Wind-Driven Seasonal Cycle of the Atlantic Meridional Overturning Circulation

Zhao, Jianrong; Johns, William E.

doi:10.1175/jpo-d-13-0144.1

Cited by 40 publications

(54 citation statements)

References 43 publications

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“…To further understand the dynamics controlling the upper layer geostrophic transport,

v_{1}^{g e o s} true(x, t true)

is separated into “barotropic”

true(v_{b t}^{g e o s} (x, t) true)

and “baroclinic”

true(v_{b c}^{g e o s} (x, t) true)

components [ Zhao and Johns , ]:

v_{b t}^{g e o s} true(x, t true) = \frac{1}{true(H_{1} + H_{2} true)} (v_{1}^{g e o s} true(x, t true) * H_{1} + v_{2}^{g e o s} true(x, t true) * H_{2}) .

v_{b c}^{g e o s} true(x, t true) = v_{1}^{g e o s} true(x, t true) - v_{b t}^{g e o s} true(x, t true) .

…”

Section: Resultsmentioning

confidence: 99%

“…This is done to better isolate the directly forced response of the AMOC, although a nonlinear version of the model is also run and compared to the linear model. The linear version of the model has also been used to study the wind‐driven seasonal cycle of the AMOC [ Zhao and Johns , ].…”

Section: Data and Numerical Modelsmentioning

confidence: 99%

“…These buoyancy related changes were found to dominate the interannual to multidecadal variations of the AMOC [ Biastoch et al ., ], while the momentum flux is responsible for higher‐frequency fluctuations. The seasonal cycle of the AMOC in the subtropics and the tropics has been shown to be mostly determined by the wind‐driven changes [ Zhao and Johns , ]. The surface wind forcing directly drives the Ekman transport and also excites Rossby waves, which propagate across the basin.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Zhao

Johns

2014

J. Geophys. Res. Oceans

Self Cite

118

107

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, when the anomalous winddriven Ekman transport also has a significant contribution. The physical mechanisms for the interannual changes of the AMOC are proposed and evaluated in a two-layer model. While the Ekman transport is linked to the North Atlantic Oscillation (NAO), the anomalous geostrophic transport involves the oceanic adjustment to surface wind forcing. In particular, the intensification and weakening of the southward interior geostrophic flow is modulated by the internal Rossby wave adjustment to the surface wind forcing. The Gulf Stream, on the other hand, is controlled by both topographic waves along the US coast and westward propagating planetary waves. Our study suggests that a large part of the observed AMOC interannual variability at 26.5 N can be explained by wind-driven dynamics.

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“…To further understand the dynamics controlling the upper layer geostrophic transport,

v_{1}^{g e o s} true(x, t true)

is separated into “barotropic”

true(v_{b t}^{g e o s} (x, t) true)

and “baroclinic”

true(v_{b c}^{g e o s} (x, t) true)

components [ Zhao and Johns , ]:

v_{b t}^{g e o s} true(x, t true) = \frac{1}{true(H_{1} + H_{2} true)} (v_{1}^{g e o s} true(x, t true) * H_{1} + v_{2}^{g e o s} true(x, t true) * H_{2}) .

v_{b c}^{g e o s} true(x, t true) = v_{1}^{g e o s} true(x, t true) - v_{b t}^{g e o s} true(x, t true) .

…”

Section: Resultsmentioning

confidence: 99%

Section: Data and Numerical Modelsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Zhao

Johns

2014

J. Geophys. Res. Oceans

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118

107

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“…Long period variability of the gyre and its associated currents has been hypothesized to be linked to changes in the Atlantic Meridional Heat Transport and interbasin exchanges (Zhao and Johns 2014). As the BC flows along the Brazilian continental margin, it shows a very distinct vertical structure and mesoscale activity.…”

