“This indicates an important turning point that each country will reach in the COVID-19 pandemic, and we predict that the United States is on course to reach this point in the coming weeks,” said , a professor of applied mathematics at the 91̽��. “It is a point of maximum strain on a country’s health and medical infrastructure.”

The new model is intended to help health officials and policymakers see at least two weeks in advance how COVID-19 will likely strain medical infrastructure in the U.S. and around the world. It relies on the number of newly diagnosed cases and the number of individuals who have recovered or died in a geographic region — whether an entire country or a subnational level like a state or province.

and , both from the First Institute of Oceanography’s Data Analysis Laboratory in Qingdao, China, created the model with Tung. Their paper describing the analysis is not yet peer reviewed but has been submitted to a journal for consideration. Their study was March 30 to the preprint site .

For the U.S., this model predicts that:

• The rate of daily new COVID-19 cases peaked on April 5-7, a projection that appears to be accurate, according to Tung
• The number of “active” COVID-19 cases, which are individuals who have been diagnosed but haven’t recovered or died, will peak on April 20, plus or minus four days, and will then slowly decline as the number of cases entering the medical system becomes less than the number of cases leaving the medical system
• The U.S. outbreak will taper off in the first week of June with projections of 710,000 total cases, but could be up to 990,000, and 28,000 deaths, but could be up to 39,000, if the current U.S. fatality rate of 4% holds

Their model also predicts that other hard-hit countries, such as Germany and Spain, have either recently peaked in active COVID-19 cases or will do so soon. The United Kingdom will not peak until the latter half of April, according to the analysis.

The model finds that the length of outbreaks will also vary by country. Germany and Italy will take a week longer than the city of Wuhan, China — the earliest epicenter — to reach their turning point in active COVID-19 cases. The United States is projected to take two weeks longer than Wuhan. Wuhan and Hubei Province were placed under strict lockdowns by the Chinese government early in the outbreak, which may explain the shorter course there, said Tung. Italy was slower to roll out lockdowns, first regionally and then nationally. The United States has no national lockdown, though a majority of states have issued stay-at-home orders.

The researchers tested the model’s efficacy using COVID-19 data from China. With an accuracy of a few days, their model predicted key events in the outbreak’s growth, spread and decline of COVID-19 in Wuhan, Hubei and the rest of China — including the peak of new cases, the peak of active cases and the subsidence of the epidemic. Wuhan’s 76-day lockdown April 8.

Scientists at the UW’s Institute for Health Metrics and Evaluation have created , which relies on other pieces of information about the pandemic, such as . In contrast, the model by Huang, Qiao and Tung uses the number of newly diagnosed cases and the number of individuals who have recovered or died. The Institute’s model primarily projects COVID-19 deaths in a region, as well as the demand for hospital resources such as ventilators.

“Our two approaches complement one another, providing the projections that health officials and governments need to understand when the maximum strain on resources is coming, and to show how the course of the pandemic depends heavily on the level of social distancing measures adopted,” said Tung.

Compared to other modeling approaches, such as widely reported from the Imperial College London, the model developed by Tung and his colleagues does not require knowledge of the infection rate or the total number of infected cases — including asymptomatic individuals. These are difficult data to collect or estimate given the relatively sparse testing for COVID-19 in many countries, and the fact that most symptom-free or mildly ill individuals are not entering the medical system for treatment.

As a result of these and other key differences, the predictions by Huang, Qiao and Tung differ significantly from Imperial College London projections of a longer outbreak with 40% to 80% of the U.S. population infected and 1.1 to 2.2 million deaths. The results from the model used by Imperial College London differed significantly because it relied on separate assumptions about COVID-19 and the predictions were generated when key parameters, such as its infection rates, were unknown, according to Tung.

“Those types of models do serve purposes, such as moving policymakers into action,” said Tung. “But once the epidemic begins, it is important to turn to data-driven models that incorporate real-time information such as diagnosed cases, recovered cases and deaths — which reflect the effects of policy decisions and the degree of compliance — so that we can more realistically project the pandemic’s course.”

For more information, contact Tung at ktung@uw.edu.

