Local Wind Stress Research Articles

Current variability by wave propagation in Todos Santos Bay, Baja California, Mexico

The temperature, velocity, and sea level fields for a summer climatological month (August) were analyzed using tridimensional and baroclinic model outputs in Todos Santos Bay (TSB), Baja California, Mexico. The numerical model was forced with wind (nonstationary), heat flux, and California Current System climatology on the open boundary. The 3- to 5-day current variability is related to a baroclinic wave traveling towards the northwest of the bay. Wave travel periodicity was due to the release of accumulated water from the Ensenada–Punta Banda Estuary (E–PBE) region. Local wind stress causes southeastward water flow and, given the TSB geometry, water accumulates in the E–PBE region. Weakening wind stress was the main cause of water release. In addition, complex empirical orthogonal function analysis found that outer TSB disturbances cause sea level variability.

Mechanisms underlying recent decadal changes in subpolar North Atlantic Ocean heat content

AbstractThe subpolar North Atlantic (SPNA) is subject to strong decadal variability, with implications for surface climate and its predictability. In 2004–2005, SPNA decadal upper ocean and sea‐surface temperature trends reversed from warming during 1994–2004 to cooling over 2005–2015. This recent decadal trend reversal in SPNA ocean heat content (OHC) is studied using a physically consistent, observationally constrained global ocean state estimate covering 1992–2015. The estimate's physical consistency facilitates quantitative causal attribution of ocean variations. Closed heat budget diagnostics reveal that the SPNA OHC trend reversal is the result of heat advection by midlatitude ocean circulation. Kinematic decompositions reveal that changes in the deep and intermediate vertical overturning circulation cannot account for the trend reversal, but rather ocean heat transports by horizontal gyre circulations render the primary contributions. The shift in horizontal gyre advection reflects anomalous circulation acting on the mean temperature gradients. Maximum covariance analysis (MCA) reveals strong covariation between the anomalous horizontal gyre circulation and variations in the local wind stress curl, suggestive of a Sverdrup response. Results have implications for decadal predictability.

Characterizing frontal eddies along the East Australian Current from HF radar observations

AbstractThe East Australian Current (EAC) dominates the ocean circulation along south‐eastern Australia, however, little is known about the submesoscale frontal instabilities associated with this western boundary current. One year of surface current measurements from HF radars, in conjunction with mooring and satellite observations, highlight the occurrence and propagation of meanders and frontal eddies along the inshore edge of the EAC. Eddies were systematically identified using the geometry of the high spatial resolution (∼1.5 km) surface currents, and tracked every hour. Cyclonic eddies were observed irregularly, on average every 7 days, with inshore radius ∼10 km. Among various forms of structures, frontal eddies associated with EAC meanders were characterized by poleward advection speeds of ∼0.3–0.4 m/s, migrating as far as 500 km south, based on satellite imagery. Flow field kinematics show that cyclonic eddies have high Rossby numbers (0.6–1.9) and enhance particle dispersion. Patches of intensified surface divergence at the leading edge of the structures are expected to generate vertical uplift. This is confirmed by subsurface measurements showing temperature uplift of up to 55 m over 24 h and rough estimates of vertical velocities of 10s of meters per day. While frontal eddies propagate through the radar domain independently of local wind stress, upfront wind can influence their stalling and growth, and can also generate large cold core eddies through intense shear. Such coherent structures are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters, influencing physical and biological dynamics, and connectivity over large distances.

Response of theKuroshioExtension path state to near‐term global warming in CMIP5 experiments with MIROC4h

AbstractIn this study, responses of the Kuroshio Extension (KE) path state to near‐term (2006–2035) global warming are investigated using a Kuroshio‐resolving atmosphere‐ocean coupled model. Under the representative concentration pathway 4.5 (RCP4.5) forcing, the KE system is intensified and its path state tends to move northward and becomes more stable. It is suggested that the local anticyclonic wind stress anomalies in the KE region favor the spin‐up of the southern recirculation gyre, and the remote effect induced by the anticyclonic wind stress anomalies over the central and eastern midlatitude North Pacific also contributes to the stabilization of the KE system substantially. The dominant role of wind stress forcing on KE variability under near‐term global warming is further confirmed by adopting a linear 1.5 layer reduced‐gravity model forced by wind stress curl field from the present climate model. It is also found that the main contributing longitudinal band for KE index (KEI) moves westward in response to the warmed climate. This results from the northwestward expansion of the large‐scale sea level pressure (SLP) field.

