Why does Ocean Heat Content diverge from GISS projections?

Guest Post by Bob Tisdale:

Why Are OHC Observations (0-700m) Diverging From GISS Projections?

INTRODUCTION

My post “NODC Corrections to Ocean Heat Content (0-700m) Part 2” illustrated the divergence between observed Global Ocean Heat Content (OHC) and the GISS projected rise. Figure 1 shows that GISS models projected a rise of 0.98*10^22 Joules per year, but, since 2003, global OHC has only been rising at 0.079*10^22 Joules per year. How could there be such a significant difference between the projection and the observed OHC data?

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Figure 1

GISS FAILS TO MODEL ENSO

Roger Pielke Sr discussed the disagreement between the GISS OHC projections and observations in his February 9, 2009 post ‘Update On A Comparison Of Upper Ocean Heat Content Changes With The GISS Model Predictions’. There he refers to a communication from James Hansen of GISS, a response to Pielke Sr and Christy, in which Mr. Hansen offers the GISS OHC projection. Refer to the linked response from Hansen here:

http://www.climatesci.org/publications/pdf/1116592Hansen.pdf

NOTE: In his response to Pielke Sr and Christy, Hansen writes, “Contrary to the claim of Pielke and Christy, our simulated ocean heat storage (Hansen et al., 2005) agrees closely with the observational analysis of Willis et al. (2004). All matters raised by Pielke and Christy were considered in our analysis and none of them alters our conclusions.” The Hansen et al (2005) paper is “Earth’s energy imbalance: Confirmation and implications.“ And the Willis et al (2004) paper is “Interannual Variability in Upper Ocean Heat Content, Temperature, and Thermosteric Expansion on Global Scales.” Link to abstract:

http://www.agu.org/pubs/crossref/2004/2003JC002260.shtml

Back to the topic of this post…

In his response to Pielke Sr and Christy, Hansen acknowledges that GISS does not account for ENSO in its models. He writes, “We note the absence of ENSO variability in our coarse resolution ocean model and Willis et al. note that a 10-year change in the tropics is badly aliased by ENSO variability.”

What Mr. Hansen fails to acknowledge is that ENSO also has significant impacts outside of the tropics.

SIGNIFICANT TRADITIONAL ENSO EVENTS CAUSE UPWARD STEP CHANGES IN OHC OF OCEAN BASINS

In my post “ENSO Dominates NODC Ocean Heat Content (0-700 Meters) Data”, I illustrated the upward step changes in OHC anomalies caused by significant traditional ENSO events such as those in 1972/73 and in 1997/98. This was done through simple comparison graphs of NINO3.4 SST anomalies, Sato Index data to illustrate the timing of explosive volcanic eruptions, and NODC (Levitus et al 2009) OHC anomaly data for individual ocean basins. Figures 2 through 4 are examples. In them, I’ve also highlighted the period GISS elected to model. Hansen explains the selection of those years in the response to Pielke and Christy linked above, “Our analysis focused on the past decade because: (1) this is the period when it was predicted that, in the absence of a large volcanic eruption, the increasing greenhouse effect would cause the planetary energy imbalance and ocean heat storage to rise above the level of natural variability (Hansen et al., 1997), and (2) improved ocean temperature measurements and precise satellite altimetry yield an uncertainty in the ocean heat storage, ~15% of the observed value, smaller than that of earlier times when unsampled regions of the ocean created larger uncertainty.”

But examination of the data illustrates variations that are caused primarily by natural variation, and much of these variations are apparent responses to ENSO, a variable that GISS does not model.

Figure 2 illustrates the monthly Tropical Indian and Pacific Ocean OHC anomaly data from January 1955 to June 2009. Note how the Tropical Indian and Pacific Ocean OHC anomaly data declines from the early-to-mid 1960s to 1973, then rises during the extended La Nina of 1973/74/75/76. And even though greenhouse gases (not illustrated) are rising from the late 1970s to 1999, there is a gradual decline in Tropical Indian and Pacific Ocean OHC anomalies. Some of this decline may be caused by the eruptions of El Chichon in 1982 and Mount Pinatubo in 1991, but their impacts are difficult to determine with the ENSO-related variability of the data. Then in 1998, Tropical Indian and Pacific Ocean OHC anomalies rise again during the multiyear La Nina that followed the significant 1997/98 El Nino. So regardless of the impacts of the El Chichon and Mount Pinatubo eruptions, the largest rises in OHC occurred during the two multiyear La Nina events associated with the El Nino events of 1972/73 and 1997/98. Also note that the period GISS elected to model captures one of these natural ENSO-induced upward step changes.

