scholarly journals Sensitivity of Coastal Currents near Point Conception to Forcing by Three Different Winds: ECMWF, COAMPS, and Blended SSM/I–ECMWF–Buoy Winds

2005 ◽  
Vol 35 (7) ◽  
pp. 1229-1244 ◽  
Author(s):  
Changming Dong ◽  
Lie-Yauw Oey
Keyword(s):  
Sea Level ◽  
Wind Stress ◽  
The Other ◽  
Wind Data ◽  
Wind Stress Curl ◽  
Near Surface ◽  
Time Integral ◽  
Point Conception ◽  
Mode 2 ◽  
Horizontal Grid

Abstract Previous observational and modeling studies have indicated the importance of finescale winds in determining the circulation near Point Conception in the Santa Maria Basin (SMB) and the Santa Barbara Channel (SBC), California. There has not been a systematic attempt, however, to analyze and quantify the sensitivity of the near-surface circulation to different wind data. Here, a regional circulation model of the SMB and SBC is driven using three wind datasets: the European Centre for Medium-Range Weather Forecasts (ECMWF; ≈ 110 km × 110 km horizontal grid), the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS; 9 km × 9 km horizontal grid), and a blended wind product that combines Special Sensor Microwave Imager (SSM/I), ECMWF, and buoy and coastal wind data and that is referred to as SEB. A springtime period (March–May 1999) in which equatorward wind dominates and wind stress curls are strong is chosen for the study. Two groups of experiments are conducted: with and without assimilating moored temperature observations. The focus is on long time scales of greater than weeks and on mean currents. Comparisons between these experiments and between model and observation show that the circulation driven by the ECMWF wind is much weaker than that by the other two winds. On the other hand, the SEB dataset shows locally intensified wind stress curls behind capes and coastal bends, whereas these wind stress curls are weak in COAMPS. It is found that these small-scale variations in the wind field force alongshore inhomogeneous pressure gradients that in turn can significantly affect the near-coast currents. The result is that modeled currents forced by SEB agree better with observations than do those produced by COAMPS. Empirical orthogonal function analyses were conducted on the near-surface currents, sea level, wind, and wind stress curl. The mode-1 current (≈40%) is unidirectional (i.e., generally equatorward or poleward) and correlates well with the mode-1 wind (≈90%). The mode-2 current (≈20%) is cyclonic in the SBC and poleward inshore and equatorward offshore in the SMB; it correlates well with mode-1 sea level (≈70%), which suggests that mode-2 currents are driven by the pressure gradient. It is significant that neither mode-2 current nor mode-1 sea level correlates well with mode-1 wind stress curl (≈70%); rather, they correlate well with the time integral of the mode-1 wind stress curl. These conclusions support a previous theoretical idea that cyclonic circulation in the SBC and the inshore currents of the SMB are both driven by alongshore pressure setup induced by the time integral of the wind stress curl, rather than by the wind stress curl itself. This idea of a pressure setup is consistent with the differences found between the currents driven by COAMPS and SEB winds.

Positive Regional Sea Level Anomalies in Southern Brazil Due to Changes in Austral Atmospheric Circulation

2021 ◽  
Author(s):  
Venisse Schossler ◽  
Francisco Aquino ◽  
Jefferson Simões ◽  
Pedro Reis ◽  
Denilson Viana
Keyword(s):  
Sea Level ◽  
Wind Stress ◽  
Southern Annular Mode ◽  
Altimeter Data ◽  
Western Boundary ◽  
Positive Trend ◽  
Boundary Current ◽  
Wind Stress Curl ◽  
Sea Level Anomalies ◽  
Regional Sea Level

