Variability of the Atlantic off‐equatorial eastward currents during 1993–2010 using a synthetic method

Abstract

[1] We have developed, validated, and applied a synthetic method to monitor the offequatorial eastward currents in the central tropical Atlantic. This method combines highdensity expendable bathythermograph (XBT) temperature data along the AX08 transect with altimetric sea level anomalies (SLAs) to estimate dynamic height fields from which the mean properties of the North Equatorial Countercurrent (NECC), the North Equatorial Undercurrent (NEUC) and the South Equatorial Undercurrent (SEUC), and their variability can be estimated on seasonal to interannual timescales. On seasonal to interannual timescales, the synthetic method is well suited for reconstructions of the NECC variability, reproduces the variability of the NEUC with considerable skill, and less efficiently describes variations of the SEUC, which is located in a region of low SLA variability. A positive correlation is found between interannual variations of the NECC transport and two indices based on an interhemispheric sea surface temperature (SST) gradient and southeasterly wind stress in the central tropical Atlantic. The NEUC is correlated on interannual timescales with SSTs and meridional wind stress in the Gulf of Guinea and zonal equatorial wind stress. This study shows that both altimetry and XBT data can be effectively combined for near-real-time inference of the dynamic and thermodynamic properties of the tropical Atlantic current system.

Interannual Variability of the Atlantic North Equatorial Undercurrent and Its Impact on Oxygen

“…Hüttl‐Kabus and Böning () also found a more northward position and higher transports between April and August and a more southward position and lower transports between September and March in their model simulation. The seasonal cycle of NEUC transport estimates in TRATL01 is also consistent with the synthesis product of Goes et al (). Similar to Goes et al (), we found a semiannual cycle of the NEUC central position along 23°W, although the timing of maxima and minima is shifted by up to 2 months.…”

“…This is in agreement with previous observational results (Bourlès et al, , ; Brandt et al, ; Schott et al, , ; Urbano et al, ). The current is weak and likely to be obscured by mesoscale activities (e.g., Goes et al, ; Weisberg & Weingartner, ) and interannual variability (Goes et al, ; Hüttl‐Kabus & Böning, ). Even with this unique data set of 21 ship sections, observations are still too sparse to identify a seasonal variability of the NEUC.…”

“…Four transport estimates exist between 35°W and 38°W during boreal spring from 1993 to 1996 (Bourlès et al, ; Schott et al, ). In the same depth range as in Goes et al () and Hüttl‐Kabus and Böning (), they vary from 1.6 to 3.6 Sv, and the NEUC position varies between 3.5°N and 5.5°N. For boreal fall there is one NEUC transport estimate of 2.5 Sv between 4°N and 6°N (Bourlès et al, ).…”

“…In a model study Hüttl‐Kabus and Böning () estimated an interannual variability of the seasonal cycle of 2 Sv, which is almost as strong as the amplitude of the seasonal cycle (3 Sv). The results of Goes et al () indicate an anticorrelation between NEUC transport variability and the Atlantic meridional mode (AMM). The AMM is characterized by a meridional interhemispheric gradient of sea surface temperature (SST) in the tropical Atlantic centered around 5° N (Nobre & Shukla, ).…”

“…Goes et al () used a synthetic method to estimate the NEUC transport between 30°W and 23°W. They combined expendable bathythermograph temperature with altimetric sea level anomalies to derive NEUC location, velocity, and transport.…”