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Timeseries and Hovmoller diagrams {#hovmoller}

notebook filename | 05-Hovmoller.Rmd

This exercise examines the marine heatwave that was observed in the North Pacific Ocean off the west coast of North America, also know as The Blob. It was first detected in late 2013 and continued to spread throughout 2014 and 2015.

The exercise demonstrates the following ERDDAP features

  • mapping data in an area of interest
  • creating a time series at a specified latitude and longitude coordinate
  • creating a Hovmoller diagram

Visualize the Blob with SST

The Blob was strong in the northern Pacific. Let's compare SST in the area after the Blob subsided and during the Blob.

Selecting the SST dataset
For this demonstration, use the Multi-scale Ultra-high Resolution sea surface temperature (MUR SST) dataset. MUR is a dataset containing data from many sensors, to increase the number of pixels that containing data. Where gaps in the measurements do exist, interpolated data fill the gaps.

  • Enter the following URL into your browser: https://coastwatch.pfeg.noaa.gov/erddap/ or Google "ERDDAP west coast".
  • In the search box type "MUR SST" and click the "Search" button.

Several MUR datasets show up in the search results. Since we will be visualizing data over many years, select the version of the dataset that contains monthly averages.

  • Locate the dataset with the title "Multi-scale Ultra-high Resolution (MUR) SST Analysis fv04.1, Global, 0.01°, 2002-present, Monthly". Alternately, you can add the dataset ID "jplMURSST41mday" to the search box to narrow your search.
  • Click on "graph" in the "Make A Graph" column to the left of the dataset.

Zoom the map in on the northern Pacific

  • Change the minimum and maximum latitude and longitude values using the latitude and longitude widgets:
    • Latitudes: 20, 60
    • Longitudes: -179, -100

Create a map for a time when Blob conditions were not present (time after 2016)

  • Change time widget to select January 2017 (2017-01-16T00:00:00Z)
  • Make sure that the "color" drop down menu has "sst" selected
  • Click "Redraw the Graph"
  • Now go to the "File Type:"' drop down menu and select ".largePng"`
  • Go to the "view the URL:" box and copy the URL.

https://coastwatch.pfeg.noaa.gov/erddap/griddap/jplMURSST41mday.png?sst[(2017-01-16T00:00:00Z)][(20.0):(60.0)][(-179.99):(-100.0)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=|||||&.bgColor=0xffccccff

  • Open a new tab in your browser and paste the URL to view the image

SST map not during the Blob

Create a map for a time during the Blob
Use one of the following methods:

  1. Modify the date of a data request with the URL
  • Copy the URL from above and paste it into a new tab on your browser
  • Change the January 2017 date (2017-01-16T00:00:00Z) to January 2014 (2014-01-16T00:00:00Z)

https://coastwatch.pfeg.noaa.gov/erddap/griddap/jplMURSST41mday.png?sst[(2014-01-16T00:00:00Z)][(20.0):(60.0)][(-179.99):(-100.0)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=|||||&.bgColor=0xffccccff

  1. Use the ERDDAP GUI
  • Return to the MUR Make A Graph page
  • Change time widget to select January 2014 (2014-01-16T00:00:00Z).
  • Make sure that the "color" drop down menu has "sst" selected.
  • Click "Redraw the Graph".
  • Now go to the "File Type:"' drop down menu and select ".largePng"`
  • Go to the "view the URL:" box and copy the URL.
  • Open a new tab in your browser and paste the URL to view the image.

SST map during the Blob

Now click back and forth between the two tabs to compare Blob to non-Blob conditions. Do you see the blob? There is a pretty subtle distinction between images on the two tabs. During Blob conditions, the isotherms should be shifted northward in the northern Pacific.

Visualize the Blob with an SST Anomaly

A better way to graphically the Blob is by looking at the temperature anomaly. Temperature anomaly datasets show the deviation of SST from a long-term mean (climatology), i.e. how is the SST different from the average value.

Selecting the SST anaomaly dataset For this demonstration, use the Multi-scale Ultra-high Resolution sea surface temperature anomaly (MUR Anom) monthly dataset (ERDDAP ID = jplMURSST41anommday).

  • In your browser, go back to the ERDDAP search page: https://coastwatch.pfeg.noaa.gov/erddap/.
  • In the search box type "jplMURSST41anommday" and click the "Search" button.
  • Only one dataset should show up. Click on "graph" in the "Make A Graph" column to the left of the dataset.

Mapping the Blob

  • Change the minimum and maximum latitude and longitude values using the latitude and longitude widgets:
    • Latitudes: 20, 60
    • Longitudes: -179, -100
  • Change time widget to select January 2014 (2014-01-16T00:00:00Z).
  • Make sure that the "color" drop down menu has "anom" selected.
  • Click "Redraw the Graph".
  • Now go to the "File Type:"' drop down menu and select ".largePng"`
  • Go to the "view the URL:" box and copy the URL.
  • Open a new tab in your browser and paste the URL to view the image.

SST map during the Blob

Now the Blob is clearly visible as an area of up to +3 degrees warmer than the long-term mean! Note how the color bar has changed. By default anomaly datasets have a color bar that differentiates between negative and positive values.

Creating a timeseries

How long did the Blob last? To find out let's make a time series of data from a point within the Blob.

  • In the Graph Type widget change from surface to lines and markers
  • In the time widget, enter a Start time of 2013-01-01, which is before the Blob started, and a Stop time of 2018-01-16T00:00:00Z.

Pick latitude and longitude coordinates that are within the Blob area.

  • Enter 45 as the latitude
  • Enter -143 as the longitude
  • Click the "Redraw the Graph" button

SST anomaly timeseries at 45N, 143W

Creating a 2D timeseries (Hovmoller) plot

The time series plot shows the progression of the Blob at a single latitude and longitude point. A better way to visualize the Blob is to create a Hovmoller diagram (https://en.wikipedia.org/wiki/Hovm%C3%B6ller_diagram), which is a two-dimensional timeseries. For this demonstration, we will plot along a longitude range, from -179E to -125E, at a latitude of 45N over time.

  • In the Graph Type widget change lines and markers back to surface
  • In the Y axis widget change latitude to time
  • Enter 2013-01-01 as the Start time
  • Enter 2018-01-01 as the Stop time
  • Enter 45 as the latitude
  • Enter -179 as the minimum longitude
  • Enter -125 as the maximum longitude
  • Click the "Redraw the Graph" button

SST anomaly Hovmoller at 45N

The plot shows that the blob moved east throughout 2014 and 2015, and then expanded across most of the eastern basin at the end of 2015.

Try this on your own

Create a Hovmoller diagram plotted against latitude rather than longitude?

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