Science: The ocean’s tallest waves are getting taller

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Waves in the stormy Southern Ocean have grown an average of 30 centimeters since 1985.

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The ocean’s tallest waves are getting taller

By Colin BarrasApr. 25, 2019 , 2:05 PM

The frigid Southern Ocean is well known for its brutal storms, which can sink ships and trigger coastal flooding on distant tropical islands. Now, a new study suggests the biggest waves there—already the world’s largest—are getting bigger, thanks to faster winds attributed to climate change.

Peter Ruggiero, a geophysicist at Oregon State University in Corvallis who was not involved in the study, calls the increase “substantial,” and says he is particularly concerned by evidence that the tallest waves are gaining height at the fastest rate. “If [those waves hit] at high tide, it could be potentially catastrophic.”

For the past 33 years, global satellites have been collecting data on ocean waves—and the winds that drive them. By bouncing energy pulses off wave crests and measuring the time those pulses take to come back, instruments called altimeters aboard satellites can measure wave height—the taller the waves, the faster the signal returns. Other satellite instruments monitor changes in the reflectivity of the ocean surface, which is reduced by wind-generated ripples, to estimate the speed of ocean winds. But interpreting the data is difficult: Different satellites can give different estimates of wind speed, for instance.

To minimize those discrepancies, physical oceanographer Ian Young at the University of Melbourne in Australia, and mathematician Agustinus Ribal at Hasanuddin University in Makassar, Indonesia, compared information from different satellites and calibrated their data against an independent data set collected by a global network of buoys floating in the ocean. When they were done, two trends stood out: Since 1985, average ocean wind speeds in most of the world have increased between 1 centimeter and 2 centimeters per second per year, leading to increases in wave height in many places.

In the Southern Ocean, the trends are particularly strong. For instance, although average wind speeds there have increased by 2 centimeters per second each year, the speed of the top 10% fastest winds has increased by 5 centimeters per second per year. And although average wave heights there have increased by just 0.3 centimeters per year, the top 10% highest has grown by an average of 1 centimeter per year—a growth of 30 centimeters since 1985, they report today in Science.

The trends could be bad news for coastal communities, which face serious risks from sea level rise and extreme storm events, Young says. If oceanic winds are stronger and waves are taller, storms could be far more damaging.

Young and Ribal have done a good job of cross-checking and double-checking data from the three different types of satellite instrument, says Ole Johan Aarnes at the University of Bergen in Norway. But, he adds, it might be “optimistic” to think that the data now contain no errors. Confirming the trends will likely require more work, he believes.

The new paper doesn’t say definitively why wave height and wind speed is changing, although Young suspects a link with climate change. Ruggiero thinks that makes sense: He points out that a recent study in Nature Communications suggests higher global temperatures related to climate change are driving an expansion of the tropics—and an increase in wind speed there. “These are the secondary effects of climate change, not the obvious ones like sea level rise,” Young says. “This is where a lot of the research emphasis is now being placed.”

https://www.sciencemag.org/news/2019/04/ocean-s-tallest-waves-are-getting-taller

WaPo: It’s been exceptionally warm in Greenland lately and ice is melting a month early

Warmth over Greenland is highlighted in a daily analysis from mid-April. (ClimateReanalyzer.org)

by Matthew Cappucci, The Capital Weather Gang, The Washington Post, April 18, 2019

You might have heard about the exceptional heat this year in the northern hemisphere and around the world. March was just declared the second warmest on record globally

Records have been shattered in Alaska. Scotland hit 70 degrees in February. Winter warmth has torched the U.K., The Netherlands, and Sweden as well — coming on the heels of Europe’s warmest year on record. But they’re not alone.

Greenland is baking, too. In fact, its summer melt season has already begun — more than a month ahead of schedule.

Marco Tedesco is a professor in atmospheric sciences at the Lamont Doherty Earth Observatory of Columbia University. He monitors behavior of the cryosphere — the part of earth’s water system that is frozen. He says melting of this extent shouldn’t begin until May. “The first melt event was detected on April 7,” he wrote in email.

Greenland melt extent in 2019, compared to normal. (National Snow and Ice Data Center)

“Air temperature anomalies were up to more than 20 degrees Celsius [36 Fahrenheit] above the mean,” noted Tedesco. His team has been eyeing Greenland’s southeast coast as ground zero for the early-season thaw. “Surface air temperature jumped to 41 degrees on April 2, up from minus-11,” he said. Temperatures dropped below freezing briefly before again soaring into the 30s, where the mercury has held steady for most of the past week.

What’s been sling-shotting this balmy air northward?

“The subtropical jet stream,” wrote Jennifer Francis, senior scientist at the Woods Hole Research Center in Falmouth, Mass. It’s teamed up with the polar jet to “transport warm, moist air from near Florida northward into southern Greenland,” she explained. “Locking this pattern in place has been a strong ridge — a northward bulge in the jet stream — just east of Greenland.”

A lack of ice cover in the Arctic Ocean north of Scandinavia gave this bubble of warmth a bit of an extra boost, intensifying its warm conveyor belt into Greenland.

