December 6, 2006

Oceans “Warming or Cooling”?

The oceans are warming, the buildup of greenhouse gases is certainly to blame, and the warmer oceans, particularly in low latitudes, are firing up bigger hurricanes than ever before – right? An article has appeared recently in Geophysical Research Letters by a team of scientists from the National Oceanography Centre at the University of Southampton, UK and the School of Earth, Ocean and Environmental Sciences at the University of Plymouth, UK. The title immediately caught our attention at World Climate Report: “Anomaly of heat content in the northern Atlantic in the last 7 years: Is the ocean warming or cooling?”

Is the sky blue? Is the Pope Catholic? Does a bear…, nevermind, you get the idea. Just the title of the article suggests to us that there could be some debate about whether the ocean is warming or cooling among oceanographers at highly respected institutions, and the fact that an excellent peer-reviewed journal like Geophysical Research Letters published the article tips us off to what should be an interesting set of findings.

Ivchenko et al. write in the first sentence of the abstract “Whether the North Atlantic Ocean is warming or cooling is an important question both in physical oceanography and climate change.” We certainly agree, but are amazed that there is a serious question to be addressed. After all, every one knows the North Atlantic Ocean, like all oceans must be warming. They note that “The Atlantic Ocean, and especially the North Atlantic ocean are an area of great scientific interest because of its influence on climate and weather in Northern Europe and in North America.” We absolutely agree.

They move on and state “There is a unique opportunity to study these questions by using the Argo profiling float data. Argo’s aim is to distribute an array of about 3000 floats (i.e. one float in every 3° square of ocean) in the ice-free regions of the World Ocean. Each float provides high quality temperature and salinity data from the surface to 2000 m.” Further, they note that “For the analysis we used the data from between 10°N and 70°N and for the period between January 1999 to December 2005.” Please note that their dataset includes the 2005 year that produced the incredible hurricane season of a year ago.

The team used the data from the Argo network to calculate an “anomaly of heat content” (AHC); they state that “The differences between the observed (Argo) values and monthly climatology were calculated by subtracting from the observed value the climatological value interpolated to the point of observation and these are called anomalies.” Importantly, they write “This climatology is based on historical hydrographic data from the end of the 1890s up to 2001.”

Basically, positive values of their AHC indicate more ocean heat content in the 1999-2005 period than the average during the 100 years prior and negative values indicate less heat content than average in the most recent period. Now for a World Climate Report highlight – they tell us “For the whole North Atlantic domain the analysis shows a negative AHC in the upper 1500 m” and “The upper 1500-m layer of the North Atlantic show persistent negative values during the whole 7 year period.” Figure 1 shows that at almost all depths, including the surface, the entire North Atlantic Ocean has less heat content from 1999-2005 than on average during the previous 100 years. However, global warmers will quickly note that the same figure shows warming, but we note that despite the warming, the values of heat content for 1999-2005 are largely still below the 100 year average.


Figure 1. Anomaly of heat content (AHC) of the North Atlantic. (A) Vertical distribution of the time averaged AHC. The horizontal bars represent one standard deviation. (B) AHC for the layer between 0 and 1500 m. The vertical bars represent one standard deviation; blue is AHC, green is a moving averaged AHC and red is the AHC filtered with a 7 point low pass filter. The magenta represents a linear regression (from Ivchenko et al., 2006)

The researchers go on to state “The southern and mid North Atlantic subdomains between 10°N and 50°N contributed mainly to the total negative value of the AHC.” Last we checked it is the lower latitude ocean conditions that control such things as hurricanes, and it is there that the negative AHC values dominate. Their heat content graph, Figure 2, further shows that in the lowest latitudes, the trend in heat content is clearly downward or non-existent. Any warming appears only north of 50°N.


Figure 2. Zonally averaged anomaly of heat content of the upper 1500 m for the 10° strips: (A) between 10° and 20°N; (B) between 20° and 30°N; (C) between 30° and 40°N; (D) between 40° and 50°N; (E) between 50° and 60°N; (F) between 60° and 70°N (from Ivchenko et al., 2006)

Somewhat surprisingly to us, they conclude “The exciting result is that the warming takes place in all the studied layers in the northern part of the domain.” To say the least, we at World Climate Report found many other aspects of their findings “exciting” – that’s why the global climate change debate is alive and well, and likely to continue well into the foreseeable future.

Reference:

Ivchenko, V. O., N. C. Wells, and D. L. Aleynik (2006), Anomaly of heat content in the northern Atlantic in the last 7 years: Is the ocean warming or cooling?, Geophysical Research Letters, 33, L22606, doi:10.1029/2006GL027691.




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