Modis "True-Color" Satellite Images

Modis "True-Color" Satellite Images

This website presents Modis Aqua and Terra satellite true-color images from the North Pacific and Western Arctic. True color images are not photographs that contain all the wavelengths a camera would capture. They are composites of data from three wavelengths measured by the satellite. These are: a red band centered at 645nm, a green band centered at 555nm, and a blue band centered at 469nm. We obtain the image files from NASA's Ocean Color browser at Goddard Space Flight Center (oceancolor.gsfc.nasa.gov). NASA also provides SeaDAS, the software we use to process the files.

The swaths observed by the Modis satellite are 2330 km wide and are divided into 5-minute granules of data. The dark seam you will see in some of the images is a small gap between two consecutive granules. The satellites are polar orbiting; that is, their track takes them near the north and south poles. A pass takes about an hour and 40 minutes to complete. Near 20Z or 21Z the satellite passes over the west coast of North America. Successive passes are farther west, so that by 00Z or 01Z, it is located over the central or western Bering Sea.

True color images share a major drawback with chlorophyll images: the wavelengths used are blocked by clouds and are only present during daylight. That means that the images we present are somewhat self-selecting. For example, we can't see what is happening to the Bering Sea ice during cloudy stormy conditions, although we can see the storm clouds and the results of the storm once the clouds part. In addition, since the images only contain information from three wavelengths, they may not show features that might be apparent at other wavelengths. It can also be difficult to decide the cause of some features in the images. For instance, sunglint, haze and frazil ice can look a lot alike, and blue-green water might be caused by sediment, coccoliths, other plankton,the shadow from a cloud, or simply low light conditions.

contour truecolor

That being said, true color images can provide qualitative information beyond the contour plots we often use to describe data. For example, a true color image of the ice cover in the Bering Sea allows us to make inferences about ice thickness and floe sizes and which way the wind is blowing, as well as ice concentration. Compare the contour plot of ice concentration on the left with the true color image from the same date on the right.

Ocean color patterns are interesting both because of the cause of the color - mainly sediment or chlorophyll - and because they often outline features in the circulation such as eddies and gradients. Frazil ice and ice floes can also be carried in, and therefore illustrate, circulation patterns such as eddies.




We have divided the images into sections according to the regions they depict. Many of the images include more than one area, and are therefore repeated in more than one section. In addition, we have divided each year's images into three-month periods; January-March,April-June,July-September, and October-December. There there is little to no light in midwinter, especially in the Arctic, so there are few images between November and February.


Images from the Bering Sea and Sea of Okhotsk:

January-March: April-June: July-September: October-December:
2000 2000 2000 2000
2001 2001 2001 2001
2002 2002 2002 2002
2003 2003 2003 2003
2004 2004 2004 2004
2005 2005 2005 2005
2006 2006 2006 2006
2007 2007 2007 2007
2008 2008 2008 2008
2009 2009 2009 2009
2010 2010 2010 2010
2011 2011 2011 2011
2012 2012 2012 2012
2013 2013 2013 2013
2014 2014 2014 2014
2015 2015 2015 2015
2016 2016 2016 2016
2017 2017 2017

Images from the Chukchi and Beaufort Seas

January-March: April-June: July-September: October-December:
2000 2000 2000 2000
2001 2001 2001 2001
2002 2002 2002 2002
2003 2003 2003 2003
2004 2004 2004 2004
2005 2005 2005 2005
2006 2006 2006 2006
2007 2007 2007 2007
2008 2008 2008 2008
2009 2009 2009 2009
2010 2010 2010 2010
2011 2011 2011 2011
2012 2012 2012 2012
2013 2013 2013 2013
2014 2014 2014 2014
2015 2015 2015 2015
2016 2016 2016 2016
2017 2017 2017

Images from the Gulf of Alaska

January-March: April-June: July-September: October-December:
2000 2000 2000 2000
2001 2001 2001 2001
2002 2002 2002 2002
2003 2003 2003 2003
2004 2004 2004 2004
2005 2005 2005 2005
2006 2006 2006 2006
2007 2007 2007 2007
2008 2008 2008 2008
2009 2009 2009 2009
2009 2009 2009 2009
2010 2010 2010 2010
2011 2011 2011 2011
2012 2012 2012 2012
2013 2013 2013 2013
2014 2014 2014 2014
2015 2015 2015 2015
2016 2016 2016 2016
2017 2017 2017

Images from the Pacific Northwest

January-March: April-June: July-September: October-December:
2000 2000 2000 2000
2001 2001 2001 2001
2002 2002 2002 2002
2003 2003 2003 2003
2004 2004 2004 2004
2005 2005 2005 2005
2006 2006 2006 2006
2007 2007 2007 2007
2008 2008 2008 2008
2009 2009 2009 2009
2010 2010 2010 2010
2011 2011 2011 2011
2012 2012 2012 2012
2013 2013 2013 2013
2014 2014 2014 2014
2015 2015 2015 2015
2016 2016 2016 2016
2017 2017 2017

Images of coccoliths in the Bering Sea

Many years, there is a coccolith (Emiliania huxleyi) bloom in the southern Bering Sea. Coccoliths are a type of phytoplankton whose shells are made of calcite. Blooms are often first visible in the outer reaches of Bristol Bay in late summer as bright turquoise patches in satellite images. In some of the images you will also see greener areas close to the coast. These areas are likely to be suspended sediment similar to those seen in Cook Inlet or near shore in the eastern Gulf of Alaska. There were several years in this time series that clouds either hid or discouraged the bloom.

Cloud Images

Of course, almost all of the images have clouds in them. However, some images show some really nice clouds. These links takes you to images of fronts,cloud streets, Van Karman vortices, smoke from forest fires, lee waves, gap winds, low clouds and fog streaming through passes and channels, and some "what's happening here?" clouds. See Gulf of Alaska Cloud Images and Bering Sea and Aleutian Islands Cloud Images