"Distinctive climate signals in reanalysis of global ocean heat content"

[LocoAko]

Journal Article: http://onlinelibrary.wiley.com/doi/10.1002/grl.50382/pdf (Disclaimer: I haven't read the actual paper myself and it is unclear to me whether it is available to the public or not since I'm on a university computer)

 

Abstract: [1] The elusive nature of the post-2004 upper ocean warming has exposed uncertainties in the ocean's role in the Earth's energy budget and transient climate sensitivity. Here we present the time evolution of the global ocean heat content for 1958 through 2009 from a new observational-based reanalysis of the ocean. Volcanic eruptions and El Niño events are identified as sharp cooling events punctuating a long-term ocean warming trend, while heating continues during the recent upper-ocean-warming hiatus, but the heat is absorbed in the deeper ocean. In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend. The warming below 700 m remains even when the Argo observing system is withdrawn although the trends are reduced. Sensitivity experiments illustrate that surface wind variability is largely responsible for the changing ocean heat vertical distribution.

 

Discussion: http://www.climatecentral.org/news/why-the-globe-hasnt-warmed-much-for-the-past-decade-15788

 

 

Why the Globe Hasn’t Warmed Much for the Past Decade

• Published: March 27th, 2013

Even the quickest glance at a graph of global temperatures makes it clear that the planet was warming sharply during the 1980s and 1990s. But while the 2000s were the hottest decade on record, the rate of warming slowed considerably after the turn of the current century — even while human emissions of heat-trapping greenhouse gas emissions have continued to grow. The question that has lingered is where’s all the extra heat going? 

 

 

The answer, according to a new paper in Geophysical Research Letters, is that a lot of it is being stored in the deep ocean, more than a half-mile down. “We normally think about global warming as what we experience on the Earth's surface,” said co-author Kevin Trenberth, of the National Center for Atmospheric Research, in an interview. If extra heat is temporarily stored elsewhere thanks to natural climate variations, we won't necessarily notice it. 

 

But sooner or later it will inevitably emerge, which means that the current slowdown in warming may well be balanced by a period of rapid warming in a few years — nobody knows how many — from now. Scientists have always said that global warming would proceed in fits and starts, not in a smooth upward trend in temperatures. This study offers one specific explanation of why that happens.

 

The natural variation in this case appears to be changes in wind patterns associated with the Pacific Decadal Oscillation, or PDO, a gradual see-sawing of ocean surface temperatures and wind patterns that goes through warm and cold phases lasting several decades. (The more familiar El Nino/La Nina oscillation, by contrast, see-saws every few years).

According to Trenberth and his colleagues, deep ocean temperatures began to rise significantly starting in about 2000, at about the same time as trade winds in the Pacific were changing in strength, in turn affecting ocean currents, all very plausibly as a result of a shift in the PDO. 

 

 

Ordinarily, heat trapped by greenhouse gases would warm the ocean’s surface water, but since warm water floats on top of colder water, the heat would have a hard time percolating to the depths. “You need something to push it down,” Trenberth said. That something could easily be strong prevailing winds, which can literally stir things up — or in this case, down.

 

Nobody can actually see this process in action; instead, Trenberth and his colleagues used sophisticated ocean-circulation models and fed in observed data about sea-surface temperatures, winds, currents and even changes in sea level, all of which affect how heat moves around. In the end, changes in the wind turned out to have the most profound effect. It’s still a circumstantial case, but, said Trenberth, “we find it very plausible that this is a real effect.”

Adding to their confidence is the fact that a similar mechanism, only in reverse, explains why 1998 remains one of the hottest years on record. “You can point to the PDO, which took extra heat out of the ocean,” Trenberth said. That pushed global warming along faster than it would naturally have happened.

Indeed, Trenberth, speculates that the PDO could also explain why temperatures rose so quickly during the 1980s and 1990s. “You can argue that the PDO was pulling heat from the ocean during that time, which is just when global warming took off. So it may well be that this natural variability has been modulating the way we see global warming for decades.”

 

In other words, the PDO is affecting how the ocean takes in the extra heat from manmade global warming, and is helping to influence the rate at which the extra heat gets released back into the atmosphere as well.

