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Formation of ice and hence more dense saltwater sinking happens in both polar regions, creating downwellings. Meanwhile, only a few spots in Antarctica have upwellings

I refer to this image: Upwelling areas https://en.wikipedia.org/wiki/Upwelling#/media/File:Upwelling_image1.jpg

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    $\begingroup$ dense salt water sinking should imply downwelling right? $\endgroup$
    – shamalaia
    Mar 15, 2018 at 8:27
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    $\begingroup$ the gulfstream flows north to the arctic and do sink there so to say there is upwelling in the arctic is wrong. $\endgroup$
    – trond hansen
    Mar 15, 2018 at 9:11
  • $\begingroup$ en.wikipedia.org/wiki/Thermohaline_circulation#/media/… $\endgroup$
    – trond hansen
    Mar 15, 2018 at 9:19
  • $\begingroup$ Upwelling happens in both polar regions along the continental shelf depending on wind direction and offshore of the shelf. The image is correct. What the other comments refer to is dense water formation in the downwelling regions along the continental shelf in both Arctic and Antarctic regions, but more predominant in the North. $\endgroup$
    – arkaia
    Mar 15, 2018 at 13:19
  • $\begingroup$ @arkaia Good comment. But concerning your edit I feel you erased one of the questions there. $\endgroup$
    – Camilo Rada
    Mar 15, 2018 at 15:12

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First, we might want to distinguish between the different types of upwelling:

  • Coastal upwelling: It is the best known form of upwelling. Winds in these systems flow parallel to the coast (with the coast to the left in the northern hemisphere or to the right in the southern hemisphere) and generate upwelling dynamics. Surface Ekman balance is setup (in deep enough waters) with water transport being to the right (left) of the wind in the northern (southern) hemisphere. The result is a deficit along the coast that requires a compensating flow in the deeper part of the water column bringing usually colder waters to the surface. If the wind blows in the opposite direction, then downwelling occurs. The figure from the question refers to coastal upwelling (here in its original website with explaining caption). There is definitely more coastal upwelling in the northern hemisphere because of the abundance of shorelines as a greater amount of landmass is present in that hemisphere.

upwelling animation Source Commons Wikipedia.

  • Equatorial upwelling: Winds from the east blow and converge along the equator as part of the Intertropical Convergence Zone (ITCZ). While the magnitude of the Coriolis acceleration is zero along the equator, Ekman transport takes place immediately north and south of the equator resulting in surface ocean divergence that requires a compensating flow of denser (nutrient-rich) water upwelled from below.

Equatorial upwelling Source

  • Southern Ocean upwelling: Strong westerly (eastward) winds dominate the atmospheric setting around Antarctica. The strong winds force the Antarctic Circumpolar Current, the strongest current in the ocean. The winds also result in an Ekman balance with northward flow in the upper part of the water column. To compensate that flow, some water flows up from greater depths. The magnitude of the Southern Ocean upwelling is still being studied, but some observations and models propose that it represents the main way for dense water from deeper areas to be reintroduced in the surface circulation. If that were the case, then it represents the largest upwelling system in the world. An alternative explanation proposes oceanic eddies as the main mechanism for dense water to reconnect with the surface circulation. Additionally, there is coastal upwelling over the Antarctic continental shelf driven by local winds that brings relatively warmer waters to the shelf contributing to local/regional ice melting. southern ocean upwelling Source

The question also mentions downwelling associated with dense water formation. Deep water forms at high latitudes (typically north and south of 60°) in both the southern and northern hemispheres. As mentioned in the question, ice contains almost no salt, so when ice forms, a salty, sub-zero brine is left behind and sinks. In the Antarctic, as the saltier water moves down, it mixes with slightly warmer intermediate water. The mixture becomes Antarctic Bottom Water (AABW), which flows northwards along the bottom in all the main oceans. In the Arctic, the northward flowing branch of the Gulf Stream (salty and warm) joins the subpolar gyre and enters the Labrador Sea where as it cools, it starts to sink. The eastern branch of the Gulf Stream begins to sink in the Nordic Seas. The resulting water masses combine to form the dense North Atlantic Deep Water (NADW) that flows south at depth. Both the creation of AABW and NADW can be considered downwelling, as there is a net downward flow in the formation areas, but they are not related to wind-driven downwelling in the vicinity of coasts.

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    $\begingroup$ Thanks! So even though the Southern Ocean upwelling is considered the largest in the world, why is the productivity of the North hemisphere subpolar water so much higher than the Southern subpolar waters ? $\endgroup$
    – Sid Datta
    Mar 16, 2018 at 1:17
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    $\begingroup$ I think you have to distinguish between production and productivity. The production of the Southern Ocean might be much larger as the area where upwelling is occurring is much more extensive. The productivity (production per unit area) might be larger in the Arctic (beware of large seasonal and interannual variabilities, nasa.gov/centers/goddard/mov/…), because of the abundance of coastlines. $\endgroup$
    – arkaia
    Mar 16, 2018 at 14:19
  • $\begingroup$ The large but relatively unproductive deeper part of the Antarctic Ocean represents about 90% of Southern Ocean production. Meanwhile, if you normalize by area, the productivity rates are smaller than in many continental shelves. $\endgroup$
    – arkaia
    Mar 16, 2018 at 14:19
  • $\begingroup$ So what you're saying, if I'm interpreting right: in the Antarctic it's generally small upwelling speeds over a large area rather than rapid speeds in focused areas? $\endgroup$
    – JeopardyTempest
    Mar 16, 2018 at 17:32
  • $\begingroup$ That is my point, but there is a lot of research on this matter. Ultimately, when there are no observations and a flux balance calculation was done, a large upwelling transport is needed to compensate the surface divergence. The issue that is being study is whether that flow is homogeneous or focused in specific regions where ocean eddies can enhance that flow. $\endgroup$
    – arkaia
    Mar 16, 2018 at 17:37

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