Going forward, “[t]hese types of patterns are expected to occur more frequently,” Francis wrote, citing climate change as the culprit. “Arctic ice cover continues to dwindle and temperatures there soar.”

But advection — the transport of air, in this case warm, from somewhere else — is just half the battle. Adding insult to injury is a shortage of cloud cover in recent weeks over Greenland. The high pressure “block” that Francis described has also helped clear the skies, allowing more sunshine to pour in and heat the ground further.

A reanalysis showing precipitation also points to where the source of warmth in Greenland is coming from. (ClimateReanalyzer.org)

“Incoming solar radiation reached a value similar to ones we observed in August last year,” wrote Tedesco. That heats the ground even more. It’s a vicious cycle of positive feedback, indicating just how unstable — and delicate — the Arctic is.

“I call this ‘melting cannibalism,” explained Tedesco. And it could get even worse, as it preconditions the ice to be more vulnerable to melting in the summer.

When snow/ice on the ground melt, they form small pools of water. That changes how reflective the surface is — a measure scientists refer to as “albedo.” Snow and ice have a very high albedo, meaning it reflects most of the incoming light that hits it. That’s why you have to wear sunglasses when you go skiing. Water, on the other hand, is a lot less shiny, which allows it to absorb more heat, a cyclical process on a local level and a driver of additional warming on the global level.

According to the National Snow and Ice Data Center, the rate of melting this early in the year has been off the charts. Satellite imagery shows several patches of extremely early melt along the coast.

View image on Twitter

Rick Thoman@AlaskaWx

Kuskokwim River ice went out at Bethel on April 13, by a week the earliest of record. Four of the past six years have been in the top ten earliest. Overall, Kusko ice is breaking about a week earlier than 90 years ago. #akwx @Climatologist49 @MarkSpringer @klshall @KYUKNews

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And it’s not just Greenland. Much of the Arctic has been baking. Ice melt in Alaska has set rivers gushing more than a month before normal in some places, setting records along the Kuskokwim River in Bethel, and triggering the earliest ice breakup along the Tanana River in Nenana.

https://www.washingtonpost.com/weather/2019/04/18/its-been-exceptionally-warm-greenland-lately-ice-is-melting-month-early/

Paul Douglas, WaPo: As oceans rapidly warm because of climate change, an urgent need to improve hurricane forecasts

Better hurricane forecasts require near-real-time, deep-ocean monitoring

Hurricane Michael on the morning of Oct. 10, 2018. (AerisWeather)

by Paul Douglas, The Washington Post, April 16, 2019

In the past two hurricane seasons, record-breaking floods have engulfed our coastal zones in the Carolinas and Texas as storms have drawn more water and grown larger from rapidly warming oceans.

As the climate system continues to warm, we will need better prediction systems so we can prepare vulnerable coastal areas for bigger, wetter and faster-strengthening hurricanes. Hurricane season is just six weeks away.

[Hurricane season is approaching. Here’s the first 2019 outlook from scientists.]

Recent studies confirm that warming of the world’s oceans is taking place faster than previously estimated — as much as 40 percent faster than the United Nations estimated in 2015.

Research confirms that roughly 93 percent of the warming from man-made greenhouse gases is going into the world’s oceans. About two-thirds is absorbed in the ocean’s top 700 meters, noted Zeke Hausfather, a climate scientist at Berkeley Earth. This is the layer from which hurricanes draw much of their energy.

In a warmer, more volatile world, accurate weather predictions will require near-real-time data, not only from the atmosphere, but from regional variations in ocean warming, according to Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research.

“The extra heat from increasing gases from carbon dioxide doesn’t necessarily go into the ocean uniformly,” Trenberth said in a recent interview. “There are increasing indications that the regional manifestations of the ocean heat content — where those spots are warmer — actually matters for subsequent climate over the next six months or so.”

Hurricane Harvey radar loop

Long radar loop of Hurricane Harvey making landfall in southeast Texas in August 2017. (AerisWeather)

Future improvements in hurricane track and intensity forecasts may rely on increasingly coupled models that combine measurements of the atmosphere with regional differences in surface and upper ocean heat content, in something close to real time.

“This is an important area of research, and I don’t think it’s being adequately capitalized at the current time,” Trenberth said, pointing out Hurricane Harvey tapped record levels of ocean heat content in the Gulf of Mexico before dumping more than 60 inches of rain near Houston.

[Because of climate change, hurricanes are raining harder and may be growing stronger more quickly]

Hurricane Florence radar loop

Satellite and radar loop of Hurricane Florence making landfall in southeast North Carolina in September 2018. (AerisWeather)

Similarly, in 2018, Hurricane Florence tapped a regional hot spot of water, where natural variability combined with a warming climate, to fuel a vast swath of historic rains across the Carolinas.

But warm temperatures at the sea surface, by themselves, were insufficient to fuel the storm.

“If there’s not the ocean heat content below the surface to help sustain that, then that high sea-surface temperature will peter out fairly quickly,” Trenberth said. “This is one of the characteristics of Florence and Harvey. Although they took a lot of heat out of the ocean, there was still plenty more heat there, prolonging the lifetime of the storm.”

