Intrinsic versus wind-forced great whirl non-seasonal variability

2024

Article référencé dans le Web of Science WOS:001169694900001

Sadhvi K., Suresh I., Lengaigne Matthieu, Izumo Takeshi, Penduff T., Molines J. M., Can A. A., Vialard Jérôme

Journal of Geophysical Research : Oceans, 2024, 129 (2), p. e2023JC020077 [21 p.] ISSN 2169-9275

The Great Whirl (GW) is a quasi-permanent anticyclonic eddy that appears every summer monsoon offshore of the Somalia upwelling. The annual cycle of the GW is well described, but deviations from its mean seasonal cycle (hereafter non-seasonal variability) have been less explored. Satellite observations reveal that the leading mode of summer non-seasonal sea-level variability in this region is associated with similar to 100-km northward or southward GW shifts from its climatological position. Northward shifts are associated with a stronger GW, and two cold, productive coastal upwelling wedges at 5 degrees N and 10 degrees N. Southward shifts are associated with a weaker GW, no wedge at 5 degrees N and a single stronger-than-usual cold and productive wedge at 10 degrees N. An eddy-permitting (25-km resolution) 50-member ensemble ocean simulation reproduces this GW variability well. It indicates that the non-seasonal GW variability has a short similar to 20 days timescale intrinsic component, associated with the GW interaction with mesoscale eddies, and a lower-frequency, similar to 100 days externally forced component. Intrinsic variability dominates at both subseasonal (two thirds of the variance) and interannual timescales (57% of the variance). The externally forced signal results from shifts in the probability distribution of the subseasonal GW position (e.g., more likely northward than southward shifted instantaneous GW positions over a season). The mechanism for this external forcing is not entirely clear, but it appears to be related to the Rossby wave response to offshore wind stress curl forcing, which evolves into a north-south dipole that projects onto the GW variability pattern. The Great Whirl (GW) is a similar to 500-km diameter oceanic eddy that forms each summer off the Somalia coast. The GW influences the temperature and biological productivity of the Somalia coastal upwelling. The GW seasonal cycle, including its northward movement at the end of the monsoon, is known. Here, we study deviations from this mean seasonal cycle. The GW exhibits fast (typically 20 days long), similar to 100-km northward or southward displacements. These displacements result from interactions with smaller neighboring oceanic eddies, and are hence the consequence of an intrinsic ocean dynamics, rather than atmospheric forcing. The GW is more pronounced when displaced northward, and the Somalia coastal upwelling has two clear "wedges" of cold, plankton-rich water at 5 degrees and 10 degrees N. When displaced southward, the GW is less pronounced, and there is a single, stronger-than-usual wedge of cold water at 10 degrees N. We also show that atmospheric forcing can induce seasonal GW displacements, by making, for example, northward events more likely than the southward. Thus, the seasonal mean GW position has both a predictable (due to atmospheric forcing) and slightly larger unpredictable (due to interactions with eddies) component. We propose that the atmospheric forcing influences the fast oscillations of the GW through slow oceanic adjustment to wind variations further east. The Great Whirl (GW) non-seasonal variability is dominated by similar to 100-km northward or southward shifts relative to its climatological position Those shifts induce sea level, surface temperature and chlorophyll signals Those GW displacements have a fast (similar to 20 days) intrinsic variability component and a slower, seasonal forced component

Limnologie physique / Océanographie physique [032]

SOMALIE

Fonds IRD [F B010089595]

fdi:010089595