Alongshore Momentum Research Articles

A Perturbative Solution for Nonlinear Stratified Upwelling over a Frictional Slope

Abstract A perturbative solution of simplified primitive equations for nonlinear weakly stratified upwelling over a frictional slope is found that resolves the vertical structure of velocity fields and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The solution uses assumptions consistent with the model proposed by Lentz and Chapman, including a steady-state, constant cross-shore density gradient, no alongshore gradients, laterally inviscid, and consideration of cross-shore advection of alongshore momentum. The solution resolves the vertical structure of velocity fields (including subsurface maxima of compensational flow, not resolved by Lentz and Chapman) and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The dynamics are similar to Lentz and Chapman; bottom stress balances alongshore wind stress in a homogeneous density ocean and is replaced by nonlinear cross-shore transport of alongshore momentum as the Burger number (S = αN/f, where α, N, and f are the bottom slope, buoyancy frequency, Coriolis frequency, respectively) increases. When the solution uses the empirical relation between cross-shore and vertical density gradients proposed by Lentz and Chapman, vorticity conservation is not satisfied and the nonlinear momentum transport estimated by the solution linearly increases with S, asymptotically matching Lentz and Chapman for S < 1. When the solution conserves interior potential vorticity, the momentum transport is proportional to S2 for S < 1 and is in better agreement with numerical simulations.

The Tilt of Mean Dynamic Topography and its Seasonality Along the Coast of the Chinese Mainland

AbstractTilt of coastal mean dynamic topography (MDT) has recently been investigated in many regions, but few studies have examined the seasonality of the MDT along the coast of the Chinese mainland. The China seas are relatively unique as they experience prominent seasonal changes in monsoons and the China Coastal Currents, which are supposed to affect the coastal MDT significantly. This study investigates the Chinese coastal MDT tilt and its seasonality based on the geodetic and ocean approaches. The two independent approaches show a good agreement in their respective estimates. The Chinese coastal MDT presents an overall northward drop and undergoes an evident seasonality with steeper slopes in winter and flatter ones in summer. A dynamical analysis based on alongshore momentum equation suggests that the alongshore MDT along the coast of the Chinese mainland is a counter balance of contributions from the alongshore wind and the coastal current‐induced bottom friction, with both of them having prominent seasonality. The overall northward drop is in fact induced primarily by the strong alongshore wind stress in winter and by coastal currents in summer, in particular for the segment between Xiamen and Zhapo. The wintertime coastal MDT is well predicted by the arrested topographic wave model, which however could not reproduce the summertime tilt. The summertime MDT is more likely affected by the open‐ocean forcing (i.e., the latitudinal difference of the Kuroshio strength) and a return flow caused by flattening the coastal tilt piled up by the previous winter monsoon.

Observations of nonlinear momentum fluxes over the inner continental shelf

Nonlinear momentum fluxes over the inner continental shelf are examined using moored observations from multiple years at two different locations in the Middle Atlantic Bight. Inner shelf dynamics are often described in terms of a linear alongshore momentum balance, dominated by frictional stresses generated at the surface and bottom. In this study, observations over the North Carolina inner shelf show that the divergence of the cross-shelf flux of alongshore momentum is often substantial relative to the wind stress during periods of strong stratification. During upwelling at this location, offshore fluxes of alongshore momentum in the surface layer partially balance the wind stress and reduce the role of the bottom stress. During downwelling, onshore fluxes of alongshore momentum reinforce the wind stress and increase the role of bottom stress. Over the New England inner shelf, nonlinear terms have less of an impact in the momentum balance and exhibit different relationships with the wind forcing. Differences between locations and time periods are explained by variations in bottom slope, latitude, vertical shear and cross-shelf exchange. Over the New England inner shelf, where moored density data are available, variations in vertical shear are explained by a combination of thermal wind balance and wind stress. An implication of this study is that cross-shelf winds can potentially influence the alongshore momentum balance over the inner shelf, in contrast with deeper locations over the middle to outer shelf.

