New Paper “Enhanced Warming Over The Global Subtropical Western Boundary Currents” By Wu Et Al 2012

figure from NOAA

There is a new paper [h/t to Peter Bühler] which examines an issue in the monitoring of long-term ocean heat trends. The paper is

Wu et al 2012: Enhanced warming over the global subtropical western boundary currents  29 January 2012 Nature: Climate Chang DOI: 10.1038/NCLIMATE1353

Peter also alerted  us to its wide coverage in the German press; see the title of the Spiegel Online article is

Golfstrom erwärmt sich noch schneller als der Atlantik [Gulf Stream warms up faster than the Atlantic]

with the highlighted text at the beginning of the article

Die Temperatur der Ozeane ist gestiegen – doch besonders rasch scheinen sich schnelle Strömungen wie der Golfstrom erwärmt zu haben. Forscher rätseln über die Gründe. Sie fordern eine Art Langzeit-EKG für die Ozeane, um künftige Veränderungen besser zu messen. [The temperature of the oceans has increased – but it seems to have warmed , in particular, surprisingly  fast in currents like the Gulf Stream. Researchers speculate about the reasons. They demand a kind of long-term ECG for the oceans to measure future changes better]

The abstract of the Nature article reads [highlight added]

“Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.”

The conclusion reads

“We conclude that the enhanced warming over the global subtropical western boundary currents in the twentieth century might be attributable to the poleward shift of their mid-latitude extensions and/or intensification in their strength. It should be noted that uncertainties remain large in terms of quantifying, detecting, and attributing the enhanced warming trends, owing to a lack of longterm observations of the western boundary currents and to varying algorithms used in SST reconstructions among reanalysis products. Some additional mechanisms, for example the enhanced upstream land warming, may also contribute to the warming over the western boundary current regions. The estimated errors of the trends presented here are also probably underestimated because they do not explicitly take into account uncertainties in observations, reconstruction algorithms, bias in data assimilations and so on.Nevertheless, the results highlight the importance of century-long datasets in detecting climate change signals. To detect future changes with confidence, a long-term monitoring network of western boundary current systems that builds on existing programmes is needed, particularly in regions of accelerated warming.”

There is an informative discussion of the accuracy of the renanalysis data that is used to make their conclusions. This text is at the end of their paper under the header of “ocean currents trends” and reads in part

“The oceanic current trends are calculated based on the century-long SODA reanalysis product26, which is based on the Parallel Ocean Program (POP) ocean model with an average horizontal resolution of 0.4 degreees  longitude x 0.25 degress latitude and with 40 vertical levels……To assess whether SODA captures variability of western boundary currents, we select two sections across the Kuroshio Current and the Gulf Stream where the transport has been routinely measured. Across the Pollution Nagasaki (PN line) section of the Kuroshio Current over the East China Sea, repeat hydrographic surveys have been conducted on a quarterly basis by the Japan Meteorological Agency since the mid-1950s. For the Gulf Stream, the transport of the Florida Current between Florida and the Bahamas near 27 N has been collected from calibration cruises and calibrated cable voltages since the early 1980s. The transports of the Kuroshio Current across the PN line, which is calculated from geostrophic balance, and the Florida Current are superimposed over these from SODA…The agreement is reasonably good, with a correlation of 0.6 and 0.65 (statistically significant at the 95% confidence level) for the Kuroshio Current and Florida Current during the observed period, respectively.”

My comments on this paper follow:

1.  The implication from the Nature article is that the enhanced warming of the western boundary currents is from greenhouse gas forcing; i.e. they write in the abstract that

“The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns…….the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate.”

However, they present in the conclusion [when these caveats should have been in the abstract] that

“It should be noted that uncertainties remain large in terms of quantifying, detecting, and attributing the enhanced warming trends…..The estimated errors of the trends presented here are also probably underestimated because they do not explicitly take into account uncertainties in observations, reconstruction algorithms, bias in data assimilations and so on.”

To not place these limitation in the abstract is somewhat disingenuous. The bottom-line conclusion of their study, which is robust, is the text

“To detect future changes with confidence, a long-term monitoring network of western boundary current systems that builds on existing programmes is needed, particularly in regions of accelerated warming”.

However a cause and effect of the observed warming, if it is actually occurring, cannot be concluded from their study to be due to just added CO2 and other greenhouse gases, when natural variability, solar influences, and non-CO2 human climate forcings (e.g. from aerosols, land use/land cover change) have also occurred in the last 100 years.

2. The second issue is that even a cursory examination of the data shows that the global surface temperature anomaly is spatially variable (e.g. see the figure at top of the page). It is also temporally variable, as exemplified by the figure below, which is from last year at about the same time of the year.

figure from NOAA 

3. The correlation of “0.6 and 0.65”, when they compare the reanalysis estimate of the ocean currents to actual observations, seems quite low, particularly considering that the observed data presumably went into creating the reanalysis. A correlation of 0.65 means only about 42% of the variance is captured by the reanalysis.

Final Comment:  I support the authors’ argument on the value of sea surface temperatures and ocean heat content as an ideal metric to diagnose global and regional warming and cooling. However, the authors seem to have bought into the erroneously narrow perspective that added greenhouse gases dominates any long terms trends that are monitored.

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