Low pressure systems (red L over northern NY & VT) are
characterized by counterclockwise circulation of winds.
This brings cold Canadian air down into the Hudson Valley.
Just goes to illustrate the difference between weather and climate. Weather is the day-to-day set of conditions in the atmosphere, climate is the average over time. In other words, climate is what you expect, weather is what you get.
So, anyway, in Friday's post I talked about how it's been unseasonably warm in the Hudson Valley this winter (at least to date). Today I want about this in more detail.
Turns out it's just not unseasonably warm here, in the first week of January, more than 1,000 temperature records were broken across the country. There was also 19% snow cover in the U.S. versus a normal for early January of closer to 50% (see This Winter's Weirdly Warm Weather Explained). Some meteorologists have dubbed these conditions "Marchuary".
So what's going on? Well, to start, it's a La Niña year. The El Niño/La Niña weather phenomenon deserves a post of its own but is basically an oscillation of high and low pressure systems across the tropical Pacific Ocean (it's also know as ENSO for El Niño Southern Oscillation). During normal conditions, there's high pressure off the west coast of South America in our winter (their summer, since seasons are reversed south of the equator) and low pressure over by Indonesia and Australia (resulting in rains).
This high pressure drives the equatorial current westward across the tropical Pacific. Cold water is pulled up from the Southern Ocean off the coast of Chile and Peru. This cold current brings plankton rich waters and fishing is good.
During an El Niño year, however, these high and low pressure systems flip (hence the word "oscillation" in ENSO) and the equatorial current reverses direction. The cold current off South America shuts down and the fish go away (causing economic hardships for fishermen in Peru and Chile). In addition, now there's low air pressure off South America resulting in rains which cause flooding and mudslides in the Andes. Over in Indonesia, they're not getting the rains and drought ensues.
Note also, in looking at the two images, how the El Niño event affects the subtropical and polar jet streams crossing North America. Our winter weather is affected by the shifting of high and low pressure systems in the tropical Pacific Ocean. These types of effects are called teleconnections by meteorologists.
That's El Niño, what then is La Niña? La Niña is basically an intensification of normal conditions. Higher high pressure off South America. This allows the eastern Pacific (off the west coasts of North and South America) to get colder than normal. This cold water evaporates less thus leading to drier atmospheric conditions. As this drier air comes off the Pacific and is carried across North America, it drops less precipitation as snow.
When there's less snow on the ground, there's also less radiation from the Sun reflected back into space. The Sun is more effectively able to warm the ground resulting in slightly higher temperatures.
That's not the whole story, however. Last year we had La Niña conditions at it was very cold and snowy (remember the various "Snowmaggedon" stories?). There are other pressure system oscillations not as well know as ENSO. The Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) also play a role in how the polar jet stream moves across North America affecting our weather.
Last year the AO and NAO were both in their negative phases and this year they're positive. This positive phase allows for a stronger, less "kinked" jet stream which is resulting in less precipitation and storms across the upper part of the country.
Bottom line is that our weather patterns are complex and interrelated. While it's tempting to look at anomalously warm weather and say "It's that damn global warming!" or look at anomalously cold weather and say "See, I told you global warming was a crock of shit!", the reality is that you can't say ANYTHING about global climate change from a single storm (e.g. Hurricane Katrina or Irene) or a couple of days or weeks of warm or cold weather. Climate is weather averaged over many years (and when you look at those records, it unquestionably supports a warming trend).