By Steve Almasy
(CNN) -- Record-breaking cold across the Midwest.
Record-breaking heat in California.
Makes you wonder what's up with Ol' Man Winter.
For example, West Coast heat made news all week -- for better or worse. California saw a record-setting winter heat wave, but on Friday, Gov. Jerry Brown delivered the bad news: He declared a drought emergency, after seeing little rain since 2012.
That announcement came two days after Monterey, California, reached 83 degrees Wednesday, smashing a record for the date by 8 degrees.
At the other extreme, it was just last week that a polar vortex delivered some of the coldest temperatures in two decades to the Midwest and Northeast.
Have Mother Nature and Ol' Man Winter gone Jekyl-and-Hyde on us?
As it turns out, there's nothing sinister at work.
It's natural variability, meteorologists say.
"It happens every year, it just varies in intensity," said Taylor Ward, a CNN weather producer.
Last year in mid-January, many Americans were wondering why it was so warm compared to most years.
But now it's the opposite -- well, for much of the country. It depends on where you live in relation to the jet stream.
The jet stream is a ribbon of wind that flows west to east high across the globe, typically at 110 mph, although its speed can often top 200 mph. Think of it as an aerial highway along which storms travel along.
But the jet stream fluctuates north and south. It's always wobbling. It drives our temperature variability daily.
One day you might be below a ridge in the jet stream and its warmth, and days later, a trough or dip will send brrrrr-utality your way.
The National Oceanic and Atmospheric Administration says climatologists in recent years observe a more wavy pattern to the jet stream, bringing outbreaks of colder weather to the areas of the Northern Hemisphere -- including the United States and Europe.
"Whether this is normal randomness or related to the significant climate changes occurring in the Arctic is not entirely clear, especially when considering individual events," NOAA says on its climate.gov website.
It's something CNN Senior Meteorologist Brandon Miller talked about last week when asked to explain the polar vortex that was leading to the coldest temperatures in many states in decades.
When asked if it was a side effect of global warming, his response was, it's under review.
"This is a hotly researched topic. In short, yes, it could be," he said. "It seems counter-intuitive that global warming could cause significant cold snaps like this one, but some research shows that it could.
"We know that different types of extreme weather can result from the overall warming of the planet, melting of the Arctic Sea ice, etc. This includes extreme distortions of the jet stream, which can cause heat waves in summer and cold snaps in winter," he said.
J. Marshall Shepherd, the president of the American Meteorological Society, says more study is needed on whether Arctic warming will have a big effect on northern climates.
In nature, air north of the jet stream is cold, and to the south it is warmer.
But the Arctic is getting warmer.
"We're relaxing or loosening that difference in temperature and that changes the jet stream and loosens it some," he told viewers of a White House video chat last week. "I think this is an area we have to look (at)...closely in the science. I think it is quite plausible."
He agreed it's too early to say with certainty.
But a study by Elizabeth Barnes of Colorado State University's department of atmospheric science found no support for the increased temperatures in the Arctic leading to additional slow-moving weather patterns.
She described the relationship between the Arctic and the weather to its south as "complex."
In the study, published last August, she said there are likely influences from other parts of the world and internal variability.
As scientists press ahead in their research, this month's jet stream reached far down to the Gulf Coast, bringing such frigidity that Midwesterners resorted to outlandish experiments, such as tossing a cup of boiled water into the outdoor air and watching it turn to snow.
Finally this week, a weather ridge in the West Coast pushed the jet stream back into Canada, to the relief of many Americans.
CNN's Michael Martinez contributed to this report.
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