Why are we so warm?
Dayton, Springfield, the Miami Valley and most of Ohio will flirt with the 60′s Wednesday evening into Thursday, continuing a trend of warmer than normal temperatures along with wet weather. Dayton was 6 degrees above normal in November and is currently 3 degrees above normal in December! This may not seem like a lot, but 3-6 degrees this time of year can mean the difference between rain and snow!
Many remember the brutal cold last December that reached to the southeast, causing disabling ice and snow storms in places such as Atlanta and Birmingham. Why is 2011 so different? Many will point to global warming as the cause…but these same people also blame global warming when temperatures drop well below normal. It’s a lazy excuse at best. Meteorologists continue to emphasize the importance of atmospheric circulations that can manipulate the jet stream. The most famous of these is the El Nino/La Nina phenomenon.
November and early December temperatures shown as a departure from normal. Notice that Ohio is 3-4 degrees above normal.
Let’s take a look at these patterns that are shaping our warmer than normal weather.
La Nina: During La Nina events, the jet stream tends to be further north (and oriented southwest to northeast) than El Nino events. Many times, this allows the jet stream to stay to our north, which keeps us in warmer weather. However, other factors can override La Nina’s positioning of the jet stream.
North Atlantic Oscillation (NAO): The NAO is one factor that is vital to the weather patterns over the eastern United States. It is a measure of the difference in pressures over Iceland and the Azores. When the NAO is positive, as it currently is, the jet stream has a more west to east component, which helps keep prolonged cold shots away. When the NAO turns negative, the jet stream dips south over the eastern U.S. This allows for cold air masses to migrate southward out of Canada. Most Nor’Easter storms happen during Negative NAO due to the storm track being “blocked” to the east of New England.
The NAO & Jet Stream illustrated. Public domain image courtesy of NASA.
Arctic Oscillation (AO): The Arctic Oscillation is often associated with the NAO. The AO is a measure of the low pressure that resides over the North Pole. When the Low is strong, the AO is said to be “positive”. The strong low tends to keep the cold located over the Arctic, allowing for seasonably warm temperatures to the south. When the low weakens, AO is said to be “negative”. The swirling of winds loosens in the Arctic, which allows intrusions of cold air into North America (along with Europe and Asia). For the past few months, the AO has been strongly in the positive phase, keeping the brutal cold air mass locked in the Arctic.
The Arctic Oscillation illustrated.
Madden Julian Oscillation (MJO): The MJO is different than the NAO and AO because it doesn’t measure the strength of a pressure system. The MJO is a conveyor belt of moisture that travels around the Equator. It can add lift and moisture to certain parts of the atmosphere, while suppressing storms in others. Because of this, it can often lead to active or inactive periods of hurricane development in the summer and fall. Different phases of the MJO can also have a widespread effect on temperatures across North America, which is illustrated below. Recently, the MJO has languished in Phase 5, which lends to a warmer than normal eastern U.S. The “Phase 5″ image below looks a lot like the “departure from normal” temperature chart posted at the top of this page, doesn’t it?
As you can see, these major influences are all aligned to give us the warmer than normal temperatures we’re experiencing. Any cold temperatures have been only temporary, which is a change from the winters of 2009 and 2010. For those of you who love the cold and snow, don’t fret. This pattern is highly unlikely to hold for the entire winter. I would expect to see a reversal and a move towards more cold and snow around mid-January….but that’s a different post for another time.