[The oceans are warming faster than we thought, and scientists suggest we brace for impact]

Like vast atmospheric washing machines, both Harvey and Florence were able to recirculate a continuous supply of deep-ocean warmth, jet fuel for hurricanes, back up to the surface — prolonging intensity and extreme rainfall rates, with catastrophic results.

Hurricane Irma is seen in a visible satellite image from Sept. 5, 2017. (NOAA/AerisWeather)

The longer lifetime and sustained intensity driven by higher ocean content are what energizes what’s known as their eyewall replacement cycles. The eyewall is the zone of towering thunderstorms surrounding the hurricane’s calm center. It’s here that you find the hurricane’s strongest winds.

During an extreme hurricane, spiral arm bands wrap around the center so intensely that the storm forms a new (outer) eyewall, cutting off moisture flow to the old eyewall, which begins to collapse. The result: The storm temporarily weakens in strength (as Katrina did just at landfall in 2005) but expands in size.

Given high ocean heat content, the hurricane can reintensify during eyewall replacement. This happened with Hurricane Irma five times, and Trenberth believes these same dynamics were witnessed with Florence. This is how storms grow bigger, more intense and longer-lived, fueled by a steady supply of ocean heat.

Stating the obvious: For weather forecasters to have any chance of accurately predicting hurricane eyewall replacement cycles, which affects storm size, intensity and severity of the subsequent storm surge, access to reliable temperature data in the upper oceans may be key.

Hurricane Katrina’s cold-water wake. Sea-surface temperature anomalies on Aug. 30, 2005. (NASA Scientific Visualization Studio)
Hurricanes are nature’s quirky, automatic pressure-relief valves, transporting excess heat and moisture away from the tropics toward the poles, leaving behind trails of cooler water. But current weather models can’t capture real-time changes in surface and deep-ocean heating.

Trenberth speculates the latest high-resolution weather models over-intensify tropical systems that assume a constant sea-surface temperature, not fully taking into account their cold wake. Accurate prediction of hurricane eyewall replacement cycles and intensification may require near-real-time, high-resolution data from the upper oceans and Gulf of Mexico.

“The European Center for Medium-Range Weather Forecasts is probably leading in terms of doing coupled weather forecasts,” Trenberth admitted, adding that the U.S. National Centers for Environmental Prediction “has been playing with this and making some progress.”

So, the challenge remains: How can we accurately predict the future when we can’t get an accurate snapshot of what’s happening right now, both atmospheric and oceanic?

Left: An Argo float. Right: An ALAMO float, a cousin of the Argo float, diving. (Coastal Studies Institute, left; Robert Todd, WHOI)
Data from the Argo network of nearly 4,000 drifting floats in the world’s oceans could be programmed to provide deep-sea temperature data more frequently, close to real time during hurricane scenarios. Right now, Argo data updates roughly every 10 days.

“In places like the gulf and East Coast, they could program those to come up and down more frequently,” Trenberth speculated. “The result would be a more comprehensive understanding of upper-ocean water temperature profiles, giving forecasters the ability to gauge whether a hurricane’s cold wake may be resupplied with additional warming at the surface, thus prolonging the storm.”

Global Argo array. (Argo Information Center)

Accurate prediction of increasingly powerful (and wet) hurricanes requires increasingly sophisticated and coupled models combining atmospheric measurements with regional hot spots of unusually warm and deep-ocean water. Hurricanes don’t follow identical tracks, because the next hurricane can “feel” where a previous hurricane has been. Trenberth’s take: “Until we properly put those kinds of factors into our models, we’re not going to quite get those tracks right.”

But regional hot spots of unusually warm, deep water may have implications beyond fueling hurricane strengthening and rainfall prediction. Nontropical cyclones, the bread-and-butter low-pressure systems that parade across our daily weather maps, also may be affected, especially the redevelopment of East Coast storms, feeding on tropical warmth and moisture.

“This can lead to some intensification, but it’s also apt to lead to development out ahead of the storm, so it affects the actual track of the storm,” Trenberth said. “It may also lead to some increase in the size of the storm.”

The aftermath of Hurricane Michael in Mexico Beach, Fla., on Feb. 24.

Weather data and high-resolution mapping are more accessible; weather model physics continues to advance and improve, but Hurricanes Harvey, Florence and Michael are reminders that the most accurate forecasts of track, intensity and rainfall require extensive, real-time analyses of ocean heat content, and not just at the surface. With rising carbon dioxide levels in the atmosphere, warming oceans fueling increasingly potent hurricanes makes real-time, deep-ocean data a necessity, not a luxury.

Tropical researchers are making great strides, but Hurricane Michael’s remarkable, last-minute intensification into a catastrophic 155-mph monster, just 2 mph shy of Category 5 strength, before leveling parts of the Florida Panhandle in October, underscores the urgency of removing biases in coupled air-sea models. Such advancements will provide meteorologists the tools necessary to improve tropical cyclone track and intensity forecasts — and save lives.

Paul Douglas is co-author of “Caring for Creation: The Evangelical’s Guide to Climate Change and a Healthy Environment” and co-founder and senior meteorologist at AerisWeather, located in the Twin Cities of Minnesota.