Shows that there are two “modes” of depth on the earth Shows that there are two “modes” of depth on the earth. One located just above sea-level and the other about 4-5 km below sea level. These are the continental and oceanic plates. Thus most of the ocean is 4-5 meters deep– and given the large gradient in area around zero it’s clear that the rise and fall of sea-level with glaciation and warming can really modify the amount of continentai

Changes in sea level over the past 250,000 yrs Highpoint of last interglacial (120,000 ya) Lowpoint of last glacial (18,000 ya) Sea-level was 130 meters lower at peak of last ice age. Note that the current ice age cycle has only been going on for maybe 3 million years. Some speculate that this has to do with the closing of the Ithums of Panama and changes in the hydrological cycle and salinity of the ocean ultimately modifying the generation of deep water and equatorial upwelling. Also you can imagine that this had an increadible effect on continental shelves– which are the most productive parts of the ocean Rise and fall primarily due to ice ages Periods of increased/decreased glaciation Increasing or decreasing the pool of water in the ocean. Note- Sea level has been below its present position for much of the last 250,000 yrs Fig 11-1, p.197

Note where the 130 m isobath is– most of the shelf was land (it is after all a continental

Shows that there are two “modes” of depth on the earth Shows that there are two “modes” of depth on the earth. One located just above sea-level and the other about 4-5 km below sea level. These are the continental and oceanic plates. Thus most of the ocean is 4-5 meters deep– and given the large gradient in area around zero it’s clear that the rise and fall of sea-level with glaciation and warming can really modify the amount of continental shelf that is ocean- - and this may have significant biogeochemical implications.

\ OF course onlong time scales (100 millions of years). This distribution is modified as the margins of the continental crusts are subducted or increased by plate tectionics

But for the purpose of this class we’ll really only consider today’s oceanic configuration. But anyone interested in climate variability going back 100’s of million or a billion years needs to consider how the variable continents would modify the heat engine of the ocean. This is essential because the vast majority of the heat in the ocean/atmospheric system is stored in the ocean. Why? It also takes about 1000 years to mix the ocean, while the atmosphere is mixed much more quickly. Thus any long term change in global temperatures are going to take time (though 1000 years is really not that long) and involve the time-scale of the ocean to reach a new equilibrium (if one is really every reached)

4km 4 km x 10,000 km is like .4 mm x 10 cm– or approximately the same dimension as a piece of paper! Yet despite the “shallowness” of the ocean it contains an incredibly rich structure of water properties and currents. In this class the water properties that we’re most interested in is the temperature and salinity– however other properties such as Nitrate, Oxygen and radionuclides are also useful in understanding ocean circulation and we will occasionally use them in classs. ~10000 km

Before we discuss the structure of the ocean– I wanted to talk about some of the properties of water. Here’s the density of fresh water as a function of temperature. It has this weird property of having a maximum density at around 4 degrees. This is why Ice floats (thought I’ve never tried this but presumably if you put a block of -1 degree ice into 25c water it would sink!). But normally ice is found it cold water and thus is the lightest form of water. This is also why fresh waters lakes overturn in both the winter and spring because before they freeze the entire lake has to be cooled to 4c before becoming thermally stratified with colder water– and eventually ice– on top. During the spring as the water warms up all the water in the lake must reach 4c before it can become thermally stratified with warmer waters at the surface. Since the cooling and heating occurs at the surface and it generates 4 degree water– it must sink the the bottom and thus completely mix the lake.

This is a T/S diagram with contours of density This is a T/S diagram with contours of density. Note that density is in sigma-T or density minus 1000. However– when we add salt– we see that the temperature of maximum density falls below the freezing point. Note the narrow range of TS properties of 90% of the ocean Thus for ocean water ice will form before it reaches maximum density. Still if it’s cold (and salty enough) it will sink the to bottom. The key ingredient is salt. If the water is is salty enough cooling it will allow it to sink to the bottom

Atlantic is saltier than Pacific Atlantic is saltier than Pacific. Where would you expect deep water to form? Where would it not form? Why do you think the ocean has this surface salinity structure?

Why

Surface density (winter) Thus the two regions with the densest surface waters are the North Atlantic and the Antarctic. This is where the deep water forms. What does this imply about deep ocean circulation? All these waters must feed the bottom of the Pacific? What does this mean about the “age” of the water in the bottom of the Atlantic vs. the bottom of the Pacific? It means that the Pacific water is older (100’s to 1000 years). What does it mean that the water TS properties remain “intact” during this passage? That mixing is weak! But other non conservative material will vary. How do you think Oxygen, CO2, Nutrients vary betwween the Atlantic and Pacific oceans?

Profiles somewhere in Pacific Ocean– must be a trench Profiles somewhere in Pacific Ocean– must be a trench. Note potential temperature and density.

Locate extrema in salinity with particular water masses Locate extrema in salinity with particular water masses. Salinity minimum is about 10 degrees where there is a clear divergences of TS values as we move up from the coldest temperatures. There are two separate clusters of points with salinity minimum –one terminates at 12-13 degrees and the other contiues all the way up to the warmest temperatures. This shows that there are two “intermediate” water masses. The terminating water mass is Antarctic intermediate water flowing up from the Antarctic coast, The other less salty is the North Pacific Intermediate Water moving south from its formation in the region in the Northern Gulf of Alaska.

Note that we can define these at increasingly finer and finer TS properties…..

TS Diagram TS Diagram.

Take one profile from 4N 16 W Today the ocean’s structure is monitored daily by a bunch of profiling floats. These profile once ad day and drift at the mercy of the currents

Here’s a profile in the eastern Atlantic Here’s a profile in the eastern Atlantic. Note salinity maximum at ~100m depth

Plotting this in a TS diagram

Difference between the “seasonal Thermocline” and the main thermocline.

Most of the ocean is COLD. Note the surface scale is exagerated

Mixing occurs via isopycnals Mixing occurs via isopycnals. And that a vertical profiles is identical to a horizontal profile. Suggestive of weak mixing- across density surfaces (diapyncal mixing) thought there has to be some!

Here we see the med outflow of saline water spreading across the entire Atlantic basin

My guess is that salinity maixmum is the med outflow

Salinity Section Pacific (180 W) Atlantic (30 W) Note how much saltier the Atlantic is

Temperature/Salinity Section Pacific (180 W) Atlantic (30 W) Cold bottom water everywhere.

Temperature Section Pacific (180 W) Atlantic (30 W)

Temperature/Oxygen Section Pacific (180 W) mL/L Atlantic (30 W) Discuss Oxygen Structure. Regions of low DO why? Why lower in Pacific? mL/L

Salinity/Nitrate Section Pacific (180 W) mM Atlantic (30 W) Discuss HNLP of in southern ocean mM

Temperature/Nitrate Section Pacific (180 W) mM Atlantic (30 W) mM