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Friday, June 14, 2013

On Ocean Heat Transport - the Stumbling Block

  1. D. Rind
  2. M. Chandler  DOI: 10.1029/91JD00009

We investigated the effect of increased ocean heat transports on climate in the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The increases used were sufficient to melt all sea ice at high latitudes, and amounted to 15% on the global average. The resulting global climate is 2°C warmer, with temperature increases of some 20°C at high latitudes, and 1°C near the equator. The warming is driven by the decreased sea ice/planetary albedo, a feedback which would appear to be instrumental for producing extreme high-latitude amplification of temperature changes. Resulting hydrologic and wind stress changes suggest that qualitatively, for both the wind-driven and thermohaline circulation, the increased transports might be self-sustaining. As such, they would represent a possible mechanism to help account for the high-latitude warmth of climates in the Mesozoic and Tertiary, and decadal-scale climate fluctuations during the Holocene, as well as a powerful feedback to amplify other climate forcings. It is estimated that ocean transport increases of 50–70% would have been necessary to reproduce the warmth of various Mesozoic (65–230 m.y. ago) climates without changes in atmospheric composition, while the 15% increase used in these experiments would have been sufficient to reproduce the general climatic conditions of the Eocene (40–55 Ma). A companion experiment indicates that increased topography during the Cenozoic (0–65 Ma) might have altered the surface wind stress in a manner that led to reduced heat transports; this effect would then need to be considered in understanding the beginning of ice ages. Colder climates, or rapid climate perturbations, might have been generated with the aid of such altered ocean transports. The large high-latitude amplification associated with ocean heat transport and sea ice changes differs significantly from that forecast for increased trace gases, for which water vapor increase is the primary feedback mechanism. The different signatures might allow for discrimination of these different forcings; e.g., the warming of the 1930s looks more like the altered ocean heat transport signal, while the warming of the 1980s is more like the trace gas effect. The actual change of ocean heat transport and deep water circulation both in the past and in the future represents a great uncertainty.

For some reason there is still an issue with the relative importance of meridional and zonal energy flux control primarily by ocean pseudo-oscillations.  The last part of this abstract has the required "is more like a trace gas effect" which is one of those funding security blurbs.  The trace gases likely have an impact but change in ocean heat capacity would have the same "look".

 I put together this chart with the GISS and Hadley Center surface station data using their monthly versions by hemisphere.  Hadley also has a tropical of 30-30 global surface temperature anomaly.  The start data 1915, was selected because it was the lowest common point of all of the data sets making it more likely to be a truly "global" event.  The noisiest data set, GISS NH was used to determine the 2 sigma or 95 percent confidence range around the mean and the linear regression extended to 2060 where the GISS loti data set regression intersects the upper 2 sigma boundary.  Be my guest, pick out the trace gas "signature".

Since the ENSO region is used as a climate variable, I found a paleo reconstruction of the Indo-Pacific Warm Pool by Oppo that shows that pre-1915, the IPWP was warming from a two sigma event circa 1700 AD.

This is the Oppo IPWP reconstruction with the ever popular Central England temperature record.  Not a perfect match but a pretty fair correlation.

Here is the full Oppo reconstruction with the Hadley Center 30-30 (tropics) splice to the end with some massaging to get the decadal "bins" centered.  That is an excellent fit.  Where do you imagine the +/- 2 sigma range would be on this chart?  That's right, the "hemisphere" monthly noise range is about equal to the normal range of natural variability at least since 0 AD.  The hemispheres are seasonally 180 degrees out of phase in the mid to upper latitudes so a change in the seasonal heat distribution produces about the same "global" surface temperature impact as a longer term ocean pseudo-oscillation would produce.  If you have enough time, these pseudo-oscillation will average to some zero impact, how long do you have to wait?

Based on the Oppo reconstruction, about 1000 years.  I can confidently say that today is warmer than it has been in about 800 years, but 900 years ago it was likely warmer.  Since the warmer and cooler never got outside the 2 sigma range, about the best accuracy you can expect with paleo reconstructions, I could also say there has been no significant change in "Global" surface temperature for the past 2000 years. 

If you are a fan of BEST land only data to make your point, since it has a larger variance or standard deviation, the 2 sigma range would be larger.  We could probably take insignificant back to the Roman optimum or the very beginning of the Holocene.

"Global" climate doesn't change much, but regional can have 20 degree swings, possibly more.  We need to get past this ocean heat transport stumbling block and cut out the statistical games if climate science is going to more forward.

Just my two sigma cents.

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