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North Atlantic cooling means climate change of a different kind coming?

Posted By Joanne Nova On May 28, 2015 @ 9:45 pm In Global Warming | Comments Disabled

Is this the way the backdown plays out? The endless warming becomes cooling, and man-made change becomes natural cycles one paper at a time? The press releases still talk of “change”! No mention that natural cycles could have been the cause of past warming, and that skeptics have been saying this for years.

Atlantic ocean, temperatures, sea level circulation index

Figure 3 | Sea-level circulation index, the NAO and the AMO on multidecadal
timescales. Shown are the accumulated sea-level index (blue), which is
representative of subpolar heat content evolution, the accumulated NAO (red,
dashed) and the AMO (black). The heat content proxy and the accumulated
NAO have been normalized. All timeseries have been 7-year low-pass filtered.
The accumulated sea-level index and accumulated NAO have been detrended.

This Nature paper will be tricky to feed into the “Panic Now!” scenario. It’s still climate change, but it’s a half a degree of cooling that might be headed your way if you live around the northern Atlantic.

UPDATE: A quick summary of the paper. McCarthy et al created a circulation index (blue line, fig 3) which appears to lead the AMO (Atlantic Multidecadal Oscillation, black line) by two years. The sea level index is generated by comparing sea levels north and south of Cape Hatteras, USA. The authors spend quite a bit of time explaining why that area is the most useful proxy for changes in ocean circulation. Their circulation index suggests the AMO has shifted to a “negative” colder phase which may last decades.

The press release is below for this tricky paper that doesn’t follow the IPCC plan. In the world of climate news it’s important that the headlines include the words “climate”, “global” and “change” and not the words “cooling”, “natural cycles” or “skeptics might be right”.

Global climate on verge of multi-decadal change

[Science Daily] A new study, by scientists from the University of Southampton and National Oceanography Centre (NOC), implies that the global climate is on the verge of broad-scale change that could last for a number of decades.

The change to the new set of climatic conditions is associated with a cooling of the Atlantic, and is likely to bring drier summers in Britain and Ireland, accelerated sea-level rise along the northeast coast of the United States, and drought in the developing countries of the Sahel region.

But global warming is still coming. One day. Sometime.

Since this new climatic phase could be half a degree cooler, it may well offer a brief reprise from the rise of global temperatures, as well as resulting in fewer hurricanes hitting the United States.

The study, published in Nature, proves that ocean circulation is the link between weather and decadal scale climatic change. It is based on observational evidence of the link between ocean circulation and the decadal variability of sea surface temperatures in the Atlantic Ocean.

So ocean circulation is the link between weather and “decadal scale climate”. Doesn’t that mean the models that didn’t include this link didn’t predict this cooling, were wrong, and overestimated the CO2? No one seems to mention that.

Lead author Dr Gerard McCarthy, from the NOC, said: “Sea-surface temperatures in the Atlantic vary between warm and cold over time-scales of many decades. These variations have been shown to influence temperature, rainfall, drought and even the frequency of hurricanes in many regions of the world. This decadal variability, called the Atlantic Multi-decadal Oscillation (AMO), is a notable feature of the Atlantic Ocean and the climate of the regions it influences.”

Doesn’t that mean that natural variation is still more important than man-made emissions, and isn’t that what skeptics have been saying for decades?

These climatic phases, referred to as positive or negative AMO’s, are the result of the movement of heat northwards by a system of ocean currents. This movement of heat changes the temperature of the sea surface, which has a profound impact on climate on timescales of 20-30 years. The strength of these currents is determined by the same atmospheric conditions that control the position of the jet stream. Negative AMO’s occur when the currents are weaker and so less heat is carried northwards towards Europe from the tropics.

The strength of ocean currents has been measured by a network of sensors, called the RAPID array, which have been collecting data on the flow rate of the Atlantic meridonal overturning circulation (AMOC) for a decade.

Dr David Smeed, from the NOC and lead scientist of the RAPID project, adds: “The observations of AMOC from the RAPID array, over the past ten years, show that it is declining. As a result, we expect the AMO is moving to a negative phase, which will result in cooler surface waters. This is consistent with observations of temperature in the North Atlantic.”

Since the RAPID array has only been collecting data for last ten years, a longer data set was needed to prove the link between ocean circulation and slow climate variations. Therefore this study instead used 100 years of sea level data, maintained by the National Oceanography Centre’s permanent service for mean sea level. Models of ocean currents based on this data were used to predict how much heat would be transported around the ocean, and the impact this would have on the sea surface temperature in key locations.

Co-author Dr Ivan Haigh, lecturer in coastal oceanography at the University of Southampton, said: “By reconstructing ocean circulation over the last 100 years from tide gauges that measure sea level at the coast, we have been able to show, for the first time, observational evidence of the link between ocean circulation and the AMO.”

Back to the paper where they discuss mechanisms, heat content, and overturning circulation:

The 7-year sea-level index leads the 7-year rate of heat content change by 2 years with a maximum correlation of 0.58
(significant at the 95%level). The reason that the accumulated sea-level index leads the large rise in heat content  from40N to 60N in the early 1990s can be interpreted by looking at maps of the heat content anomaly evolution. Heat content builds downstream of the intergyre region from the mid-1980s to the mid-1990s (Fig. 2b). This heat content anomaly is then observed downstream in the subpolar gyre in the late 1990s and early 2000s (Fig. 2c), indicating that the sea-level index could provide an early indication of subpolar heat content change.

They are quietly saying that natural cycles are are important drivers of sea level changes on the US East coast as well as Greenland ice sheets.

In recent years, the sea-level index (Fig. 1d) indicates that the AMOis again transitioning to a negative phase, consistent with observations of a reduced overturning circulation8.

Using the sea-level difference between subtropical and subpolar gyres, we have developed and validated a proxy for ocean circulation in the intergyre region. This represents a mechanism for ocean heat transport to the subpolar gyre and heat content changes there. When observations exist, heat content changes have coincided with the major phase transitions of theAMO, confirming that ocean circulation plays a key role in decadal Atlantic variability. The ocean responds to NAO forcing with changes in ocean circulation: on decadal timescales, the ocean integrates NAO forcing and returns it to the atmosphere as the AMO. This is implicitly the Bjerknes compensation that had previously
been seen in air–sea fluxes29. The sea-level difference provides an indicator of ocean circulation changes that precede phase changes in the AMO, thus explaining why, as the positive AMO declines4, sea-level rise is accelerating north ofCapeHatteras9,10.While Greenland ice sheet melt has been linked with accelerating sea-level rise in recent years, the fact that the period of accelerated sea-level rise from the 1950s to the 1970s10 (as well as the current period of sea-level rise) coincides with a declining AMO indicates that multi-decadal fluctuations in ocean circulation play a key role. In this framework, sea-level rise along the US east coast becomes entwined with the effects of the AMO on climate.

This paper will no doubt be valuable if the weather cools over Europe in the next ten years.

Reference:

Gerard D. McCarthy, Ivan D. Haigh, Joël J.-M. Hirschi, Jeremy P. Grist, David A. Smeed. (2015) Ocean impact on decadal Atlantic climate variability revealed by sea-level observations. Nature; 521 (7553): 508 DOI: 10.1038/nature14491

PS: Thank you to Peter from Adelaide who sent two letters this week. Both received gratefully. – Jo

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