Lynne Talley, 1997
Back to SIO 210 index. Reading and study questions
The following lectures concern specific circulation and water property distributions in each of the oceans. We use a combination of dynamic topographies and water property maps and vertical sections to say what the circulation is, what waters are renewed and hopefully something about the rate of renewal. A number of figures were already presented in the previous lectures, showing meridional vertical sections and vertical profiles of temperature and salinity. Also shown previously were the wind forcing fields, surface temperature and surface salinity. A large number of vertical sections can be perused using the online Vertical Section Atlas.
We begin our study of regional circulation with the North Pacific because it has the weakest thermohaline forcing of any of the ocean basins and is thus the best basin to first grasp principles of wind-driven circulation. While there is thermohaline forcing in all oceans, and undoubtedly upwelling as a result of the global thermohaline forcing, there are no deep water sources in the North Pacific and the intermediate water sources are weak. Thus the N. Pacific is actively ventilated to no more than 2000 meters depth, which coincides with the depth of the wind-driven circulation. Strong ventilation due to surface outcrops in the open North Pacific affects only the top 1000 meters, and is directly tied to the wind-driven circulation.
The wind forcing for the North Pacific consists of westerlies at latitudes north of about 30°N in the mean, and trades to the south. Within the trades occurs the Intertropical Convergence Zone (ITCZ), centered at 10°N. The dominant pattern of atmospheric circulation north of 30N is associated with the Aleutian Low. The mean winds create Ekman divergence north of about 40°N, Ekman convergence between 15°N and 40°N, and Ekman divergence between 5°N and 15°N. These patterns drive Sverdrup transport which is northward in Ekman divergence regions and southward in Ekman convergence regions. Because the return flow for this interior ocean transport must be in western boundary currents, this produces cyclonic circulation in the north (the subpolar gyre), anticyclonic circulation at mid-latitudes (subtropical gyre), and a meridionally narrow cyclonic circulation centered at 10°N. The western boundary currents associated with these three gyres are: East Kamchatka Current/Oyashio, Kuroshio, and Mindanao Current. These surface circulation patterns are clearly evident in any surface dynamic topograpy relative to greater depth - e.g. Wyrtki, Reid and Arthur.
At 1000 meters depth, the subtropical gyre differs clearly from the surface gyre - while its northern and western sides appear in the same location as at the surface, its southern boundary is considerably farther north than at the surface, and its eastern portion is weak or non-existent. This poleward shrinkage of the subtropical gyre was first documented by Reid and Arthur and is a feature of the subtropical gyre in every ocean basin.
Vertical changes in the subpolar and tropical cyclonic gyres are not as apparent as in the subtropical gyre, probably because of the very narrow meridional extent of both gyres. The subpolar gyre would extend much farther north but for the presence of Alaska; its extension into the Bering Sea is considerably complicated by the Aleutian Arc. In dynamic topographies, it appears to have little variation in shape with depth although its existence below about 2000 meters is doubtful. The tropical gyre is also very narrow due to the wind pattern which causes it, and its vertical penetration is also not clear.
Interior flow nomenclature. The eastward flow of the northern subtropical and southern subpolar gyres is referred to as the North Pacific Current. It is not a uniform eastward flow but is punctuated by zonal fronts with somewhat intensified flow which occur at remarkably unchanging latitudes despite strong seasonal and interannual changes in forcing. The two principal fronts are the Subarctic Front at around 40-42°N andthe Subtropical Front at around 30-32°N. It is likely that these are eastward extensions of the separated Oyashio and Kuroshio respectively, but it is also clear that several semi-permanent fronts arise from both of these separated currents.
The westward flow of the southern subtropical gyre and northern tropical gyre is referred to as the North Equatorial Current. The NEC appears to be more intense in the tropical circulation than in the subtropical circulation. The eastward flow on the south side of the tropical gyre is the North Equatorial Countercurrent; despite its narrowness it is very swift and carries a large transport. The equatorial currents are described in a later lecture.