Cross‐Shore Structure of Tidally Driven Alongshore Flow Over Rough Bathymetry

AbstractA tidally driven alongshore flow on the western coast of O'ahu Hawai'i is examined using velocity measurements from an autonomous underwater vehicle (AUV) along with time series observations of the alongshore pressure gradient. Depth‐averaged velocities over the forereef shelf are reconstructed from AUV‐based velocity observations as a function of cross‐shore distance assuming a sinusoidal tidal periodicity. Ensemble phase averages of the alongshore pressure gradient and velocities from multiple AUV surveys reveal characteristics akin to an oscillatory boundary layer, with the nearshore flow leading the offshore flow in phase and with a corresponding velocity attenuation at shallower depths. Analysis of the depth‐averaged alongshore momentum equation indicates that the cross‐shore structure and evolution of the tidal boundary layer is well described by a balance between the local acceleration, the barotropic pressure gradient, and bottom drag. This primary balance allows estimation of the drag coefficient as a function of cross‐shore distance over depths spanning from 24 to 6 m. Results indicate that drag coefficients range from 0.004 to 0.010 [±0.002] over a 600 m cross‐shore forereef section. These estimates are in good agreement with previous results obtained at the 12 m isobath using fixed observations and compare favorably with roughness estimates from LIDAR and AUV‐based mapping. Roughness data suggest that larger scales, with wavelengths of O(10 m), play a more significant role than smaller meter‐scale roughness in determining the drag on the tidal flow.

The Influence of A Cross‐Reef Channel On the Wave‐Driven Setup and Circulation at Ipan, Guam

AbstractThe influence of a deep (30 m), narrow (30 m) cross‐shore channel on the circulation and wave‐induced setup over a shallow (∼0.5 m) and wide (∼400 m) shore‐attached fringing reef is examined using field measurements collected at Ipan, Guam. Mean currents on the reef flat over a 7‐week study period during mid and high tides when the reef is submerged are directed toward the channel with the alongshore component of the current increasing with proximity to the channel. The cross‐shore component of the reef flat current is directed onshore at the sensors in the far‐field of the channel with a weak offshore flow at the current meter located closest to the channel (∼760 m to the north). Low‐frequency fluctuations of the alongshore reef flat current and offshore channel current are significantly correlated and with the incident significant wave height. Mean and low‐frequency fluctuating currents are forced by the spatially variable wave‐driven setup, modulated by tidal elevation, which creates a pressure gradient over the reef flat due to the channel where waves do not break. The dominant alongshore momentum balance on the reef flat is between the pressure gradient and bottom stress, with an inferred drag coefficient of CD ∼ 0.01. A simple analytical model is presented that is consistent with the observations and delineates the near‐ and far‐field of the channel as a function of the aspect ratio of the reef. Observations from a longer deployment of channel currents are highly correlated with incident wave height in distinct tidal level bands.

Alongshore momentum balance over shoreface-connected ridges, Fire Island, NY,

Hydrodynamic and hydrographic data collected on the inner shelf of Fire Island, NY, over a region of shoreface-connected ridges (SFCRs), are used to describe wind-driven circulation over uneven topographies along relatively straight coastlines. The data reveal a predominantly alongshore flow, under westward wind forcing, with localized offshore current veering over the SFCR crests and an onshore veering over the adjacent troughs. Alongshore momentum balance analysis of the observations reveals that total acceleration (i.e., local and advective) and bottom stress are balanced by wind stress and pressure gradient. A numerical model is used to verify the experimentally derived results and to analyze the cross-shore momentum balance which is not resolved by the experimental data. Analysis of the numerical and field data reveal that the total alongshore pressure gradient force can be considered as the sum of a local and a regional component. The former is correlated with the alongshore advective acceleration that develops over the crest of the ridges resembling a Bernoulli-like pressure-flow relationship. The regional alongshore pressure gradient is related to wind stress with which it maintains a strong, negative relationship. Over the ridge crests the cross-shore veering of the flow is due to mass conservation, associated with a balance between cross-shore advective acceleration and local cross-shore pressure gradient. Under variable wind conditions the regional pressure gradients are more important, as compared to quasi steady-state conditions which increase the influence of the local scale pressure gradient. A time scale of 6 h is found as the temporal scale required for the local pressure gradients to make a significant contribution to the momentum balance.

