Haida Eddies are episodic, clockwise rotating ocean eddies that form during the winter off the west coast of British Columbia's Haida Gwaii and Alaska's Alexander Archipelago. These eddies are notable for their large size, persistence, and frequent recurrence. Rivers flowing off the North American continent supply the continental shelf in the Hecate Strait with warmer, fresher, and nutrient-enriched water. Haida eddies are formed every winter when this rapid outflow of water through the strait wraps around Cape St. James at the southern tip of Haida Gwaii, and meets with the cooler waters of the Alaska Current. This forms a series of plumes which can merge into large eddies that are shed into the northeast Pacific Ocean by late winter, and may persist for up to two years.
Haida eddies can be more than 250 km in diameter, and transport a mass of coastal water approximately the volume of Lake Michigan over 1,000 km offshore into the lower nutrient waters of the northeast Pacific Ocean. These "warm-core rings" transport heat out to sea, supplying nutrients (particularly nitrate and iron) to nutrient depleted areas of lower productivity. Consequently, primary production in Haida eddies is up to three times higher than in ambient waters, supporting vast phytoplankton-based communities, as well as influencing zooplankton and icthyoplankton community compositions.
The Haida name is derived from the Haida people native to the region, centered on the islands of Haida Gwaii (formerly known as the Queen Charlotte Islands).
Historical observations
Due to their large size, it was not until the satellite era that scientists were able to observe the full scale and life cycles of Haida eddies. Their extent is such that an ocean liner can move through the eddy without observing its borders, so accurate records did not exist until the late 1980s.
Between 1985 and 1990, the first US research mission to study changes in sea surface height using radar altimetry (an instrument used to measure the ocean surface height using a radar pulse in reference to a geoid), was conducted by the US Navy using the Geodetic/Geophysical Satellite (GEOSAT). The primary focus was to study fronts, eddies, winds, waves, and tides; each of these processes produce a change in sea surface height of several meters. In 1986, researchers Gower and Tabata observed clockwise eddies in the Gulf of Alaska using GEOSAT - the first satellite observation of Haida eddies. In 1987, the Ocean Storms program deployed 50 drifters to examine intertidal oscillations and mixing during fall storms and observed eddies propagating westward. Also in 1987, researchers Richard Thomson, Paul LeBlond, and William Emery observed that ocean drifters deployed in the Gulf of Alaska at 100–120 meters below the surface had stopped their eastward motion and actually began to move westward counter to the predominant current. The researchers attributed the unexpected motion to eddies dragging the buoys westward from their path at approximately 1.5 cm/s.
In 1992, Haida eddies were observed by researchers Meyers and Basu as positive sea surface height anomalies using TOPEX-POSEIDON, an altimetry-based satellite platform (like GEOSAT). They specifically noted an increase in the number of Haida eddies during the 1997/1998 El Niño winter. Haida eddy altimetry observations were further supplemented by European Remote Sensing satellites, ERS1 and ERS2. In 1995 Richard Thomson, together with James Gower at the Institute of Ocean Sciences in British Columbia, discovered the first clear evidence of eddies along the entire continental margin using temperature maps from infrared observations using National Oceanic and Atmospheric Administration (NOAA) satellites. Satellite observations coupled with drifter observations have allowed scientists to resolve physical and biogeochemical structures of Haida eddies.
Formation General circulation
Ocean circulation in the region begins with the transport of waters eastward along the North Pacific Current, also known as the "West Wind Drift", which forms the northern branch of the anticyclonic (clockwise rotation of fluids in Northern Hemisphere) North Pacific subtropical gyre. The North Pacific current approaches the continental US and bifurcates into the southward flowing California Current and the northward flowing Alaska Current. The latitude of this bifurcation is dependent on changes in the midlatitude (30-60° latitude) westerly atmospheric wind patterns, which is the primary forcing on the ocean's circulation in this region. These westerly winds oscillate around 45°N and can have variable wind speeds. Changes in these winds are based on the large-scale atmospheric circulation which has seasonal (summer/winter), interannual (ENSO), and decadal (Pacific Decadal Oscillation, or PDO) variability. The northwestward Alaska Current then feeds into the westward Alaskan Coastal Current, and eventually into the Alaskan Stream; together these make up the cyclonic (counterclockwise rotating) subpolar Alaskan gyre, where Haida eddies are found.
