On the Influence of Wind-Stress Curl on Low-Frequency Shelf Water Flow

AbstractHigh-frequency Doppler radar (HFDR) and acoustic Doppler current profiler (ADCP) time-series observations during the Philippine Straits Dynamics Experiment (PhilEx) were analyzed to describe the mesoscale currents in Panay Strait, Philippines. Low-frequency surface currents inferred from three HFDR (July 2008–July 2009), reveal a clear seasonal signal concurrent with the reversal of the Asian monsoon. A mesoscale cyclonic eddy west of Panay Island is generated during the winter northeast (NE) monsoon. This causes changes in the strength, depth, and width of the intraseasonal Panay coastal (PC) jet as its eastern limb. Winds from QuikSCAT and from a nearby airport indicate that these flow structures correlate with the strength and direction of the prevailing local wind. An intensive survey in 8–9 February 2009 using 24 h of successive cross-shore conductivity–temperature–depth (CTD) sections, which in conjunction with shipboard ADCP measurements, show a well-developed cyclonic eddy characterized by near-surface velocities of 50 cm s−1. This eddy coincides with the intensification of the wind in between Mindoro and Panay Islands, generating a positive wind stress curl in the lee of Panay, which in turn induces divergent surface currents. Water column response from the mean transects show a pronounced signal of upwelling, indicated by the doming of isotherms and isopycnals. A pressure gradient then is set up, resulting in the spin up of a cyclonic eddy in geostrophic balance. Evolution of the vorticity within the vortex core confirms wind stress curl as the dominant forcing.

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ENSO-correlated fluctuations in ocean bottom pressure and wind-stress curl in the North Pacific

Ocean Science ◽

10.5194/os-7-685-2011 ◽

2011 ◽

Vol 7 (5) ◽

pp. 685-692 ◽

Cited By ~ 15

Author(s):

D. P. Chambers

Keyword(s):

Wind Stress ◽

North Pacific ◽

Low Frequency ◽

Ocean Model ◽

Ocean Bottom ◽

Bottom Pressure ◽

Wind Stress Curl ◽

Ocean Bottom Pressure ◽

The North ◽

The North Pacific

Abstract. We examine the output of an ocean model forced by ECMWF winds to study the theoretical relationship between wind-induced changes in ocean bottom pressure in the North Pacific between 1992 until 2010 and ENSO. Our analysis indicates that while there are significant fluctuations correlated with some El Niño and La Niña events, the correlation is still relatively low. Moreover, the ENSO-correlated variability explains only 50 % of the non-seasonal, low-frequency variance. There are significant residual fluctuations in both wind-stress curl and ocean bottom pressure in the region with periods of 4-years and longer. One such fluctuation began in late 2002 and has been observed by the Gravity Recovery and Climate Experiment (GRACE). Even after accounting for possible ENSO-correlated variations, there is a significant trend in ocean bottom pressure in the region, equivalent to 0.7 ± 0.3 cm yr−1 of sea level from January 2003 until December 2008, which is confirmed with steric-corrected altimetry. Although this low-frequency fluctuation does not appear in the ocean model, we show that ECMWF winds have a significantly reduced trend that is inconsistent with satellite observations over the same time period, and so it appears that the difference is due to a forcing error in the model and not an intrinsic error.

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The Influence of Periodic Forcing on the Time Dependence of Western Boundary Currents: Phase Locking, Chaos, and Mechanisms of Low-Frequency Variability

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0113.1 ◽

2016 ◽

Vol 46 (4) ◽

pp. 1117-1136 ◽

Cited By ~ 3

Author(s):

Andrew E. Kiss ◽

Leela M. Frankcombe

Keyword(s):

Time Dependence ◽

Time Scales ◽

Wind Stress ◽

Phase Locking ◽

Low Frequency ◽

Period Doubling ◽

Nonlinear Effects ◽

Western Boundary ◽

Boundary Current ◽

Wind Stress Curl

AbstractIn this study an idealized gyre is put into a temporally periodic state by a steady wind stress curl forcing, and its nonlinear response to variable forcing is investigated by a detailed parameter survey varying the time-mean component of the wind and the amplitude and frequency of a periodic component. Periodic wind variations exceeding ~0.5% profoundly affect the western boundary current (WBC) time dependence, yielding regime diagrams with intricately interleaved regions of phase locking, quasiperiodicity, and chaos. In phase-locked states, the WBC period is locked to a rational multiple of the forcing period and can be shifted far outside its natural range. Quasiperiodic states can exhibit long intervals of near-synchrony interrupted periodically by brief slips out of phase with the forcing. Hysteresis and a period-doubling route to chaos are also found. The nonlinear WBC response can include variability at long time scales that are absent from both the forcing and the steadily driven current; this is a new mechanism for the generation of low-frequency WBC variability. These behaviors and their parameter dependence resemble the Devil’s staircase found in the “circle map” model of a periodically forced nonlinear oscillator, but with differences attributable to higher-dimensional dynamics. These nonlinear effects occur with forcing amplitudes in the observed range of the annual wind stress curl cycle and therefore should be considered when inferring the cause of observed WBC time scales. These results suggest that studies omitting either forcing variation or nonlinearity provide an unrealistically narrow view of the possible origins of time dependence in WBCs.

