A key factor contributing to catastrophes along the Pacific Coast from floods in California to deadly wildfires in Chile is hotter ocean temperatures. However, the debate is ongoing about whether this is due to El Niño or climate change, or a combination of both. Some scientists believe they see evidence of both contributing to the recent extreme weather. Others argue that it’s too early to definitively attribute the intensity of the atmospheric rivers hitting the Pacific Coast to climate change. One thing that can be confirmed is that ocean temperatures offshore are noticeably warmer than usual. The warmer oceans and air allow the atmosphere to hold more moisture, around 4% more moisture for every degree of warming. In California, for the second consecutive winter, massive amounts of water are being dumped by atmospheric rivers, which can be 200-300 miles wide, transporting significant moisture across hundreds of miles. However, there is a fine line between too much and not enough, as the recent torrential rain and high winds have caused widespread destruction in the state, resulting in three fatalities. Los Angeles experienced the third wettest two-day rainfall dating back to 1877, with up to 10 inches of rain in some locations – almost half of the city’s normal seasonal rainfall. It is evident that the “rivers in the sky” will become wetter and more intense in the future, but the extent to which today’s warmer water and air temperatures are due to natural climate variability, such as El Niño or La Niña, versus changing climate, still lacks consensus among scientists.
What’s happening with the warm oceans?Ocean temperatures have been steadily rising globally for years, and the last three decades have been the warmest on record. There are indications that this warming is accelerating, with the oceans absorbing much of the excess heat energy retained by greenhouse gas emissions. As a result, surface and deeper ocean temperatures are increasing. The average global sea surface temperatures between the latitudes of 60 South and 60 North have been hovering at 21.1 degrees Celsius this week, matching a record set in the previous year.
The warmer ocean temperatures are in line with expectations following a major El Niño. This phenomenon weakens trade winds along the Pacific Ocean off the South American coast, pushing warmer water eastward and northward along the Pacific Coast of the United States. It has led to temperatures off the California coast being approximately 3.5 degrees warmer than usual.
Is climate change making atmospheric rivers wetter?Climate change is projected to increase the moisture content and precipitation carried by atmospheric rivers, especially on mountain slopes. Additionally, warmer sea surface temperatures, stronger winds, and lower relative humidity are expected to lead to wetter atmospheric rivers. The Intergovernmental Panel on Climate Change and other studies have predicted increased precipitation in atmospheric rivers in the future, although this trend is not yet evident in observations, according to Gershunov. However, a rapid study conducted by scientists from the international consortium ClimaMeter suggested that floods affecting coastal California in recent years have been up to 15% wetter and windier, attributing most of that increase to human-driven climate change and a modest role played by natural climate variability.
‘Rivers in the sky’:What exactly is an atmospheric river?According to Davide Faranda, a researcher in climate physics at the French National Center for Scientific Research, their findings suggest that the extreme characteristics of California floods are linked to human-driven climate change and a subtle influence of natural climate variability.
Heat waves rage in South AmericaIntense heat waves have been sweeping across much of South America, including Chile, Argentina, Paraguay, and Colombia. The temperature in Santiago, Chile’s capital city, reached 99.1 degrees on Jan. 31, the third highest temperature in 112 years of recording. In addition to the heat, drought, low humidity, and strong winds have contributed to catastrophic fires in the region. At least 130 people have died in Chile, with hundreds more missing in the Valparaiso region including the coastal town of Viña del Mar.
Bárbara Tapia Cortés from WMO’s Regional Office for the Americas highlighted that while El Niño is a natural phenomenon, recent events are occurring in the context of worsening climate change. The current El Niño episode is likely to intensify existing heat, leading to more extreme weather events.