Wind and Current Forcing Combine to Drive Strong Upwelling in the Agulhas Current

AbstractStrong upwelling events inshore of the Agulhas Current close to 33.5°S are investigated. These events are important to the exchange of shelf and slope waters, potentially enhancing primary productivity and advecting larvae offshore. Using hydrographic observations, this study shows that a wind-driven upwelling event and a current-driven upwelling event can each advect central waters more than 130 m upward, resulting in a maximum 9°C cooling at 50-m depth over the continental shelf and surface cooling greater than 4°C. The authors use satellite data to assess the frequency and forcing mechanisms of similar cold events from January 2003 through December 2011, defining cold events as days when the sea surface temperature (SST) anomaly is significantly correlated with a local current or wind forcing. The authors identify 47 events with an average length of 2.2 days and SST anomaly of −1.6°C, corresponding to an average 13 days of surface cold events along the Agulhas Current front per year. This study uses combined EOF analysis to characterize these cold events based on four highly correlated forcing mechanisms: alongshore wind speed, wind stress curl, current meandering, and current speed over the slope. The authors find that meanders act in combination with upwelling-favorable winds to force the strongest cold events, while upwelling-favorable winds alone, possibly primed by Ekman veering, force weaker cold events. Most significantly, it is found that the frontal curvature of warm Agulhas Current meanders couples with the atmosphere to drive local wind stress curl anomalies that reinforce upwelling.

The Effects of Mesoscale Ocean–Atmosphere Coupling on the Quasigeostrophic Double Gyre

We investigate the effects of sea surface temperature (SST)-dependent wind stress on the wind-driven quasigeostrophic (QG) double gyre. The main effects are to reduce the strength of the circulation and to shift the inter-gyre jet to the south. The SST front across the inter-gyre jet induces a zonal wind stress anomaly over the jet that accelerates the southern flank of the jet and decelerates the northern flank. This local wind stress anomaly causes the jet to shift southwards. Shifting the jet south, away from the peak wind stress, reduces the net power input to the ocean circulation. Allowing the wind stress to depend on the difference between the atmospheric and oceanic velocity also reduces the net wind power input, and has a larger impact than SST dependence. When wind stress depends only on SST, the impact on the circulation is stronger than when wind stress depends on both SST and ocean surface velocity. Ocean surface velocity dependence leads to direct extraction of mesoscale energy by the winds. In contrast, SST dependence leads to injection (extraction) of mesoscale energy in the subtropical (subpolar) gyres, with almost complete cancellation because of the symmetric wind field.

Alongcoast structure and interannual variability of seasonal midshelf water properties and velocity in the Northern California Current System

AbstractMoored sensors were maintained for ∼5 years on the northern California Current System (CCS) midshelf. The alongcoast sensor array spanned the area of influence of the plume from the Columbia River, several submarine canyons, as well as a coastal promontory where the equatorward coastal jet frequently separates from the shelf. Upwelling‐favorable wind stress magnitude decreases poleward by more than a factor of three over the latitudinal range and shelf width varies by a factor of two. In spite of the alongcoast structure in setting, both seasonal and interannual patterns in subsurface layer water properties were remarkably similar at all sites. Higher in the water column, freshwater forcing was substantial. Because of the near surface freshwater input, seasonal sea surface and subsurface temperatures were almost perfectly out of phase in the northernmost CCS, with a mid water column inversion in winter. Year to year differences in subsurface layer wintertime water properties were similar to spatial and temporal patterns of wind stress variability: little alongcoast structure except in salinity, but pronounced interannual differences strongly related to local wind stress. Summertime wind and subsurface property patterns were the opposite of those in winter: pronounced alongcoast wind stress structure, but little or no alongcoast or interannual variability in water properties, and only a weak relationship to local wind stress. Summertime interannual water property variability, including source waters, was shown to be more consistent with “remote forcing” via larger scale wind stress rather than with local wind stress, particularly in the northernmost CCS.

