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