The Fiordland Current, southwest New Zealand: mean, variability, and trends

ABSTRACT The mean, interannual variability, decadal variability, and longer-term trends in the across-track geostrophic velocity of the poleward-flowing boundary current along the Fiordland coast (the Fiordland Current, FC) are examined over the altimeter record (September 1992–May 2017). In the mean, the FC is southwestward, with stronger velocities in an inner current near the coast and weaker velocities in an outer current further offshore. Examination of the alongshore momentum balance suggests that the mean flow is driven by a poleward downsloping alongshore pressure gradient. At interannual time-scales, variability in the FC was correlated with the alongshore wind stress, indicating that an equatorward (poleward) wind stress drives an equatorward (poleward) velocity anomaly. At decadal time-scales, variability in the FC was correlated with the South Pacific wind stress curl, indicating that as the wind stress curl increases (decreases) the poleward flow in the FC weakens (strengthens), consistent with the response of a traditional eastern boundary current. Long-term trends show a weakening outer current and strengthening inner current. Increasing flow in the inner current may result in increased transport of subtropical water around southern New Zealand.

An Alongshore Momentum Budget Over a Fringing Tropical Fore‐Reef

AbstractExisting momentum budgets over coral reefs have predominantly focused on cross‐reef dynamics, lacking analysis of alongshore processes. To complement existing cross‐reef research and enhance our understanding of forcing variability at the semidiurnal period, this study examines the σ–coordinate, depth‐averaged alongshore momentum budget over a fore‐reef as a function of tidal phase. The observations were gathered over a 3‐week timespan, between the 12‐ and 20‐m isobaths of a Hawaiian fringing reef system, focusing on two moorings on the 12‐m isobath, where median drag coefficients estimated from log fits are CD=0.0080[−0.002,+0.004] and CD=0.0023[−0.0006,+0.0009]. Analysis at one location shows that the unsteadiness, barotropic pressure gradient, and bottom drag are equally important, and their combination is sufficient to close the momentum budget. However, bottom drag is less important at the second mooring; the difference between unsteadiness and pressure gradient suggests that advective acceleration plays a significant role.

Remote Forcing of Shelf Flows by Density Gradients and the Origin of the Annual Mean Flow on the Mid‐Atlantic Bight

AbstractIn the annual mean, a southwestward along‐isobath flow exists in the Mid‐Atlantic Bight. It has been shown to be driven neither by local winds, which are too weak and in the wrong direction to explain the flow, nor by local density gradients, which are too weak to explain the flow. Prior work has established that the mean flow is part of a local alongshore momentum balance between bottom friction and along‐isobath pressure gradient; but that work does not explain the origin of the pressure gradient. A simple model of shelf flows driven by density anomalies elsewhere on the shelf is developed. This model is used to argue that the annual average flow is consistent with remote forcing by density gradients on the shelf along the southern edge of the Laurentian Channel and the Scotian Shelf. These density gradients are largely caused by freshwater input from the Saint Lawrence River, and their effect when integrated over the Laurentian Channel, Scotian Shelf, and Gulf of Maine shows only a weak seasonal signal.

Mechanisms of the intensification of the upwelling-favorable winds during El Niño 1997–1998 in the Peruvian upwelling system

