Wind-forced Upwelling Research Articles

On the nature of wind-forced upwelling and downwelling in Mackenzie Canyon, Beaufort Sea

Data from a four-mooring array deployed across the shelf and slope of Mackenzie Canyon, from 2016 to 2018, are used to characterize wind-driven upwelling and downwelling events and investigate their impact on the transport of water masses. We distinguish between coastal upwelling/downwelling forced by the along-coast wind stress, and canyon upwelling/downwelling associated with the wind stress curl in the Canada Basin. During coastal upwelling (downwelling), the isopycnals slope upward (downward) above the shelf bathymetry, and the normally westward-flowing shelf current is strengthened (reversed). During canyon upwelling (downwelling), the isopycnals slope upward (downward) progressing onshore relative to the background condition, and the cyclonic recirculation in the canyon is stronger (weaker), with an up-canyon (down-canyon) component on the upper slope. During periods of simultaneous coastal and canyon upwelling, Atlantic Water from the basin can be fluxed onto the shelf. These occurrences are dictated by the combination of three factors: the coastal upwelling strength, canyon upwelling strength, and the background depth of the offshore Pacific Water-Atlantic Water interface. Dense winter water is formed on the shelf by enhanced air-sea buoyancy flux during freeze-up and subsequent openings in the ice. During simultaneous coastal and canyon downwelling this water be fluxed offshore into the canyon, with the potential to ventilate the interior halocline.

On the nature of wind-forced upwelling in Barrow Canyon

Using time series from a mooring deployed from 2002 to 2004 near the head of Barrow Canyon, together with atmospheric and sea-ice data, we investigate the seasonal signals in the canyon as well as aspects of upwelling and the wind-forcing that drives it. On average, the flow was down-canyon during each month of the year except February, when the up-canyon winds were strongest. Most of the deep flow through the head of the canyon consisted of cold and dense Pacific-origin winter water, although Pacific-origin summer waters were present in early autumn. Over the two-year study period there were 54 upwelling events: 33 advected denser water to the head of the canyon, while 21 upwelled lighter water due to the homogeneous temperature/salinity conditions during the cold season. The upwelling occurs when the Beaufort High is strong and the Aleutian Low is deep, consistent with findings from previous studies. Most events resulted in the reintroduction of Pacific Winter Water onto the Chukchi Shelf, rather than advection of Atlantic Water. Overall there were more than twice as many upwelling events during the cold season as the warm season due to the seasonal strengthening of the winds. An analysis of temporally integrated measures revealed a statistically significant relationship between the atmospheric forcing and upwelling response, such that stronger storms tend to result in stronger upwelling. Finally, it is demonstrated that upwelling typically corresponds to the occurrence of the Northeast Chukchi Polynya.

Inferring cetacean population densities from the absolute dynamic topography of the ocean in a hierarchical Bayesian framework.

We inferred the population densities of blue whales (Balaenoptera musculus) and short-beaked common dolphins (Delphinus delphis) in the Northeast Pacific Ocean as functions of the water-column’s physical structure by implementing hierarchical models in a Bayesian framework. This approach allowed us to propagate the uncertainty of the field observations into the inference of species-habitat relationships and to generate spatially explicit population density predictions with reduced effects of sampling heterogeneity. Our hypothesis was that the large-scale spatial distributions of these two cetacean species respond primarily to ecological processes resulting from shoaling and outcropping of the pycnocline in regions of wind-forced upwelling and eddy-like circulation. Physically, these processes affect the thermodynamic balance of the water column, decreasing its volume and thus the height of the absolute dynamic topography (ADT). Biologically, they lead to elevated primary productivity and persistent aggregation of low-trophic-level prey. Unlike other remotely sensed variables, ADT provides information about the structure of the entire water column and it is also routinely measured at high spatial-temporal resolution by satellite altimeters with uniform global coverage. Our models provide spatially explicit population density predictions for both species, even in areas where the pycnocline shoals but does not outcrop (e.g. the Costa Rica Dome and the North Equatorial Countercurrent thermocline ridge). Interannual variations in distribution during El Niño anomalies suggest that the population density of both species decreases dramatically in the Equatorial Cold Tongue and the Costa Rica Dome, and that their distributions retract to particular areas that remain productive, such as the more oceanic waters in the central California Current System, the northern Gulf of California, the North Equatorial Countercurrent thermocline ridge, and the more southern portion of the Humboldt Current System. We posit that such reductions in available foraging habitats during climatic disturbances could incur high energetic costs on these populations, ultimately affecting individual fitness and survival.

