Deep Ocean Thermohaline Circulation

Name: _____________________ OCN 201 (Physical Oceanography) Deep Ocean (Thermohaline) Circulation Introduction Circulation in the ocean is unified through the “global conveyor belt” which connects surface and deep ocean circulation, transporting heat and salt on a global scale. Deep ocean circulation is driven primarily by slight differences in seawater density that are caused by variations in temperature and salinity. Thus, deep ocean circulation is referred to as thermohaline (temperature-salinity) circulation . Thermohaline circulation involves the creation and movement of unique water masses . These are large homogeneous volumes of water that possess a characteristic range of temperature and salinity. Most deep waters masses form at high latitudes at the ocean surface, where they acquire their unique temperature and salinity. In the high latitudes of the North Atlantic, for example, the salty waters from the Gulf Stream are chilled, resulting in the formation of sea ice. Salt excluded from the formation of the ice, further enhances the salinity of the water. This cold, salty water is denser than the surrounding water and thus sinks, becoming North Atlantic Deep Water (NADW). Eventually, the water reaches a depth where the surrounding area has the same density and the water mass begins to flow along “horizontally” channeled by sub-marine features. These waters gradually warm and mix with overlying waters as they flow towards lower latitudes rising slowly at the rate of only a few meters per year. In the deep, waters move slowly in comparison to the well-defined gyres of surface currents. Water at the bottom of the Pacific can be 1500 years old and may take as long as 1,000 years to move through the conveyor. The identification of these water masses allows scientists to monitor the transport of water on a global scale. Assignment Welcome to the middle of the Atlantic Ocean! You are a Physical Oceanographer, embarking on your first research cruise to learn about different water masses. You want to know how cold, dense, and saline the North Atlantic Ocean is at different depths. You’re so excited, you forget to bring your Dramamine ® , and you get super seasick for the 2 months you’re out at sea. Don’t worry – you were still able to collect your water samples (your data). Now, it’s time to analyze your data! In this exercise, you will analyze water samples collected from the North Atlantic Ocean. Each sample, although from the same geographic location, has been collected from a different water depth. Using the information provided for temperature and salinity, you will determine resulting density of the water and propose an appropriate water mass name. Table 1 provides information regarding the characteristics of the major water masses. You will use this information to help you identify the water mass for each of your samples. Table 1. Water Mass Identification Chart

Name: _____________________ OCN 201 (Physical Oceanography) The following data table (Table 2) provides information about your water samples. On the Density T-S Diagram (Figure 1) plot the temperature and salinity values given below and label each data point with its water depth. In Table 2 below, record the density you have determined. Using Table 1, propose an appropriate water mass name for each sample based upon the density you have determined; the temperature and salinity values will help clarify the information. Record this name in the space provided in Table 2 below. Table 2. North Central Atlantic Ocean Sample Data Water Mass Name Temperature Range ( ° C) Salinity Range (‰) Density (g/mL) Antarctic Bottom Water (AABW) 0.0 34.6-34.8 1.0275 – 1.0280 Antarctic Intermediate Water (AAIW) 3.0-6.0 34.1-34.3 1.0270 – 1.0275 N. Atlantic Central Surface Water (NACSW) 9.0-17.0 35.1-36.3 1.0265 – 1.0270 Mediterranean Intermediate Water (MIW) 9.0-14.0 35.6-36.5 1.0275 – 1.0280 North Atlantic Deep Water (NADW) 3.0-6.0 34.1-34.4 1.0275 – 1.0280 Depth (m) Temperature ( ° C) Salinity (‰) Density (g/ mL) Water Mass Name 100 15.0 36.0 1.0265 NACSW 500 4.0 34.2 1.0272 AAIW 1000 10.0 35.8 1.0275 MIW 2000 4.0 34.9 1.0277 AABW

Name: _____________________ OCN 201 (Physical Oceanography) Now, you must label your water masses in Figure 2 . This diagram shows a cross-section of the ocean, including your sample site. Label each area , based on your determinations in Table 2, according to depth with the appropriate water mass abbreviation. See Table 1 for water mass names and abbreviations. Figure 2 Answer the following questions: 1. The dominant water mass in this region is the Atlantic ocean. 2. Is the saltiest, most dense water always at the bottom of the ocean? Why? Yes the water with the highest salinity is going to be at the bottom of the ocean, because at the surface of the ocean the salt does not get absorbed. The saltier water sinks and since the sun only

Name: _____________________ OCN 201 (Physical Oceanography) warms up the surface of the ocean. The water is a lot cooler at the ocean floor, meaning that the water molecules move at a slower pace which means that the saltiest water has a higher density. 3. What does thermohaline circulation mean? How long does the deep water take to circulate around the world? It’s the movement of water that circulates from one ocean to another, based on the temperature and salinity. It takes roughly 1000 years for the water to circulate the world. 4. How is deep water formed? When fresh water glaciers form and the salt is excluded, the salt makes that region of water dense, so it sinks and starts the thermohaline circulation. EXTRA CREDIT (Up to 9 points) Figure 3 *lighter grey = warm surface current *darker grey = cold deep current At the surface, the current is driven by wind, while the deep ocean is driven by thermohaline circulation. Thermohaline Circulation is constantly considered the “Global Ocean Conveyor Belt” of water currents. Sure, we can think about it this way because once the deep water is formed at the surface and moves down to the deep ocean, beginning to circulate, it stays there