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Now, a team of climatologists, led by researchers at the University of California, Riverside, posits that the recent widening of the tropical belt is primarily caused by multi-decadal sea surface temperature variability in the Pacific Ocean. This variability includes the Pacific Decadal Oscillation (PDO), a long-lived El Niño-like pattern of Pacific climate variability that works like a switch every 30 years or so between two different circulation patterns in the North Pacific Ocean. It also includes, the researchers say, anthropogenic pollutants, which act to modify the PDO.

Study results appear March 16 in Nature Geoscience.

“Prior analyses have found that climate models underestimate the observed rate of tropical widening, leading to questions on possible model deficiencies, possible errors in the observations, and lack of confidence in future projections,” said Robert J. Allen, an assistant professor of climatology in UC Riverside’s Department of Earth Sciences, who led the study. “Furthermore, there has been no clear explanation for what is driving the widening.”

Now Allen’s team has found that the recent tropical widening is largely driven by the PDO.

“Although this widening is considered a ‘natural’ mode of climate variability, implying tropical widening is primarily driven by internal dynamics of the climate system, we also show that anthropogenic pollutants have driven trends in the PDO,” Allen said. “Thus, tropical widening is related to both the PDO and anthropogenic pollutants.”

Widening concerns

Tropical widening is associated with several significant changes in our climate, including shifts in large-scale atmospheric circulation, like storm tracks, and major climate zones. For example, in Southern California, tropical widening may be associated with less precipitation.

Of particular concern are the semi-arid regions poleward of the subtropical dry belts, including the Mediterranean, the southwestern United States and northern Mexico, southern Australia, southern Africa, and parts of South America. A poleward expansion of the tropics is likely to bring even drier conditions to these heavily populated regions, but may bring increased moisture to other areas.

Widening of the tropics would also probably be associated with poleward movement of major extratropical climate zones due to changes in the position of jet streams, storm tracks, mean position of high and low pressure systems, and associated precipitation regimes. An increase in the width of the tropics could increase the area affected by tropical storms (hurricanes), or could change climatological tropical cyclone development regions and tracks.

Belt contraction

Allen’s research team also showed that prior to the recent (since ~1980 onwards) tropical widening, the tropical belt actually contracted for several decades, consistent with the reversal of the PDO during this earlier time period.

“The reversal of the PDO, in turn, may be related to the global increase in anthropogenic pollutant emissions prior to the ~ early 1980s,” Allen said.

Analysis

Allen’s team analyzed IPCC AR5 (5th Assessment Report) climate models, several observational and reanalysis data sets, and conducted their own climate model experiments to quantify tropical widening, and to isolate the main cause.

“When we analyzed IPCC climate model experiments driven with the time-evolution of observed sea surface temperatures, we found much larger rates of tropical widening, in better agreement to the observed rate–particularly in the Northern Hemisphere,” Allen said. “This immediately pointed to the importance of sea surface temperatures, and also suggested that models are capable of reproducing the observed rate of tropical widening, that is, they were not ‘deficient’ in some way.”

Encouraged by their findings, the researchers then asked the question, “What aspect of the SSTs is driving the expansion?” They found the answer in the leading pattern of sea surface temperature variability in the North Pacific: the PDO.

They supported their argument by re-analyzing the models with PDO-variability statistically removed.

“In this case, we found tropical widening — particularly in the Northern Hemisphere — is completely eliminated,” Allen said. “This is true for both types of models–those driven with observed sea surface temperatures, and the coupled climate models that simulate evolution of both the atmosphere and ocean and are thus not expected to yield the real-world evolution of the PDO.

“If we stratify the rate of tropical widening in the coupled models by their respective PDO evolution,” Allen added, “we find a statistically significant relationship: coupled models that simulate a larger PDO trend have larger tropical widening, and vice versa. Thus, even coupled models can simulate the observed rate of tropical widening, but only if they simulate the real-world evolution of the PDO.”

Future work

Next, the researchers will be looking at how anthropogenic pollutants, by modifying the PDO and large scale weather systems, have affected precipitation in the Southwest United States, including Southern California.

“Future emissions pathways show decreased pollutant emissions through the 21st century, implying pollutants may continue to drive a positive PDO and tropical widening,” Allen said.

Story Source:

The above story is based on materials provided by University of California – Riverside. The original article was written by Iqbal Pittalwala. Note: Materials may be edited for content and length.

