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Introductory Oceanography (OCNG 251) Study Guide: Part 1

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Introductory Oceanography (OCNG 251) Study Guide: Part 1 Introductory Oceanography (OCNG 251) Study Guide: Part 1 This half session dealt with the construction of all conditions responsible for the observed global circulation patterns in the World Ocean. In a sense, we started the course from the very end, trying to build an ocean and understanding its physical structure (the water part in this case). The objective of the entire session is to understand the concept behind Figure 1 below: Figure 1: General circulation pattern of the ocean. Surface currents are indicated in red while deep currents are presented in blue. In Figure 1, one can see that there is a link between surface circulation (red) and deep circulation (blue). Of course, to conserve mass, there must be a link between these wo circulation patterns. Areas of “deep water formation” will transfer water from the surface to the deep ocean whereas water returns to the surface via zones of upwelling. Transfer from the surface to the deep ocean will occur due to densification (increased density) of surface water (mostly through cooling but also through some increased salinity during ice formation and salt concentration in seawater). Upwelling will occur through physical transfer from current formation (Ekman circulation in eastern ocean basins) and as water is pushed up continental slope (like when the North Atlantic Deep Water is pushed up the slope of the Antarctic continent). These features are all shown in Figure 1 with areas of deep water formation as purple dots (North and South Atlantic), and areas of upwelling with blue‐to‐red arrows (eastern regions of ocean basins). Also note that surface currents are characterized by circular patterns, called gyres, in each oceanic basin for each hemisphere (Atlantic and Pacific each have 2 gyres, whereas the Indian has only 1). The entire purpose of the first session was thus to bring all the elements necessary to comprehend the processes responsible for the ocean circulation illustrated in Figure 1. These elements are: OCNG 251 (Dr. P. Louchouarn) – Study Guide #1 2 ‐ Systems and cycles. Specifically, how mass and energy cycle through different section of a system (from the micro‐ to macro‐scales). In this section we emphasized notions of reservoir, flux, source/sink, residence time, steady state, as well as positive and negative feedback mechanisms. ‐ Physical properties of water and, in particular, how temperature and salinity affect the density of seawater. We also focused on heat capacity to explain the temperature changes different media experience (i.e. atmosphere vs. ocean, continents, vs. oceans, etc) when subjected to a gain or loss of heat. ‐ Heat budget of the earth, particularly with respect to the unbalance in incoming short wave radiations and outgoing long wave radiations that is observed in inter‐tropical vs. high latitude zones. ‐ Atmospheric circulation, as it is driven by that same unbalance in the earth heat budget and affected by the earth’s rotation ( Coriolis). The interplay of these processes then leads to global as well as seasonal wind patterns (e.g. easterlies/westerlies and monsoons, respectively). ‐ Surface ocean circulation, driven itself by the wind drag of constant winds and affected by coriolis, vorticity, and geostrophic forces. Except for the effect of local winds, the general surface ocean circulation follows the atmospheric High/Low distribution pattern with circular motion (gyres) in each ocean basin. The circulation is clockwise in the north hemisphere, and counterclockwise in the south hemisphere. ‐ Deep ocean circulation, driven by density formation in high latitude zones. Surface water can undergo large increases in density due to an interplay of salinity and temperature changes. When warm water cools, its density increases markedly. Similarly, when water increases in salinity, its density increases as well. The cooling of surface seawater in northern latitudes (e.g. sub‐Arctic seas and around Antartica) leads to an increase in its density and thus vertical transfer of water towards the deep ocean. Similarly, during sea ice formation, the expulsion of salts from the forming ice results in brine formation (increase in salinity in sea waters) and thus an increase in the water density. These processes lead to deep water‐mass formation, each with specific density conditions that help or prevent their mixing in the deep ocean. ‐ Global ocean circulation. The surface and deep ocean circulations are tied at both “ends” where surface water cools (at high latitudes) to form deep waters, and where deep waters are upwelled towards to surface (mostly on eastern boundaries of oceans) to reintegrate the surface circulation loops and eventually reach the cooling sites for another cycle. On average, a full ocean circulation cycle takes several hundred years to complete (~500 yrs) but this “mixing speed” is variable and can accelerate or decelerate depending on the rate of deep water formation (cooling, salinity changes) and