Hurricane season got quiet (so far) & Sahel got wet

Recent news articles from the Washington Post “Why hurricane season is suddenly quiet — and what’s in store” and “A rare deluge could hit parts of the Sahara that almost never get rain this time of year” discussed two seemingly unconnected climate conditions around the tropical North Atlantic and West Africa. For the former, some scientists blamed the Atlantic Niña (i.e., cold sea surface temperature anomalies in the eastern equatorial Atlantic) developed in the past three months. See related discussions in the climate.gov event tracker article “Atlantic Niña on the verge of developing. Here’s why we should pay attention” and an update “Four things to know about a possible Atlantic Niña“. For the latter (a wetter-than-normal condition over the Sahel region), the Madden–Julian Oscillation (MJO), a patch of tropical thunderstorms that formed across the Indian Ocean and moved slowly eastward across the Pacific, South America, and the Atlantic, has been brought up as a potential driver. During Aug 15 – 20, the MJO was in Phase 2, the phase in which the West African region is typically under a wetter-than-normal condition (see MJO rainfall composites). However, it continued only for a brief period (5 ~ 6 days) and was a fairly unremarkable event (see CPC’s MJO monitoring page). In the next two weeks (Sep 1 – 14), the MJO is expected to be in Phases 4 and 5 (see CPC’s MJO forecast page). During those MJO phases, the center of convection is around the Maritime Continent; thus the Sahel region is either unaffected or drier-than-normal (see MJO rainfall composites). So, it is still unclear why the Sahel region is (will be) wetter than normal and whether it is linked to the quiet hurricane season (so far).

The current (Aug 29, 2024) operational sea surface temperature (SST) anomalies in the tropical Atlantic (Featured image) show the remnant of Atlantic Niña in the deep tropical Atlantic, as well as the lingering warm SST anomalies in the northern edge of the main development region (MDR) for Atlantic hurricane in and around the West African coast (~ 20°N). Such a strong anomalous meridional SST gradient in the region could promote increased convection around 20°N. The implied moisture convergence in the region could further promote a dryer-than-normal condition (i.e., anomalous moisture divergence) in the deep tropical North Atlantic. Therefore, the anomalous meridional SST gradient could produce a northward shift in the Inter Tropical Convergence Zone (ITCZ) rainband, possibly causing “a rare deluge over the parts of the Sahara that almost never get rain this time of year”. For the same reason, African easterly waves, as known as the seeds for Atlantic hurricanes, are occurring significantly farther north than usual, and thus entering the Atlantic at the relatively cooler northern edge of the MDR, potentially decreasing the probability of tropical cyclones forming off the West African coast.

In summary, the remnant of Atlantic Niña in the deep tropical Atlantic and the lingering warm SST anomalies in the northern edge of the MDR produced a strong anomalous meridional SST gradient across the tropical North Atlantic near the West African coast. The anomalous SST gradient in turn may have caused (or contributed to) both the quiet hurricane season (so far) and the rare wet conditions over the Sahel desert region. See Kim et al. (2023), Vallès‐Casanova et al. (2020), and the references in those papers for the relationships between Atlantic Niño/Niña, Cape Verde hurricanes, ITCZ, and West African rainfall.

It is important to be reminded that (1) the peak Atlantic hurricane activity typically occurs around mid-September, (2) a tropical cyclone may develop anywhere in the tropical North Atlantic away from the West African coast, including the Gulf of Mexico, Caribbean Sea, and off the east coast of Florida, and (3) the Atlantic hurricane season is still expected to be above average according to the latest update from NOAA CPC. So, it is important to pay close attention to the watches and warnings from the National Hurricane Center and your local weather office.

Image Credit: https://www.ospo.noaa.gov/Products/ocean/sst/anomaly/

Kim, D., Lee, SK., Lopez, H. Foltz, GR., Wen, C., West, R. & Dunion, J. (2023). Increase in Cape Verde hurricanes during Atlantic Niño. Nature Communications, 14, 3704. https://doi.org/10.1038/s41467-023-39467-5

Vallès‐Casanova, I., Lee, S.‐K., Foltz, G. R., & Pelegrí, J. L. (2020). On the spatiotemporal diversity of Atlantic Niño and associated rainfall variability over West Africa and South America. Geophysical Research Letters, 47, e2020GL087108.  https://doi.org/10.1029/2020GL087108