. The cold anomaly may inhibit development in this area, but there is an area of warmer water stretching from 120W out into the Central Pacific to around 160W where tropical cyclone development will be favored. SSTs in the Gulf of Mexico and Caribbean Sea are right about normal for this time of year, while temperatures in the tropical Atlantic Ocean are running slightly warmer than normal, especially in the waters near the Cabo Verde Islands off the west coast of Africa (an area where tropical systems begin to get organized). There is also an area of cooler water in the Sargasso Sea (a region of the Atlantic Ocean east of the Gulf Stream), which could serve to weaken any tropical cyclones aimed at the US East Coast.

The features mentioned above are very useful in the days leading up to the potential development of a tropical system, however, many are not reliably predictable on a long enough timescale to be useful in preparing a seasonal outlook. Therefore, in developing our forecast, we need to lean on features that are more predictable at longer lead times (such as ENSO) and what occurred in past years where similar conditions existed (analogs).

The Bottom Line

Based upon the available data, our prediction is for slightly below-normal tropical activity in the Atlantic Basin, and for above-normal tropical activity across the Eastern and Central Pacific hurricane basins. For reference, a normal year in the Atlantic (Pacific) Basin includes 12 (16) named storms, 6 (9) hurricanes, and 2.5 (4) major hurricanes (category 3 or higher). It must be noted that even in years when hurricane activity is minimal, it is still possible for a major hurricane to impact the United States. It is important for those living in hurricane-prone areas to be prepared for a landfalling tropical system, even in years when below-normal tropical activity is expected.