Warming Pacific Drives Global Temperatures

Unprecedented changes in the Pacific Ocean lie behind record global surface temperatures this year and the abortive El Nino Pacific Ocean warming event and may also spell the end of the global warming pause, according to two leading climate scientists.

US space agency NASA and other organisations have reported record global average surface temperatures over the last six months or so and these are believed to be connected to a warming in the sea surface temperature (SST) of the Pacific Ocean, especially in the north, according to Axel Timmermann of the University of Hawaii and Kevin Trenberth of the US National Center for Atmospheric Research.

Data from the UK Meteorological Office and from the US National Oceanic and Atmospheric Administration indicate that the surface waters of the Pacific Ocean have never been warmer than they have been over the last few months. The northern hemisphere summer of 2014 has seen the highest global mean sea surface temperatures recorded since systematic measuring started. Sea surface temperatures have exceeded those of the record-breaking 1998 El Nino year.

Timmermann used the phrase “quite remarkable” to describe what has been happening and Trenberth called the events “extraordinary”. Both men told reportingclimatescience.com that they believe this warming may mean the end of the so called pause, or hiatus, in global warming – which, according to some measures, has been going on since the mid-1990s.

The Pacific Ocean switched from a long-term cooling phase that it entered in the late 1990s to a warming phase earlier this year leading to the record sea surface temperatures that have been measured across the region and to changes in weather patterns, according to Timmermann. These changes appear to be linked with the Interdecadal Pacific Oscillation (IPO), a long term cyclic pattern of sea surface temperature variations that affect both the northern and southern Pacific, he said.

The change in the IPO from a negative cooling phase to a positive warming phase is thought to have accompanied the recent unprecedented warming of northern Pacific waters. “From around the end of January there was a huge warm water build up off the Alaskan coast and a warming of the north Pacific,” Timmermann told reportingclimatescience.com. “Quite remarkable. I have not seen anything like this before in the northern Pacific”.

NCAR’s Trenberth linked events to changes in the Pacific Decadal Oscillation (PDO) which is similar to the IPO in that it is a long-term cyclic variations in sea surface temperatures. “The fact is that there has been extraordinary warming in Alaskan waters and there were changes throughout the Pacific,” explained Trenberth. “The key thing is that the PDO is recorded as having changed phase,” he explained.

Timmermann is very clear about what this means for global warming. “Over the last 15 years or so – the period of the global warming hiatus – the Pacific has been anomalously cold and there has been a very strong negative IPO,” explained Timmermann. “This has now stopped. The negative IPO has stopped. This is the same as saying the global warming hiatus has stopped.”

Trenberth believes that the end of the pause “very much relates to whether or not the PDO has indeed switched” adding that the kind of change that has been seen this year is “exactly, the sort of thing we would be looking for” to indicate that the pause had ended. He pointed out that the data shows that the PDO has moved from a negative (cooling) state to a positive (warming) state but it is still uncertain as to whether this is just a blip or whether it represents a more long-term flip. The PDO index in October was high and positive.

Both men stress that there are a number of different factors at play. However, the sequence of events that has contributed to the current situation is becoming clear. The negative IPO phase in place since the late 1990s led to very strong equatorial trade winds – which are separate from the classical trade winds that blow across the Pacific north and south of the equator. “These cooling equatorial trade winds were so strong that they sucked up water from the eastern equatorial Pacific and moved it west,” said Timmermann.

The result was that equatorial Pacific cooled and the sea level in the western Pacific rose much faster than the global average rate – and the cooling Pacific sea surface waters cooled the atmosphere above them and so caused the pause. “So the trade winds intensified, the equatorial Pacific cooled, sea levels in the west rose and this all goes together with the global warming hiatus,” explained Timmermann.

This period of strong equatorial trade winds came to an end at the beginning of 2014 resulting in a warming in the northern Pacific and especially along Alaskan coastal waters.

Next, a series of waves of warm water – known as Kelvin waves – moved across the Pacific from the west near Indonesia to the east and these were interpreted as signs that an El Nino Pacific Ocean warming event may be about to take place. Trenberth explained to reportingclimatescience.com that, as a result of this movement of water, sea levels rose in the central and eastern Pacific and fell back in the western Pacific. However, this warm water did not trigger the expected full blown El Nino.

Instead, the warm water moved across the Pacific until it hit the western coast of the Americas and moved north and south – warming coastal waters along the west coast of North America as far north as Oregon and causing a warming of the north eastern Pacific Ocean waters from April through to September.

Finally, over the last three months or so the classical trade winds started to weaken too. Normally they act to cool the ocean. This time the trade winds weakened considerably and the cooling ceased almost completely. This intensified warming in the central Pacific.

So during 2014 the waters of the northern, north eastern and central Pacific have all warmed significantly. This has had the effect of raising the global average surface temperature to record levels which will almost certainly result in 2014 being reported as the warmest year on record.

Trenberth explained that there have been widespread impacts on the weather as storm systems – such as the recent Typhoon Nuri – moved further northwards than usual, drought conditions in California persist while a mass of cold air moved south across central and eastern North America in November.

Meanwhile waters off the coast of Hawaii reached 29oC or 30oC through the summer, according to Timmermann, causing corals to die and bleach. “We have seen temperature anomalies of 4oC in some area – very extreme. This warming is bad news for salmon fisheries and also for coral. Fish and sea life are experiencing this year what we are projecting for 100 years time,” said Timmermann.

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