The physical processes driving the wind intensification in a coastal band of ~ 100 km off Peru during the intense 1997–1998 El Nino (EN) event were studied using a regional atmospheric model. A simulation performed for the period 1994–2000 reproduced the coastal wind response to local sea surface temperature (SST) forcing and large scale atmospheric conditions. The model, evaluated with satellite data, represented well the intensity, seasonal and interannual variability of alongshore (i.e. NW–SE) winds. An alongshore momentum budget showed that the pressure gradient was the dominant force driving the surface wind acceleration. The pressure gradient tended to accelerate the coastal wind, while turbulent vertical mixing decelerated it. A quasi-linear relation between surface wind and pressure gradient anomalies was found. Alongshore pressure gradient anomalies were caused by a greater increase in near-surface air temperature off the northern coast than off the southern coast, associated with the inhomogeneous SST warming. Vertical profiles of wind, mixing coefficient, and momentum trends showed that the surface wind intensification was not caused by the increase of turbulence in the planetary boundary layer. Moreover, the temperature inversion in the vertical mitigated the development of pressure gradient due to air convection during part of the event. Sensitivity experiments allowed to isolate the respective impacts of the local SST forcing and large scale condition on the coastal wind intensification. It was primarily driven by the local SST forcing whereas large scale variability associated with the South Pacific Anticyclone modulated its effects. Examination of other EN events using reanalysis data confirmed that intensifications of alongshore wind off Peru were associated with SST alongshore gradient anomalies, as during the 1997–1998 event.

Circulation and water properties in the landfast ice zone of the Alaskan Beaufort Sea

Moorings, hydrography, satellite-tracked drifters, and high-frequency radar data describe the annual cycle in circulation and water properties in the landfast ice zone (LIZ) of the Alaskan Beaufort Sea. Three seasons, whose duration and characteristics are controlled by landfast ice formation and ablation, define the LIZ: “winter”, “break-up”, and “open-water”. Winter begins in October with ice formation and ends in June when rivers commence discharging. Winter LIZ ice velocities are zero, under-ice currents are weak (~5cms−1), and poorly correlated with winds and local sea level. The along-shore momentum balance is between along-shore pressure gradients and bottom and ice-ocean friction. Currents at the landfast ice-edge are swift (~35cms−1), wind-driven, with large horizontal shears, and potentially unstable. Weak cross-shore velocities (~1cms−1) imply limited exchanges between the LIZ and the outer shelf in winter. The month-long break-up season (June) begins with the spring freshet and concludes when landfast ice detaches from the bottom. Cross-shore currents increase, and the LIZ hosts shallow (~2m), strongly-stratified, buoyant and sediment-laden, under-ice river plumes that overlie a sharp, ~1m thick, pycnocline across which salinity increases by ~30. The plume salt balance is between entrainment and cross-shore advection. Break-up is followed by the 3-month long open-water season when currents are swift (≥20cms−1) and predominantly wind-driven. Winter water properties are initialized by fall advection and evolve slowly due to salt rejection from ice. Fall waters and ice within the LIZ derive from local rivers, the Mackenzie and/or Chukchi shelves, and the Arctic basin.

Observations and 3D hydrodynamics-based modeling of decadal-scale shoreline change along the Outer Banks, North Carolina

Long-term decadal-scale shoreline change is an important parameter for quantifying the stability of coastal systems. The decadal-scale coastal change is controlled by processes that occur on short time scales (such as storms) and long-term processes (such as prevailing waves). The ability to predict decadal-scale shoreline change is not well established and the fundamental physical processes controlling this change are not well understood. Here we investigate the processes that create large-scale long-term shoreline change along the Outer Banks of North Carolina, an uninterrupted 60km stretch of coastline, using both observations and a numerical modeling approach. Shoreline positions for a 24-yr period were derived from aerial photographs of the Outer Banks. Analysis of the shoreline position data showed that, although variable, the shoreline eroded an average of 1.5m/yr throughout this period. The modeling approach uses a three-dimensional hydrodynamics-based numerical model coupled to a spectral wave model and simulates the full 24-yr time period on a spatial grid running on a short (second scale) time-step to compute the sediment transport patterns. The observations and the model results show similar magnitudes (O(105m3/yr)) and patterns of alongshore sediment fluxes. Both the observed and the modeled alongshore sediment transport rates have more rapid changes at the north of our section due to continuously curving coastline, and possible effects of alongshore variations in shelf bathymetry. The southern section with a relatively uniform orientation, on the other hand, has less rapid transport rate changes. Alongshore gradients of the modeled sediment fluxes are translated into shoreline change rates that have agreement in some locations but vary in others. Differences between observations and model results are potentially influenced by geologic framework processes not included in the model. Both the observations and the model results show higher rates of erosion (∼−1m/yr) averaged over the northern half of the section as compared to the southern half where the observed and modeled averaged net shoreline changes are smaller (<0.1m/yr). The model indicates accretion in some shallow embayments, whereas observations indicate erosion in these locations. Further analysis identifies that the magnitude of net alongshore sediment transport is strongly dominated by events associated with high wave energy. However, both big- and small- wave events cause shoreline change of the same order of magnitude because it is the gradients in transport, not the magnitude, that are controlling shoreline change. Results also indicate that alongshore momentum is not a simple balance between wave breaking and bottom stress, but also includes processes of horizontal vortex force, horizontal advection and pressure gradient that contribute to long-term alongshore sediment transport. As a comparison to a more simple approach, an empirical formulation for alongshore sediment transport is used. The empirical estimates capture the effect of the breaking term in the hydrodynamics-based model, however, other processes that are accounted for in the hydrodynamics-based model improve the agreement with the observed alongshore sediment transport.

