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Home , Abyssal plain, Central Indian Ridge, Continental shelf, East Pacific Rise, Echo sounding, Mixed layer

INTRODUCTION TO PHYSICAL

INSTRUCTOR: Weiqing Han Professor ATOC, the University of Colorado (CU) UCB 311 Boulder, Co 80309 Phone:303-735-3079 Fax:303-492-3524 Email:[email protected] August 2019

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Chapter 1. Basins and properties of

1.1 Description of basins & relation to oceanic processes and climate

Land

Shelf Open Ocean

Slope

(ridges and ) Rise Abyssal

Fig. 1.1. Schematic diagram showing ocean basin features. Ocean basin--common features. Each basin has its own features that distinguish itself from other ocean basins; however they do have some common aspects. Each basin consists of (i) ``the '' - a shallow ledge next to the edge of each (0~200m), (ii) ``the continental slope'' - from the edge of the continental shelf, the floor slopes towards the (200m~3000m), (iii) ``the '' - at the foot of the slope, settle out of the gradually to form a much shallower slope, and (iv) ``the '' - the floor of the deep sea (>3000m). About 76.7% of the world's are occupied by the Abyssal plain, 7.4% by Continental Shelf, and 15.9% by Continental Slope and rise. All these features are called ``''. The Abyssal are also filled with ridges and trenches. The deepest is the in the Pacific, which is ~11km deep.

The Inter-ocean ridges are: Mid-Atlantic ridge--SW Indian Ridge-- --Pacific Antarctic Ridge--.

The (PO). The PO is the largest of all oceans. In the it spans a zonal distance of ~20,000 km from Malacca (4.66ºN, 99.55ºE, Indonesia) to Panama city (8.54° N, 80.78°W, Panama). Its meridional extent between Bering Strait and Antarctic is over 15,000 km. With all its adjacent it covers ~178 x 106 km2 and represents 40% of the world ocean surface, equivalent to the area of all . Without its part, the PO is ~147 x 106 km2, about twice the area of the , and it does not have deep water sources. This vast ocean facilitates strong

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air-sea coupling, and is the home for the most prominent mode of climate variability at interannual timescale: the El Niño and Southern Oscillation (ENSO), with the continental effect (i.e., ) being overall small except for some marginal seas (e.g., the South Sea in the western Pacific). Its mean depth is about 4270m. Ridges: Pacific Antarctic Ridge, East Pacific Rise, Emperor Hawaiian Ridge, Rise Nazca Ridge. (Fig. 1.2).

Fig. 1.2. Basin and bathymetry of the world’s oceans. The . Different from the PO, the Atlantic extends both into the and Antarctic region, giving it a total meridional extent. In comparison, its zonal largest extent, between the of and the of northwest Africa, spans a little more than 8300km. It has the largest number of adjacent seas. The large N-S extent favors deep water formation, because at higher surface water in some regions is cold enough to sink during winter . Also, its adjacent sea—the --produces Mediterranean water mass. The small zonal distance suggests its close contact with the lands, and thus continental monsoon effect is larger than that of the Pacific. Both air/sea interaction and monsoon are important for the ocean/atmosphere variability. Including all adjacent seas, the Atlantic covers ~106 x 106 km2. Its mean depth is about 3300m. The major ridge is the Mid-Atlantic Ridge. (Fig. 1.2)

The Indian Ocean. It is small compared to the Pacific and Atlantic Oceans. Its N-S extent is ~9600km from the Antarctic to the inner of Bengal and spans 7800 km in the E-W direction between S. Africa and W. . Including the Southern Ocean part it covers ~74 x 106 km2. The Indian Ocean is connected to the Pacific Ocean via the Indonesian Throughflow, the only low pathway for the warm and fresher water from the Pacific to enter the Indian Ocean, which serves as the upper branch of the global thermohaline conveyor belt (Fig. 1.3; also see a short video at: https://en.wikipedia.org/wiki/Thermohaline_circulation). The Indian Ocean is interesting

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in that its northern boundary is located in the tropics. One consequence of this basin geometry is that it is subject to the strong monsoon influence due to land/ocean heating contrast. The monsoon means strong seasonally reversing surface winds. Subject to the monsoon wind forcing, the dynamics can affect the Bay of Bengal circulation and even the Arabian Sea, through coastally trapped waves that propagate around the and westward radiating Rossby waves. 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, an ocean-atmosphere coupled mode of climate variability at interannual timescale. Its mean depth is around 3800m. Ridges: SW Indian Ridge--Central Indian Ridge--Southeast Indian Ridge, (also see Fig.1.2).

Fig. 1.3. The global thermohaline conveyer belt.

The . The Arctic Ocean (Fig. 1.4) covers the north polar region, and 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 . The area of sea ice covered region varies with season. The major connection of the Arctic with the three oceans is through the Atlantic, where a 1700 km wide opening exists along a large oceanic sill running from the Greenland across to Iceland, the Faroe and Scotland. Minor openings to the Atlantic Ocean exist through the Canadian . The connection with the Pacific Ocean through Bering Strait is narrow and shallow (45m deep and 85km wide). In the Arctic and North Atlantic Oceans, sea ice melting and freezing affect deep water formation and therefore the global . This aspect will be discussed in the section. The Arctic Ocean consists of a few Mediterranean seas and is separated into a few basins by Ridges. The Canadian basin has a depth of 3600-3800m; the Makarov

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basin has a mean depth of 3900m; the Amundsen Basin had a depth of 4300-4500m; and the Nansen Basin has a depth 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 can flow into the Arctic, affecting the Arctic climate (Fig. 1.4).

