Click the link to read the briefing on the Western Water Assessment website:

La Niña continued to influence regional weather during May with cooler than average temperatures throughout most of the region and above average precipitation for northern Wyoming. June 1st SWE was much above normal in much of Wyoming, near normal in northern Colorado and east of the Continental Divide and generally below normal or completely melted in Utah. June 1st seasonal streamflow forecasts were below to much-below normal for the Upper Colorado River and Great Basins. While drought remains present in 92% of the region, drought conditions significantly improved in northern Wyoming, but worsened in Utah and southern Colorado.

May precipitation was generally below normal in the Intermountain West. The driest conditions were observed in southern Utah and southwest Colorado where many locations saw no measurable precipitation. May precipitation was above normal in northwestern Wyoming, central Wyoming and locations along the Continental Divide in Colorado. Due to mostly below normal May temperatures, many mid-to-high elevation locations saw significant snowfall.

Regional temperatures were near-to-below normal during May. In northern Utah, northwestern Colorado and western Wyoming, temperatures were 2-4 degrees below normal.

June 1st snow water equivalent (SWE) was generally above normal in the northern portion of the region and completely melted out or below normal in southwestern Colorado and southern Utah. Near normal June 1st SWE was observed in northern Colorado, northern Utah and southern Wyoming despite below average May 1st SWE in many of these locations. In much of Wyoming, June 1st SWE conditions ranged from 126% – 705% of normal. Improvements in June SWE conditions compared to May1st SWE conditions were due to below normal temperatures and areas of much above average precipitation.

June 1st seasonal streamflow forecasts are below to much-below normal in the Upper Colorado River and Great Basins. Seasonal runoff forecasts were highest, but still below normal, for the Upper Colorado, Upper Gunnison, and Upper Yampa River basins. Seasonal runoff forecasts are much-below normal for all other river basins. Many locations in Utah and western Colorado, where temperatures were near normal and precipitation was below normal, dust storms caused significant dust accumulation on snowpack and speeded melt. Inflow forecasts for all major regional reservoirs are much-below normal with Green Mountain (79% normal) and Blue Mesa (68%) Reservoirs faring best and Lake Powell, Flaming Gorge, McPhee and Navajo Reservoirs are forecasted to have less than 55% normal inflow volumes.

Drought conditions continue across 92% of the region. Overall, drought conditions improved in the northern portion of the region and deteriorated in the south. Cooler and wetter conditions led to a two-category improvement of drought conditions in northern Wyoming and a large decrease in the coverage of extreme (D3) drought. D3 drought developed in southwestern Wyoming during May. Drought conditions worsened in western Colorado with the emergence of D1 drought along the Colorado River and expansion of D2 and D3 drought. Drought was removed in portions of Larimer and Routt Counties in northern Colorado. In Utah, D3 drought emerged over large areas of the state, now covering 72% of its area.

La Niña conditions continued during May with eastern Pacific Ocean temperatures averaging 1ºC below normal. La Niña conditions are expected to persist at least through June and there is a greater than 50% probability of La Niña continuing through early winter. A typical La Niña weather pattern is forecasted to continue during June with a higher probability of wetter and cooler than normal conditions for northern Wyoming. The NOAA seasonal forecast for June-August predicts a 60-70% probability of above average temperatures for most of the region and an increased probability of below normal precipitation for Wyoming, most of Colorado and northern Utah. There is an increased probability of above average summer precipitation in southern Arizona, suggesting the possibility of a stronger than normal monsoon.

Click here to access the paper on the Nature website (Bryam Orihuela-Pinto, Matthew H. England & Andréa S. Taschetto). Here’s the abstract:

Climate projections suggest a weakening or collapse of the Atlantic Meridional Overturning Circulation (AMOC) under global warming, with evidence that a slowdown is already underway. This could have significant ramifications for Atlantic Ocean heat transport, Arctic sea ice extent and regional North Atlantic climate. However, the potential for far-reaching effects, such as teleconnections to adjacent basins and into the Southern Hemisphere, remains unclear. Here, using a global climate model we show that AMOC collapse can accelerate the Pacific trade winds and Walker circulation by leaving an excess of heat in the tropical South Atlantic. This tropical warming drives anomalous atmospheric convection, resulting in enhanced subsidence over the east Pacific and a strengthened Walker circulation and trade winds. Further teleconnections include weakening of the Indian and South Atlantic subtropical highs and deepening of the Amundsen Sea Low. These findings have important implications for understanding the global climate response to ongoing greenhouse gas increases.

Click the link to read “A huge Atlantic ocean current is slowing down. If it collapses, La Niña could become the norm for Australia” on The Conversation website (Matthew England, Andréa S. Taschetto & Bryam Orihuela-Pinto):

Matthew England, UNSW Sydney; Andréa S. Taschetto, UNSW Sydney, and Bryam Orihuela-Pinto, UNSW Sydney

Climate change is slowing down the conveyor belt of ocean currents that brings warm water from the tropics up to the North Atlantic. Our research, published today in Nature Climate Change, looks at the profound consequences to global climate if this Atlantic conveyor collapses entirely.