How deep does the wind-driven circulation extend in the interior of the North Pacific's subtropical region? Using patterns of properties on isopycnals, it is possible to trace a subtropical gyre down to about 2000 meters, with poleward shrinkage throughout this depth. Potential vorticity maps on isopycnals show regions of homogenized potential vorticity which shrink poleward with depth and disappear around 2000-2500 meters. These homogenized regions are presumed to indicate the location of the wind-driven circulation based on Rhines and Young (1982). As noted next, the western boundary current (Kuroshio) does not have the same depth limitations.
Western boundary currents in the North Pacific. The Kuroshio is the western boundary current of the subtropical gyre. Its transport is 60-70 Sv with large seasonal variations. It arises at the western boundary in the bifurcation of the North Equatorial Current; the southward flow is the Mindanao Current and the northward flow the Kuroshio. It passes along the coast of Taiwan and west of the Ryukyu Islands. The western boundary in this region is actually the broad shelf of the East China Sea rather than a continent, and so some of the Kuroshio's transport is actually up on the shelf, although the main core of flow remains in the deep channel. The Kuroshio turns eastward and emerges through Tokara Strait. A small portion remains west of Japan, entering the Japan Sea as the Tsushima Current; surface drifter measurements suggest that the actual continuity of flow into the Japan Sea is marginal.
After passing through Tokara Strait, the Kuroshio continues eastward and passes through the Izu Ridge just south of Japan. Between Tokara Strait and the Izu Ridge, the Kuroshio exists in one of two modes - it either flows due eastward or undergoes a large southward meander. This bimodality appears to be due to the wave-guide nature of the two bounding ridges. When the Kuroshio is in the large meander state, its transport is usually reduced compared to when it follows the "straight" (progressive meandering) path. Relationship of this to weather in Japan, STMW formation?? The Kuroshio state appears to be 70% (?) straight, 30% (?) large meander. An index of its state is maintained by whom and how do you get it??
The Kuroshio separates from the land at the southeastern corner of Honshu. At this location it often undergoes a large northward meander, which often produces a warm core ring. Even though Tokara Strait is but xx meters deep
depth of Kuroshio
tight recirculation
East Kamchatka Current/Oyashio. EKC arises out of the Bering Sea through Near Strait (?). Transport southward along Kamchatka is approximately xx Sv. Part of the transport enters the Okhotsk Sea (around 5 Sv) where it is greatly transformed in properties and emerges with different T/S/O2 characteristics. The current emerges primarily at Bussol' Strait where it joins the EKC. South of this point the western boundary current is referred to as the Oyashio. It flows southward along the remaining Kuril Islands, along the coast of Hokkaido and separates at the southern end of Hokkaido. Favorite diagrams, density compensation, barotropic nature. Separates at different location from Kuroshio. Eastward extension into Subarctic Front.
The subarctic circulation appears to be composed of four nearly separate cyclonic cells: one in the Gulf of Alaska, one in the western subarctic region, and one in each of the Bering and Okhotsk Seas. Each of these has a western boundary current of sorts - in teh Gulf of Alaska it is the Alaskan Stream, which appears to be mainly a northern boundary current except that the coastline has enough slant that it acquires western boundary current identity. This strong current evaporates at the southernmost point of the Aleutians, with some flow turning northward into the Bering Sea, some turning back eastwards. The western boundary currents of the Bering Sea and western subarctic gyres have been referred to already. In the Okhotsk Sea there also occurs a western boundary current - the East Sakhalin Current.
Eastern boundary currents of the North Pacific. Bifurcation of NPC, turning into the California Current and the eastern limb of the subpolar gyre.
Water masses and ventilation of the subtropical gyre. Subduction (salinity maximum, salinity minima, tritium/salinity/potential vorticity signatures on isopycnals). LPS model. Density range of subducting waters. Creation of two water "masses".