Figure 1.4. The Arctic Ocean

1.2. Properties of seawater o Pure water is composed of H2O. The maximum is at 4 C rather than at freezing point 0oC. This is because the polymer-like chains of up to 8 molecules due to the polar of the water molecule, is a function of temperature. The major difference between seawater and pure water is in seawater. The physical properties of pure water in fluid dynamics studies are functions of pressure (P) and temperature (T), while those of

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seawater are functions of P, T, and (S). Due to salinity, the freezing point of seawater 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. In , we often use cgs units since currents are in the of cm/s rather than m/s. P in cgs: dyn/cm2. 1 Pascal = 1 N/m2. [ 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 pressure gradient is the change of pressure 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, which will be given later in ) The total vertical variation in pressure in the ocean is thus from near zero (surface) to ~10,000 dbar (deepest).

Temperature. An important physical property of seawater is its temperature. It was one of the first ocean parameters studied. (SST) variations are important for driving , and thus important for understanding climate variability. Units: oC and oK (Kelvin); 0oC=273.16K, and noC=(273.16+n) K.

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; therefore, it is important to measure 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 do we have constant proportions? Salt comes from weathering of continents, deep sea vents, etc; the input is very slow (in the order of 100,000 years) compared with the mixing rate om the ocean (~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 is (Cl 55% of all dissolved material), the second largest is ion (Na 30.6%), followed by many other constituents. Actually, there is a slight variation in the proportions, and recommendations were made to formulate new definitions of salinity that depend on the actual constituents. In 2010, a new standard for the properties of seawater - the Thermodynamic Equation of Seawater 2010 (TEOS-10) - was introduced. A significant change compared with the past practice is that TEOS-10 uses Absolute Salinity SA (mass fraction of salt in seawater) as opposed to Practical Salinity SP (which is essentially a measure of the conductivity of seawater) to

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describe the salt content of seawater. Ocean salinity now has the units of g/kg (see http://www.teos-10.org).

Units. In the original definition, salinity units were o/oo (parts per thousand). This was replaced by the ``practical salinity unit (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 based on conductivity and is not precisely related to the mass of dissolved material. Salinity units for TEOS-10 is g/kg.

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. Physical oceanographers are particularly interested in salinity and temperature of seawater because they are the properties that 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 surfaces with constant .

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 , Appendix 3.)

Density units (mks): kg m-3; cgs: g cm-3.

Discussion. Freshwater density is ~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 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. So we should be careful when use these tracers.

Sound in the sea. In clear ocean water, 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 to detect objects. of 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) --detect ocean depth. They send sound pauses; after the sound pauses hit the ocean bottom and are reflected back, it takes time (t). Using the mean speed C, we

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have depth D=(C*t)/2, where D is depth and t is time. (b) --echo sounder. The in water is approximately 1500 m/s. It depends on pressure, temperature and salinity. The higher the pressure, the higher the sound speed (in a sense that the water is more ``rigid" and so the speed increases). The higher the temperature, the higher the sound speed. (Molecules at higher temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly.) The higher the salinity the faster the sound speed. In real ocean however salinity changes are small (~1psu) but temperature and pressure changes are large. Therefore, sound speed varies primarily with the variations of pressure and temperature (https://dosits.org/tutorials/science/tutorial-speed/).

SONAR (SOund Navigation And Ranging) can be used to detected submarine or school of fishes. (Eco-sounder: emits narrow sound beam pauses approximately horizontally and reflect back after hitting objects. Can turn 360 degrees, and reach hundreds of meters distance.) (c) SOFAR . 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 attains maximum at the surface and bottom, with a minimum in between (Figs 1.5a-b). 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. The SOFAR channel functions as a waveguide. 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 to the layer of minimum sound speed. (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).

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Fig. 1.5a. 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 (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).

Physical oceanography application 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. Since 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 the oceans. (ATOC project; program ended a few years ago; http://aog.ucsd.edu/thermometry/index.htm). (ii) US Navy Sound surveillance System--array of used by the Navy for deep ocean surveillance during the cold war. (iii) Tracking of vessels in distress-Before GPS the SOFAR channel was used for locating the and aircrafts in distress. (iv) Humpback whales may use the SOFAR channel to communicate since they migrate thousands of kilometers.

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Sound! speed profile calculated Typical T & S profiles; From the T&S profiles on the left

Fig. 1.5b. (Left) typical temperature and salinity profiles; (right) Vertical profile of sound speed based on the T and S from the left panel. The sound speed is calculated using the following formula from Wilson’s (1960) equation: U=1449 + 4.6T – 0.055T2 + 0.0003T3 + 1.39( S – 35) +0.017D, where U is the speed (m/s), T is the temperature (° C), S is the salinity (psu), and D is depth (m).

Light in the Sea: Absorption and penetration. When the visible light penetrates into the water (0.4-0.8µm, 1µm=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 z, and k is the vertical attenuation coefficient of seawater. For the clearest ocean water: k is small (close to 0.02/m), and � = �� = � × 37% is at 50m. For the turbid coastal water, however, k is large (close to 2/m), and � = �� = � × 37% is at 0.5m. This light penetration is attracting more and more oceanographers' attention, since it will affect the SST, biological activities, and climate. [NOTE: Depth z at where � = �� is often referred to as the ``e-folding'' scale of light attenuation.]

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