Abstract Pressure gradients and winds play an important role in Southern Hemisphere (SH) sea levels, which are currently associated with the positive trend of the Southern Annular Mode (SAM). This study investigated regional sea level anomalies (SLAs) in the southern coast Brazil using altimeter data (1993–2019), post-processed by the X-TRACK (CTOH/LEGOS). We observed a negative SLA from 1993 to 2009 and a positive SLA from 2010 to 2019, with upward trends throughout the evaluation period. We analyzed wind stress curl, pressure, and wind fields at sea level (FNMOC and ERA 5, respectively) in addition to sea surface temperature and height anomalies (SSTA/SSHA-OISST) in the South Atlantic Ocean (SAO) for 1993–2009 and 2010–2019. In relation to the first period, the second shows the enhancement in Hadley and Walker cells and trade winds, in addition to greater SSTA and SSHA in SAO. The SAO subtropical gyre and zonal winds at 45°S contribute to the intensification of the western boundary current. A greater pressure gradient between the SAO surface and the southeast of South America is noteworthy. Regionally, the positive SAM brings an increase in sea level to the study area, caused by greater wind stress and variability in heat flows.

The role of oceanic internal instabilities on the Great Whirl interannual variability 

2021 ◽  
Author(s):  
Kwatra Sadhvi ◽  
Iyyappan Suresh ◽  
Takeshi Izumo ◽  
Jérôme Vialard ◽  
Matthieu Lengaigne ◽  
...  
Keyword(s):  
Interannual Variability ◽  
Sea Level ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Offshore Wind ◽  
Intrinsic Variability ◽  
Wind Stress Curl ◽  
Sea Level Anomalies

<p>The Great Whirl (GW) is a quasi-permanent anticyclonic eddy that appears every summer monsoon in the western Arabian Sea off the horn of Africa. It generally forms in June, peaks in July-August, and dissipates afterward. While the annual cycle of the GW has been previously described, its year-to-year variability has been less explored. Satellite observations reveal that the leading mode of summer interannual sea-level variability in this region is associated with a typically ~100-km northward or southward shift of the GW. This shift is associated with coherent sea surface temperature and surface chlorophyll signals, with warmer SST and reduced marine primary productivity in regions with positive sea level anomalies and vice versa. Eddy-permitting (~25 km) and eddy-resolving (~10 km) ocean general circulation model simulations reproduce the observed pattern reasonably well, even in the absence of interannual variations in the surface forcing. This implies that the GW interannual variability partly arises from oceanic internal instabilities. Ensemble oceanic simulations further reveal that this stochastic oceanic intrinsic variability and the deterministic response to wind forcing each contribute to ~50% of the total GW interannual variability in July-August. The deterministic part appears to be related to the oceanic response  to Somalia alongshore wind stress and offshore wind-stress curl variations during the monsoon onset projecting onto the GW structure, and getting amplified by oceanic instabilities. After August, the stochastic component dominates the GW variability.</p>

scholarly journals Response of Near-Inertial Shear to Wind Stress Curl and Sea Level

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Jing Gao ◽  
Jianing Wang ◽  
Fan Wang
Keyword(s):  
Sea Level ◽  
Wind Stress ◽  
Deep Ocean ◽  
Wind Stress Curl ◽  
Sea Level Anomalies ◽  
Ocean Response ◽  
Downward Propagation ◽  
Shear Energy ◽  
And Sea Level

AbstractNear-inertial waves (NIWs) contain a pronounced portion of shear energy in the internal wave field and is of great importance to deep ocean mixing. However, accurate simulation of NIWs remains a challenge. Here we analyzed 3-year long mooring observation of velocity profiles over 80–800 m to study the responses of near-inertial downward shear to varying wind stress curls and sea level anomalies (SLAs). It is demonstrated that moderate (even weak) cyclone makes more contributions to enhanced shear below the pycnocline than very strong cyclone. Because very strong curl can stall the downward propagation of large shear. The large positive and negative SLAs cause the accumulation of large shear in the lower and upper parts of the pycnocline through inducing downwelling and upwelling motions, respectively. Time variation of near-inertial shear was strongly influenced by cases of large curls and interannual variation of SLA, and thus did not follow the seasonal variation of wind stress. Our analyses suggest that matched fields of wind stress curl and SLA, and well representing the ocean response to moderate cyclone are needed in simulating the role of NIWs on mixing.

scholarly journals Low-Frequency Surface Currents and Generation of an Island Lee Eddy in Panay Island, Philippines