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The Influence of Sloping Mean Density Surfaces on Low-Frequency Shelf Water Flow

Journal of Physical Oceanography ◽

10.1175/1520-0485(1987)017<0507:tiosmd>2.0.co;2 ◽

1987 ◽

Vol 17 (4) ◽

pp. 507-517 ◽

Cited By ~ 2

Author(s):

Allan J. Clarke ◽

Manuel Lopez

Keyword(s):

Water Flow ◽

Low Frequency ◽

Shelf Water

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ENSO-correlated fluctuations in ocean bottom pressure and wind-stress curl in the North Pacific

Ocean Science Discussions ◽

10.5194/osd-8-1631-2011 ◽

2011 ◽

Vol 8 (4) ◽

pp. 1631-1655

Author(s):

D. P. Chambers

Keyword(s):

Wind Stress ◽

North Pacific ◽

Low Frequency ◽

Satellite Observations ◽

Ocean Bottom ◽

Bottom Pressure ◽

Wind Stress Curl ◽

Ocean Bottom Pressure ◽

The North ◽

The North Pacific

Abstract. We examine the magnitude of ENSO-correlated variations in wind-stress curl and ocean bottom pressure in the North Pacific between 1992 until 2010, using satellite observations and model output. Our analysis indicates that while there are significant fluctuations correlated with some El Niño and La Niña events, the correlation is still relatively low. Moreover, the ENSO-correlated variability explains only 50 % of the non-seasonal, low-frequency variance. There are significant residual fluctuations in both wind-stress curl and ocean bottom pressure in the region with periods of 4-years and longer. One such fluctuation began in late 2002 and has been observed by the Gravity Recovery and Climate Experiment (GRACE). Even after accounting for ENSO variations, there is a significant trend in ocean bottom pressure in the region, equivalent to 0.7 ± 0.3 cm yr−1 of sea level from January 2003 until December 2008, which is confirmed with steric-corrected altimetry. Although this low-frequency fluctuation does not appear in an ocean model, we show that the winds used to force the model have a significantly reduced trend that is inconsistent with satellite observations over the same time period.

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Seasonal variability of SST fronts and winds on the southeastern continental shelf of Brazil

Ocean Dynamics ◽

10.1007/s10236-019-01310-1 ◽

2019 ◽

Vol 69 (11-12) ◽

pp. 1387-1399 ◽

Cited By ~ 3

Author(s):

Huan-Huan Chen ◽

Yiquan Qi ◽

Yuntao Wang ◽

Fei Chai

Keyword(s):

Continental Shelf ◽

Wind Stress ◽

Seasonal Variability ◽

Detection Algorithm ◽

Coupling Coefficients ◽

Seafloor Topography ◽

Wind Stress Curl ◽

Large Coupling ◽

The Impact ◽

Sst Fronts

Abstract Fourteen years (September 2002 to August 2016) of high-resolution satellite observations of sea surface temperature (SST) data are used to describe the frontal pattern and frontogenesis on the southeastern continental shelf of Brazil. The daily SST fronts are obtained using an edge-detection algorithm, and the monthly frontal probability (FP) is subsequently calculated. High SST FPs are mainly distributed along the coast and decrease with distance from the coastline. The results from empirical orthogonal function (EOF) decompositions reveal strong seasonal variability of the coastal SST FP with maximum (minimum) in the astral summer (winter). Wind plays an important role in driving the frontal activities, and high FPs are accompanied by strong alongshore wind stress and wind stress curl. This is particularly true during the summer, when the total transport induced by the alongshore component of upwelling-favorable winds and the wind stress curl reaches the annual maximum. The fronts are influenced by multiple factors other than wind forcing, such as the orientation of the coastline, the seafloor topography, and the meandering of the Brazil Current. As a result, there is a slight difference between the seasonality of the SST fronts and the wind, and their relationship was varying with spatial locations. The impact of the air-sea interaction is further investigated in the frontal zone, and large coupling coefficients are found between the crosswind (downwind) SST gradients and the wind stress curl (divergence). The analysis of the SST fronts and wind leads to a better understanding of the dynamics and frontogenesis off the southeastern continental shelf of Brazil, and the results can be used to further understand the air-sea coupling process at regional level.

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On the Role of Bottom Pressure Torques in Wind-Driven Gyres

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0147.1 ◽

2021 ◽

Vol 51 (5) ◽

pp. 1441-1464

Author(s):

Andrew L. Stewart ◽

James C. McWilliams ◽

Aviv Solodoch

Keyword(s):

Wind Stress ◽

Bottom Friction ◽

Bottom Pressure ◽

Wind Stress Curl ◽

Previous Theory ◽

Model Experiments ◽

Basin Scale ◽

Vorticity Budget ◽

Gyre Circulation

AbstractPrevious studies have concluded that the wind-input vorticity in ocean gyres is balanced by bottom pressure torques (BPT), when integrated over latitude bands. However, the BPT must vanish when integrated over any area enclosed by an isobath. This constraint raises ambiguities regarding the regions over which BPT should close the vorticity budget, and implies that BPT generated to balance a local wind stress curl necessitates the generation of a compensating, nonlocal BPT and thus nonlocal circulation. This study aims to clarify the role of BPT in wind-driven gyres using an idealized isopycnal model. Experiments performed with a single-signed wind stress curl in an enclosed, sloped basin reveal that BPT balances the winds only when integrated over latitude bands. Integrating over other, dynamically motivated definitions of the gyre, such as barotropic streamlines, yields a balance between wind stress curl and bottom frictional torques. This implies that bottom friction plays a nonnegligible role in structuring the gyre circulation. Nonlocal bottom pressure torques manifest in the form of along-slope pressure gradients associated with a weak basin-scale circulation, and are associated with a transition to a balance between wind stress and bottom friction around the coasts. Finally, a suite of perturbation experiments is used to investigate the dynamics of BPT. To predict the BPT, the authors extend a previous theory that describes propagation of surface pressure signals from the gyre interior toward the coast along planetary potential vorticity contours. This theory is shown to agree closely with the diagnosed contributions to the vorticity budget across the suite of model experiments.

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