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INTRODUCTION TO PHYSICAL OCEANOGRAPHY

INSTRUCTOR: Weiqing Han Associate Professor ATOC, University of Colorado (CU) UCB 311 Boulder, Co 80309 Phone:303-735-3079 Fax:303-492-3524 Email:Weiqing.han@colorado.edu

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Chapter 1. Basins and properties of sea water

1.1 Description of ocean basins

Fig. 1.1. Schematic diagram showing ocean basin features. Ocean basin--common features. As we shall see later in this course, each basin has different features, but they do have various things in common. Each basin is composed of (i) ``the continental shelf'' - a shallow ledge next to the edge of each continent (0-200m), (ii) ``the continental slope'' - from the edge of the continental shelf, the sea floor slopes towards the deep sea (200m-3000m), (iii) ``the continental rise'' - at the foot of the slope, sediments settle out of the water gradually to form a much shallower slope, and (iv) ``the abyssal plain'' - the flat floor of the deep sea (>3000m). About 76.7% of the world's oceans are occupied by the Abyssal plain, 7.4% by Continental Shelf, and 15.9% by Continental Slope and rise. All these features are called ``topography''. The Abyssal plains are also filled with ridges and trenches. The deepest trench is the Mariana trench in the Pacific, which is about 11km deep. Inter-ocean ridges: Mid-Atlantic ridge--SW Indian Ridge--Central Indian Ridge--Southeast Indian Ridge--Pacific Antarctic Ridge--East Pacific Rise. Pacific Ocean (PO). The PO is the largest of all oceans. In the tropics it spans a zonal distance of 20,000 km from Malacca strait to Panama. Its meridional extent between Bering Strait and Antarctic is over 15,000 km. With all its adjacent seas it covers 178 x 106 km2 and represents 40% of the world ocean surface, equivalent to the area of all continents. Without its southern ocean part the PO 147 x 106 km2, about twice the area of the Indian Ocean, and it does not have deep water source. This vast ocean facilitates strong air-sea coupling--ENSO, with continental monsoon effect small. Its mean depth

Shelf

Slope

Rise Abyssal Plain

Open Ocean

Land

(ridges and trenches)

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is about 4270m. Ridges: Pacific-Antarctic Ridge, East Pacific Rise, Emperor Hawaiian Seamounts Ridge, Chile Rise, Nazca Ridge. (Fig. 1.2, Fig. 1.3)

Figure 1.2. Adapted from Regional Oceanography.

Atlantic Ocean. Different from the PO, the AO extends both into the Arctic and Antarctic region, giving it a total meridional extent. In comparison, its zonal largest extent, between the Gulf of Mexico and the coast of northwest Africa, spans little more than 8300km. It has the largest number of adjacent seas. The large N-S extent favors deep water formation. Also, its adjacent sea--Mediterranean sea--produces Mediterranean water mass. The small zonal distance suggests its close contact with the lands, and thus continental monsoon effect is large. Both air/sea interaction and monsoon are important for the ocean/atmosphere variability. Including all adjacent seas, AO covers 106 x 106 km2. It mean depth is about 3300m. Mid-Atlantic Ridge. (Figs 1.2 and 1.3)

Indian Ocean. Smallest of all oceans. N-S extent: 9600km from Antarctic to the inner Bay of Bengal and spans 7800 km in the E-W direction between S. Africa and W. Australia. Including the SO it covers 74 x 106 km2. Connect to the Pacific Ocean via Indonesian Throughflow. The Indian Ocean is interesting in that its northern boundary is located in the tropics. One consequence of this basin geometry is that it is subject to the

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strong monsoon influence due to land/ocean heating contrast. The monsoon--means strongly seasonal varying winds. Subject to the monsoon wind forcing, the equatorial wave dynamics can affect the Bay of Bengal circulation and the Arabian Sea, due to westward radiating Rossby waves and coastal Kelvin waves that propagate around the coast. Indian Ocean air/sea interaction is not as well understood as the other two oceans, although people generally agree with the recently discovered Indian Ocean Dipole Mode--air/sea coupled mode at interannual timescale. Its mean depth is around 3800m. Ridges: SW Indian Ridge--Central Indian Ridge--Southeast Indian Ridge, Ninety East Ridge. (Also see Figs 1.2 and 1.3.)