2019 ◽  
Vol 49 (3) ◽  
pp. 765-787 ◽  
Author(s):  
Charina Lyn Amedo-Repollo ◽  
Xavier Flores-Vidal ◽  
Cedric Chavanne ◽  
Cesar L. Villanoy ◽  
Pierre Flament
Keyword(s):  
Wind Stress ◽  
Doppler Radar ◽  
Low Frequency ◽  
Acoustic Doppler Current Profiler ◽  
Cyclonic Eddy ◽  
Surface Currents ◽  
Wind Stress Curl ◽  
Near Surface ◽  
Geostrophic Balance ◽  
Mesoscale Currents

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.

scholarly journals Causal Mechanisms of Sea-level and Freshwater Content Change in the Beaufort Sea

2021 ◽  
Author(s):  
Ichiro Fukumori ◽  
Ou Wang ◽  
Ian Fenty
Keyword(s):  
Sea Ice ◽  
Sea Level Rise ◽  
Sea Level ◽  
Wind Stress ◽  
Beaufort Sea ◽  
Ekman Transport ◽  
Dynamic Adjustment ◽  
Near Surface ◽  
Ice Melt ◽  
Causal Mechanisms

AbstractIn the Arctic’s Beaufort Sea, the rate of sea-level rise over the last two decades has been an order of magnitude greater than that of its global mean. This rapid regional sea-level rise is mainly a halosteric change, reflecting an increase in Beaufort Sea’s freshwater content comparable to that associated with the Great Salinity Anomaly of the 1970s in the North Atlantic Ocean. Here we provide a new perspective of these Beaufort Sea variations by quantifying their causal mechanisms from 1992 to 2017 using a global, data-constrained ocean and sea-ice estimate of the Estimating the Circulation and Climate of the Ocean (ECCO) consortium. Our analysis reveals wind and sea-ice jointly driving the variations. Seasonal variation mainly reflects near-surface change due to annual melting and freezing of sea-ice, while interannual change extends deeper and mostly relates to wind-driven Ekman transport. Increasing wind stress and sea-ice melt are, however, equally important for decadal change that dominates the overall variation. Strengthening anticyclonic wind stress surrounding the Beaufort Sea intensifies the ocean’s lateral Ekman convergence of relatively fresh near-surface waters. The strengthening stress also enhances convergence of sea-ice and ocean heat that increase the amount of Beaufort Sea’s net sea-ice melt. The enhanced significance at longer time-scales of sea-ice melt relative to direct wind forcing can be attributed to ocean’s advection and mixing of melt-water being slower than its dynamic adjustment to mechanical perturbations. The adjustments’ difference implies that the sea-ice-melt-driven diabatic change will persist longer than the direct wind-driven kinematic anomaly.

scholarly journals Influence of Long-Term Changes in the Large-Scale Sea Level Pressure Field on the Wind Regime and the Wind Stress Curl in the Black Sea

2021 ◽  
Vol 37 (2) ◽  
Author(s):  
I. G. Shokurova ◽  
A. A. Kubryakov ◽  
M. V. Shokurov ◽  
◽  
◽  
...  
Keyword(s):  
Sea Level ◽  
Black Sea ◽  
Wind Stress ◽  
Mediterranean Region ◽  
The Black Sea ◽  
Wind Regime ◽  
Wind Stress Curl ◽  
Sea Level Pressure ◽  
Level Pressure

Purpose. The paper is aimed at studying the relationship between the wind regime and the wind stress curl in the Black Sea, on the one hand, and the long-term changes in the sea level pressure fields in winter months, on the other. Methods and Results. The data on wind speed and sea level pressure in January – February from the NCEP/NСAR reanalysis for 1948–2018 are used. Based on the 6-hour data, the synoptic conditions accompanied by high and low values of the wind stress curl in the sea were determined. The synoptic situations in which a vast anticyclone is located north and northeast of the sea, and the area of low pressure – to the southwest of the sea in the Mediterranean region, are accompanied by the northeast and east winds, and by the cyclonic curl predominance. On the contrary, passing of the cyclones to the north of the sea and increase of pressure to the southwest are followed by the westerly and southwesterly winds, and by the anticyclonic curl predominance. Extremely high monthly mean values of the cyclonic curl were observed in those years, when the area occupied by the Siberian anticyclone increased and expanded westward, so that the Black Sea was on the southwestern periphery of its spur. Extremely low values of the anticyclonic curl were noted when the Azores anticyclone area expanded to the Mediterranean region. The wind stress curl changes on the multidecadal scales have shown its relation to the global changes in the field of the sea level pressure and the sign of the pressure anomalies at the low latitudes. Conclusions. The opposite sign of the surface pressure anomalies taking place to the northeast and southwest of the sea is accompanied by the highest values of the wind stress curl.