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

AbstractAlong various stretches of the Antarctic margins, dense Antarctic Bottom Water (AABW) escapes its formation sites and descends the continental slope. This export necessarily raises the isopycnals associated with lighter density classes over the continental slope, resulting in density surfaces that connect the near-freezing waters of the continental shelf to the much warmer circumpolar deep water (CDW) at middepth offshore. In this article, an eddy-resolving process model is used to explore the possibility that AABW export enhances shoreward heat transport by creating a pathway for CDW to access the continental shelf without doing any work against buoyancy forces. In the absence of a net alongshore pressure gradient, the shoreward CDW transport is effected entirely by mesoscale and submesoscale eddy transfer. Eddies are generated partly by instabilities at the pycnocline, sourcing their energy from the alongshore wind stress, but primarily by instabilities at the CDW–AABW interface, sourcing their energy from buoyancy loss on the continental shelf. This combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth CDW layer, sustaining a local maximum in the eddy kinetic energy over the slope and balancing the Coriolis force associated with the shoreward CDW transport. The resulting slope turbulence self-organizes into a series of alternating along-slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets. Cross-shore variations in the potential vorticity gradient cause the jets to drift continuously offshore, suggesting that fronts observed in regions of AABW down-slope flow may in fact be transient features.

Wind relaxation and a coastal buoyant plume north of Pt. Conception, CA: Observations, simulations, and scalings

AbstractIn upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer and inner‐shelf mooring locations throughout the study. A poleward‐propagating plume of Southern California Bight water that increases shelf water temperatures by 5°C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface‐trapped and slope‐controlled dynamics. Plume momentum balances are distinct between the offshore (&gt;30 m depth) region where the plume is surface‐trapped, and onshore of the 30 m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5°C high, potentially affecting plume propagation distance and persistence.

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.

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.

Tilt of mean sea level along the Pacific coasts of North America and Japan

AbstractThe tilt of coastal mean sea level with respect to an equipotential surface is estimated using two fundamentally different approaches. The geodetic approach is based on tide gauge and GPS observations, and a model of the geoid. The ocean approach uses a high‐resolution, dynamically based ocean model to estimate mean dynamic topography. Along the Pacific coast of North America the two approaches give similar large‐scale profiles with a minimum at about 40°N and a maximum in the northern part of the Gulf of Alaska. Along the Pacific coast of Japan the geodetically determined coastal sea levels indicate an eastward drop of about 20 cm along the south coast and a further northward drop across Tsugaru Strait. Both of these features are reproduced by the ocean models. An analysis of the alongshore momentum balance suggests that alongshore wind stress acting over the inner shelf is the primary driver of the mean sea level profile along the coast of North America. Several large‐scale features are explained using arrested topographic wave theory. A similar momentum analysis, and an additional study of time variability of sea level and circulation, suggests that the Kuroshio is the main driver of the mean sea level tilt along the south coast of Japan. Discrepancies in the alongshore tilt of sea level estimated by the geodetic and ocean approaches along both coasts are discussed in terms of errors in the ocean and geoid models.