Section: Introductionmentioning

confidence: 99%

An assessment of the Brazil Current baroclinic structure and variability near 22° S in Distinct Ocean Forecasting and Analysis Systems

et al. 2016

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The Brazil Current (BC) is the Western Boundary Current of the South Atlantic subtropical gyre, the dominant dynamic feature in the South Atlantic Ocean. The importance of this current lies in that it is the main conduit of subtropical waters to higher latitudes in the South Atlantic Ocean. This study assesses the structure and variability of the BC across the nominal latitude of 22°S using data from the high-density eXpendable BathyThermograph (XBT) AX97 transect and from three numerical ocean models with data assimilation. This XBT transect was implemented in 2004 and represents one of the longer-term monitoring systems of the BC in existence. The goal of this work is to enhance the understanding of the temporal and spatial variability of the ocean dynamics in the southwestern South Atlantic Ocean by using a suite of hydrographic observations and numerical model outputs. In the present work, 37 XBT transect realizations using data collected between 2004 and 2012 are used. Daily outputs covering the same time period are evaluated from Hybrid Coordinate Ocean Model with the Navy Coupled Ocean Data Assimilation (HYCOM-NCODA) with a 1/12°horizontal resolution, and GLORYS2V3 and FOAM, both with a 1/4°horizontal resolution. These Ocean Forecasting and Analysis Systems (OFAS) are able to capture the mean observed features in the 22°S region, showing a BC core confined to the west of 39°W and an Intermediate Western Boundary Current between the depths of 200 and 800 m. However, the OFAS tend to overestimate the mean BC baroclinic volume transport across the AX97 reference transect and underestimate its variability. The OFAS show that the coastal region between the coastline and the western edge of the AX97 transect plays an important role in the mean BC total transport, contributing to up to 23 % of its value, and further that this transport is not sampled by the XBT observations with its current sampling strategy. In order to understand the variability of the BC, a statistical classification of the BC is proposed, with the identification of three different events: weak, intermediate, and strong.

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“…Even though the mean AMOC state must be sustained by a stable source of NADW production, variations in the AMOC transport are forced by both buoyancy fluxes and surface wind stress. In fact, more studies increasingly point to the surface wind as the primary driver for seasonal‐to‐decadal variability of the AMOC transport in the subtropics [ Biastoch et al ., ; Kanzow et al ., ; Xu et al ., ; Zhao and Johns , ; Polo et al ., ]. Numerical models that are forced only by wind stress are able to reproduce a seasonal cycle of AMOC that compares well with observations [ Zhao and Johns , ].…”

Section: Introductionmentioning

confidence: 99%

Local and remote wind stress forcing of the seasonal variability of the Atlantic Meridional Overturning Circulation (AMOC) transport at 26.5°N

Yang

2015

JGR Oceans

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The transport of the Atlantic Meridional Overturning Circulation (AMOC) varies considerably on the seasonal time scale at 26.5 N, according to observations made at the RAPID-MOCHA array. Previous studies indicate that the local wind stress at 26.5 N, especially a large wind stress curl at the African coast, is the leading contributor to this seasonal variability. The purpose of the present study is to examine whether nonlocal wind stress forcing, i.e., remote forcing from latitudes away from 26.5 N, affects the seasonal AMOC variability at the RAPID-MOCHA array. Our tool is a two-layer and wind-driven model with a realistic topography and an observation-derived wind stress. The seasonal cycle of the modeled AMOC transport agrees well with RAPID-MOCHA observations while the amplitude is in the lower end of the observational range. In contrast to previous studies, the seasonal AMOC variability at 26.5 N is not primarily forced by the wind stress curl at the eastern boundary, but is a result of a basin-wide adjustment of ocean circulation to seasonal changes in wind stress. Both the amplitude and phase of the seasonal cycle at 26.5 N are strongly influenced by wind stress forcing from other latitudes, especially from the subpolar North Atlantic. The seasonal variability of the AMOC transport at 26.5 N is due to the seasonal redistribution of the water mass volume and is driven by both local and remote wind stress. Barotropic processes make significant contributions to the seasonal AMOC variability through topography-gyre interactions.

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Wind-Driven Seasonal Cycle of the Atlantic Meridional Overturning Circulation

Cited by 40 publications

References 43 publications

Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

An assessment of the Brazil Current baroclinic structure and variability near 22° S in Distinct Ocean Forecasting and Analysis Systems

Local and remote wind stress forcing of the seasonal variability of the Atlantic Meridional Overturning Circulation (AMOC) transport at 26.5°N

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