Coastal Upwelling Supplies Oxygen-Depleted Water to the Columbia River Estuary

Low dissolved oxygen (DO) is a common feature of many estuarine and shallow-water environments, and is often attributed to anthropogenic nutrient enrichment from terrestrial-fluvial pathways. However, recent events in the U.S. Pacific Northwest have highlighted that wind-forced upwelling can cause naturally occurring low DO water to move onto the continental shelf, leading to mortalities of benthic fish and invertebrates. Coastal estuaries in the Pacific Northwest are strongly linked to ocean forcings, and here we report observations on the spatial and temporal patterns of oxygen concentration in the Columbia River estuary. Hydrographic measurements were made from transect (spatial survey) or anchor station (temporal survey) deployments over a variety of wind stresses and tidal states during the upwelling seasons of 2006 through 2008. During this period, biologically stressful levels of dissolved oxygen were observed to enter the Columbia River estuary from oceanic sources, with minimum values close to the hypoxic threshold of 2.0 mg L−1. Riverine water was consistently normoxic. Upwelling wind stress controlled the timing and magnitude of low DO events, while tidal-modulated estuarine circulation patterns influenced the spatial extent and duration of exposure to low DO water. Strong upwelling during neap tides produced the largest impact on the estuary. The observed oxygen concentrations likely had deleterious behavioral and physiological consequences for migrating juvenile salmon and benthic crabs. Based on a wind-forced supply mechanism, low DO events are probably common to the Columbia River and other regional estuaries and if conditions on the shelf deteriorate further, as observations and models predict, Pacific Northwest estuarine habitats could experience a decrease in environmental quality.

Influence of ocean winds on the pelagic ecosystem in upwelling regions

Upwelling of nutrient-rich, subsurface water sustains high productivity in the ocean's eastern boundary currents. These ecosystems support a rate of fish harvest nearly 100 times the global mean and account for >20% of the world's marine fish catch. Environmental variability is thought to be the major cause of the decadal-scale biomass fluctuations characteristic of fish populations in these regions, but the mechanisms relating atmospheric physics to fish production remain unexplained. Two atmospheric conditions induce different types of upwelling in these ecosystems: coastal, alongshore wind stress, resulting in rapid upwelling (with high vertical velocity, w); and wind-stress curl, resulting in slower upwelling (low w). We show that the level of wind-stress curl has increased and that production of Pacific sardine (Sardinops sagax) varies with wind-stress curl over the past six decades. The extent of isopycnal shoaling, nutricline depth, and chlorophyll concentration in the upper ocean also correlate positively with wind-stress curl. The size structure of plankton assemblages is related to the rate of wind-forced upwelling, and sardine feed efficiently on small plankters generated by slow upwelling. Upwelling rate is a fundamental determinant of the biological structure and production in coastal pelagic ecosystems, and future changes in the magnitude and spatial gradient of wind stress may have important and differing effects on these ecosystems. Understanding of the biological mechanisms relating fisheries production to environmental variability is essential for wise management of marine resources under a changing climate.

Correcting for Beam Spread in Acoustic Doppler Current Profiler Measurements

Abstract Spatial homogeneity assumptions inherent in the conversion of directly measured acoustic Doppler current profiler (ADCP) beam to Cartesian coordinates for the Janus configuration are investigated. These assumptions may be adequate for large-scale flows, such as tidal currents and wind-forced upwelling. However, for high-frequency features, such as internal solitons and turbulence, the velocity fields may vary over scales comparable to the divergence of the acoustic beams. Equations are derived for beam spreading, and it is shown that a first-order correction can be applied to improve velocity measurement accuracy. Two cases are examined. First, the effects of the spatial and temporal convolution inherent in beam spreading from the Janus configuration ADCP are applied to a model internal solitary wave. It is shown that the corrected vertical velocities have deviations of less than 2 mm s−1 for distances up to 30 m from the transducer face and are approximately 3 times more accurate than the uncorr...

Circulation over the northeastern New Zealand continental slope, shelf and adjacent Hauraki Gulf, during spring and summer

The physical oceanography of the northeastern New Zealand slope, shelf and adjacent Hauraki Gulf is investigated, to determine the forcing of the along- and cross-shelf non-tidal residual circulation. Cross-correlation and empirical orthogonal function analyses of current meter data clearly separated wind-forced upwelling and downwelling Ekman dynamics in the shelf circulation, from dynamics over the outer shelf and slope, which were dominated by longer-period variability of the East Auckland Current (EAUC). Moored thermistor and satellite temperature data mapped three inner-to-mid-shelf upwellings in early spring to early summer. Between early and late summer, however, upwelling ceased and there was a marked transition toward downwelling and strongly stratified conditions on the shelf and in Hauraki Gulf. The offshore extent and temporal response of the upwellings to winds were in approximate agreement with theory. Moored thermistor and CTD data obtained during four research voyages showed the internal hydrological structures of the shelf and slope watermasses from early spring to late summer. While the forcing for shelf circulation arose largely from local winds, far-field effects associated with the EAUC caused shallow (<60 m depth) subtropical surface water intrusions across the shelf/slope boundary in late summer and near-bed onshore flows at the shelf edge, possibly through bottom Ekman-layer effects. These results describe the physical dynamics underpinning the chemical and biological ecosystem responses to seasonally varying upwelling and downwelling, and surface water intrusion, described in an accompanying paper (Continental Shelf Res. (2004), this volume).

