AMERICAN METEOROLOGICAL SOCIETY The Maury Project Density-Driven Ocean Circulation TEACHER’S GUIDE A The Maury Project This guide is one of a series produced by The Maury Project, an initiative of the American Meteorological Society and the United States Naval Academy. The Maury Project has created and trained a network of selected master teachers who provide peer training sessions in precollege physical oceanographic education. To support these teachers in their teacher training, The Maury Project develops and produces teacher's guides, slide sets, and other educational materials. For further information, and the names of the trained master teachers in your state or region, please contact: The Maury Project American Meteorological Society 1200 New York Avenue, NW, Suite 500 Washington, DC 20005 This material is based upon work supported by the National Science Foundation under Grant No. ESI-9353370. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation. © 2018 American Meteorological Society (Permission is hereby granted for the reproduction of materials contained in this pub- lication for non-commercial use in schools on the condition their source is acknowl- edged.) i Forward This guide has been prepared to introduce fundamental understandings about the guide topic. This guide is organized as follows: Introduction This is a narrative summary of background information to introduce the topic. Basic Understandings Basic understandings are statements of principles, concepts, and information. The basic understandings represent material to be mastered by the learner, and can be especially helpful in devising learning activities and in writing learning objectives and test items. They are numbered so they can be keyed with activities, objectives and test items. Activities These are related investigations. Each activity typically provides learning objectives, directions useful for presenting and completing the activity and questions designed to reinforce the learning objectives. Information Sources A brief list of references related to the guide topic is given for further reading. ii If we except the tides, and ... those that may be creat- ed by the wind, we may lay it down as a rule that all the currents of the ocean owe their origin to difference of specific gravity between sea water at one place and sea water at another; for wherever there is such a difference, whether it be owing to difference of temperature or to dif- ference of saltiness, etc., it is a difference that disturbs equilibrium, and currents are the consequence. Matthew Fontaine Maury from The Physical Geography of the Sea. 1855. iii Introduction: Density-Driven Ocean Circulation The oceans are layered systems. Winds, waves and currents stir the ocean surface to depths of a few hundred meters to form a mixed layer having relatively uniform temperature and salinity. Below the mixed layer in the low and mid-latitudes, there are other layers which together make up the remaining 80% of the ocean’s volume. These layers include: intermediate water immediately below the mixed layer; deep water extending from below the intermediate water to near the ocean bottom; and bottom water in contact with the sea floor. Density (mass per unit volume) differences of seawater, however small, drive the layered circulation of the ocean. Surface sea water made more dense by cooling, becoming saltier, or by mixing, sinks to depths where its density is the same as surrounding water at the same level. There it spreads horizontally, leafing between waters of lesser density above and greater density below. It will continue spreading outward, at a very slow pace compared to surface ocean currents, as more water of the same density arrives from above. The densities of seawater at the ocean surface can increase by cooling when energy is lost to the atmosphere through radiation, conduction and evaporation. Densities also increase when seawater salt concentrations increase in regions where evapora- tion exceeds precipitation and runoff. Greater seawater densities also occur in isolat- ed polar regions when salt is left behind in the remaining water when sea ice forms. Since density variations driving the deep circulation are due to differences in tem- perature (thermal) and salinity (haline), the deep ocean circulation is also referred to as thermohaline circulation. There are only a few locations worldwide where surface seawater can become dense enough to sink to the ocean depths. In most of these source regions, denser water is produced by air-ocean interaction over marginal seas which have limited exchange with the moderating influence of the open ocean. The intense cooling or evaporation over these basins, partially separated from the open ocean by land or submarine barriers, produces dense seawater that sinks. The water flows over the shallow sill leading to the open ocean and descends along the continental shelf and slope to reach its final depth. During its descent along the rough bottom, the water may be physically mixed with lighter seawater, thus forming less dense intermediate and deep layers. Also, there are open ocean source regions where surface mixing produces water of greater density. The tendency of water to seek its own density level within the ocean leads to the formation of water masses. Water masses are bodies of water that are fairly uniform in their densities and are identifiable from their temperature, salinity and other char- 1 acteristics. Since waters of different density are slow to mix, water masses maintain their narrow temperature and salinity ranges. This permits their identification and tracing long after they have left their source regions. Water masses are named for both their source region and the ocean layer in which they are found. Antarctic Bottom Water, the densest water mass, forms in the cold Antarctic ice shelves and flows slowly northward along the sea floor, into all the ma- jor ocean basins. As part of a slow thermohaline, or density-driven circulation, it may not return to the surface for hundreds or even thousands of years. Scientists study the connections between density driven ocean currents and wind driven surface currents. One area of research centers on how small changes in either salt or heat content could dramatically change the direction of ocean flows and thus influence weather and climate on local, regional and global scales. These whole-ocean circulation studies should help scientists to better understand the role that the oceans play in global cycles. 2 Basic Understandings The Role of the Sun 1. The Sun is the ultimate source of energy that brings about the circulation of the ocean. 2. Because of astronomical and atmospheric factors, ocean surfaces in tropical regions receive more of the sun’s energy over the course of a year than do those at higher latitudes. 3. In tropical regions, the radiant energy received from the Sun exceeds the ra- diant energy lost from Earth to space. At higher latitudes, radiation loss from Earth is greater than the solar input. 4. The imbalance between radiant energy gains and losses at different latitudes results in a poleward flow of heat that is almost entirely accomplished by atmo- spheric motions. The Layered Ocean 1. The downward transfer of heat from the warm ocean surface is caused mainly by the physical mixing of waters. 2. The uppermost few hundred meters are well stirred by winds, waves and cur- rents to form a mixed layer having relatively uniform temperature and salinity. 3. Below the mixed layer, in low and mid-latitudes, there is a layer in which the temperature decreases rapidly with increasing depth. This permanent feature commonly extends from the base of the mixed layer to a depth of about 1000 meters and is called the thermocline. 4. Extending below the thermocline are other layers which make up 80% of ocean’s water volume and throughout which the temperatures are within a few degrees of the freezing temperature. These layers include: intermediate water, nearest to the thermocline; deep water, extending to near the ocean bottom; and bottom water, in contact with the sea floor. 3 Density-Driven Circulation 1. Small differences in seawater densities drive the circulation of the deep ocean. 2. Water that is denser than the surrounding surface water will sink until it reaches a level at which it is denser than the water above but less dense than the wa- ter below. There, it will spread horizontally as more water of the same density arrives from above. 3. This density increase can happen by cooling through energy loss to the atmo- sphere above via radiation, conduction and evaporation, by becoming more salty from water loss via evaporation to the atmosphere, or by mixing. 4. In polar regions, the formation of sea ice can also increase seawater salinity and density. Ice formation leaves behind dissolved substances which concen- trate in the seawater that remains. 5. Seawater salinity is increased in locations where water loss by evaporation from the ocean surface exceeds water gain from precipitation. This occurs most significantly in sub-tropical regions. 6. Sinking denser seawater displaces less dense seawater, which eventually returns to the surface and, once at the surface, eventually flows to a region of cooling, evaporation, freezing, or mixing. Thermohaline Circulation 1. Since the density variations driving the deep circulation are due to differences in temperature (thermo) and salinity (haline), the deep ocean circulation is also referred to as a thermohaline circulation. 2. Thermohaline circulation is initiated at the ocean surface by temperature and salinity conditions that produce higher density water, which sinks and slowly spreads horizontally at depths determined by water densities. 3. The speed of a thermohaline current is a small compared to the speed of wind-driven surface currents. 4. Water leaving the surface as part of the thermohaline circulation may not return to the surface for hundreds or thousands of years.