upwelling (wind strength, atmospheric pressure oscillation). ‐ Earth climate balance. The relationship between atmospheric and ocean circulation, help redistribute heat from zones of surplus radiation (inter‐tropical zones) to zones of deficit (high latitudes). In low latitudes, the majority of the heat transfer occurs through ocean circulation, whereas atmospheric circulation is responsible for most of the heat transfer in mid‐ to high latitudes. Event such as hurricanes are rapid and natural “pressure valve” processes that transfer large amounts of heat from the inter‐tropical zones to mid‐latitude regions. OCNG 251 (Dr. P. Louchouarn) – Study Guide #1 3 1) Systems and cycles Some Definitions Transport and transformation processes within definite reservoirs: Carbon, Rock, Water Cycles Reservoir: (box, compartment: M in mass units or moles) An amount of material defined by certain physical, chemical, or biological characteristics that can be considered homogeneous: O2 in the atmosphere; carbon in living organic matter in the Ocean; ocean water in surface water masses. Flux: (F) The amount of material transferred from one reservoir to another per unit time (per unit area): The rate of evaporation of water from the surface ocean; the rate of deposition of inorganic carbon (carbonates in marine sediments); the rate of contaminant input to a lake or a bay Source: (Q) A flux of material into a reservoir Sink: (S) A flux of material out of a reservoir Budget: A balance sheet of all sources and sinks of a reservoir. If sources and sinks balance each other and do not change with time, the reservoir is in steady‐state (M does not change with time). If steady‐state prevails, then a flux that is unknown can be estimated by its difference from the other fluxes Turnover time: The ratio of the content (M) of the reservoir to the sum of its sinks (S) or sources (Q). The time it will take to empty the reservoir if there aren’t any sources. It is also a measure of the average time an atom/molecule spends in the reservoir. Cycle: A system consisting of two or more connected reservoir, where a large part of the material is transferred through the system in a cyclic fashion Feedback: All closed and open systems respond to inputs and have outputs. A feedback is a specific output that serves as an input to the system. Negative Feedback (stabilizing): The system’s response is in the opposite direction as that of the output. An example given in class is the increased reflection of solar radiation (albedo) from upper level clouds. Increased heat evaporation clouds increased albedo lowered incoming radiation decreased overall heat. Positive Feedback (destabilizing): The system’s response is in the same direction as that of the output. An example given in class is the increased trapping of infrared radiation from lower level clouds. Increased heat evaporation clouds increased I.R. trapping increased overall heat. We also sent some time on the concept of residence time (a concept we will be using also in the second section of this course to explain the salt composition of seawater and biogeochemical cycles). Residence Time is a high probability that a certain fraction of a substance (atoms or molecules) forming the reservoir (M) will be of a certain age (mean age of the element when it leaves the reservoir). The residence time of water in the atmosphere is very short (~10‐20 days). The residence time of water in the Oceans is much longer (~4000 years). However, the residence time in different components of the atmosphere and oceans, and therefore the time of exchange between these different reservoirs, vary widely. OCNG 251 (Dr. P. Louchouarn) – Study Guide #1 4 Figure 2: Time exchange for exchange of air and water between the atmosphere and ocean. Advantages of Cycle Approach • Provides overview of fluxes, reservoir contents, and turnover time • Gives a basis for quantitative modeling • Helps to estimate the relative magnitudes of natural and anthropogenic fluxes • Stimulates questions such as: Where is the material coming from?, where is it going next? • Helps identify gaps in knowledge Disadvantages of Cycle Approach • Analysis, by necessity, superficial. Little or no insight into what goes inside the reservoir (“black box”) • Gives false impression of certainty. Often, at least one of the fluxes is derived from balance considerations (may be erroneous!) • Analysis based on average quantities that cannot always be easily measured because of spatial and temporal variations, as well as other factors. 2) Physical properties of water Water molecule: Dipole Uneven charge Hydrogen bonds! (DNA anyone?) Higher energy requirement for change of state (solid to liquid, liquid to gaseous) than similar molecules. Make sure you can explain the figure below: Figure 3: Melting and boiling temperatures for water and a series of molecules with similar chemical composition. OCNG 251 (Dr. P. Louchouarn) – Study Guide #1 5 The structure of the water molecule thus leads to very high energy requirements for changes of state (Latent Heat), in particular for changes between liquid to gaseous state.
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