Summertime coastal current reversal opposing offshore forcing and local wind near the middle east coast of Korea: Observation and dynamics

AbstractA 6 year long current measurement at a buoy station off the middle east coast of Korea reveals an equatorward reversal of coastal current in summer opposing poleward local wind stress and offshore boundary current. The current reversal extends about 40 km offshore from the coast and is concurrent with warming and freshening of water column. Estimates of the depth‐averaged alongshore momentum balance suggest a major balance between the alongshore pressure gradient and the lateral friction. Sources of the pressure gradient for the summertime current reversal are identified as the alongshore buoyancy gradient driven by the wind curl gradient and the prevalence of warmer and lower salinity water in the north. Alongshore pressure gradient and velocity induced by the wind curl gradient are quantified, which yields the observed seasonal current reversal.

Modelling wave–current interactions off the east coast of Scotland

Abstract. Densely populated coastal areas of the North Sea are particularly vulnerable to severe wave conditions, which overtop or damage sea defences leading to dangerous flooding. Around the shallow southern North Sea, where the coastal margin is lying low and population density is high, oceanographic modelling has helped to develop forecasting systems to predict flood risk. However, coastal areas of the deeper northern North Sea are also subject to regular storm damage, but there has been little or no effort to develop coastal wave models for these waters. Here, we present a high spatial resolution model of northeast Scottish coastal waters, simulating waves and the effect of tidal currents on wave propagation, driven by global ocean tides, far-field wave conditions, and local air pressure and wind stress. We show that the wave–current interactions and wave–wave interactions are particularly important for simulating the wave conditions close to the coast at various locations. The model can simulate the extreme conditions experienced when high (spring) tides are combined with sea-level surges and large Atlantic swell. Such a combination of extremes represents a high risk for damaging conditions along the Scottish coast.

Annual Sea Level Changes on the North American Northeast Coast: Influence of Local Winds and Barotropic Motions

Abstract Understanding the relationship between coastal sea level and the variable ocean circulation is crucial for interpreting tide gauge records and projecting sea level rise. In this study, annual sea level records (adjusted for the inverted barometer effect) from tide gauges along the North American northeast coast over 1980–2010 are compared to a set of data-assimilating ocean reanalysis products as well as a global barotropic model solution forced with wind stress and barometric pressure. Correspondence between models and data depends strongly on model and location. At sites north of Cape Hatteras, the barotropic model shows as much (if not more) skill than ocean reanalyses, explaining about 50% of the variance in the adjusted annual tide gauge sea level records. Additional numerical experiments show that annual sea level changes along this coast from the barotropic model are driven by local wind stress over the continental shelf and slope. This result is interpreted in the light of a simple dynamic framework, wherein bottom friction balances surface wind stress in the alongshore direction and geostrophy holds in the across-shore direction. Results highlight the importance of barotropic dynamics on coastal sea level changes on interannual and decadal time scales; they also have implications for diagnosing the uncertainties in current ocean reanalyses, using tide gauge records to infer past changes in ocean circulation, and identifying the physical mechanisms responsible for projected future regional sea level rise.

Alongshore momentum transfer to the nearshore zone from energetic ocean waves generated by passing hurricanes

AbstractWave and current measurements from a cross‐shore array of nearshore sensors in Duck, NC, are used to elucidate the balance of alongshore momentum under energetic wave conditions with wide surf zones, generated by passing hurricanes that are close to and far from to the coast. The observations indicate that a distant storm (Hurricane Bill, 2009) with large waves has low variability in directional wave characteristics resulting in alongshore currents that are driven mainly by the changes in wave energy. A storm close to the coast (Hurricane Earl, 2010), with strong local wind stress and combined sea and swell components in wave energy spectra, has high variability in wave direction and wave period that influence wave breaking and nearshore circulation as the storm passes. During both large wave events, the horizontal current shear is strong and radiation stress gradients, bottom stress, wind stress, horizontal mixing, and cross‐shore advection contribute to alongshore momentum at different spatial locations across the nearshore region. Horizontal mixing during Hurricane Earl, estimated from rotational velocities, was particularly strong suggesting that intense eddies were generated by the high horizontal shear from opposing wind‐driven and wave‐driven currents. The results provide insight into the cross‐shore distribution of the alongshore current and the connection between flows inside and outside the surf zone during major storms, indicating that the current shear and mixing at the interface between the surf zone and shallow inner shelf is strongly dependent on the distance from the storm center to the coast.

