2019 Atlantic Hurricane Season Outlook: April Atlantic Discussion

In addition to the state of the ENSO (see previous article), meteorologists also monitor the state of the Atlantic Ocean itself in order to predict how much tropical activity may develop during the hurricane season. Factors monitored include sea surface temperature anomalies, pressure patterns, and tropical wave activity. Most factors are currently indicating a less active season is coming, but others suggest it could produce more activity.

In terms of sea surface temperatures, the setup is not quite ideal for an active season at the moment (Fig 1). The MDR currently consists of relatively average SSTAs, with a belt of slightly cooler anomalies stretching from the Lesser Antilles towards the Canary Islands. This borders on the higher SSTAs found within the Sargasso Sea between the Greater Antilles and the Gulf Stream, with anomalies generally running at around +1-1.5C. The Caribbean is currently average and the Gulf of Mexico is running above average near the loop current, but below average towards Mexico and Texas. In addition, waters north of the Gulf Stream consist of anomalies around -2C, particularly near Maritime Canada (not pictured). This type of pattern is technically a -AMO signature, as the anomalies closer to North America are running higher than those throughout the tropics, across western Africa, and south of Greenland and Iceland. As of March, the AMO index was around -1.87. This also signifies a negative Atlantic tripole pattern, as the anomalies within the subtropics are generally higher than those within the tropical and polar regions, meaning that more convective heating and lift is centered over the subtropics rather than tropics.

Figure 1: Current sea surface temperature anomalies as of April 18, revealing a general misplacement of warn SSTAs within the Atlantic Basin. (NOAA OSPO)

Climate models are in a general agreement that the MDR, Caribbean Sea, and Gulf of Mexico may anomalously warm slightly into the summer, but likely not much more than a half of a degree on average. However, the most drastic difference shown is that anomalies around the Gulf Stream will rise drastically, possibly by as much as +4C, as ridging intensifies and the mid-latitude jet lifts northward. While this would set the AMO pattern into a more neutral state, the tripole pattern would further favor the subtropics, concentrating convergence and lift north of the MDR. In turn, sinking air and convective outflow surrounding the subtropical convection tends to prevent and shear any thunderstorm activity within the tropics, essentially reversing the regular Atlantic Hadley Circulation. In addition, the increased convection would lower pressures across the subtropics, causing the subtropical ridge to weaken and allow environmental pressures to rise across the tropics (Fig 2). All of these factors suggest that tropical waves, depending on their initial ferocity, may struggle to maintain convection in the tropics and would normally have to wait to develop further once they reach the southwestern Atlantic, assuming they are able to survive.

Figure 2: Forecast mean sea-level pressure anomalies during the summer, indicating higher-than-normal pressures likely in the tropical Atlantic. (ECMWF Seasonal Forecasts)

Despite these factors, there are some factors that favor activity, although not as pronounced as those that are against activity. As the subtropical ridge is expected to be weaker during the summer, the flow around it would also weaken as well. This is due to, in addition to the enhanced convection, a negative North Atlantic Oscillation beginning to take hold. A -NAO induces weaker troughing shifted towards Greenland and weaker ridging just off the Iberian Peninsula. This will cause the center of ridging to shift farther away from the tropics, causing trade winds to slow. Slower easterly trades across the tropical Atlantic would cause vertical speed shear to reduce significantly and allow developing circulations to close off along tropical wave axes. As a result, positive zonal flow anomalies will shift to the northern Atlantic and lighten trades across the tropics (Fig 3). However, this would only apply for the MDR. The Caribbean would still experience higher zonal wind anomalies because convective inflow leading into the Pacific would still be enhanced, as the El Nino remains into the summer and increases convective coverage there. As a result, the trades speed up markedly near the Lesser Antilles. In addition to westerly convective outflow in the upper-levels, the Caribbean will likely be a hostile place for tropical convection to prosper.

Figure 3: CFS 850mb zonal wind anomalies for the summer, showing slightly lower trade winds across most of the tropical Atlantic. (Tropical Tidbits)

While it is a bit too early to state anything concrete about the state of the African monsoon season, precipitation anomalies have been running above average for most of tropical west Africa (Fig 4). As a result, the West African Monsoon (WAM) appears to be more amplified than normal so far. Because this is a monsoonal pattern, it is likely that these high precipitation anomalies will persist into the summer. The ITCZ would also likely benefit, as higher convective activity over western Africa generally is conjoined with a stronger convergence zone south of 10N. This indicates that tropical waves will likely be more robust than usual as they exit the coastline due to a more amplified WAM. However, the enhancement of this convection may also enhance the African Easterly Jet, which will transport dust from the Sahara Desert into the Atlantic Ocean. The question is how much of this dust will be picked up and how long it will stick around within the Saharan Air Layer. This will likely remain unanswered until after the hurricane season begins.

Figure 4: Surface precipitable water anomalies for the African continent during March, indicating higher-than-normal PWAT anomalies across west Africa. (ESRL Daily Mean Composites)

Overall precipitation anomalies indicate that the western Atlantic will see higher-than-normal rainfall throughout the summer and early autumn, while the tropics and Gulf of Mexico will be notably drier-than-average (Fig 5). This mostly follows from sea surface temperatures anomalies, but also takes into account how strong the trades and ENSO will be to forecast the overall likelihood of tropical activity. Areas forecast to see higher precipitation are also in areas where tropical development is favored with this climatological setup. Most other global climate models generally agree with this general solution, with a wet subtropical Atlantic and dry tropical Atlantic.

Figure 5: CFS Precipitation anomalies for July through September in the Atlantic Ocean, with higher precip anomalies in the subtropics and lower in the tropics. (Tropical Tidbits)

In years such as this, there is much more uncertainty with forecasting the state of the Atlantic than there is with the ENSO, as there are simply more factors to contend with for the Atlantic. Not all factors that contend with the state of the Atlantic are mentioned here, but simply just the primary ones. I will publish a second article re-assessing the state of the Atlantic during the middle of May as more information and data comes out, hopefully diminishing uncertainty.

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