page_title: Haida Eddies
section_title: General circulation
hierarchical_section_title: Haida Eddies / Formation / General circulation
caption_attribution_description: The North Pacific Gyre, a clockwise-swirling vortex of ocean currents comprising most of the northern Pacific Ocean
context_page_description: 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 to nutrient depleted areas of lower productivity.
context_section_description: 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.
caption: North Pacific Current splitting into southward California Current and northward Alaska Current (bifurcation in image occurring around 45°N). Haida eddies occur within the subpolar Alaska Gyre to the north of the Pacific Current. Arrows indicate the direction of the current.
exif: {}