A Modeling Study of Eulerian and Lagrangian Aspects of Shelf Circulation off Duck, North Carolina

The effects of wind-forced upwelling and downwelling on the continental shelf off Duck, North Carolina, are studied through experiments with a two-dimensional numerical primitive equation model. Moored and shipboard measurements obtained during August‐November 1994 as part of the Coastal Ocean Processes (CoOP) Inner Shelf Study (ISS) are used for model‐data comparisons. The model is initialized with realistic stratification and forced with observed wind and heat flux data. Both strongly stratified and weakly stratified conditions, found during August and October, respectively, are studied. August is characterized by fluctuating alongshelf wind direction, and October is dominated by downwelling-favorable winds. The across-shelf momentum balance is primarily geostrophic on the continental shelf. The alongshelf momentum balance is mainly between the Coriolis force and vertical diffusion with additional contributions from the local acceleration and nonlinear advection terms. The model solutions are utilized to acquire detailed information on the time- and space-dependent variability of the across-shelf circulation and transport and to investigate the dependence of this circulation on the seasonal change in stratification. When the stratification breaks down, as in October, the across-shelf transport is reduced significantly in comparison with the theoretical Ekman transport for large wind stress values. The paths of individual model water parcels are traced using two methods: calculation of Lagrangian trajectories and time evolution of three Lagrangian label fields. The August period produces complex Lagrangian dynamics because of the switching between upwelling and downwelling winds. The October period illustrates a mean downwelling response that advects parcels across and along the shelf and vertically.

Wind-Forced Upwelling: The Role of the Surface Mixed Layer

Abstract A numerical study is made of the dynamics of the circulation that arises from forcing by a steady, uniform alongshore wind over a uniform zonal shelf in the Southern Hemisphere. The results show that most of the upwelling is confined to the region inshore of the shelf break. Over the slope and within the region of wind forcing the circulation is found to be dominated by the growth of both an anticyclonic and a cyclonic eddy, an offshore flow, and downwelling. Two new mechanisms are identified as being responsible for these features of the upwelling circulation. First, the offshore advection of alongshore momentum (ρ0uυx) within the surface mixed layer (SML) is shown to accelerate the alongshore current near the eastern end of the region of wind forcing, leading to a sharpening of the cross-shelf gradient of sea level and an excess in the alongshore transport. This excess transport acts to raise sea level, leads to downwelling below the base of the SML, and causes the growth of the anticyclonic ed...

Kinematics of Inner Shelf Motions during the Summer Stratified Season off New Jersey*

Subinertial currents on a wide (∼100 km), shallow (∼20 m), but nevertheless vertically stratified shelf off the Atlantic seaboard of the United States are investigated at spatial scales of about 20 km in the alongshore and 10 km in the across-shore direction. During the summer of 1996 the inner shelf off New Jersey was stratified due to both temperature and salinity that varied vertically by more than 12°C and 4 psu, respectively. Upwelling favorable winds and an intermittent buoyancy-driven Hudson coastal current impact this stratification inshore of the 15-m isobath. Waters offshore were always stratified except during the passage of Tropical Storm Bertha. Mean currents are weak because wind-forced upwelling and buoyancy-forced downwelling events occurred about evenly during the observational study period. At monthly to daily timescales currents always veered counterclockwise with depth in a bottom Ekman-layer sense by more than 30° inshore and 50° offshore. Complex empirical orthogonal function (CEOF) analyses revealed that these veering angles are contained in the first mode that explains 80%–85% of the total variance at individual locations. It also explains 72% of the variance of 44 current time series of an across-shore section. The veering constitutes a robust feature that cannot be rationalized by Ekman dynamics in shallow water alone. The authors hypothesize that the veering represents a frictional response common to both upwelling and downwelling events. The CEOF analysis does not separate wind from buoyancy forcing. The two forcing mechanisms thus appear to be dynamically coupled. Nevertheless, the first two CEOFs suggest distinctly different circulation regimes: For positive and negative temporal amplitudes mode 1 represents a wind-forced upwelling and a buoyancy-forced downwelling circulation while mode 2 represents the lateral shear of the flow field. Synoptic maps of surface currents from ocean surface current radar reveal realizations of these event types.