In winter, the location of the North Pacific Current bifurcation is approximately 45°N, which is 5° south of where it bifurcates in the summer at approximately 50°N. This has implications as to what water is moved into the Alaskan subpolar gyre. In winter, when the splitting of the current is more south, fresh, warmer waters from river input from the Columbia (47°N) and Fraser (49°N) rivers are transported north. This shift in the North Pacific current location leads to winter currents transporting relatively warmer water poleward from a lower latitude than in the summer. Although the northern branch of the subtropical gyre shifts south in the winter, the subpolar gyre does not shift location, but intensifies in its circulation. This intensification brings a greater volume of water from the south into the subpolar gyre, which again is dependent on the magnitude of atmospheric circulation. For example: the Aleutian Low is a persistent low pressure system over the Gulf of Alaska that can fluctuate on decadal timescales, producing the PDO. If this system is relatively strong during winter, there will be an increase in northward transport of waters along the Alaskan current from southerly winds. Haida eddies have been documented to form predominantly in the winter when bifurcation is south, and favorable atmospheric conditions are met to intensify the subpolar gyre. With these conditions, Haida eddy formation has also been documented to occur from baroclinic instabilities from alongshore wind reversals, equatorial Kelvin waves, and bottom topography. Baroclinic instabilities form when tilting or sloping of isopycnals (horizontal lines of constant density) form. Baroclinic instabilities from alongshore wind reversals occur when a persistent wind along the coast changes direction. For example: in the Gulf of Alaska average winds travel from the south, poleward (termed southerly winds), but during a wind reversal the winds will abruptly shift to a northwesterly wind (coming from the northwest), and the coastal current that was being pushed north will now be pushed south. This change in direction causes rotation in an originally northward flowing current, which results in tilting isopyncals. Kelvin waves that form along the equator are able to travel along the west coast of North America to the Gulf of Alaska, where their presence can cause disruptions in the poleward current and form baroclinic instabilities. Bottom topography, the third formation process of Haida eddies, can occur because the Alaska current will interact with hills or rock formations below the surface, and this can cause baroclinic instabilities.
General physical attributes Haida eddies possess common physical characteristics that are dependent on the attributes of the water that is being transported, and how that influences the overall structure. Haida eddies are characterized as relatively long-lived, transient (departure from the average ocean current along the coast), medium-sized (mesoscale) ocean eddies that rotate clockwise (anti-cyclonic), and possess a warm, less-saline core, relative to the surrounding waters. These warm waters within the eddy are attributed to the baroclinic clockwise motion that results in a piling up of water near the center, and a downward displacement of surface water to depth (downwelling). This phenomenon is referred to as Ekman pumping, resulting from a conservation of mass, vertical velocity, and the Coriolis force. Downwelling of water from convergence produces what is called 'dynamic height anomalies' between the center and the surrounding waters. The anomaly is calculated by taking the difference between the surface of interest, for example the middle of a Haida eddy, and a reference point (in oceanography it is in reference to the geopotential surface, or the geoid). Haida eddies are capable of producing dynamic height anomalies between the center and the surrounding waters of 0.12-0.35 m.
Ekman pumping of surface waters, coupled with northward transport of warm waters (from location of bifurcation), dampens the temperature gradient from the surface down to 300 m, so that water temperature within the eddy is warmer below the surface than typical conditions. Stratification increases between these warmer, less-saline vortices and the surrounding waters by effectively depressing background lines of constant temperature (isotherms) and salinity (isohalines) (shown in figure). This makes them an ideal vehicle to transport coastal water properties into the Gulf of Alaska because of reduced mixing with surrounding waters.
As Haida eddies break away from the coast into the subpolar gyre, they transport water properties such as temperature, salinity and kinetic energy. A common water mass in the area is the Pacific Subarctic Upper Water (PSUW) mass with conservative (constant through time and space) properties of salinity (32.6-33.6 psu) and temperature (3-15 °C). PSUW moves into the Alaska Current from the North Pacific Current and may be mixed via Haida eddies into the subpolar gyre. Fresh (low salinity) water from rivers are mixed into Haida eddies. They are also able to exchange potential energy and momentum from the coastal mean current, a process that takes energy away from the coastal current and advects it toward the middle of the gyre. On average, the Gulf of Alaska experiences 5.5 Haida eddies per year, with a typical eddy characterized by a dynamical height of approximately 0.179 m, propagation speed of 2 km per day, average core diameter of 97 km, total volume of approximately 3,000 to 6,000 km3, and a duration of 30 weeks.
Biogeochemical and nutrient dynamics Biogeochemical dynamics in Haida eddies are typically characterized by highly productive, yet relatively nutrient depleted surface waters, that may be replenished by diffusion and mixing from nutrient abundant sub-surface core waters. This nutrient exchange is also often facilitated by seasonal fluctuations in the surface mixed layer depth (~20 m in winter, up to 100 m in summer), bringing the low-nutrient surface waters in contact with the nutrient-rich core waters as the mixed layer deepens. Upon eddy formation in winter, surface water concentrations are high in nutrients including nitrate, carbon, iron, and others that are important for biological production. However, they are quickly consumed by phytoplankton through spring and summer, until fall when the now reduced nutrient concentrations can be slowly replenished by mixing with the sub-surface core waters. The net effect of Haida eddies on macronutrients and trace metal micronutrients is that of offshore transport of materials from coastal waters to open ocean, increasing offshore primary productivity inside the eddy formation site.
Dissolved iron