Features and variability of the South China Sea western boundary current from 1992 to 2011

Different from the traditional definition of the South China Sea western boundary current (SCSWBC), in this paper, only the southwestward and southward currents along the northern and western slopes in the SCS, which are closely associated with the basin-wide wind stress curl, are defined as the SCSWBC, while the flows on the southwestern shelf driven directly by the local wind stress are regarded as part of the shelf circulation. Using a new reanalysis dataset of the SCS in conjunction with the in situ and remote sensing data, the main features and variability of the SCSWBC from 1992 to 2011 were studied. Dictated by the prevailing monsoonal winds and in- and outflows, the SCSWBC in winter extended the full length of the western slope and reached its maximum intensity off the southeast coast of Vietnam, while in summer the main body of the SCSWBC was limited to the northern half of the western slope and merged with the northward coastal current to form the Vietnam Offshore Current (VOC) at about 12° N. Moreover, the respective seasonal patterns of the SCSWBC showed pronounced interannual variations in its structure, including the axis, the width, and the maximum depth. The strength of the SCSWBC, with the transport of −11.8 ± 3.5 Sv in winter and −3.0 ± 1.6 Sv in summer off the central coast of Vietnam, also varied significantly from year to year. It was demonstrated that the monsoonal forcing over the SCS, the interannual variability of which was closely associated with El Nino events, played an important role in modulating the interannual variability of the SCSWBC, whereas the influence from the upper-layer Luzon Strait transport was secondary.

Interannual Variability of the Mindanao Current/Undercurrent in Direct Observations and Numerical Simulations

AbstractThe interannual variability of the boundary currents east of the Mindanao Island, including the Mindanao Current/Undercurrent (MC/MUC), is investigated using moored acoustic Doppler current profiler (ADCP) measurements combined with a series of numerical experiments. The ADCP mooring system was deployed east of the Mindanao Island at 7°59′N, 127°3′E during December 2010–August 2014. Depth-dependent interannual variability is detected in the two western boundary currents: strong and lower-frequency variability dominates the upper-layer MC, while weaker and higher-frequency fluctuation controls the subsurface MUC. Throughout the duration of mooring measurements, the weakest MC was observed in June 2012, in contrast to the maximum peaks in December 2010 and June 2014, while in the deeper layer the MUC shows speed peaks circa December 2010, January 2011, April 2013, and July 2014 and valleys circa June 2011, August 2012, and November 2013. Diagnostic analysis and numerical sensitivity experiments using a 2.5-layer reduced-gravity model indicate that wind forcing in the western Pacific Ocean is a driving agent in conditioning the interannual variability of MC and MUC. Results suggest that westward-propagating Rossby waves that generate in the western Pacific Ocean (roughly 150°–180°E) are of much significance in the interannual variability of the two boundary currents. Fluctuation of Ekman pumping due to local wind stress curl anomaly in the far western Pacific Ocean (roughly 120°–150°E) also plays a role in the interannual variability of the MC. The relationship between the MC/MUC and El Niño is discussed.

Interannual variability in the South‐East Atlantic Ocean, focusing on the Benguela Upwelling System: Remote versus local forcing

AbstractWe investigate the respective roles of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on coastal variability in the South‐East Atlantic Ocean extending up to the Benguela Upwelling System (BUS) over the 2000–2008 period. We carried out a set of six numerical experiments based on a regional ocean model, that differ only by the prescribed forcing (climatological or total) at surface and lateral boundaries. Results show that at subseasonal timescales (<100 days), the coastal oceanic variability (currents, thermocline, and sea level) is mainly driven by local forcing, while at interannual timescales it is dominated by remote equatorial forcing. At interannual timescales (13–20 months), remotely forced Coastal‐Trapped Waves (CTW) propagate poleward along the African southwest coast up to the northern part of the BUS at 24°S, with phase speeds ranging from 0.8 to 1.1 m.s−1. We show that two triggering mechanisms limit the southward propagation of CTW: interannual variability of the equatorward Benguela Current prescribed at the model's southern boundary (30°S) and variability of local atmospheric forcing that modulates the magnitude of observed coastal interannual events. When local wind stress forcing is in (out) of phase, the magnitude of the interannual event increases (decreases). Finally, dynamical processes associated with CTW propagations are further investigated using heat budget for two intense interannual events in 2001 and 2003. Results show that significant temperature anomalies (±2°C), that are mostly found in the subsurface, are primarily driven by alongshore and vertical advection processes.

