2019 Atlantic Hurricane Season Outlook: April ENSO Discussion

Hurricane Season is now less than 2 months away, and forecasters are beginning to focus their attention on the Atlantic and its surroundings to forecast how much activity we can expect in the upcoming season. The primary factor that most forecasters look towards to forecast the upcoming hurricane season is the state of the ENSO region. Right now, there exist multiple factors that favor Atlantic activity based around this, but, at the moment, there are more factors that may inhibit activity.

As of now, we are in an El Nino state across the equatorial Pacific. Anomalies are generally running about +0.7C above average across the Nino 3.4 region, and are above +0.5C across all other ENSO regions as well (Fig 1). This has been present since last year, but are on an increase now after a period of slightly lower anomalies during the winter. Below the surface, a strong subsurface Kelvin wave is slowly emerging towards the surface, with core anomalies running as much as +6C, making this a very strong Kelvin wave (Fig 2). This will allow surface anomalies to become better defined as Relatively weak easterly trades coming off South America and frequent westerly wind bursts are currently resulting in a weakening of the Pacific Walker Cell. This is reflected in the current 90-day Southern Oscillation Index value of -6.84 and a March average of -6.48 (Fig 3). These negative values indicate a weaker than average PWC, which allows equatorial trades across the Pacific to slow and allows SSTAs to rise as a result. So, at least in the short-term, the El Nino may intensify past +1.0C anomalies.

Figure 1: Current global sea surface temperature anomalies as of April 4, revealing a pronounced El Nino and ill-defined PDO signature. (NOAA OSPO)
Figure 2: Subsurface equatorial temperature anomalies, revealing a pronounced Kelvin wave underneath the core of the El Nino. (Climate Prediction Center)
Figure 3: Southern Oscillation Index values for the last 4 weeks, revealing a slight recent increase. (Long Paddock)

Normally, an El Nino such as this at this time of year would gradually intensify into the summer and fall, greatly inhibiting Atlantic tropical activity. However, there does exist some indication that the current El Nino may not be able to get much stronger than it is now. The Pacific Decadal Oscillation, which is signified through a horseshoe-shaped region of anomalies off the west coast of North America, is usually running above average all throughout when an El Nino such as this is present, with a cold region of water near and east of Japan. However, there is a marked region of cool anomalies located off the West Coast of the United States and an area of generally average SSTAs off the coast of Japan that is breaking the usual pattern (Fig 1). As a result, the El Nino may not reach its fullest potential in spreading widespread convective energy. In addition, climate models are indicating that the ENSO is likely to not get much warmer into the summer and beyond (Fig 4). The current Kelvin wave is emerging much earlier than most, and waters behind it are relatively cooler, though still above average, which is why most of these models do not show a significant change in the ENSO, a feature that is normally seen either positively or negatively. This is indicating that the actual surface anomalies won’t be as high as they otherwise would have been, even though the core of the Kelvin wave always remains below the surface. In addition, the 30-day SOI values have been on an increase lately (Fig 3). While this may be a short-term occurrence, it’s possible that it could led to a large enough increase in the strength of the PWC to increase trades and lower ENSO anomalies if it persists. The 90-day average has just begun increasing, though, so if this would be occurring, it would be in its early stages of development.

Figure 4: Spread of dynamical and statistical models for the ENSO through the end of the year, indicating no marked change in the current state (International Research Institute and Climate Prediction Center)

When an El Nino of this caliber is present during the hurricane season, it normally isn’t large enough of an inhibitor to completely shut off activity. It does, however, affect which portions of the basin will see higher activity than others. The Caribbean Sea, particularly the central and eastern Caribbean, is usually the least hospitable location in the basin, as convective outflow causes westerly winds aloft, creating a high amount of vertical directional shearing (Fig 5); this disrupts thunderstorm development by removing the convective tops and tilting the vertical column of rising air in a developing wave. However, the Gulf of Mexico, subtropical Atlantic, and eastern MDR are normally favored, as wind shear is markedly lower and results in converging winds with other typical synoptic features such as the mid-latitude jet and subtropical ridge. But with most of the tropics unable to generate sustained convection, tropical waves usually are choked off before they can get going, unless they can survive into the western Atlantic or Gulf of Mexico, that is. Convective energy still remains adequate in the Atlantic, however, as the Pacific Walker Cell isn’t quite strong enough to overpower the Atlantic Walker Cell. Once anomalies surpass a full degree above normal, however, convective forcing and lift is heavily concentrated in the Eastern Pacific Basin, leaving the Atlantic with comparatively little available fuel for use.

Figure 5: Average Caribbean wind shear (850-200mb) from July 1 to October 15 during years when ENSO anomalies are between +0.5 and +1.0. (ESRL Daily Mean Composites)

As of now, it appears likely that an El Nino will persist into the summer and fall, but the question as of now is how strong it will be. If it does not cross into moderate territory, or past +1.0, then there is still a chance that the Atlantic could produce similar levels of activity to last year, or a slightly above average year just based off ENSO alone. Activity is also dependent on the state of the Atlantic itself, which will be discussed in a later article towards the middle of the month.

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