While many aspects of the movie are unrealistic, oceanographers are concerned about the long-term stability of the Atlantic Ocean circulation, and previous studies show that it has slowed dramatically in the past decade. New research from the 91̽�� and the Ocean University of China finds the slowdown is not caused by global warming but is part of regular, decades-long cycle that will affect temperatures in coming decades.

The was published July 18 in Nature.

“Climate scientists have expected the Atlantic overturning circulation to decline long-term under global warming, but we only have direct measurements of its strength since April 2004. And the decline measured since then is 10 times larger than expected,” said corresponding author , a 91̽��professor of applied mathematics with an adjunct appointment in atmospheric sciences.

“Many have focused on the fact that it’s declining very rapidly, and that if the trend continues it will go past a tipping point, bringing a catastrophe such as an ice age. It turns out that none of that is going to happen in the near future. The fast response may instead be part of a natural cycle and there are signs that the decline is already ending.”

The results have implications for surface warming. The current’s speed determines how much surface heat gets transferred to the deeper ocean, and a quicker circulation would send more heat to the deep Atlantic. If the current slows down, then it will store less heat, and Earth will be likely to see air temperatures rise more quickly than the rate since 2000.

“The global climate models can project what’s going to happen long-term if carbon dioxide increases by a certain amount, but they currently lack the capability to predict surface warming in the next few decades, which requires a knowledge of how much the excess heat trapped by greenhouse gases is being absorbed by the oceans,” Tung said.

The , or AMOC, is a conveyor belt that brings surface water northward in the Atlantic; from there, the heavier salty water sinks and returns at depth from the Labrador and Nordic seas, near the North Pole, all the way south to the Southern Ocean. Most people are interested in what happens at the surface — the Gulf Stream and associated Atlantic currents carry warmer water north, bringing mild temperatures to Western Europe.

But the new paper argues that the most important step, from a climate perspective, is what happens next. In the North Atlantic, the saltier water from the tropics sinks almost a mile (1,500 meters). As it does, it carries heat down with it away from the surface.

Changes in the strength of the AMOC affect how much heat leaves our atmosphere. The new study uses a combination of data from Argo floats, ship-based temperature measurements, tidal records, satellite images of sea-surface height that can show bulges of warm water, and recent itself to suggest that its strength fluctuates as part of a roughly 60- to 70-year, self-reinforcing cycle.

When the current is faster, more of the warm, salty tropical water travels to the North Atlantic. Over years this causes more glaciers to melt, and eventually the freshwater makes the surface water lighter and less likely to sink, slowing the current.

When the AMOC is in a slow phase, the North Atlantic becomes cooler, ice melt slows, and eventually the freshwater melt source dries up and the heavier saltier water can plunge down again, which speeds up the whole circulation.

The new study argues that this current is not collapsing, but is just transitioning from its fast phase to its slower phase – and that this has implications for heating at the surface.

From 1975 to 1998, the AMOC was in a slow phase. As greenhouse gases were accumulating in the atmosphere, Earth experienced distinct warming at the surface. From about 2000 until now, the AMOC has been in its faster phase, and the increased heat plunging in the North Atlantic has been removing excess heat from the Earth’s surface and storing it deep in the ocean.

“We have about one cycle of observations at depth, so we do not know if it’s periodic, but based on the surface phenomena we think it’s very likely that it’s periodic,” Tung said.

The new paper supports the authors’ showing that since 2000, during which observations show a slowdown in surface warming, heat has accumulated deep in the Atlantic Ocean. The new study shows this is the same period when Atlantic overturning circulation was in its fast phase.

Recent measurements of density in the Labrador Sea suggest the cycle is beginning to shift, Tung said. That means that in coming years the AMOC will no longer be sending more of the excess heat trapped by greenhouse gases deep into the North Atlantic.

“The good news is the indicators show that this slowdown of the Atlantic overturning circulation is ending, and so we shouldn’t be alarmed that this current will collapse any time soon,” Tung said. “The bad news is that surface temperatures are likely to start rising more quickly in the coming decades.”

