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Wednesday, January 22, 2014

Warming may spike when Pacific Decadal Oscillation moves to a positive phase

We are currently in a negative phase of the Pacific Decadal Oscillation, a cycle with phases that can last between 15 to 30 years. During a negative phase waters are cooler and heat mixes into deeper levels of the north Pacific ocean. When the next phase change ocurrs to a positive PDO we are likely to see some of the stored ocean heat released spiking atmospheric temperatures.

The Pacific Decadal oscillation was only identified by scientists in the 1990s, and we only have accurate observational data for identifying it going back some 50-60 years. Nathan J Mantua and SR Hare from the University of Washington in Seattle described the operation of the Pacific Decadal Oscillation in the Journal of Oceanography in 2002 (Mantua & Hare 2002 PDF). Read more about the PDO from the Climate Impacts Group from University of Washington.

Wikipedia describes the basics of the PDO cycle:

The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs. It shifts phases on at least inter-decadal time scale, usually about 20 to 30 years.

The Pacific Decadal Oscillation affects the cycles of the more frequent El Niño Southern Oscillation (ENSO). During the PDO negative phase ENSO responds with periods of La Niña predominating, while a PDO positive phase El Niño patterns are more likely to occur.

Wikipedia again on how the PDO impacts our climate variability:

During the positive phase the wintertime Aleutian low is deepened and shifted southward, warm/humid air is advected along the North American west coast and temperatures are higher than usual from the Pacific Northwest to Alaska but below normal in Mexico and the Southeastern United States.

Winter precipitations are higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States but reduced over Canada, Eastern Siberia and Australia.

McCabe et al. showed that the PDO along with the AMO strongly influence multidecadal droughts pattern in the United States, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase in both cases if the PDO is associated with a positive AMO.

A number of studies have identified a possible link between the PDO and decadal East Asian Monsoon variability and the associated summer rainfall and temperature changes over East China. (See YU Lei, 2013 - PDF). Increased rainfall and decreased summer temperature is also observed over the Indian subcontinent during the negative phase.

We entered a negative phase of the PDO in 1998 after the El Niño of 1998 that set record setting global temperatures which climate sceptics like to cherry pick their start dates from for spreading confusion on temperature trends.

Signs the Pacific Decadal Oscillation becoming weaker but more frequent

When the Pacific Decadal Oscillation switches to a positive phase, which may happen relatively soon, we are likely to see more El Niño cycles and an outburst of heat from the ocean pushing up atmospheric temperatures. If you think the heat was bad in the record setting Australian temperatures of 2013 in a neutral ENSO period, they could be further elevated by the next strong El Niño in the positive phase of the Pacific Decadal Oscillation.

“The 1997 to ’98 El Niño event was a trigger for the changes in the Pacific, and I think that’s very probably the beginning of the hiatus,” says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. According to this theory, the tropical Pacific should snap out of its prolonged cold spell in the coming years.“Eventually, it will switch back in the other direction.” Trenbeth told Nature in a news article on the missing heat.

Modelling results published January 2014 show that the "North Pacific ocean decadal variability, its dominant mode (i.e., PDO), and atmospheric decadal variability, have become weaker under global warming, but with PDO shifting to a higher frequency." (Changfang Fang et al 2014) in a paper titled The impact of global warming on the pacific decadal oscillation and the possible mechanism.

Recent studies by Scientists are also identifying that El Niño Southern Oscillation (ENSO) is likely to intensify and become more frequent with global warming. Research published in January 2014 indicates that Global warming is doubling the risk of Extreme El Ninos.

Update: 22 January

Hansen on global temperature update for 2013

James Hansen, Makiko Sato and Reto Ruedy
In their latest paper released on 21 January 2014 on Global Temperature Update Through 2013 (PDF) summarises the current state of global temperature trends including detailing the importance of the current cooling in the tropical Pacific and the prospects for new global temperature records in 2014 or 2015 if an El Niño eventuates:

"The rate of global warming is slower in the past decade than in the prior three decades. Slower growth of net climate forcings and cooling in the tropical Pacific Ocean both contribute to the slower warming rate, with the latter probably the more important effect. The tropical Pacific cooling is probably unforced variability, at least in large part. The trend toward an increased frequency of extreme hot summer anomalies over land areas has continued despite the Pacific Ocean cooling. The “bell curves” for observed temperature anomalies show that, because of larger unforced variability in winter, it is more difficult in winter than in summer to recognize the effect of global warming on the occurrence of extreme warm or cold seasons. It appears that there is substantial likelihood of an El Niño beginning in 2014, and as a result a probable record global temperature in 2014 or 2015."

Coral growth bands extend history of Pacific Decadal Oscillation phases

A new study by researchers at the University of Queensland have identified the signs of the PDO phases in flood marks from Queensland's Fitzroy River contained in coral growth bands.

Caption: Figure 2. Time series of G/B anomalies for the period 1921–2011.
(A) Monthly G/B (luminescence) anomalies for all six cores. (B) Number of cores used to construct the composite record. (C) Long term G/B composite record, with major flood events registered by instrumental records. Colour dashed lines under the profile denote the highest individual flood events registered at Rockhampton, the nearest gauging station to the river mouth (data from Water Division Brisbane, Bureau of Meteorology, Station 039264). Colours refer to gauge height in m. Shaded areas correspond to La Niña periods.
Source: doi:10.1371/journal.pone.0084305.g002

“We have demonstrated that this oscillation pattern is a key driver of river runoff impacting on the Great Barrier Reef,” Dr Rodriguez-Ramirez said. “Our study suggests that this oscillation will influence the frequency and intensity of future extreme events such as floods – as well as ecological processes – in the region.”

This may provide a useful proxy measurement for extending the identification of PDO phases back in time by up to 400 years. The study examined century-old coral colonies – “bommies” – around the Keppel Islands, 50km from the Fitzroy River mouth.

Co-author Professor Jian-xin Zhao, also from UQ’s School of Earth Sciences, said “This is a significant advance in our understanding of climate variability.”

Like the growth rings in trees that can give year by year seasonal information on a range of environmental factors, corals can provide a similar history and clues to environmental conditions when viewed under ultraviolet light.

“As a result of this research, we now know that corals from this area have great potential for reconstructing even longer-term historical evolution of the Pacific Decadal Oscillation,” Co-author Professor John Pandolfi, from UQ’s School of Biological Sciences, said.

“Some bommies are five to six metres tall and 400 years old. They can provide us with much longer-term information than is available from satellite-based data, which goes back only 40-50 years. This will help refine future models for predicting this important climate phenomenon.”

Researchers from NIOZ Royal Netherlands Institute for Sea Research and The University of Western Australia’s Oceans Institute/School of Earth and Environment also worked on the research.

A similar study of corals near Hainan that was published May 2013 in the Journal of Geophysical Research: Oceans, identified Variations in the Pacific Decadal Oscillation since 1853 in a coral record from the northern South China Sea (Wenfeng Deng et al 2013). Comparison of a variety of proxies from different locations can help piece together the history of past PDO cycles.


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