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= California Current draft = - add source for six major upwelling currents -- add Portugal Current -- Talley -- or say 4 major EBUs -- Garcia-Reyes et al.

- The California Current region is economically important to coastal communities through fisheries as well as shipping ports and ecotourism. (cite Fisheries Economics, sydeman) Climate change may impact the California Current in many ways, including warming temperatures, intensified upwelling, seasonal hypoxia, acidification, increased harmful algal blooms, and disruptions to fisheries, marine mammals, and seabirds. (cite asch, bakun 2015)

Physical Properties
First paragraph:

The California Current is 50-100 kilometers wide and flows at a maximum surface velocity of 40 to 80 centimeters per second, extending to a depth of about 300 meters. It is located 200-300 kilometers offshore on average, but moves seasonally. The current is made up of narrow areas of fast flow called jets within a broad area of slow flow, with two prominent zones of faster flow, one near the coast and one offshore (checkley and barth, huyer 1998, hickey 1979). The characteristics of the California Current vary along the coast, and it can be separated into a northern region north of Cape Mendocino, a southern region south of Point Conception, and a central region between the two.

last paragraph:

Finally, the coastal jet is a strongly seasonal component of the California Current that occurs 5 to 25 kilometers from shore. The jet is closely linked to winds and flows southward in the spring and summer, with the maximum speed moving progressively offshore. Especially in winter, flow near the coast can vary greatly, even reversing, on a timescale of days as winds switch from north to southward. In the winter, the coastal jet is replaced by northward surface flow?

The current has two components, both of which are ultimately driven by atmospheric pressure and winds. The first component is the southward flow of the North Pacific subtropical gyre, a large clockwise flow in the North Pacific. The California Current is fed by the North Pacific Current on the north side of the gyre. This part of the flow is driven by large-scale winds in the Pacific, which cause water to build up in the center of the gyre. This pressure distribution causes a surface flow to the south on the eastern side of the gyre through the geostrophic balance, which describes how pressure and the rotating earth combine to drive ocean currents. The buildup of water in the center of the gyre also drives downward motion of water called Ekman pumping. The squeezing of water columns by this downward motion causes southward Sverdrup transport throughout the gyre as the water columns move southward to maintain their angular momentum. The second component is the coastal upwelling system, driven by alongshore winds. These winds occur because the prevailing winds in the midlatitudes, the westerlies, are deflected to the south by the North American continent. In the Northern Hemisphere, water is transported to the right of the wind direction due to the rotation of the earth in a process called Ekman transport. When winds blow to the south along the coast, this moves water offshore. The movement of the water creates a pressure gradient with lower pressure along the coast, driving geostrophic surface flow to the south. The movement of water offshore also pulls water up from below, causing upwelling.

The characteristics of the current vary along the coast, and it can be separated into a northern region north of Cape Mendocino, a southern region south of Point Conception, and a central region between the two.

Related currents
The California Current is part of a system of currents that occur along the West coast of North America, including the California Current, the California Undercurrent, the Davidson Current (sometimes called the Inshore Countercurrent), the Southern California Countercurrent (also called the Southern California Eddy), and the coastal jet. The California Undercurrent flows northward underneath and inshore of the California Current, carrying tropical Pacific water with a warm, salty, low oxygen signature. The undercurrent flows at speeds of 10 cm/s or faster and is about 20 km wide. Its core is located at 250 m depth and it extends to 1000 m depth. The undercurrent occurs because the water along the coast has layers of density that change the pressure gradient at depth, leading to a reversal in the flow direction through a process called the thermal wind. Inshore of the California Current is the Davidson Current, also called the Inshore Countercurrent, which flows to the north in the fall and winter north of Point Conception. This current is the surface expression of the undercurrent, which gets closer to the surface during the winter and deepens in summer. The Southern California Eddy originates at an eastward bend in the California Current near 32°N, where a northward flow splits off from the main current and moves into the Southern California Bight. This flow is seasonally variable, but there is northward flow near the shore most of the year in the northern part of the bight, and this flow can connect with the Davidson Current. Finally, the coastal jet is a strongly seasonal current occurring 5 to 25 km from shore. The direction of the jet is closely linked to winds, and it tends to be strongest in spring. It flows northward in winter, close to shore, and flows southward in the spring and summer, with the maximum speed moving progressively offshore. Especially in winter, the flow can vary greatly, even reversing, on a timescale of days as winds switch from north to southward.