Examination of Winter Circulation in a Northern Gulf of Mexico Estuary

Numerical model experiments were conducted to examine how estuarine circulation and salinity distribution in the Calcasieu Lake Estuary (CLE) of southwest Louisiana respond to the passage of cold fronts. River runoff, local wind stress, and tides from December 20, 2011, to February 1, 2012, were included as input. The experiments showed an anticyclonic circulation in the eastern CLE, a cyclonic circulation in West Cove, and a saltwater conduit in the navigation channel between these circulation cells. Freshwater from the river and wetlands tends to flow over the shallow shoals toward the ocean, presenting a case of the conventional estuarine circulation with shallow water influenced by river discharge and with weak tidally-induced motion, enhanced by wind. The baroclinic pressure gradient is important for the circulation and saltwater intrusion. The effect of remote wind-driven oscillation plays an important role in circulation and salinity distribution in winter. Unless it is from the east, wind is found to inhibit saltwater intrusion through the narrow navigation channel, indicating the effect of Ekman setup during easterly wind. A series of north-south oriented barrier islands in the lake uniquely influenced water level and salinity distribution between the shallow lake and deep navigation channel. The depth of the navigation channel is also crucial in influencing saltwater intrusion: the deeper the channel, the more saltwater intrusion and the more intense estuarine circulation. Recurring winter storms have a significant accumulated effect on the transport of water and sediment, saltwater intrusion, and associated environmental and ecosystem effects.

Monitoring Oregon Coastal Harmful Algae: Observations and implications of a harmful algal bloom-monitoring project

The accumulation of domoic acid (DA) and saxitoxins (STX), phycotoxins produced by some species of Pseudo-nitzschia and Alexandrium, respectively, in coastal food webs are a focus of research on the West Coast of the United States due to the deleterious effects they have on coastal ecosystems and economies. Results are presented from the 2007–2012 Monitoring Oregon Coastal Harmful Algae (MOCHA) project, the Oregon coast's first HAB monitoring and research program. Both historical toxin databases and more detailed case-study observations of individual HAB events are compiled to provide the first detailed overview of HAB occurrence in this region. These results are also presented in the context of informing future HAB monitoring in this and other upwelling regimes affected by STX and DA. A 2009–2010 warming event was associated with the greatest HAB activity during the MOCHA project, including anomalously high sea surface temperatures and shellfish harvesting closures due to STX and DA in 2009 and 2010, respectively. In regards to HAB monitoring, it is shown that (1) razor clams are a more sensitive indicator of DA than mussels; (2) water column concentrations of particulate domoic acid greater than 103ngL−1 can be used as a threshold for early-warning of shellfish DA toxicity and (3) approximately bi-weekly, or shorter, monitoring of Alexandrium in the surf zone and/or offshore can provide advance notice of STX contamination of shellfish. Both of the latter two metrics gain added value when coupled with local wind stress, a proxy of downwelling/relaxation events that facilitate greater interaction between offshore blooms and shellfish.

Interannual variability of thermal front west of Luzon Island in boreal winter

Interannual variability of thermal front west of Luzon Island during the winter of 1993–2013 is examined with the method of singular value decomposition (SVD) and a suite of satellite measurements in this paper. It is found that both the area and intensity of the thermal front west of Luzon Island show apparent interannual variability. Further study based on SVD shows that the interannual variability of the thermal front is highly associated with El Nino and Southern Oscillation (ENSO), and the correlation coefficient between Nino3.4 index and the first Principal Component (PC1) of thermal front can reach–0.65. The mechanism can be described as follows. In El Nino (La Nina) years, the East Asian winter monsoon (EAWM) is weakened (enhanced), inducing weaker (stronger) local wind stress curl (WSC) west of Luzon Island, and resulting in weakened (enhanced) Luzon cold eddy, which finally leads to the weakening (enhancement) of the thermal front.

