@article{fdi:010089595,
  title = {{I}ntrinsic versus wind-forced great whirl non-seasonal variability},
  author = {{S}adhvi, {K}. and {S}uresh, {I}. and {L}engaigne, {M}atthieu and {I}zumo, {T}akeshi and {P}enduff, {T}. and {M}olines, {J}. {M}. and {C}an, {A}. {A}. and {V}ialard, {J}{\'e}r{\^o}me},
  editor = {},
  language = {{ENG}},
  abstract = {{T}he {G}reat {W}hirl ({GW}) is a quasi-permanent anticyclonic eddy that appears every summer monsoon offshore of the {S}omalia upwelling. {T}he annual cycle of the {GW} is well described, but deviations from its mean seasonal cycle (hereafter non-seasonal variability) have been less explored. {S}atellite 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. {N}orthward shifts are associated with a stronger {GW}, and two cold, productive coastal upwelling wedges at 5 degrees {N} and 10 degrees {N}. {S}outhward 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}. {A}n eddy-permitting (25-km resolution) 50-member ensemble ocean simulation reproduces this {GW} variability well. {I}t 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. {I}ntrinsic variability dominates at both subseasonal (two thirds of the variance) and interannual timescales (57% of the variance). {T}he 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). {T}he mechanism for this external forcing is not entirely clear, but it appears to be related to the {R}ossby wave response to offshore wind stress curl forcing, which evolves into a north-south dipole that projects onto the {GW} variability pattern. {T}he {G}reat {W}hirl ({GW}) is a similar to 500-km diameter oceanic eddy that forms each summer off the {S}omalia coast. {T}he {GW} influences the temperature and biological productivity of the {S}omalia coastal upwelling. {T}he {GW} seasonal cycle, including its northward movement at the end of the monsoon, is known. {H}ere, we study deviations from this mean seasonal cycle. {T}he {GW} exhibits fast (typically 20 days long), similar to 100-km northward or southward displacements. {T}hese displacements result from interactions with smaller neighboring oceanic eddies, and are hence the consequence of an intrinsic ocean dynamics, rather than atmospheric forcing. {T}he {GW} is more pronounced when displaced northward, and the {S}omalia coastal upwelling has two clear "wedges" of cold, plankton-rich water at 5 degrees and 10 degrees {N}. {W}hen displaced southward, the {GW} is less pronounced, and there is a single, stronger-than-usual wedge of cold water at 10 degrees {N}. {W}e also show that atmospheric forcing can induce seasonal {GW} displacements, by making, for example, northward events more likely than the southward. {T}hus, the seasonal mean {GW} position has both a predictable (due to atmospheric forcing) and slightly larger unpredictable (due to interactions with eddies) component. {W}e propose that the atmospheric forcing influences the fast oscillations of the {GW} through slow oceanic adjustment to wind variations further east. {T}he {G}reat {W}hirl ({GW}) non-seasonal variability is dominated by similar to 100-km northward or southward shifts relative to its climatological position {T}hose shifts induce sea level, surface temperature and chlorophyll signals {T}hose {GW} displacements have a fast (similar to 20 days) intrinsic variability component and a slower, seasonal forced component},
  keywords = {{G}reat {W}hirl ; non-seasonal variability ; sea level ; cold wedges ; {S}omalia ; upwelling ; intrinsic oceanic variability ; {SOMALIE}},
  booktitle = {},
  journal = {{J}ournal of {G}eophysical {R}esearch : {O}ceans},
  volume = {129},
  numero = {2},
  pages = {e2023{JC}020077 [21 p.]},
  ISSN = {2169-9275},
  year = {2024},
  DOI = {10.1029/2023jc020077},
  URL = {https://www.documentation.ird.fr/hor/fdi:010089595},
}