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United States flags are displayed on flood-damaged homes in the Breezy Point section of Queens, N.Y., Wednesday, Nov. 28, 2012. (AP Photo/Mark Lennihan)

This story originally appeared on Climate Central.

When Hurricane Sandy struck the Northeast, it was not the storm’s winds but the massive pile of water those winds pushed in front of the storm that wreaked the most havoc, inundating coastal areas in 3 to 9 feet of water, causing billions in damages, and leaving dozens dead. In general, this storm surge poses a far greater threat to lives and property than winds when hurricanes and tropical storms hit.

It’s with this in mind that the National Hurricane Center (NHC) has developed an experimental system that maps the projected storm surge in areas under threat from tropical cyclones — just in time for this hurricane season, which began on Sunday, June 1.

Sandy was no different than most other tropical cyclones, the generic term for hurricanes and typhoons, with the major proportion of its damage caused by storm surge. In the 50-year period from 1963 to 2012, storm surge accounted for nearly half of all tropical cyclone deaths in the U.S., according to a recent study in the Bulletin of the American Meteorological Society. That’s far more than the 8 percent the study attributed to the high winds many assume to be the biggest danger in a hurricane. Yet wind speed continues to be the main measure of a cyclone’s strength, determining when a tropical storm becomes a hurricane and what “category” ranking a hurricane receives.

It’s this unappreciated threat from storm surge that prompted the NHC to begin issuing experimental storm surge flooding maps for all tropical cyclones that threaten U.S. coasts starting with the 2014 hurricane season. The maps will show how high a particular storm’s surge is expected to get in different areas and how far inland it could go.

“This is a really desperately needed update,” said Jamie Rhome, a storm surge specialist with the NHC who is heading the mapping effort.

Adding to the need for such a warning mechanism is the fact that the population along coastal areas has been rising, putting more people in harm’s way. And rising sea levels — the result of the ocean warming and ice melting due in part to warming global temperatures — are increasing the storm surge threat. Some research has also suggested that Atlantic basin storm could become more intense, if less frequent, which could effect the level of storm surge coastal communities experience.

While Sandy brought threat of storm surge to the public’s attention, it was an earlier storm that jump-started the NHC’s efforts to produce inundation maps: Hurricane Ike, which hit Galveston, Texas, on Sept. 13, 2008, as a Category 2 hurricane and brought storm surge that reached 15-20 feet above normal tides.

An example of a new storm surge mapping tool unveiled by the National Hurricane Center as an experimental product during the 2014 season. This map shows how a hypothetical Hurricane ‘X’ might impact Ft. Myers, Fla., with different levels of storm surge color coded. Credit: NHC/NOAA

Rhome said that the effort began shortly after Ike, when social scientists were brought in to help run focus groups and tell the hurricane researchers what kind of tool would be most useful to the public. Based on that feedback, Rhome and his team spent the last few years refining the maps.

Sandy just “shined a big spotlight on the project” and helped expedite its release, Rhome told Climate Central. Recent advances in computer models and the growing availability of detailed topographical data, both critical for telling how high waters could rise, have also helped bring the project to fruition and the resulting maps will now be tested over the next two hurricane seasons.

Each map will show the area under threat from storm surge, with color-coded ratings showing how high the water could get above the ground in different areas. The scale runs from blue to red and represents water rising anywhere from up to 3 feet to more than 9 feet above ground.

The maps are based on several factors including how big the incoming storm is, its intensity, and the track it is expected to take since winds on the right side of a storm will push water toward shore, while the winds on the left side will push it away. The ratings also take into account astronomical tides and the elevation of the land under threat.

Maps will be issued for a given storm when a hurricane watch or warning is issued, and in some cases for a tropical storm watch or warning, when any place along the U.S. Gulf or East Coast is under threat.

Throughout the two-year experimental run, Rhome and his colleagues will see how the maps perform in showing where the surge actually occurs. And if the season is as uneventful for the U.S. as it was in 2013? Then the team will create “fake” storms to see what the models predict.

“There’ll still be ways to test it,” Rhome said.

Rhome and his team will also be looking for feedback from officials and the public. The NHC has a survey set up on its site where users can leave comments.

So far, the feedback “has been incredibly if not overwhelmingly positive,” Rhome said.

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