>The KNMI Climate Explorer webpages contain a wealth of data. The start page is here:
http://climexp.knmi.nl/start.cgi?someone@somewhere
The Monthly Observation data selection page is here:
http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere
Included are Cloud Cover data sets from CRU, ICOADS, and ISCCP. This post provides graphs of Ocean Cloud Cover for the globe and numerous subsets based on ICOADS 2-degree enhanced data. The CDC’s webpage is here:
http://www.cdc.noaa.gov/cdc/data.coads.2deg.html
Though the data does run back as far as 1800, it is sporadic until 1950, which is why the following graphs begin in 1950. The KNMI versions are updated as needed or as requested, so the cloud cover data ends mid-year in 2007. This is the reason the graphs of annual data end in 2006. (It would be nice to see what’s happened over the past two years, but I have not requested an update since this is only a blog, not serious data analysis.) The graphs that follow are also based on the raw okta data, not anomalies. An okta is a unit of cloudiness measurement, where a 0 oktas equals clear sky and 8 oktas equals complete cloud cover. OKTAS DO NOT DIFFERENTIATE BETWEEN CLOUD TYPE.
I will do a batch of anomaly comparisons in an upcoming post.
GLOBAL
Figure 1 illustrates annual Global Ocean Cloud Cover from 1950 to 2006. The trend in cloud cover appears steepest from 1950 to 1960. The upward trend decreases from 1960 to the mid-1970s, then increases again until its peak in 1996, the year before the 1997/98 El Nino. After a dip and rebound, the Global Ocean Cloud Cover dropped steadily from 1998 to 2006.
http://i42.tinypic.com/20ixkle.jpg
Figure 1
SOUTHERN HEMISPHERE
The Ocean Cloud Cover for the Southern Hemisphere is shown in Figure 2. From 1960 to its peak in 1998, the underlying trend appears exponential. Afterwards, Southern Hemisphere Ocean Cloud Cover dropped through 2006.
http://i43.tinypic.com/femmwo.jpg
Figure 2
NORTHERN HEMISPHERE
In Figure 3, the Ocean Cloud Cover for the Northern Hemisphere is shown. Cloud cover rose quickly from 1950 to 1961. The trend flattened significantly from 1961 to 1983 until the year after the El Chichon volcanic eruption. The flat period coincidentally agrees the period of increased volcanic activity. Northern Hemisphere Ocean Cloud Cover rose again, with a minor fluctuation, until 1992. The severe drop in 1993 appears to be the result from the Mount Pinatubo eruption. Ocean Cloud Cover for the Northern Hemisphere then seems to react to ENSO until 2006.
http://i40.tinypic.com/11snxj6.jpg
Figure 3
TROPICS
Figure 4 shows the Tropical Ocean Cloud Cover from 1950 to 2006. The underlying trends are difficult to pick out, but Tropical Ocean Cloud Cover appears to accelerate upwards in the early to mid-1980s, peak in 1996 before the 97/98 El Nino, then peak secondarily in 1999 and 2001, before starting to decrease until 2006.
http://i40.tinypic.com/sxoo0j.jpg
Figure 4
COMPARISONS
Figure 5 illustrates Ocean Cloud Cover for the Northern and Southern Hemispheres and Globally. Southern Hemisphere Cloud cover is significantly less than that of the Northern Hemisphere.
http://i42.tinypic.com/20kacuf.jpg
Figure 5
The Ocean Cloud Cover for the Atlantic, Pacific, and Indian Oceans are shown in Figure 6. The Atlantic Cloud Cover is slightly less than the Pacific, but the Indian is significantly less than the other two.
http://i44.tinypic.com/wvb9rs.jpg 
Figure 6
East and West Tropical Pacific Ocean Cloud Cover data are illustrated in Figure 7. Before 1986, there was a substantial difference between the two data sets, but in 1987, the Eastern Tropical Pacific Cloud Cover rose, reducing the difference.
http://i39.tinypic.com/2m7aoag.jpg
Figure 7
What happened in 1987?
CLOSING COMMENT
I found the apparent impact (assumed based on timing) of volcanic aerosols on Northern Hemisphere Ocean Cloud Cover trends, and only the Northern Hemisphere trends to that extent, to be the most interesting find in the ocean cloud cover data that I’ve downloaded so far.