Figure 1.3

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Figure 1.4

Arctic Ocean. Covers the arctic north polar region, is the smallest of the world’s oceans. It covers an area of about 14 x 106 km2. Much of the ocean is covered by sea

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ice. The area of sea ice covered region varies with season. The major connection of the Arctic Seas with the three oceans is to the Atlantic Ocean where a 1700 km wide opening exists along a large oceanic sill running from Greenland across to Iceland, the Faroe Islands and Scotland. Minor openings to the Atlantic Ocean exist through the Canadian Archipelago. The connection with the Pacific Ocean through Bering Strait is narrow and shallow (45m deep and 85km wide). Arctic Ocean, sea ice melting and freezing, affects deep water formation and thus global thermohaline circulation. This aspect will be discussed in the thermodynamics section. It is composed of Mediterranean seas and separated into a few basins by Ridges. Canadian basin: depth of 3600-3800m, Makarov basin 3900m in depth, Amundsen Basin, depths of 4300-4500m, and the Nansen Basin, depths of 3800m--4000m. The Ridges between these basins are the Alpha and Mendeleyev Ridge system , the Lomonossov Ridge, and the Arctic Mid-Ocean Ridge (Nansen Ridge). Atlantic warm water flows to Arctic--affects Arctic climate. (Fig. 1.4)

1.2. Properties of sea water Pure water is composed of H2O. Maximum density is at 4oC rather than at freezing point 0oC.This is because the polymer-like chains of up to 8 molecules due to the polar nature of the water molecule, as a function of temperature. The difference between sea water and pure water is: salt in the sea water. The physical properties of pure water relevant to fluid dynamics studies are functions of pressure (P) and temperature (T), while those of sea water are functions of P, T, and salinity (S). Due to salinity, the freezing point of sea water is approximately -2oC. Pressure. Pressure is the force per unit area exerted by water (or air in the atmosphere) on either side of the unit area. Units: The units of force are: 1N=kg.m/s2, which is from Newton's Law F = ma. The units of pressure are: N/m2. We often use cgs units since currents are in the order of cm/s rather than m/s. P in cgs: dyn/cm2. 1 Pascal = 1 N/m2. [Atmospheric pressure is usually measured in bars. 1 bar = 106 dyn /cm2 = 105 Pascal. Ocean pressure is usually measured in decibars. 1 dbar = 10-1 bar = 105 dyn/cm2 = 104 Pascal. Description. The force due to pressure comes from the difference in pressure from one point to another - i.e. the``pressure gradient force" since the gradient is the change over distance. The force is in the direction from high to low pressure, hence we say the force is oriented ``down the pressure gradient". The pressure at a given depth depends on the mass of water lying above that depth. (Hydrostatic equation will be given in class.) The total vertical variation in pressure in the ocean is thus from near zero (surface) to 10,000 dbar (deepest). Temperature. An important physical characteristic of sea water is its temperature. It was one of the first ocean parameters studied. SST variation is important for driving atmospheric circulation, and thus important for understanding climate variability. Units: oC. Kelvin. 0oC=273.16K. noC=(273.16+n) K.

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Salinity. Salinity is roughly the number of grams of dissolved matter per kilogram of seawater. This was the original definition, and at one time salinity was determined by evaporating the water and weighing the residual. The dissolved matter in seawater affects its density, hence the importance of measuring salinity. The "law" of constant proportions (Dittmar, 1884), formalized the observation that the composition of the dissolved matter in seawater does not vary much from place to place. Why constant proportions? Salts come from weathering of continents and deep-sea vents, etc - the input is very very slow (order 100000 years) compared with the mixing rate of the whole ocean (which is order 1000 years). Thus it is possible to measure just one component of the dissolved material and then estimate the total amount of dissolved material (salinity). This approach was used until the 1950's. The main constituent of sea salt is Chlorine ion (Cl 55% of all dissolved material), the second largest is sodium ion (Na 30.6%), followed by many other constituents (see Pickard and Emery Chapter 3). In actuality, there is a slight variation in the proportions, and recommendations are underway to formulate new definitions of salinity which depend on the actual constituents - this may likely take the form of geographically-dependent tables of corrections to the quantity which is measured (usually conductivity). Units. In the original definition, salinity units were o/oo (parts per thousand). This was replaced by the ``practical salinity unit" or psu. Most recently, the recommendation of the Scientific Committee for Oceanic Research (SCOR) working group on salinity is that salinity be unitless, as the measurement is now based on conductivity and is not precisely related to the mass of dissolved material. The total amount of salt in the world oceans does not change except on the longest geological time scales. However, the salinity does change, in response to freshwater inputs from rain and runoff, and freshwater removal through evaporation. Density. The physical oceanographer is particularly interested in the salinity and temperature of sea water because they are characteristics which help to identify a particular water body and also because, together with pressure, they determine density ( ). Density is important because water parcels basically move along isopycnic surfaces, the surface with constant densities. Seawater density depends on temperature, salinity and pressure. Colder water is denser. Saltier water is denser. High pressure increases density. The dependence is nonlinear. An empirical equation of state is used, based on very careful laboratory measurements. (See Gill, Appendix 3.) Density units (mks): kg m-3; cgs: g cm-3.

