INTRODUCTION TO PHYSICAL OCEANOGRAPHY 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 1 Chapter 1. Basins and properties of seawater 1.1 Description of ocean basins & relation to oceanic processes and climate Land Shelf Open Ocean Slope (ridges and trenches) Rise Abyssal Plain 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 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 ``bathymetry''. The Abyssal plains are also filled with ridges and trenches. The deepest trench is the Mariana trench in the Pacific, which is ~11km deep. The Inter-ocean ridges are: Mid-Atlantic ridge--SW Indian Ridge--Central Indian Ridge Southeast Indian Ridge--Pacific Antarctic Ridge--East Pacific Rise. The 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 (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 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 is ~147 x 106 km2, about twice the area of the Indian Ocean, and it does not have deep water sources. This vast ocean facilitates strong 2 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., monsoon) being overall small except for some marginal seas (e.g., the South China Sea in the western Pacific). Its mean depth is about 4270m. Ridges: Pacific Antarctic Ridge, East Pacific Rise, Emperor Hawaiian Seamounts Ridge, Chile Rise Nazca Ridge. (Fig. 1.2). Fig. 1.2. Basin and bathymetry of the world’s oceans. The Atlantic Ocean. Different from the PO, the Atlantic 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 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 latitudes surface water in some regions is cold enough to sink during winter season. Also, its adjacent sea—the Mediterranean sea--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 Bay of Bengal and spans 7800 km in the E-W direction between S. Africa and W. Australia. 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 latitude 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 3 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 equatorial wave dynamics can affect the Bay of Bengal circulation and even the Arabian Sea, through coastally trapped waves that propagate around the coasts 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, Ninety East Ridge (also see Fig.1.2). Fig. 1.3. The global thermohaline conveyer belt. The Arctic Ocean. 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 sea ice. 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 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). In the Arctic and North Atlantic Oceans, sea ice melting and freezing affect deep water formation and therefore the global thermohaline circulation. This aspect will be discussed in the thermodynamics 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 4 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 density 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 nature of the water molecule, is a function of temperature. The major difference between seawater and pure water is salt in seawater. The physical properties of pure water in fluid dynamics studies are functions of pressure (P) and temperature (T), while those of 5 seawater are functions of P, T, and salinity (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 physical oceanography, 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 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 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 property of seawater is its temperature. It was one of the first ocean parameters studied. Sea surface temperature (SST) variations are important for driving atmospheric circulation, 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).