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Figure 2

Figure 3 illustrates the long-term OHC anomaly data for the South Pacific. The South Pacific OHC anomalies oscillate at or near 0 GJ/sq meter from 1971 to 1996 even though greenhouse gas emissions are increasing. The dip between the late 1960s and 1970 could be related to the volcanic eruption in 1963. If so, then the period of relatively flat OHC anomalies could be extended further back in time. What is certain is that there was a shift in South Pacific OHC anomalies, an upward step, in response to the 1997/98 El Nino. This happened, of course, during the period modeled by GISS.

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Figure 3

Like the Tropical Indian and Pacific Ocean OHC anomalies, the South Indian Ocean OHC anomalies decrease until the early 1970s, then rise in two steps in response to the La Nina events associated with the El Nino events of 1972/73 and 1997/98. Note the response of the South Indian Ocean OHC anomalies to the 1991 Mount Pinatubo eruption. Without that decline, the South Indian Ocean OHC anomalies are relatively flat though greenhouses gases are rising, similar to the South Pacific OHC data. And once more, the period GISS modeled captures the ENSO-induced rise associated with the 1997/98 El Nino.

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Figure 4

BUT ENSO RELEASES HEAT FROM THE TROPICAL PACIFIC

If ENSO events release heat from the tropical Pacific to the atmosphere, how then could they cause upward step changes in the OHC of other ocean basins?

During El Nino events, warm waters in the Pacific Warm Pool shift eastward to release heat that has been stored since the last La Nina event. Some of this warm water returns to the Pacific Warm Pool during the subsequent La Nina; some of it is transported to nearby ocean basins. This transport of warm water causes the OHC in those nearby oceans to rise. ENSO events also cause changes in Hadley and Walker circulation, changes in wind stress, and changes in cloud cover outside of the tropical Pacific. GCMs that do not model ENSO cannot account for these changes and cannot estimate their impacts on SST and OHC.

NORTH ATLANTIC OHC IS ALSO GOVERNED BY NATURAL VARIABLES

Over the past 50+ years, North Atlantic OHC anomalies rose at a rate that almost tripled the rise in global OHC anomalies. Refer to Figure 5. I discussed and illustrated the natural factors that impact the long-term North Atlantic OHC anomaly trends in the post “North Atlantic Ocean Heat Content (0-700 Meters) Is Governed By Natural Variables”. These natural variables include ENSO, the North Atlantic Oscillation (NAO), and Atlantic Meridional Overturning Circulation (AMOC). Unfortunately, the NODC OHC data only extends back to 1955. It is therefore impossible to determine how much of the excessive rise in the North Atlantic is related to AMOC.

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Figure 5

The Tropical North Atlantic OHC anomalies, Figure 6, show responses to ENSO events that are similar to the Tropical Indian and Pacific Ocean OHC data, Figure 2, except the tropical North Atlantic variations are imposed on what appears to be an AMOC-related positive trend. The period modeled by GISS included the response of the Tropical North Atlantic OHC anomalies to the 1997/98 El Nino.

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Figure 6

Also discussed and illustrated in my post “North Atlantic Ocean Heat Content (0-700 Meters) Is Governed By Natural Variables”, Lozier et al (2008) in “The Spatial Pattern and Mechanisms of Heat-Content Change in the North Atlantic” identifies the North Atlantic Oscillation as the driver of decadal OHC variability in the high latitudes of the North Atlantic. Link:

http://www.sciencemag.org/cgi/content/abstract/319/5864/800?rss=1

The decadal variations in the NAO (inverted and scaled) do appear to agree with the High-Latitude North Atlantic OHC anomalies, Figure 7, until the aftermath of the 1997/98 El Nino. Note again that the period that GISS elected to model captures this NAO-related rise in High-Latitude North Atlantic OHC anomalies. Did the GISS model include the NAO in its analysis of OHC? I can find no mention of it in Hansen et al (2005) “Earth’s energy imbalance: Confirmation and implications.“

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Figure 7

It appears that the North Atlantic OHC anomalies peaked in 2005. Are they on a multidecadal decline now? If the North Atlantic OHC is, in fact, governed by the same processes that cause the multidecadal variations in North Atlantic SST anomalies known as the Atlantic Multidecadal Oscillation (AMO), this would have a major impact on the GISS projections. Did GISS include this natural variability in its model? I can find no reference to it in Hansen et al (2005) “Earth’s energy imbalance: Confirmation and implications.“

IN SUMMARY

It appears the reason OHC observations are diverging from the GISS projection is GISS failed to recognize the impact of natural variables such as AMOC, the NAO, and ENSO on OHC. GISS assumed the rise in OHC from 1993 to 2003 was caused by anthropogenic forcings, when, in fact, there is little evidence to support this in the OHC data of the individual ocean basins. In order for OHC anomalies to rise in agreement with the GISS projection, there would have had to have been another significant traditional El Nino followed by a multiyear La Nina, and there would have had to have been another shift in the NAO, and there would have had to have been a continued rise in North Atlantic OHC anomalies. Unfortunately for GISS (and for the IPCC who relies on GCMs that fail to model natural variables properly), these natural variables have not cooperated.