The first author is at the Ocean University of China and Qingdao National Laboratory of Marine Science and Technology. The study was funded by the U.S. National Science Foundation, the Natural Science Foundation of China, the National Key Basic Research Program of China and a Frederic and Julia Wan Endowed Professorship.

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For more information, contact Tung at ktung@uw.edu or 206-685-3794.

NSF: AGS-1262231; NSFC: 41330960, 41776032; Nat’l Key BPRC: 2015CB953900

More than a dozen theories have now been proposed for the so-called global warming hiatus, ranging from air pollution to volcanoes to sunspots. New research from the 91̽�� shows that the heat absent from the surface is plunging deep in the north and south Atlantic Ocean, and is part of a naturally occurring cycle. The is published Aug. 22 in .

Subsurface ocean warming explains why global average air temperatures have flatlined since 1999, despite greenhouse gases trapping more solar heat at the Earth’s surface.

“Every week there’s a new explanation of the hiatus,” said corresponding author , a 91̽��professor of applied mathematics and adjunct faculty member in atmospheric sciences. “Many of the earlier papers had necessarily focused on symptoms at the surface of the Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find the underlying cause.”

The results show that a slow-moving current in the Atlantic, which carries heat between the two poles, sped up earlier this century to draw heat down almost a mile (1,500 meters). Most of the previous studies focused on shorter-term variability or particles that could block incoming sunlight, but they could not explain the massive amount of heat missing for more than a decade.

“The finding is a surprise, since the current theories had pointed to the Pacific Ocean as the culprit for hiding heat,” Tung said. “But the data are quite convincing and they show otherwise.”

Tung and co-author of the Ocean University of China, who was a 91̽��visiting professor last year, used recent observations of deep-sea temperatures from that sample the water down to 6,500 feet (2,000 meters) depth, as well as older oceanographic measurements and computer reconstructions. Results show an increase in heat sinking around 1999, when the rapid warming of the 20^th century stopped.

“There are recurrent cycles that are salinity-driven that can store heat deep in the Atlantic and Southern oceans,” Tung said. “After 30 years of rapid warming in the warm phase, now it’s time for the cool phase.”

Rapid warming in the last two and a half decades of the 20^th century, they proposed in an earlier , was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface. When observations show the ocean cycle flipped, in about 2000, the current began to draw heat deeper into the ocean, working to counteract human-driven warming.

The cycle starts when saltier, denser water at the surface northern part of the Atlantic, near Iceland, causes the water to sink. This changes the speed of the huge current in the Atlantic Ocean that circulates heat throughout the planet.

“When it’s heavy water on top of light water, it just plunges very fast and takes heat with it,” Tung said. Recent observations at the surface in the North Atlantic show record-high saltiness, Tung said, while at the same time, deeper water in the North Atlantic shows increasing amounts of heat.

The oscillations have a natural switch. During the warm period, faster currents cause more tropical water to travel to the North Atlantic, warming both the surface and the deep water. At the surface this warming melts ice. This slowly makes the surface water there less dense and after a few decades puts the brakes on the circulation, setting off a 30-year cooling phase.

The authors dug up historical data to show that the cooling in the three decades between 1945 to 1975 – which caused people to worry about the start of an ice age – was during a cooling phase. (It was thought to have been caused by air pollution.) Earlier records in Central England the 40- to 70-year cycle goes back centuries, and other records show it has existed for millennia.

Changes in Atlantic Ocean circulation historically meant roughly 30 warmer years followed by 30 cooler years. Now that it is happening on top of global warming, however, the trend looks more like a staircase.

This explanation implies that the current slowdown in global warming could last for another decade, or longer, and then rapid warming will return. But Tung emphasizes it’s hard to predict what will happen next.

A pool of freshwater from melting ice now sitting in the Arctic Ocean, for example, could overflow into the North Atlantic to upset the cycle.

“We are not talking about a normal situation because there are so many other things happening due to climate change,” Tung said.

The research was funded by the U.S. National Science Foundation and the National Natural Science Foundation of China.

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For more information, contact Tung at 206-685-3794 or tung@amath.washington.edu.

Grant numbers: NSF AGS-1262231 and NNSF China: 41330960 and 41176029.