Elsevier

Continental Shelf Research

Volume 89, 15 October 2014, Pages 24-37
Continental Shelf Research

Research papers
The response of large outflows to wind forcing

https://doi.org/10.1016/j.csr.2013.11.006Get rights and content

Highlights

  • A numerical model is used to study the role of winds in the evolution of buoyant coastal plumes.

  • A conceptual model for the upwelling of plumes over a slopping bottom is developed.

  • Two parameters are introduced to assess the plume across- and alongshelf behavior.

  • Plume responses to upwelling-favorable winds will depend on their discharge magnitude.

  • Plumes with similar wind forcing but distinct outflows can exhibit “synoptic” or “seasonal” behaviors.

Abstract

A numerical model is used to gauge the impact of winds on the evolution of coastal plumes generated by a variety of inlet outflows. The analysis is summarized by a conceptual model that accounts for the formation of surface and bottom mixed layers and tilting of the plume front. It also provides the basis for a two parameter classification of upwelling. The first parameter indicates when a wind event is capable of fully exporting plume waters offshore. The second determines when winds can overcome the plume buoyancy-driven flow. These indices help to explain why larger outflows tend to be less susceptible to upwelling. During an upwelling event, large plumes tend to maintain their structure, while smaller systems are commonly detached and dispersed offshore. The onset of downwelling events often reorganizes large plumes, thus promoting their net downshelf displacement. In contrast smaller systems frequently restart their formation, consequently limiting their downshelf penetration. The addition of long-term fluctuations, superimposed to the synoptic wind forcing, suggests a mechanism for typical seasonal to interannual variability commonly observed for large discharges.

Introduction

Coastal outflow plumes are important agents for the transport of organisms, sediments and chemical substances in the coastal ocean (Garvine, 1995, Hill, 1998, Henrichs et al., 2000). Formed by the freshwater supply from estuaries, they typically propagate along the coast as buoyancy-driven flows, but also respond to tides (Garvine, 1999), winds (Lentz and Largier, 2006) and ambient currents (García Berdeal et al., 2001, Fong and Geyer, 2002, Avicola, 2003). Of primary concern here is the impact of alongshelf winds on the development and dispersal of plumes. Observations indicate that plumes' responses are dominantly governed by Ekman dynamics (Fong et al., 1997, Hallock and Marmorino, 2002). The onshore transport associated with downwelling-favorable winds tends to keep plumes against the coast, usually promoting their intrusion in the form of narrow coastal jets (Rennie et al., 1999, Moffat and Lentz, 2012). Upwelling-favorable winds, on the other hand, promote offshore and upshelf transport. Plumes typically evolve as thin surface layers that mix and stretch offshore (e.g. Fig. 5.4 in Rennie, 1998 and Fig. 9 in Lentz, 2004). Eventually many plumes detach and move offshore (Fong and Geyer, 2001, Houghton et al., 2004).

The role of winds in coastal circulation is of considerable theoretical and practical interest. Chao (1987) demonstrated how highly asymmetric responses occur for upwelling- and downwelling-favorable configurations. Fong and Geyer (2001) developed a conceptual model for the cross-shelf response to upwelling and prediction of the plume leading edge thickness and velocity of propagation. Lentz (2004) explicitly included the process of entrainment to Fong and Geyer's model for the upwelling of plumes. Whitney and Garvine (2005) studied the influence of winds on the alongshelf circulation with an index that compared the strength of buoyancy- and wind-driven currents. Kourafalou et al. (1996a) and Zhang et al. (2009) described the freshwater pathways to the deep ocean.

Most of these modeling studies have scaled the problem for small to medium rivers from ~250 to 2500 m3 s−1. These are the discharges observed for systems like the Chesapeake and Delaware, which typically extend from 100 to 150 km from the bay mouth and 15 to 20 m over a steep inner-shelf (Boicourt, 1973, Wong and Münchow, 1995, Le Vine et al., 1998). Few studies have considered larger discharges, although there are important point-source mid-latitude outflows. Of the top twenty global largest discharges, for example, fourteen are located in latitudes greater than 20° (Hovius, 1998). These discharges vary from 7600 to 31 000 m3 s−1 and include rivers such as the Columbia, Mississippi and St. Lawrence in North America, the Yenisei in Russia and the Yangtze in China. The Rio de la Plata, for example, discharges ~23000m3 s−1 and forms a large plume that reaches ~60m depth, extends ~100km offshore and ~750km alongshelf (Guerrero et al., 1997, Möller et al., 2008). Fig. 1 summarizes these scales.