The sensitivity of Southeast Pacific heat content to changes in ocean structure

2021 ◽  
Author(s):  
Dan Jones ◽  
Emma Boland ◽  
Andrew Meijers ◽  
Gael Forget ◽  
Simon Josey ◽  
...  
Keyword(s):  
Objective Function ◽  
Wind Stress ◽  
Heat Content ◽  
Global Ocean ◽  
Wind Stress Curl ◽  
Adjoint Sensitivity ◽  
Mode Water ◽  
Near Surface ◽  
Basin Scale ◽  
Southeast Pacific

<p>The Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on timescales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally-constrained, physically-consistent global ocean state estimate (i.e. ECCOv4) to examine how changes in ocean properties can affect the heat content both in the mixed layer and in the recently ventilated subsurface, focusing on the Southeast Pacific. First, we carry out a comprehensive adjoint sensitivity study using near-surface heat content as the objective function, highlighting the locations and timescales with the largest potential to affect the properties of relevant subduction regions. Next, we use a set of numerical tracer release experiments to identify the subduction and export pathways from the surface into the subsurface thermocline, thereby defining the recently ventilated interior. Using the tracer distribution to define our objective function, we employ an adjoint method to calculate temporally-evolving sensitivity maps that highlight the processes, locations, and timescales that are potentially most relevant for changing the heat content of the recently ventilated Pacific. In order to examine the full nonlinear response, we use the adjoint sensitivity fields to design a set of forward, nonlinear perturbation experiments. We find surprisingly weak sensitivities to high latitude wind stress and heat flux, and relatively high sensitivities to wind stress curl in subpolar latitudes. Despite the localized nature of mode water subduction hotspots, changes in basin-scale density gradients are an important controlling factor on heat distribution in the Southeast Pacific.</p>

The Role of Wind Stress Curl in Jet Separation at a Cape

2007 ◽  
Vol 37 (11) ◽  
pp. 2652-2671 ◽  
Author(s):  
Renato M. Castelao ◽  
John A. Barth
Keyword(s):  
High Resolution ◽  
Numerical Model ◽  
Spatial Structure ◽  
Wind Stress ◽  
Coastal Upwelling ◽  
Separation Process ◽  
Wind Forcing ◽  
The Other ◽  
Wind Stress Curl

Abstract A high-resolution numerical model is used to study the importance of spatial variability in the wind forcing to the separation of a coastal upwelling jet at a cape. An idealized topography and wind field based on observations from the Cape Blanco (Oregon) region are used. Several simulations are investigated, with both the intensity and the spatial structure of the wind forcing varied to isolate the importance of the observed intensification in the wind stress and wind stress curl magnitudes to the separation process. A simulation using a straight coast confirms that the presence of the cape is crucial for separation. Wind stress intensification by itself, with zero curl, does not aid separation. The wind stress curl intensification south of the cape, on the other hand, is important for controlling details of the process. Because the positive wind stress curl drives upwelling, isotherms in the offshore region tilt upward, creating a pressure gradient that sustains an intensification of the southward velocities via the thermal wind balance. This aids jet separation via continuity and by creating potential vorticity contours that track far offshore of the cape. The timing of the separation is dependent on the intensity of the wind stress curl (stronger curl leads to earlier separation), while how far offshore the jet is deflected depends on the offshore extent of the region of positive curl close to the coast (increasing the extent increases the deflection).

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