The Summertime Low-Level Jet and Marine Boundary Layer Structure along the California Coast

This paper examines the strong, summertime northerly low-level jet (LLJ) that frequently exists along the California coast. The persistent synoptic-scale pressure distribution (North Pacific high to the west, thermal low to the east) and baroclinity created by the juxtaposition of the heated continent and the cool marine layer produce the mean structure of this LLJ. Strong diurnal thermal forcing, coupled with topographic influences on the flow, modulate the jet structure, position, and intensity. A mesoscale model is used to examine many of the complex facets of the LLJ flow dynamics. Several sensitivity studies, in addition to a control experiment, aid in this investigation. Principal findings of this study include the following. The pronounced east–west slope of the marine planetary boundary layer (MPBL) is not due primarily to colder SST values along the coast. Dynamically forced low-level coastal divergence, coupled with synoptic-scale divergence, appears to be dominant in determining MPBL inversion slope and profoundly impacts the coastal stratus distribution. Maximum baroclinity occurs in midafternoon, whereas the LLJ maximum occurs in the evening. An analytical treatment of the dynamics shows that diurnal variation of the jet-level baroclinity, coupled with inertial and friction effects, explain this jet timing. In a no-terrain simulation, the jet is broader, somewhat weaker, and tilts more to the west than in our control case. A deeper boundary layer occurs over the location of the Central Valley of California in the no-terrain simulation than in the more realistic control run. Consequently, a delay in time of maximum baroclinity aloft occurs in the no-terrain case, and the LLJ maximum occurs later as compared to the control. The core of the jet, which resides in the inversion capping the MPBL, lowers and moves toward the coast during the day and lifts and moves farther away from the coast at night. Meso-β-scale structure of the LLJ along the coast is forced by the topography of points and capes. Thee mesoscale model simulation has supercritical flow, showing expansion fan characteristics, in the MPBL around Cape Mendocino. Model results are consistent with mountain wave theory in that a near-surface wind speed maximum and pressure minimum are modeled on the lee side of Cape Mendocino. The LLJ maxima in the lee of points and capes produce local maxima in surface stress. The position of these wind stress maxima correlate well with the location of cold pools observed in the SST, implying that locally enhanced, wind-forced upwelling plays a major role in the creation of such cold SST patches.

The Role of Indonesian Throughflow in a Global Ocean GCM

The effect of the Indonesian Throughflow on the World Ocean circulation is examined by a series of experiments with a global ocean GCM. The principal objective is to gain an understanding of how ocean flows respond to the throughflow, and how these changes result in changes in the pattern of surface heat flux and sea surface temperature. Four model runs are conducted. The first run features an open Indonesian passage through which a nonzero net throughflow is permitted. The second run features a complete blockage of the Indonesian passage. The third run is designed to isolate the effects of purely buoyancy-driven throughfiow: the Indonesian passage is open but the net volume transport is required to be zero. The fourth run is designed to isolate the effects of nonzero net throughflow on the Indian Ocean, independent of interocean buoyancy differences: the Indonesian passage is open but the throughflow water is cooled and salted toward profiles characteristic of the east Indian Ocean in the absence of throughflow. Comparison of the first and second runs shows that the throughflow generally warms the Indian Ocean and cools the Pacific. However, large changes in the surface temperature and heat flux are restricted to certain well-defined regions: the Agulhas Current/outflow, the Leeuwin Current region off western Australia, the Tasman Sea, the equatorial Pacific, and two bands in the midlatitude South Pacific. In contrast, large subsurface temperature changes are widespread across both oceans. Heat budget analysis indicates that the large surface responses are dependent on the subsurface temperature change being brought to the surface, either by strong wind-forced upwelling (as in the equatorial Pacific) or by vigorous mixing in convective mixed layers (as in the other regions). Over most of both oceans, such mechanisms are absent and surface heat-flux changes are small (a few W m−2). There, subsurface temperature perturbations are largely insulated from the surface and may extend via direct advection or baroclinic wave propagation. The additional beat is released upon encounter with upwelling or a convective mixed layer, which may be far removed from the source of the perturbation. Atlantic and far Southern Ocean effects are mostly very small, possibly because of our use of restoring upper boundary conditions. The third and fourth runs break the throughflow into its baroclinic and barotropic components. The baroclinic (buoyancy-driven) component affects surface beat flux strongly in the Leeuwin Current region but relatively weakly in the Agulhas Current and Tasman Sea. The barotropic component has the converse effect. Interocean heat exchange is discussed; the full throughflow transports a net 0.63 petawatts out of the Pacific Ocean, which represents about one-third of the total heat input into the model's equatorial Pacific.