Despite many similarities these coastal currents have markedly different responses to wind. While upwelling-favorable winds were shown to reverse and detach the Delaware (Münchow and Garvine, 1993), observations from Plata suggest that individual upwelling events should rarely destroy the plume (Pimenta et al., 2008). Plata also exhibits large seasonal variability. During the winter, the plume extends for 1200 km from the estuary mouth, while during the summer its typical downshelf intrusion is ~400km (Möller et al., 2008) (Fig. 1). Although this migration has been attributed to winds, the specific role of synoptic and seasonal wind components in the evolution of coastal plumes remains unclear.1

Our primary goal here is to quantify the role of the discharge magnitude in the response of coastal plumes to wind forcing. The approach is to perform numerical simulations with a three-dimensional hydrodynamic model implemented to a simplified shelf and estuary bathymetry. Model details are given in Section 2. Motivated by Delaware and Plata observations we study a wide range of plumes that vary from inner-shelf to mid-shelf density fronts. These experiments are described in Section 3. Plume responses to constant upwelling-favorable winds are explored in Section 4, while the theory and conceptual model are presented in Section 5. Varying upwelling and downwelling wind simulations are explored in Section 6, where the responses to synoptic forcing are described for different outflow magnitudes. Section 7 presents a summary and concluding remarks.

Section snippets

Model description

The Princeton Ocean Model (POM, Blumberg and Mellor, 1987) is used to study the effect of inlet discharge magnitude on the wind response of coastal plumes. POM is a finite-difference hydrostatic model in terrain-following coordinates that treats non-linear time dependent flows in three-dimensions. This model is widely used in process-oriented and realistic simulations, see Mellor (2004) and Kourafalou et al. (1996b) for the governing equations, numerical schemes or physical parameterizations.

Inlet discharge base cases

Before studying the influence of winds on the development of buoyancy-driven currents, it is appropriate to review some basic characteristics of coastal plumes. It is stressed that we do not attempt to reproduce the entrainment rates or geometry of the Delaware and Plata estuaries. Rather, the inlet outflow and its density anomaly are adjusted to obtain plumes of comparable sizes to those systems.

Six discharge magnitudes, varying between 20 000 and 640 000 m3 s−1 are simulated, so that the inlet

Plume response to constant upwelling winds

The effect of upwelling-favorable winds over previously developed plumes is explored in this section. As our numerical simulations are performed with f<0, these refer to eastward (positive) wind stress. Inlet discharges are constant, and winds are linearly spun up from rest over one inertial period tf=2π/f. After spin-up, winds are constant during the rest of the simulation.

Conceptual model for plume upwelling

The fundamental dynamics of coastal plumes can be codified by simplified physics as described by Fong and Geyer (2001), Lentz and Helfrich (2002), and Whitney and Garvine (2005). Here we propose a conceptual model for plume related upwelling, but include the presence of a sloping bottom and the effect of the tilt of the density front over the buoyancy driven flow. The model plumes are initially represented by a two-dimensional narrow front positioned at the depth hp, distance L from the coast

Plume response to varying alongshelf winds

Upwelling winds will export the plume waters upshelf and offshore, possibly erasing the plume signature, while downwelling winds will transport surface waters back to shore, potentially restructuring the plume buoyancy. These competing mechanisms arise from the natural variability of winds. Yet the plume overall response to synoptic forcing also should depend on the discharge magnitude. This issue is explored here by applying winds that vary between upwelling- and downwelling-favorable

Summary and conclusions

The effect of upwelling-favorable winds on the evolution of mid-latitude buoyant plumes with bottom contact was investigated using a three dimensional hydrodynamic model with an idealized estuary and shelf bathymetry. Emphasis was given to plume response to winds as a function of the outflow magnitude.

Simulations demonstrated that upwelling winds typically export plume water offshore through the surface mixed layer, while dense waters intrude below the plume through the bottom mixed layer. The

Acknowledgments

F.M.P. acknowledges the Brazilian CAPES Foundation (BEX 2242/03-6), the University of Delaware Sea-Grant program and Propesq (UFRN, Edital 03/2011) for his support. A.D.K. acknowledges the support of the Mary. A.S. Lighthipe endowment to the University of Delaware. We would like to acknowledge Pablo Huq, Derek Fong, Andreas Münchow and Steven Lentz for constructive comments to an earlier version of this paper. The original article was also improved through the helpful remarks of two anonymous

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