A quiet start The Atlantic hurricane season is underway, but activity has been unusually limited so far. With just three named storms since the season began in June, many people could be forgiven for forgetting that hurricane season is already in progress.
The Atlantic Basin has produced only three named storms, all of which were weak and highly disorganized tropical cyclones. Forecasters expect just one or two additional systems to develop by mid-August as the season continues to face several unfavorable conditions.
Key factors include this year’s strengthening El Niño, which is driving exceptionally warm sea-surface temperatures across a critical region of the Pacific Ocean. A persistent layer of dry, dusty air from Africa is also spreading across the Atlantic, further limiting tropical development.
Together, these large-scale weather patterns are suppressing tropical cyclone formation across the Atlantic Basin. Forecasters currently expect one of the mildest Atlantic hurricane seasons in years, with fewer than 10 named storms likely to develop under the prevailing conditions.
Experts have been surprised by the rapid intensification of El Niño this summer. The climate pattern occurs when equatorial waters in the central and eastern Pacific Ocean become warmer than average, triggering major changes in weather patterns around the world.
El Niño is officially defined by water temperatures in key equatorial regions rising at least 0.5 degrees Celsius, or 0.9 degrees Fahrenheit, above average for several consecutive months. Current measurements show that parts of the Pacific are at least 1.5 degrees Celsius warmer than normal, while some areas are experiencing temperature anomalies as high as 4 degrees Celsius.
These unusually warm Pacific waters generate widespread rising air and thunderstorms, which alter upper-level wind patterns. The resulting atmospheric changes send stronger-than-normal winds eastward toward the Atlantic Basin. This increased wind shear can prevent tropical disturbances from developing into organized storms.
Thunderstorms provide the foundation for tropical systems. When strong winds blow at different speeds or directions higher in the atmosphere, they disrupt the structure of developing thunderstorms and make it difficult for tropical cyclones to form and strengthen.
The same climate pattern is contributing to increased tropical cyclone activity across the Pacific. By early August, the western Pacific typhoon season was busier than any season in nearly 50 years. Four storms reached Category 5 intensity, including Shinraku in April and Babi, Dolphin, and Genevieve in July. The central and eastern Pacific are also active, with the U.S. National Hurricane Center forecasting above-normal activity in those regions into the fall.
El Niño is not the only factor suppressing Atlantic hurricane activity this year. The West African monsoon typically sends thunderstorms westward over the Atlantic, where some disturbances can develop into tropical storms or hurricanes. This year’s monsoon has been weaker than usual, with below-average rainfall in areas including Sierra Leone and Guinea, producing fewer strong tropical disturbances.
Dry air from the Sahara Desert is also reducing the chances of Atlantic tropical development. A persistent plume of dusty air has been moving from east to west across the Atlantic, acting like an atmospheric lid that limits thunderstorm growth and suppresses tropical cyclone formation.
Combined, these conditions have produced the least active Atlantic hurricane season since at least 2013. Forecasters use cumulative cyclone energy, or ACE, to measure a season’s overall storm intensity and duration. An average Atlantic hurricane season produces an ACE value of 122.5. So far this season, the ACE total is just 3.1—about 23 percent of the typical mid-August value.
Still, a quiet hurricane season does not mean there is no risk. El Niño years can produce powerful and destructive hurricanes even when overall conditions are unfavorable. Tropical Storm Arthur may have broken Louisiana’s 24-hour rainfall record after more than 29 inches of rain fell near Cottonport. Historically, Hurricanes Camille, Ivan, Charley, Michael, Idalia, and Lee all developed during El Niño years. Coastal residents should continue monitoring the Atlantic hurricane forecast and maintain an emergency plan in case conditions change quickly.
Source: www.wired.com