Subtropical mode water. "Mode" means large volume on a volumetric T-S diagram (illustrate). Thick layer of 16-19C water occurs in the Kuroshio recirculation region (Masuzawa, 1969). Temperature of thickest layer decreases eastward. Occurs in region of very large surface heat loss. Thick layer must be there due to dynamics of the recirculation (geostrophic flow decreasing with depth), but homogeneity of the layer and interannual variations indicate importance of the large heat loss as well. Relation to large meander state of the Kuroshio.
Figure. Potential temperature at 155E (WOCE section P10) and Sigma theta at 155E (WOCE section P10) illustrating the North Pacific Subtropical Mode Water just south of the Kuroshio.
Water masses and ventilation of the subpolar gyre. No subduction in the subpolar gyre in the same sense as in the subtropical gyre since region of upwelling rather than downwelling. Surface densities are higher in the west than in the east, and are highest in the Okhotsk Sea, along Hokkaido and just south of Hokkaido. The highest surface densities are coincident with input of saline waters from the Japan Sea - through Soya Strait into the Okhotsk Sea and through Tsugaru Strait into the region south of Hokkaido.
Sea ice formation in the Okhotsk, ventilation of denser waters, production of NPIW.
Surface layer of the subpolar gyre - in the east is a thick warm layer proceeding around the Alaskan gyre. West of the dateline, in the Western Subarctic Gyre, is usually found a temperature minimum layer in the summer. Associated with the T min layer is very high oxygen saturation in the summertime, due to capping by surface warm water and slight warming of the subsurface T min layer. (calculate T increase needed to create the supersaturation...) In Japanese literature the temperature minimum layer is referred to as the dichothermal layer. Below the T min naturally there must be a temperature maximum layer, referred to as the mesothermal layer in Japanese literature. The maximum temperature indicates that this water must have a substantial component which comes from either the east or the south since otherwise it would have acquired the low temperature of the surface layer. In the Okhotsk Sea, the temperature minimum layer is much thicker and much deeper, reflecting the deep ventilation that occurs there.
NPIW formation in the mixed water region. Weak ventilation of waters below the S min density through denser water formation in the Okhotsk Sea - reaching to about 2000 meters. Consider all of this NPIW.
Deep waters of the North Pacific. Oxygen minimum layer which lies at about 1500 meters. This is clearly within the depth range of NPIW ventilation, albeit very weak ventilation. The oxygen minimum is intensified within the bowl of the subtropical/subpolar gyres. I don't understand its maintenance.
Deep waters below the wind-driven circulation. The Pacific Deep Water occupies most of the water column between 1500 and about 4000 meters. It characteristic is a deep silica maximum layer, whose lateral origin is in the northeastern Pacific. It is separated from the bottom Antarctic Bottom Water (also known as Lower Circumpolar Water) by a so-called "benthic front" in the southern and western North Pacific. The densest water enters the North Pacific across the equator in the west, through Samoan Passage (S. Pacific). This water flows northward and splits in the western N. Pacific, with a portion flowing eastward south of Hawaii and a portion continuing northward, possibly through Wake Passage but in a generally broad flow.
Silica on a near-bottom isopycnal reveals two separate deep circulations, which appear to be anticyclonic rather than cyclonic; one concentrated north of the latitude of the Hawaiian Island and the other to the south. Densest water enters the northern North Pacific along the western boundary, primarily channelled by the deep trenches along the western and northern boundaries.
Carbon-14 dating of deep waters - use some figure for this - Stuiver
The deep waters above the Lower Circumpolar Water also exhibit two large anticyclonic cells based on silica distributions. These suggest that outflow of the deep water is in the eastern North Pacific, to the equator.
Connection of the North Pacific to the global ocean. Reid and Lynn isopycnal plots suggest the influence of NADW on a global scale. The freshest water on the characteristic isopycnal is in the North Pacific, if one ignores the narrow band around the Antarctic. This freshening indicates the importance of vertical mixing despite the assumption of isopycnal movement of waters. The high salinity of the NADW creates a relatively high circumpolar water which spreads northward in the Pacific to become the bottom water of the North Pacific. This is freshened and warmed (upwells) in the North Pacific.