Why isn't it raining in California?

CaliforniaLess rainy season in winter

A high pressure area that literally sticks over the North Pacific and triggers a weather blockade. It keeps winter storms and bands of clouds from reaching California. It is unusually warm and dry there when it should be cold and humid.

This weather situation during the rainy season in California has accumulated more and more in the last four decades and increases the risk of persistent droughts in the sunny state - as in the past four years. That was the result of a study by Californian climate researchers from Stanford University. Among them Daniel Swain:

"This high-pressure blockade is characterized by changes in the wind at high altitude. We know it as the jet stream. It usually brings precipitation and storms to California during the rainy season. In recent winters, the jet stream has not dodged as far south. Instead, it shifted north. And the rain , which California normally gets, fell further north. As a result, Alaska, for example, last experienced its snowy and rainiest winter to date. "

Investigation of weather conditions from seven decades

Swain and his colleagues scanned weather data for the past seven decades. They were particularly interested in California's hottest, coldest, driest and wettest years during this time. And for the weather patterns that prevailed under these conditions.

According to the researchers, there was a clear trend: The persistent winter high pressure area over the North Pacific has become more common - and with it particularly hot and dry years in California.

"The increase in temperatures due to climate change increases the risk of droughts anyway. Here we have an additional factor: The air circulation in the atmosphere is also changing. There are more frequent weather patterns that are associated with extremely low rainfall in California."

The pattern of large-scale air currents such as the jet stream is shifting. This also shifts the highs and lows. Or they remain in a place like the high pressure wedge off the California coast for an unusually long time. This is also due to the increasing warming of the earth's atmosphere, as can be assumed.

California will have to prepare for another drought

But that doesn't mean that there won't be a drop of rain in Los Angeles or San Francisco in the future. There will continue to be winters in which the jet stream can see California and it rains a lot, say the researchers. But the question is: Can this even compensate for a precipitation deficit as large as the one that has prevailed in California since the most recent record drought?

This winter it rained again. That was due to the El Nino climate phenomenon. California always brings rain. But Daniel Swain does not consider the record drought to be over. It has been weighing on the sunny state since 2012:

"Although the El Nino was very strong this time, the rainfall in southern California was well below average. That has not cured the drought we are suffering from. And we have to ask ourselves whether this winter might already be the signature of the future We don't know if El Ninos California will bring enough rain at all. "

This also applies to the north of the sunny state. There is the Sierra Nevada. California relies on heavy snowfall in this mountainous region. And the meltwater that runs off the mountain peaks in spring.

But even here the winter did not bring the longed-for relief, as Daniel Swain says. Despite the strong El Nino, 20 percent less snow fell than the long-term average. So California's water deficit will persist for the time being.