 

If that’s the case, then global temperatures are poised for another rapid rise when the PDO see-saws out of its current phase and begins pulling heat back out of the ocean — something that’s inevitable sooner or later, although nobody knows precisely when it might happen. When it does, the question will no longer be where all the extra heat has gone, but where’s all the extra heat coming from. 

 

But the answer is likely to be exactly the same.

[AvantHiatus]

Appears to be a plausible theory regarding the slower warming trend of surface temperatures. I think it's unprecedented that the Earth warmed as much as it did in the 1980's and 1990's considering these events occur on generally longer timescales.

[dan11295]

If the PDO is a factor, along with reduced solar irradiance and preponderance of La Nina events within the past few years, that combination would be a good explanation as to why on the surface temperatures have not appeared to increase. Also shows even more why 5-10 year time scales aren't very useful for long term climate trends. It is possible that the overall climate sensitivity to CO2 has been exaggerated a tad due to the rapid warm up from 1980-1998.

[ben4vols]

I think it's unprecedented that the Earth warmed as much as it did in the 1980's and 1990's considering these events occur on generally longer timescales.

 

Was there not a decent warming event centered around the 1920's? 

[forkyfork]

[The_Global_Warmer]

Great read. Thank You very much.

[donsutherland1]

Many thanks for posting this paper. Last year, in one of the discussions, I noted that a simple linear model that takes into consideration the PDO helps explain the current pause in the rate of the warming. This paper is a serious work and it provides important insight into that possible angle. With the cold PDO, part of the heat from the energy imbalance that would otherwise be boosting surface temperatures more than has been the case recently, is currently being stored.

[bluewave]

This paper follows a similar one published in 2011 which has a full version free of charge online.

 

 

http://www.cgd.ucar....2011etalNCC.pdf

 

 

There have been decades, such as 2000–2009, when the observed globally averaged surface-temperature time series shows little positive or even slightly negative trend1 (a hiatus  period). However, the observed energy imbalance at the top-of-atmosphere for this recent decade indicates that a net energy flux into the climate system of about 1 W m−2 7 (refs 2,3) should be producing warming somewhere in the  system4,5 . Here we analyse twenty-first-century climate-model simulations that maintain a consistent radiative imbalance at

the top-of-atmosphere of about 1 W m−2 as observed for the 11 past decade. Eight decades with a slightly negative global mean surface-temperature trend show that the ocean above 300 m  takes up significantly less heat whereas the ocean below 300 m  takes up significantly more compared with non-hiatus decades.  The model provides a plausible depiction of processes in the  climate system causing the hiatus periods, and indicates that a  hiatus period is a relatively common climate phenomenon and

may be linked to La Niña-like conditions.

 

 

The time series of globally averaged surface temperature from  all five climate-model simulations show some decades with little or no positive trend (Fig. 1a), as has occurred in observations  (Supplementary Fig. S1 top). Running ten year linear trends of globally averaged surface temperature from the five model ensemble  members reveal hiatus periods (Fig. 1a) comparable to observations  (Supplementary Fig. S1 middle). Using the first ensemble member as an example, the overall warming averaged over the century is about +0.15 ◦ C per decade. However, the decades centred around  2020, 2054, 2065, 2070, and several decades late in the century  show either near zero or slightly negative trends in that ensemble  member. We choose two ten year periods in this ensemble member when the globally averaged surface temperature is negative, that is, less than −0.10 ◦ C over the decade (Fig. 1a), and six similar  periods that meet the same criterion from the other four ensemble  members, to form an eight-member composite of hiatus periods.

 

http://www.scienceda...10918144941.htm

 

 

To track where the heat was going, Meehl and colleagues used a powerful software tool known as the Community Climate System Model, which was developed by scientists at NCAR and the Department of Energy with colleagues at other organizations. Using the model's ability to portray complex interactions between the atmosphere, land, oceans, and sea ice, they performed five simulations of global temperatures.

The simulations, which were based on projections of future greenhouse gas emissions from human activities, indicated that temperatures would rise by several degrees during this century. But each simulation also showed periods in which temperatures would stabilize for about a decade before climbing again. For example, one simulation showed the global average rising by about 2.5 degrees Fahrenheit (1.4 degrees Celsius) between 2000 and 2100, but with two decade-long hiatus periods during the century.