Upwelling
Alongshore winds drive upwelling in the California Current system. Upwelled water comes from 150-200 m depth and is defined by its high nutrient content and low temperature, producing a region of cool surface water 80-300 km from shore and fueling high productivity in the coastal zone. The offshore motion of water is not uniform, but rather a complicated, swirling pattern of eddies and jets. These jets take the form of squirts, which end offshore, and meanders, which circle back to the coast. The shape of the coastline influences where offshore-moving filaments tend to occur.

Upwelling in the current is driven by two processes, both linked to winds. The first process is the Ekman transport of water offshore, driven by southward flow along the coast. Because the coast is a solid boundary, this transport requires that water move up from below to replace the water moved offshore. Similarly, if winds blow northward along the coast, water is transported towards the shore, causing downwelling. The second process leading to upwelling is the curl of the wind stress, which describes the degree of rotation of the winds. When the speed of the southward wind increases moving offshore, the curl of the wind is positive and the Ekman transport increases moving away from the coast, which causes Ekman pumping and enhances upwelling. The speed of upwelling driven by wind stress curl is slower than that of coastal upwelling, but the process increases the width of the upwelling area along the coast.

The extent and strength of upwelling varies strongly with seasons, especially in the northern part of the current, as the atmospheric pressure patterns that drive winds move around. In the winter, north of Cape Mendocino, winds shift from southward to northward and there is downwelling instead of upwelling. South of Cape Mendocino, winds support upwelling during the entire year. However, winds are weaker in the Southern California Bight. The upwelling system is more sensitive to local winds in the northern part of the current compared to the southern part. The shape of the coastline and the ocean bottom, including features like capes and canyons, also affects upwelling by causing jets of water to move offshore.

Variability
The California Current varies on several different timescales, from seasonal to interannual to decadal. Changes in the current are tied to changes in atmospheric pressure and winds which drive responses in the ocean.

The major driver of seasonal variability is changes in winds and wind stress curl. In winter, the current is located farther offshore, with minimal upwelling and northward flow along the coast associated with the Davidson Current. In summer, an upwelling front develops at the coast, where there is a sharp change in water properties at the junction between upwelled water at the coast and surface water offshore. This front moves offshore as the season progresses. Upwelling indices are used to assess the strength of upwelling, and are calculated based on winds or associated Ekman transport. Another metric to understand the strength of upwelling in a given year is the cumulative wind stress for the full upwelling season (Barth et al. 2007, Schwing et al. 2006). The strongest upwelling occurs from April to July, around 34°N, offshore of Point Conception. The onset of upwelling in the northern and central parts of the current, called the "spring transition", and the change to downwelling, called the "fall transition", vary from year to year with winds. When upwelling starts later than usual, the water is warmer than usual and impacts are felt throughout the California Current ecosystem.

- spring transition -- Huyer and Smith 1978; Huyer et al 1979; ;Lentz 1987

On interannual time scales, the El-Niño Southern Oscillation (ENSO) also impacts the California Current. During ENSO events, coastal Kelvin waves propagate northward along the coast from the equator, changing the vertical location of sharp changes in properties like water temperature (the thermocline) and nutrients. This, in turn, changes the properties and composition of upwelled water, and when nutrient content is lower than usual, productivity is also lower than usual. During an El Niño event, water tends to be warmer and saltier and sea level higher than usual, with stronger flow to the north along the coast in the fall and winter. La Niña produces the opposite effect. ENSO can also change the California Current by affecting atmospheric pressure in the North Atlantic and by causing more "tropical" water to be carried northward along the coast. El Niño events vary in their impact on the current, depending on when they arrive and pre-existing water conditions.

Moving north along the coast, more of the variability in the current can be explained by decadal changes. However, the drivers and characteristics of these changes are not fully understood. The Pacific Decadal Oscillation and the North Pacific Gyre Oscillation are two patterns of variability in the Pacific that account for some of the observed changes in the California Current system.

PDO, NPGO -- Di Lorenzo et al. 2008, Steele, 2004; Mantua et al. 1997 (PDO)

Other periodic events such as marine heatwaves can impact the California Current.

Climate change may lead to a delayed start to the upwelling season, as well as stronger upwelling later in the season.