If you are not familiar with the term, an atmospheric river (AR) is a narrow band of atmospheric moisture and they are responsible for the majority of moisture movement outside of the tropics. They form in the lower atmosphere and can move with wind currents for thousands of miles, lasting many days. When they strike obstacles, such as mountain ranges, the uplift causes the moisture to condense and precipitate out. That is why they cause so much precipitation in California - they travel in from the Pacific and strike the Sierra Nevada mountains. Take a look at this graphic showing the amount of precipitable water in the atmosphere and you can clearly see the atmospheric river heading into California.
|Source: CCI Reanalyzer|
Note how the AR passes over Hawaii, hence the nickname "Pineapple Express." Also, notice how much more moisture there is in the tropics. You can also see how the AR ends on the West Coast and doesn't move inland very much. West of the mountains will get rain, but the areas east of the mountains will remain dry. While some rain is good, ARs are capable of bringing catastrophic amounts. An AR in 1861-2 brought 43 straight days of rain to the West Coast and turned the interior valleys into lakes, causing massive amounts of damage and killing thousands.
Is this the end of the drought? We can hope so, but we'll have to wait and see to be sure. Hopefully, we won't see anything like the AR of 1861.
So, now the question is, is the drought a naturally-occurring event or because of climate change? The answer, as is typically the case, a mixture of both. California has had droughts in the past and will continue to have them in the future. This drought may have been a naturally occurring event. There are reports going both ways. But, the one thing that is consistent is that global warming certainly made this drought much worse than ones in the past. What studies have shown is that there have been droughts in the past that had even less rain than this current one, but they were all much cooler. This current drought has been a 'hot drought' and California has experienced record high temperatures (remember that the next time someone tells you East Coast temperatures mean the planet is cooling) at the same time the rain failed.
In short, the drought was caused by what has become known as the 'ridiculously resilient ridge.' This ridge caused the wind currents to go north, parallel to the West Coast, preventing moisture from reaching the coastal regions. The currents went up into Alaska and Canada before turning south and plunging into Mid-America, leading to our cooler than average winters in the east these last couple of years. There is debate if this ridge was natural or caused by climate change. But, drought was exacerbated by the high temperatures and the higher temperatures were most certainly caused by global warming.
What that means is that we can expect the naturally occurring drought cycle going forward, but they will look more and more like this last one. Even if this AR brings relief, California needs to make plans because this is going to happen again.