EE107 Lab 4 Ocean Circulation - S

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Boston University *

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107

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Electrical Engineering

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Dec 6, 2023

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EE 107: Introduction to Climate and Earth System Science Lab #4 Name: Date: 23 Oct 2023_______________ Section: ______________ Laboratory # 4: Ocean Circulation In this lab, you will study dynamics of the world's oceans. In particular, you will learn what processes are responsible for the circulation of the upper surface of the oceans, versus those responsible for deep water circulation. Key Concepts: Surface Winds, Currents, Coriolis Effect, Salinity, thermohaline circulation Key Skills: Latitude and Longitude Wind-Generated Surface Currents activity adapted from Indianapolis Public Schools 1

EE 107: Introduction to Climate and Earth System Science Lab #4 Introduction Surface winds are those that are in contact with Earth’s surface. As surface winds blow across the ocean, friction drags the surface water in the general direction the air is moving. Adjacent water flows in to replace water removed and also is transported by the wind. In areas of prevailing winds, a constant flow of water is established. Large-scale, unidirectional flow of water produced by the wind is called a wind-generated current . Speed of a wind-generated current is greatest at the surface and decreases with depth because energy transfer downward in the water column is inefficient. Where currents collide with land, they are deflected and flow along the landmass as a boundary current . Each hemisphere has three major wind belts and three windless zones between them. The major surface winds are Trade Winds (0° to 30°), Westerlies (30° to 60°), and Polar Easterlies (60° to 90°). Winds are named for the direction from which they originate. For example, westerlies blow from the west to the east. As we will learn in the next lab a apparent force the Coriolis force causes the major winds to deflect to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. We call this deflection Coriolis effect Water propelled by the wind is also affected by Coriolis effect. In the north Atlantic Ocean, the Trade Winds produce the North Equatorial Current that flows westward from Africa to South America. Similarly, the Westerlies produce the North Atlantic Current that flows eastward from North America toward Europe. In each hemisphere of each ocean, the wind-driven and boundary currents combine to form a circular path called a gyre . Surface currents do not extend to great depths, but can influence deeper layers of water in several ways. In areas that are relatively shallow, surface currents can induce a weak current in the denser waters at depth. Where strong, persistent winds develop, currents move surface water away 2

EE 107: Introduction to Climate and Earth System Science Lab #4 from the land and denser underlying water may be drawn upward to replace the surface water. This is called upwelling . This denser deep water is typically very nutrient rich, and can aid the productivity of fisheries where upwelling exists. In contrast, if winds drive water toward a landmass, surface waters will accumulate and ultimately sink. This phenomenon is called downwelling . A phenomenon known as Ekman transport is important particularly in areas of coastal upwelling. Ekman transport is the net movement of surface water (a balance between the Coriolis Effect and drag forces) due to wind. It causes surface water to move at right angles to the prevailing wind direction. In the Northern Hemisphere, Ekman transport causes surface water to move 90 degrees to the right of the wind direction, and in the Southern Hemisphere, 90 degrees to the left of the wind direction. Northern Hemisphere Ekman Transport: Southern Hemisphere Ekman Transport: Wind blows move to the left Deep water circulation (i.e. thermohaline circulation ) is generated by density differences between water masses. It can produce both horizontal and vertical flow. Water density increases as water becomes cooler or more saline. Water will sink or rise to a level such that all water below it is denser and all water above it is less dense (stratification). The water mass then flows outward in all unobstructed directions, sinking below any less dense and rising above any denser water masses it encounters. Several forms of thermohaline circulation occur in the oceans. In subpolar to polar regions, cold, dense water descends to the bottom of the ocean and slowly flows towards the equator. This phenomenon is a result of sea ice formation at the poles. A process known as brine rejection occurs when ice forms from seawater: as ice forms, salts are concentrated in the surrounding water, increasing its salinity, and in turn its density. As these currents slowly flow, they mix with adjacent water masses and eventually lose their identity. Excess evaporation in a restricted sea can also produce dense saline water. Circulation through the Strait of Gibraltar, the entrance to the Mediterranean Sea, is an example of this form of thermohaline circulation. Wind Surface Water Surface Water Wind Wind Surface Water Surface Water Wind 3

EE 107: Introduction to Climate and Earth System Science Lab #4 Salty and dense water has a higher rate than ci Sink in colder and salter Water sinks near colder temp then passes the equator Definitions:  Halocline – the boundary between less salty water and saltier water (i.e. where there is a rapid change in salinity with a small change in depth.  Thermocline – the boundary between warmer water and cooler water (i.e. where there is a rapid change in temperature with a small change in depth.  Pycnocline – the boundary between less dense water and denser water (i.e. where there is a rapid change in density with a small change in depth). There are 3 pycnoclines before the red water is added – 2 haloclines and 1 thermocline.  Upwelling – the rising of less dense water through denser water.  Downwelling – the sinking of denser water through less dense water.  Advection – the fast mixing of 2 different waters, in this demonstration caused by downwelling, upwelling, and also the sinking of the red water to the bottom due to its being introduced from a higher level. Part 1. Deep Water Circulation For this exercise we need to work together as a Team!!! Record the names of the members of your team and exchange contact information in case it is needed. Make sure that you exchange the results, you will be graded as a team! Team Members in your group ____________________________________________________________________________________________ ____________________________________________________________________________________________ ____________________________________________________________________________________________ ____________________________________________________________________________________________ _______________________________________________________________________________________ Deep Water Circulation Activity The purpose of this exercise is to demonstrate how density differences (due to temperature and salinity differences) can lead to ocean circulation. Read carefully the instructions and THINK about the 4

EE 107: Introduction to Climate and Earth System Science Lab #4 processes that will occur, MAKE PREDICTIONS for the motions of fluids, and WATCH and make OBSERVATIONS of the Exercise Materials : Room temperature fresh and salt water, blue, yellow, and red food coloring, fish tank, vials, platform, boiling water, temperature probe Exercise summary :  Set up a fish tank filled with tap water . This water should be “fresh” and have essentially zero percent salinity, and should be “clear” and free of any coloring.  Prepare the following fluids that will be added to the tank: o A: a beaker of salty water , by dissolving _______ grams of salt into ______ milliliters of room temperature water. Part of this solution should be poured into a vial and dyed with food color . The vial (and salty water it contains) will be laid horizontally on a platform at one end of the tank. (part A in the figure below) o B: a beaker of warm (nearly boiling) water with no salt . Part of this solution will be poured into a flask and dyed with a different food color than the salty water . The open flask will be placed at the bottom of the tank (Part B in the figure below) at the opposite end as the vial in part A o C: a mixed solution , containing 200 mL of solution form part A, and 200 ml of the solution from part B. This fluid will be dyed with a different food color than the salty or hot water and poured gently (pour over a plastic ruler at the side) into the water tank near its center.  Make predictions about what will happen to each of these fluids as they are introduced to the tank. 5

EE 107: Introduction to Climate and Earth System Science Lab #4 If water is salty water is dense water will sink Questions : 1) Fill in the table below to indicate the initial conditions of each of the fluids in the demonstration. a) Calculate the ‰ salinity of each fluid . We will calculate a ‰ (parts per thousand) salinity as the mass of salt (g) per volume of water (mL) using the following equation: ‰ salinity = grams of salt / mL of water * 1000 (for example, a solution with 35g of salt in 1000 mL water has 35‰ salinity). 40g 1000ml =40% b) Measure the temperature of each solution c) Note the color of each solution 6

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