We found the collapse of this system – called the Atlantic meridional overturning circulation – would shift the Earth’s climate to a more La Niña-like state. This would mean more flooding rains over eastern Australia and worse droughts and bushfire seasons over southwest United States.

East-coast Australians know what unrelenting La Niña feels like. Climate change has loaded our atmosphere with moister air, while two summers of La Niña warmed the ocean north of Australia. Both contributed to some of the wettest conditions ever experienced, with record-breaking floods in New South Wales and Queensland.

Meanwhile, over the southwest of North America, a record drought and severe bushfires have put a huge strain on emergency services and agriculture, with the 2021 fires alone estimated to have cost at least US$70 billion.

Earth’s climate is dynamic, variable, and ever-changing. But our current trajectory of unabated greenhouse gas emissions is giving the whole system a giant kick that’ll have uncertain consequences – consequences that’ll rewrite our textbook description of the planet’s ocean circulation and its impact.

What is the Atlantic overturning meridional circulation?

The Atlantic overturning circulation comprises a massive flow of warm tropical water to the North Atlantic that helps keep European climate mild, while allowing the tropics a chance to lose excess heat. An equivalent overturning of Antarctic waters can be found in the Southern Hemisphere.

Climate records reaching back 120,000 years reveal the Atlantic overturning circulation has switched off, or dramatically slowed, during ice ages. It switches on and placates European climate during so-called “interglacial periods”, when the Earth’s climate is warmer.

Since human civilisation began around 5,000 years ago, the Atlantic overturning has been relatively stable. But over the past few decades a slowdown has been detected, and this has scientists worried.

Why the slowdown? One unambiguous consequence of global warming is the melting of polar ice caps in Greenland and Antarctica. When these icecaps melt they dump massive amounts of freshwater into the oceans, making water more buoyant and reducing the sinking of dense water at high latitudes.

Around Greenland alone, a massive 5 trillion tonnes of ice has melted in the past 20 years. That’s equivalent to 10,000 Sydney Harbours worth of freshwater. This melt rate is set to increase over the coming decades if global warming continues unabated.

A collapse of the North Atlantic and Antarctic overturning circulations would profoundly alter the anatomy of the world’s oceans. It would make them fresher at depth, deplete them of oxygen, and starve the upper ocean of the upwelling of nutrients provided when deep waters resurface from the ocean abyss. The implications for marine ecosystems would be profound.

With Greenland ice melt already well underway, scientists estimate the Atlantic overturning is at its weakest for at least the last millennium, with predictions of a future collapse on the cards in coming centuries if greenhouse gas emissions go unchecked.

The ramifications of a slowdown

In our study, we used a comprehensive global model to examine what Earth’s climate would look like under such a collapse. We switched the Atlantic overturning off by applying a massive meltwater anomaly to the North Atlantic, and then compared this to an equivalent run with no meltwater applied.

Our focus was to look beyond the well-known regional impacts around Europe and North America, and to check how Earth’s climate would change in remote locations, as far south as Antarctica.

The first thing the model simulations revealed was that without the Atlantic overturning, a massive pile up of heat builds up just south of the Equator.

This excess of tropical Atlantic heat pushes more warm moist air into the upper troposphere (around 10 kilometres into the atmosphere), causing dry air to descend over the east Pacific.

The descending air then strengthens trade winds, which pushes warm water towards the Indonesian seas. And this helps put the tropical Pacific into a La Niña-like state.

Australians may think of La Niña summers as cool and wet. But under the long-term warming trend of climate change, their worst impacts will be flooding rain, especially over the east.

We also show an Atlantic overturning shutdown would be felt as far south as Antarctica. Rising warm air over the West Pacific would trigger wind changes that propagate south to Antarctica. This would deepen the atmospheric low pressure system over the Amundsen Sea, which sits off west Antarctica.

This low pressure system is known to influence ice sheet and ice shelf melt, as well as ocean circulation and sea-ice extent as far west as the Ross Sea.

A new world order

At no time in Earth’s history, giant meteorites and super-volcanos aside, has our climate system been jolted by changes in atmospheric gas composition like what we are imposing today by our unabated burning of fossil fuels.

The oceans are the flywheel of Earth’s climate, slowing the pace of change by absorbing heat and carbon in vast quantities. But there is payback, with sea level rise, ice melt, and a significant slowdown of the Atlantic overturning circulation projected for this century.

Now we know this slowdown will not just affect the North Atlantic region, but as far away as Australia and Antarctica.

We can prevent these changes from happening by growing a new low-carbon economy. Doing so will change, for the second time in less than a century, the course of Earth’s climate history – this time for the better.

Matthew England, Scientia Professor and Deputy Director of the ARC Australian Centre for Excellence in Antarctic Science (ACEAS), UNSW Sydney; Andréa S. Taschetto, Associate Professor, UNSW Sydney, and Bryam Orihuela-Pinto, PhD Candidate, UNSW Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.