All Oceans Were Not Created Equal

In “Where it’s at!” I confessed my undying devotion to Antarctic waters. And fair enough – the Southern Ocean flows right around the globe, is virtually impossible to monitor because of ice and cloud, gets whipped by the strongest winds on earth and has the highest mean wave height in the world. Take that, The Pacific, you giant pansy.

But there’s something else that makes the Southern Ocean special: it’s essentially vertically coherent.

When you imagine tropical oceans, you presumably think of sparkling blue WARM waters. The truth is, only a thin layer gets heated by incoming solar radiation. Even at the equator, below the surface (~50m) a transition zone called the thermocline is all that separates you and your piña colada from hypothermia (~1000m). The significance of this is that in tropical and subtropical seas, you have a skin of warm water that behaves very differently to the body of water it sits atop. In slightly more scientific terms: flow is geostrophic (balanced between Coriolis force and horizontal pressure gradient) across the permanent thermocline, and wind-driven ocean currents are thus limited to the surface layer.

Ocean layers at depth – note the bathymetry (mountains and valleys on the seafloor) (http://www.srh.noaa.gov/jetstream/ocean/layers_ocean.htm)

I’ve just set up a page where you can pester me with questions so if you want to know more, feel free to ask. If you don’t care about the physical properties of different oceans, however, all you need to know is that in the Southern Ocean, this permanent thermocline doesn’t exist. I’m simplifying things, but I find it convenient to imagine a slab of water up to 2km deep that thunders around the Antarctic continent at tremendous pace. Of course, the sea floor isn’t flat so whenever the bottom of this slab encounters a mountain or a valley, it trips and destabilises the entire water column.

Seafloor topography (http://www.ibcso.org/)

This sounds pretty obvious but it’s a really unusual feature in the oceanography world, and it endows the Southern Ocean with some unique properties.

For one, it allows for localised upwelling wherever a seamount (think: underwater mountain peak) trips up the current. You may have read a few articles about iron fertilisation experiments in the Southern Ocean, but this happens naturally in areas such as these. (As an aside, iron fertilisation doesn’t work). Little green cells need sunlight and an array of nutrients to grow, and iron is very important for the process of photosynthesis. Wherever the Antarctic circumpolar current hits prominent features on the sea floor, iron-rich sediments get upwelled to the surface layers where the phytoplankton live!

Little green cells + Sunlight + Iron = photosynthesis and growth. Hey presto! Phytoplankton bloom!

Every significant topographic feature in the Southern Ocean is associated with phytoplankton blooms

Of course, as much as all oceans were not created equal, this means that even my beloved Southern Ocean has areas that, wellll… don’t really interest me. The area upstream of Marion really fascinates me because the current hits a seamount AND the tail-end of the Mid-Atlantic ridge, generating some cool phytoplankton dynamics. Of course, I’m not going to be able to measure this because of the drama I explained in “Missing the Boat”, but let us not dwell. All I need to do is find an EQUALLY awesome region to study… or maybe several.

I went to a great talk last night, drank a healthy measure of wine afterwards and came up with an idea for my PhD at around 2am. I presented this mad plan to Lars-the-guru this morning and he didn’t say much… But he did give me a high-five. Next post explaining everything coming soooon!

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