Local wind effect on the Kuroshio path state off the southeastern coast of Kyushu

Small meanders of the Kuroshio southeast of Kyushu, Japan, are conventionally known to form in winter or early spring, but rarely in other seasons. This study examines the seasonal nature of the small meander formation as it relates to the seasonal monsoon winds over the East China Sea, based on observational data analyses, theoretical considerations and numerical experiments. Positional data of the Kuroshio path indicate that the small meander experiences phase locking with the seasonal cycle, as is conventionally thought, although its intensity is modulated on decadal time scales. Monthly mean data of the surface geostrophic current and wind stress fields indicate that the southwestward wind blowing against the Kuroshio in the autumn and early winter causes an inshore-ward shift of the Kuroshio in the northern Okinawa Trough. This effect acts to strengthen the convoluted pattern of the Kuroshio path around Kyushu, and to develop the small meander southeast of the same island. Assuming that the Kuroshio is a surface geostrophic jet with a double-exponential velocity profile, the observed evidence is well explained by a combination of Ekman layer dynamics and quasi-geostrophic dynamics. More specifically, Ekman pumping due to nonlinear Ekman divergence over the jet decreases (increases) the surface velocity of the offshore (inshore) side of the jet, resulting in the inshore-ward shift of the jet. A two-layer shallow water model with idealized topography shows that the response of the modeled Kuroshio to the local wind stress is consistent with observational and theoretical results.

Strongly-sheared wind-forced currents in the nearshore regions of the central Southern California Bight

Contrary to many previous reports, winds do drive currents along the shelf in the central portion of the Southern California Bight (SCB). Winds off Huntington Beach CA are the dominant forcing for currents over the nearshore region of the shelf (water depths less than 20m). Winds control about 50–70% of the energy in nearshore alongshelf surface currents. The wind-driven current amplitudes are also anomalously high. For a relatively weak 1dyne/cm2 wind stress, the alongshelf surface current amplitudes in this region can reach 80cm/s or more. Mid-depth current amplitudes for the same wind stress are around 30–40cm/s. These wind-driven surface current amplitudes are much larger than previously measured over other nearshore shelf regions, perhaps because this program is one of the few that measured currents within a meter of the surface. The near-bed cross-shelf currents over the nearshore region of the Huntington Beach shelf have an Ekman response to winds in that they upwell (downwell) for down (up) coast winds. This response disappears further offshore. Hence, there is upwelling in the SCB, but it does not occur across the entire shelf. Subthermocline water in the nearshore region that may contain nutrients and plankton move onshore when winds are southeastward, but subthermocline water over the shelf break is not transported to the beach. The currents over the outer shelf are not predominately controlled by winds, consistent with previous reports. Instead, they are mainly driven by cross-shelf pressure gradients that are independent of local wind stress.

Vertical and horizontal structures of the North Pacific subtropical gyre axis

Mean structures of the North Pacific subtropical gyre are investigated using Argo float observations and several wind stress products, with a particular focus on the gyre axis, which is defined as a large-scale boundary between eastward motions on the northern part of the subtropical gyre and westward motions on the southern part. Structures of the gyre axis are different between the regions east and west of about 180°. In the west, the gyre axis is affected by the strong Kuroshio Extension jet, showing discrepancy with features expected from the Sverdrup balance and ventilated thermocline theory. In the east, depth-integrated geostrophic circulation displays the two gyre axes at about 25°N and 30°N in association with a local cyclonic wind stress curl anomaly. It is expected from the Sverdrup balance that the two gyre axes would extend to the west across the subtropical gyre, but the actual geostrophic circulation exhibits only one gyre axis around 30°N to the west. The vertical structure of the subtropical gyre exhibits a remarkable northward shift of the gyre axis with depth in the east, consistent with the southward decrease in the thickness of density layers around the gyre axis, while the northward shift is less obvious in the west, where the gyre axis lies along the southern flank of the Kuroshio Extension. The gyre shift is relatively smaller in the central mode water (CMW) layer due to the horizontal uniformity of the CMW, suggesting a dynamic effect of the CMW on large-scale circulations of the subtropical gyre.