Investigation of the North Brazil Current retroflection and North Equatorial Countercurrent variability

Fonseca, Carlos A.; Goñi, Gustavo; Johns, William E.; Campos, Edmo

doi:10.1029/2004gl020054

Cited by 48 publications

(53 citation statements)

References 19 publications

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“…In total the NECC transport above σ θ = 27.1 kg/m 3 sums up to 11.5 Sv in October/November 2002. This is in good agreement to the estimate of Fonseca et al [2004], who described for the region west of 30°W an annual cycle of the NECC with a maximum transport of 12 Sv occurring in November.…”

Section: Resultssupporting

confidence: 92%

Water masses and currents in the upper tropical northeast Atlantic off northwest Africa

2005

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Recent current measurements in the tropical eastern North Atlantic reproduce the components of the large scale flow field. However, the observations as well as the 1/12°‐FLAME model computations indicate that a lot of eddy scale variability is superimposed on the mean flow field. Despite of the disturbance by variability the signature of the Guinea Dome is well present. In November 2002 the Guinea Dome transport from direct observations was about 2.8 Sv above σθ = 25.8 kg/m3 and 4 Sv between σθ = 25.8 and 27.1 kg/m3. The oxygen minimum in the shadow zone comprises the central water and the Antarctic Intermediate Water (AAIW) layers and is located between the equatorial current system and the North Equatorial Current. The North Equatorial Counter‐ and Undercurrents at 3° to 6°N are major oxygen sources for the central water layer of the low‐oxygen regions in the northeastern tropical Atlantic. A second, northern North Equatorial Countercurrent (nNECC) band exists at 8° to 10°N. The nNECC carries oxygen rich water from the southern hemisphere eastward but with an admixture of water from the northern hemisphere. A float at 200 m depth was spreading eastward in the North Equatorial Undercurrent (NEUC), at 28°W it shifted northward into the nNECC, and then was trapped in the Guinea Dome region for more than 3 years. The model indicates the region 22° to 32°W as the area of exchange between the NECC/NEUC and the nNECC bands. In the AAIW layer the northern Intermediate Countercurrent acts as oxygen source for the oxygen minimum zone.

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Section: Resultssupporting

confidence: 92%

Water masses and currents in the upper tropical northeast Atlantic off northwest Africa

2005

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“…(a) A thermocline-intensified ring observed in December 1998 that reveals maximum swirl velocities below 100 m, and (b) a surface-intensified ring observed in February 1999 that reveals maximum swirl velocities above 75 m. In (a) warm (Ͼ28.4°C) and low-salinity (Ͻ35.4) Amazon-Orinoco River plume water is revealed in the topmost 30-40 m centered over the core of the spinning ring. From 8 yr of altimetry data, the average northerly position of the NBC retroflection is 6.7°N Ϯ 1.8° (Fonseca et al 2004). 7a).…”

Section: ) Hurricane Joyce 2000supporting

confidence: 56%

Amazon and Orinoco River Plumes and NBC Rings: Bystanders or Participants in Hurricane Events?

Ffield

2007

Journal of Climate

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ABTRACTThe Amazon and Orinoco River plumes and North Brazil Current (NBC) rings are investigated during the 1 June through 30 November Atlantic hurricane season to identify their impact on upper-ocean temperatures in the region and to draw attention to their potential role in hurricane maintenance and intensification. The analysis uses ocean temperature and salinity stratification data, infrared and microwave satellite-derived sea surface temperature (SST) data, and Atlantic tropical storm and hurricane tracks data. The Amazon-Orinoco River plume spreads into the western equatorial Atlantic Ocean forming an extensive (0°-20°N, 78°-33°W) 10-60-m-thick buoyant surface layer associated with the warmest surface temperatures (up to ϩ3°C) in the region due to the freshwater barrier layer effect. At times the warm Amazon-Orinoco River plume is bisected by cool-surface NBC rings. For the 1960 to 2000 time period, 68% of all category 5 hurricanes passed directly over the historical region of the plume, revealing that most of the most destructive hurricanes may be influenced by ocean-atmosphere interaction with the warm plume just prior to reaching the Caribbean. Statistical analyses of tropical Atlantic SSTs and tropical cyclone wind speeds reveal a significant and unique relationship between warm (cool) SSTs in the Amazon-Orinoco River plume and stronger (weaker) tropical cyclone wind speeds between 35°and 55°W. This implies that warmer (cooler) plume SSTs due to increased (decreased) river discharge may directly contribute to a more (less) vigorous hurricane season.

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“…The second is the retroflection latitude of the NBC-NBUC, which is around 68-78N (Fonseca et al 2004). The first is the bifurcation latitude of the NEC, which is determined by the wind-driven circulation and is around 108-128N.…”

Section: Sensitivity To the Subsurface Temperature Conditionsupporting

confidence: 57%

Effect of Atlantic Meridional Overturning Circulation Changes on Tropical Atlantic Sea Surface Temperature Variability: A 2½-Layer Reduced-Gravity Ocean Model Study

2010

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Previous coupled climate model simulations reveal that a dipole-like SST pattern with cooler (warmer) temperature over the north (south) tropical Atlantic emerges in response to a slowdown of the Atlantic meridional overturning circulation (AMOC). Using a 2½-layer reduced-gravity ocean model, a systematic investigation into oceanic processes controlling the tropical Atlantic sea surface temperature (SST) response to AMOC changes by varying the strength of northward mass transport at the open boundaries was conducted. It is found that the North Brazil Current (NBC) reverses its direction in response to a shutdown of the AMOC. Such a circulation change causes a decrease in upper equatorial ocean stratification and warming in the Gulf of Guinea and off the coast of Africa. These findings point to the importance of oceanic dynamics in the equatorial SST response to AMOC changes. Sensitivity experiments further show that the SST response relates nonlinearly to AMOC changes. The strength of the SST response increases dramatically when the AMOC strength falls below a threshold value. This nonlinear threshold behavior depends on the position of a subsurface temperature gradient forming along the boundary between the northern subtropical gyre and the tropical gyre that interacts with the western boundary current. The analysis suggests that, in order for the oceanic dynamics to have a dominant influence on tropical Atlantic SST in response to AMOC changes, two conditions must be satisfied: 1) the AMOC must weaken substantially so that the NBC flows equatorward, permitting water mass exchange between the northern subtropical and tropical gyres, and 2) the subsurface temperature front must be located in an optimal location where subsurface temperature anomalies induced by AMOC change are able to enter the equatorial zone.

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Investigation of the North Brazil Current retroflection and North Equatorial Countercurrent variability

Cited by 48 publications

References 19 publications

Water masses and currents in the upper tropical northeast Atlantic off northwest Africa

Water masses and currents in the upper tropical northeast Atlantic off northwest Africa

Amazon and Orinoco River Plumes and NBC Rings: Bystanders or Participants in Hurricane Events?

Effect of Atlantic Meridional Overturning Circulation Changes on Tropical Atlantic Sea Surface Temperature Variability: A 2½-Layer Reduced-Gravity Ocean Model Study

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