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Discussion. Freshwater density is about 1000 kg/m3. Typical densities for seawater are only slightly higher: 1020 to 1050 kg/m3, with most of this range being due to pressure. The range of densities at the sea surface is about 1020 to 1029 kg/m3. Tracers. Dissolved oxygen content, concentration of nutrients (Nitrate, phosphate, silicate, etc) can be used as Tracers for water masses. But they are non-conservative. Biological processes may change the concentration of oxygen or nutrients without any movement of the water mass, should be careful. Sound in the sea. In clear ocean water, sunlight may be detectable (with instruments) down to 100m but the range at which man can see details of objects in the sea is rarely more than 50m and usually less. Therefore, we use sound to detect objects. [Frequencies of sounds which are of interests in the ocean range from 1Hz to thousands of kHz. Wavelength: 1500km for 1Hz to 7cm for 200kHz. Most instruments use 10~100 kHz, wavelengths 14 to 1.4cm.] (a) Echo sounding--detect ocean depth. Send sound pauses, after they hit the ocean bottom and are reflected back, it takes time (t). Using mean speed C, we have D (D=(C t)/2). (b) Sonar--echo sounder. The speed of sound in water is approximately 1500 m/s. It depends on pressure and on temperature. The higher the pressure, the higher the sound speed (in a sense, the water is more ``rigid" and so the speed increases). The higher the temperature, the higher the sound speed. SONAR (SOund Navigation And Ranging) can be used to detected submarine or school of fishes. (Eco-sounder: emit narrow sound beams pauses approximately horizontally and reflect back. Can turn 360 degrees, and reach hundreds of meters distance.) (c) SOFAR channel. In most areas of the ocean, the warm water at the surface and the high pressure at the bottom produce a sound speed profile which is maximum at the surface and bottom, with a minimum in between. This sound speed minimum (near surface at high latitudes to over 1000m in mid-and low latitudes) is referred to as the SOFAR (SOund Fixing And Ranging) channel. Where there is a sound speed minimum, it functions as a wave guide. If you send out sound beams at this depth at hundreds of hertz frequencies with moderate angles from the horizontal (straight up and down beams won't be channeled), refraction makes the sound waves channeled. SOFAR. (Sound tends to Refract toward the speed minimum). Where temperature is low or inverted near the surface, then there is no surface maximum in sound speed and the SOFAR channel is found at the sea surface (typical of the subpolar and polar regions). Use of SOFAR channel: (i) Acoustic Thermometry of Ocean Climate (ATOC)-measuring global warming using the speed of sound in the ocean. Scientists proposed to place sound sources across the ocean using the SOFAR channel. As we learned, the speed of sound in water depends heavily on temperature. So how fast the sound arrives at the other side would depend on the temperature of the ocean basin. In this way scientists can measure temperatures of oceans. (ATOC project; program ended a few years ago; http://aog.ucsd.edu/thermometry/index.htm). (ii) US Navy Sound surveillance System--array of hydrophones used by the Navy for deep ocean surveillance during the cold war. (iii) Tracking of vessels in distress-Before GPS the

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SOFAR channel was used for locating the ships and aircrafts in distress. (iv) Humpback whales may use the SOFAR channel to communicate since they migrate thousands of kilometers.

Fig. 1.5. Schematic diagram for SOFAR channel. Horizontal sound paths in the SOFAR channel (top) and near the surface if the surface layer is well mixed (bottom). The diagram at the left gives the vertical sound velocity profile for each case; note the very different vertical scales. The red circle indicates the sound source. The principle of sound propagation is that sound rays always bend towards the region of lower sound speed. This produces a sound channel near 1000 m depth (top) and a shadow zone below the surface mixed layer (bottom) to which sound from the source cannot penetrate. Since sound rays are reversible, this also means that sound produced in the shadow zone cannot be heard by a sound detector placed at the location of the red circle. (Adapted from www.es.flinders.edu.au/.../ figures/fig5a4.html; © 1996 M.Tomczak)

Light in the Sea: absorption and penetration. When the visible light penetrates into the water (0.4-0.8 , 1 =10-6m, from violet to red), most of them is absorbed within the upper a few meters. Light attenuation law: where is the ``shortwave radiation'' at the surface, and is the shortwave radiation at depth , and is the vertical attenuation coefficient of the water. Clearest ocean water:

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is small (0.02/m, at 50m), for turbid coastal water, is large

(close to 2/m), at 0.5m). This light penetration is attracting more and more oceanographers' attention, since it will affect the SST, biological activities, and thus

climate. [NOTE: Depth at where is often referred to as the ``e-folding'' scale of light attenuation.]