A CLOSING NOTE ABOUT THE IMPACTS OF ANTHROPOGENIC GREENHOUSE GASES ON OHC

I was once asked by a blogger at another website, “What is the source of the energy necessary to raise SSTs?” I have revised my response to include OHC.

The ultimate source of energy necessary to raise SSTs would be an increase in solar irradiance, regardless of whether the increase in solar irradiance resulted from variations in the solar cycle, or from changes in cloud cover, or from a reduction in stratospheric volcanic aerosols. The impact of shortwave radiation (visible light) on SST depends on factors such as the turbidity of the water and sea surface albedo, which in turn depend on other variables including wind speed and chlorophyll concentration. Downwelling shortwave radiation reaches ocean depths of a few hundred meters. Therefore, changes in downwelling shortwave radiation would have a significant impact on OHC.

An increase in downward longwave (infrared) radiation caused by anthropogenic greenhouse gas emissions, on the other hand, can only warm the top few centimeters of the oceans. So an increase in downward longwave (infrared) radiation only warms the top few centimeters while downwelling shortwave radiation (visible light) warms the top few hundred meters.

However, it has been argued by AGW proponents that through mixing caused by waves and wind stress turbulence, the downward longwave (infrared) radiation would warm the mixed layer of the ocean. This in turn would affect the temperature gradient between the mixed layer and skin, dampening the outward flow of heat from the ocean to the atmosphere. The end result: OHC would rise due to an increase in downward longwave (infrared) radiation caused by increases in greenhouse gas emissions.

The OHC data illustrated in this post provide little support for the argument that downward longwave (infrared) radiation causes OHC to rise. OHC anomalies for the Tropical Indian and Pacific Oceans and for the South Indian and South Pacific Oceans show little upward trend from the early 1970s to the late 1990s. The only significant rises in OHC for those datasets occur in response to significant traditional ENSO events.

To emphasize this, the North Pacific OHC anomaly graph, Figure 8, illustrates a long-term decline in OHC from the late 1950s to the late 1980s, followed by a sudden upward shift. This long-term decline does not appear to be consistent with arguments that accelerating greenhouse gas emissions cause OHC to rise. The upward shift in the late 1980s appears to be associated with the 1986/87/88 El Nino. This El Nino is one of the two El Nino events since 1976 that caused upward step changes in SST anomalies of the East Indian and West Pacific Oceans and in the TLT anomalies of the Mid-To-High Latitudes of the Northern Hemisphere–the second being the 1997/98 El Nino. Why did the 1986/87/88 ENSO event cause the upward step in North Pacific OHC anomalies? Or was it caused by a shift in some other natural variable?

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Figure 8

As illustrated in this post, the impacts of natural variables such as ENSO, NAO, and AMOC dominate short-term and long-term OHC variability. ENSO events also cause upward step changes in SST and TLT anomalies, as noted above. These impacts on SST and TLT anomalies were discussed and illustrated in my posts:

1.Can El Nino Events Explain All of the Global Warming Since 1976? – Part 1

2.Can El Nino Events Explain All of the Global Warming Since 1976? – Part 2

3.RSS MSU TLT Time-Latitude Plots…Show Climate Responses That Cannot Be Easily Illustrated With Time-Series Graphs Alone

If and when GCMs like those used by GISS, and in turn by the IPCC, are capable of reproducing ENSO events and their multiyear aftereffects on SST, TLT, and OHC anomalies, they may be capable of determining “Earth’s energy imbalance: Confirmation and implications.“ At present, they are not.

SOURCES

The NINO3.4 SST anomaly data is based on HADISST data available through the KNMI Climate Explorer. The NODC OHC data is also available through Climate Explorer:

http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere

Sato Index data is available through GISS:

http://data.giss.nasa.gov/modelforce/strataer/

Specifically:

http://data.giss.nasa.gov/modelforce/strataer/tau_line.txt