ERTH 110 lab 8 EC

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California State University, Chico *

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110

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Geography

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Jan 9, 2024

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Lab 8: Atmospheric and ocean circulation (C HANGED FROM P ROF . K AREN G ROVE , S AN F RANCISCO S TATE U NIVERSITY ) Objective The objective of this lab is to view data that illustrate how the ocean circulates. Oceanic circulation greatly influences climate, which affects us all. The atmosphere and ocean work together to absorb heat and redistribute it from one part of the globe to another. Otherwise, the tropics would get hotter and hotter, and the polar regions would get colder and colder. The ocean circulates by surface currents that are driven mainly by the wind, and by deep currents that are driven mainly by density contrasts in the water produced by temperature and salinity variations. Because the wind drives surface currents, we must also look at how the atmosphere circulates. In this lab we will primarily investigate evidence of surface currents. Because of the large scale of oceanic currents, observations from satellites are often used to study their characteristics and variations. Background Ekman transport and currents Before you start you should remember and be able to predict in which directions the surface currents are flowing. In order to do this, you need information about the atmosphere and the water: Figure 1: Illustration of processes at high- and low-pressure regions In warm areas, air rises from the surface and leads to a low-pressure zone. In colder areas, sinking air is associated with a high-pressure zone (Figure 1). Hence at the surface there is a pressure gradient from high to low pressure. This pressure gradient leads to the movement of air from high- to low-pressure regions. However, the Coriolis force leads to a reflection of the moving air to the right on the northern hemisphere and to the left on the southern hemisphere. Furthermore, Oceans have a greater heat capacity than land because the specific heat of water is greater than that of dry soil and because mixing of the upper ocean results in a much larger mass 1

of water being heated than land. This causes land areas to heat more rapidly and to higher temperatures and also to cool more rapidly and to lower temperatures, compared to oceans. Hence, the difference in the heat capacity leads to a switch of high and low pressure zones between day and night and between seasons. Now, wind moves water, but the overall water transport is not in the direction of the wind. Friction between each layer of water molecules leads to an overall water transport that is 90 o the right on the northern hemisphere and to the left on the southern hemisphere. This change in direction is again a result of the Coriolis force. This water transport is called Ekman transport (Figure 2): Figure 2: Ekman transport model. Ekman transport pushes water away, which leads to a change in sea level. The sea level is higher in the direction of the water transport, and lower in the region where the water was pushed from (Figure 3). 2

Figure 3: Ekman transport and changes in sea level. This means that the weight of the water column will be higher on the "piled up" side of the basin, and gravity will act to pull the water back down the slope in the direction it came. But remember, rather than simply moving back and forth in response to the force of wind versus gravity, the Coriolis effect causes the flow to be deflected either to the right (in the Northern Hemisphere) or to the left (in the Southern Hemisphere). This deflection of water leads to geostrophic surface currents Part I. Determine Ekman transport and geostrophic currents on an Earth without land You can use your own colors or type of arrows, but then provide a legend or follow instruction below 1. The black arrows present the general wind directions in the different latitudes 2. Draw with a red pencil the direction of the Ekman transport for the different wind directions. (6 points) 3. Determine the areas of convergence (C)and divergence (D). Areas of convergence is where water would pile up, and a region of divergence is where water would be pushed away. Label with a black pencil (5 points) If you cannot draw this directly on the word document, draw the required image on a piece of paper, take a picture and copy and paste here. 3

Part II: Identify wind, Ekman transport and currents if land is present and during different seasons So now you have determined the direction of the general surface currents, the wind and the Ekman transport if there is no land present. But what happens when land is present? Again you can choose your own color or even symbols. The colors I mentioned are just suggestions. a. Consider Peru. In general, there is a barometric high on the Ocean and barometric low on land. Please draw the direction of the wind (black), Ekman transport (red) and geostrophic current (green) next to the image. (3 points)  Write H and L for high and low pressure zones  Use a dashed line to indicate in which direction wind tries to blow (wind as a result of the pressure gradient)  Peru is on the Southern hemisphere, hence wind is 90 o deflected to the left from the pressure gradient  Now draw the Ekman transport which is 90 o to the left of the wind direction  In rough approximation, are 45 o to the left of the wind direction  Determine if upwelling or downwelling is occurring off the coast of Peru. If you cannot draw directly into this word document, draw a picture and paste the picture here. 4 Land Ocean

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