Modeled alongshore circulation and force balances onshore of a submarine canyon

AbstractAlongshore force balances, including the role of nonlinear advection, in the shoaling and surf zones onshore of a submarine canyon are investigated using a numerical modeling system (Delft3D/SWAN). The model is calibrated with waves and alongshore flows recorded over a period of 1.5 months at 26 sites along the 1.0, 2.5, and 5.0 m depth contours spanning about 2 km of coast. Field observation‐based estimates of the alongshore pressure and radiation‐stress gradients are reproduced well by the model. Model simulations suggest that the alongshore momentum balance is between the sum of the pressure and radiation‐stress gradients and the sum of the nonlinear advective terms and bottom stress, with the remaining terms (e.g., wind stress and turbulent mixing) being negligible. The simulations also indicate that unexplained residuals in previous field‐based estimates of the momentum balance may be owing to the neglect of the nonlinear advective terms, which are similar in magnitude to the sum of the forcing (pressure and radiations stress gradients) and to the bottom stress.

On the temporal memory of coastal upwelling off NW Africa

AbstractWe use a combination of satellite, in situ, and numerical data to provide a comprehensive view of the seasonal coastal upwelling cycle off NW Africa in terms of both wind forcing and sea surface temperature (SST) response. Wind forcing is expressed in terms of both instantaneous (local) and time‐integrated (nonlocal) indices, and the ocean response is expressed as the SST difference between coastal and offshore waters. The classical local index, the cross‐shore Ekman transport, reproduces reasonably well the time‐latitude distribution of SST differences but with significant time lags at latitudes higher than Cape Blanc. Two nonlocal indices are examined. One of them, a cumulative index calculated as the backward averaged Ekman transport that provides the highest correlation with SST differences, reproduces well the timing of the SST differences at all latitudes (except near Cape Blanc). The corresponding time lags are close to zero south of Cape Blanc and range between 2 and 4 months at latitudes between Cape Blanc and the southern Gulf of Cadiz. The results are interpreted based on calculations of spatial and temporal auto and cross correlations for wind forcing and SST differences. At temporal scales of 2–3 weeks, the alongshore advection of alongshore momentum compensates for interfacial friction, allowing the upwelling jet and associated frontal system to remain active. We conclude that the coastal jet plays a key role in maintaining the structure of coastal upwelling, even at times of relaxed winds, by introducing a seasonal memory to the system in accordance with the atmospheric‐forcing annual cycle.

What dynamics drive future wind scenarios for coastal upwelling off Peru and Chile?

The dynamics of the Peru–Chile upwelling system (PCUS) are primarily driven by alongshore wind stress and curl, like in other eastern boundary upwelling systems. Previous studies have suggested that upwelling-favorable winds would increase under climate change, due to an enhancement of the thermally-driven cross-shore pressure gradient. Using an atmospheric model on a stretched grid with increased horizontal resolution in the PCUS, a dynamical downscaling of climate scenarios from a global coupled general circulation model (CGCM) is performed to investigate the processes leading to sea-surface wind changes. Downscaled winds associated with present climate show reasonably good agreement with climatological observations. Downscaled winds under climate change show a strengthening off central Chile south of 35°S (at 30°S–35°S) in austral summer (winter) and a weakening elsewhere. An alongshore momentum balance shows that the wind slowdown (strengthening) off Peru and northern Chile (off central Chile) is associated with a decrease (an increase) in the alongshore pressure gradient. Whereas the strengthening off Chile is likely due to the poleward displacement and intensification of the South Pacific Anticyclone, the slowdown off Peru may be associated with increased precipitation over the tropics and associated convective anomalies, as suggested by a vorticity budget analysis. On the other hand, an increase in the land–sea temperature difference is not found to drive similar changes in the cross-shore pressure gradient. Results from another atmospheric model with distinct CGCM forcing and climate scenarios suggest that projected wind changes off Peru are sensitive to concurrent changes in sea surface temperature and rainfall.