Variability of the Atlantic Ocean North Equatorial Counter Current from
15 years of ADCP Observations and GLORYS12V1 Reanalysis
Abstract
Ocean current observations from different seasons of 15 years of
vessel-mounted Doppler Current Profiler, merely from the PIRATA program,
and drifters-derived near-surface currents are used to describe the
structure, the variability of the North Equatorial Countercurrent (NECC)
at 38{degree sign}W in the tropical Atlantic. Then used to validate
the GLORYS12V1 ocean reanalysis, showing that the inferred NECC’s
characteristics present reliable realism. This allows further analysis
of the NECC’s seasonal and interannual variabilities over the full
reanalysis period (1993-2018). The NECC presents an annual cycle of
northward migration driven by the wind field with a two-cores structure.
It exhibits a single branch, the sNECC, from December to June. With the
addition of a second branch, the nNECC, the rest of the year. The sNECC
starts the seasonal cycle in June, then grows northward, driven by the
northward migration of the ITCZ with a three-month time lag. The nNECC
core appears in August, migrates northward, then vanishes in
December/January around 12{degree sign}N. From January to May the
sNECC decays, with northward displacement toward 7-9{degree sign}N,
driven by the second zero of the wind stress curl. From year to year,
the NECC shows significant variations in relation with the tropical
Atlantic zonal and meridional climate modes. First, positive NECC
transport anomalies appear with “warm” meridional mode and zonal mode
“Atlantic Niña” phases. Second, positive anomalies appear with weak
meridional “cold” phase, and “Atlantic Niño” increasing zonal mode.
Third, negative NECC anomalies occur with “cold” meridional phase and
a negative zonal mode.