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This entry might ramble a bit; mainly it’s an update on some material culled from GRL over the last couple of months.

Several climate blogs are focussing on the issue of the ARGO floats and the measurement of Ocean Heat Content (OHC) and its associated anomalies, Rabett Run, RealClimate, Stoat, Climate Science, all have ongoing discussions of this issue. The discussions revolve around the recently noted errors in the measuring system and its implications for a proper assessment of the global heat budget.

Several commentators have made reference to the GRACE satellite experiment, expressing the hope that results from this will help confirm whether or not the global heat budget can be balanced, and whether it will give a result consistent with the output of the Atmosphere-Ocean Global Climate Models (AOGCMs).

But it might not be that simple. An example of why this might be the case is the paper, published last month, by Chambers et. al. The abstract reads:

The Gravity Recovery and Climate Experiment (GRACE) was designed to measure variations in the Earth’s gravity field from space at monthly intervals. Researchers have used these data to measure changes in water mass over various regions, including the global oceans and continental ice sheets covering Greenland and Antarctica. However, GRACE data must be smoothed in these analyses and the effects of geocenter motions are not included. In this study, we examine what effect each of these has in the computation of ocean mass trends using a simulation of ice melting on Greenland, Antarctica, and mountain glaciers. We find that the recovered sea level change is systematically lower when coefficients are smoothed and geocenter terms are not included. Assuming current estimates of ice melting, the combined error can be as large as 30–50% of the simulated sea level rise. This is a significant portion of the long-term sea level change signal, and needs to be considered in any application of GRACE data to estimating long-term trends in sea level due to gain of water mass from melting ice.

What I would point to in this is the highlighted sentence. The paper is not about OHC as such, but does illustrate the complexity of working with the data and the need to apply caution to any conclusions based on the use of smoothed data. No doubt, the scientists who do this work will take all due care in framing the terminology of their conclusions, but it does sort of suggest that using GRACE as a litmus test for the accuracy of the ARGO network is likely to be fraught with difficulty.

Four more papers have caught my eye:

Retroflection of part of the east Greenland current at Cape Farewell: GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L07609, doi:10.1029/2006GL029085, 2007, by Holliday et. al.;

Recent trends in Arctic Ocean mass distribution revealed by GRACE: GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L07602, doi:10.1029/2006GL029016, 2007, Morison et. al.;

Changes in the pool of Labrador Sea Water in the subpolar North Atlantic: GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L06605, doi:10.1029/2006GL028959, 2007, Kieke


Interannual variability of the upper ocean carbon cycle in the northeast Atlantic Ocean; GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L07608, doi:10.1029/2006GL028145, 2007, Gonzales-Davila

The first three papers all deal with different aspects of freshwater distribution in the North Atlantic, the fourth with the separate, but connected, issue of the ocean carbon balance. There has long been speculation that changes in climate will have some impact in the future on the ocean circulation (I avoid calling it the Thermohaline Current, or THC, deliberately). A neat piece of analysis last year from Curry at Woods Hole Oceanographic Institute analysed the idea of a slow-down in the North Atlantic part of the OC. This suggested that there could be impacts from a substantial freshening in the area bounded by the Greenland-Shetland shelf, which is roughly where the Irminger Current sits.

The first paper shows a retroflection of the Irminger current away from the West Greenland (Labrador) area, back to the Irminger, of up to 5.1Sv (Svalbard units). This is a vast flow of water. The overall transport to Labrador, another key site for the overturning circulation reduces from 17+ Sv to 11+ Sv.

The second paper reveals freshening of the Arctic bottom water. Other research has suggested that this is primarily caused by influx from the Bering Sea into the Beaufort gyre, but the question here is about the potential outflow through the Fram Strait, which also feeds into the Greenland-Shetland shelf area.

The third paper shows changes in the Labrador mixing which seem to be consistent with those shown in the first paper.

Why might this be important? Taken together, and with the fourth paper appended to it, what seems readily apparent is that some of the freshening process which might precede a slow-down of the ocean circulation appears to be already under way. This is consistent with model out from the AOGCMs, but the time horizon appears to be way ahead of that suggested by the models, which consistently refuse to show a major slow-down before about 2060.

I hope some of the scientists who are involved in studying climate change and climate impacts are working on this matter already. It worries me.


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April 2007