The main aim of Part D of this mid-term exercise is to answer a series of questions based on compilation of ocean temperature and salinity data that enable evaluation of the following hypothesis, posed as a question:
- Is the thermocline in ocean gyres lowered by the process of downwelling associated with Ekman transport and geostrophic flow?
This page also provides links to the resources, previously utilized in Module 7 and in the Phase I Group Activities, Part C, needed to accomplish the task.
The Levitus atlas (Levitus Atlas (Links to an external site.)) provides access to temperature and salinity data that can be displayed as maps or profiles. Here the focus is on the Annual (year-round average) data option. Follow the sequence below to access the data as a global map of ocean surface temperature:
- First select annual (ANNUAL (Links to an external site.)) and on the next webpage choose the option “Temperature” under Datasets and variables
- In the series of tabs above the green ellipses denoting SOURCES – LEVITUS94 – ANNUAL – temp select Views and then the image to the far right labeled “colors with coasts”.
- The image presented is a color-coded annual average surface temperature map (temperature (Links to an external site.)).
- Contours can be added to provide numerical values for temperatures by the using the pull-down menu options in the bottom right and selecting colors|contours.
The figure below provides an illustration of the appearance of the the sea surface temperature of the North Pacific (N. Pacific (Links to an external site.)) with the Levitus Atlas, which is the region of focus for this part of the exercise.
It is also possible to use the Levitus Atlas to plot depth profiles of temperature at specified locations, such as the Pacific Ocean at 30°N,140°E (temperature (Links to an external site.)). Changing the values for latitude and longitude in the windows above the graph and redrawing it provides comparable data profile for any location in the ocean, although resetting the depth and temperature range may be necessary. These figures provide a visual guide to the depth of the thermocline across the North Pacific, as shown in the image below and as described and illustrated in the subsequent video.
Question 1: In the North Pacific at co-ordinates of 30°N, 120°W what is the approximate water temperature at 500 m depth and how does the temperature at this depth change at 10° longitude intervals in proceeding westward at 30°N to 130°W, 140°W, 150°W, 160°W, 170°W, 180°, 170°E, 160°E, 150°E, and 140°E?
A strong answer will specify the temperature at 500m depth for each of these locations, thereby defining the changes in temperature observed in moving westward at 30°N from 120°W to 140°E.
(Please note that W longitudes, such as 130°W and 140°W, represent locations in the eastern Pacific and E longitudes, such as 150°E, and 140°E, represent locations in the western Pacific)
Question 2: Describe the overall trend in temperatures at 500 m depth in the North Pacific in proceeding westward at 30°N from 120°W to 130°E. Do these ocean temperature data for the North Pacific support or refute the hypothesis that the thermocline in ocean gyres is lowered by the process of downwelling associated with Ekman transport and geostrophic flow?
A strong answer will summarize variations in temperature at 500m depth for this transect across the North Pacific at 30°N and explain whether the observed longitudinal variations in temperature profiles with depth are consistent with the hypothesis.
Question 3: Bonus Question (5 points)
Do changes in the shape of the temperature profile with depth from the surface to ~ 700 m over the longitudinal profile of the North Pacific provide supporting evidence for a lowered thermocline in the western North Pacific? Is there other evidence for vertical transport of water in the western North Pacific that might also confirm or refute the hypothesis?
A strong answer will assess whether longitudinal changes in the depth profile of temperatures from the surface to 700m across the North Pacific provide support for the to the hypothesis and suggest how other data that reflect the vertical movement of water might contribute to the confirmation or refutation of the hypothesis.