Cold Wave

 

  The Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM) and its recent homologue, the North Atlantic Oscillation (NAO) are the most prominent modes of variability in the Northern Hemisphere winter climate.

  The Artic oscillation(AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is an index of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude, and it is characterized by pressure anomalies of one sign in the Arctic with the opposite anomalies centered about 37–45N. The AO is believed by climatologists to be causally related to, and thus partially predictive of, weather patterns in locations many thousands of miles away, including many of the major population centers of Europe and North America.

  The degree to which Arctic air penetrates into middle latitudes is related to the AO index, which is defined by surface atmospheric pressure patterns. When the AO index is positive, surface pressure is low in the polar region. This helps the middle latitude jet stream to blow strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar region. When the AO index is negative, there tends to be high pressure in the polar region, weaker zonal winds, and greater movement of frigid polar air into middle latitudes. 

• Positive Phase (AO+)

  Under positive Arctic Oscillation conditions, the polar vortex is strong and air pressure at the North Pole is lower than average (strong). In comparison, air pressure at mid-latitudes is higher (weaker). Because air travels from areas of high to areas of low pressure the resulting flow is towards the Arctic. This influx increases the speed of the polar circulation, the westerlies, and northeasterly trade winds. Stronger winds work to tightly encircle and confine the coldest air to the high latitudes.

• Negative Phase (AO-)

  When the polar vortex is weaker—often as a result of milder air invading the Arctic region from neighboring Siberia—air pressure at high latitudes is less (higher) and the AO enters a negative phase. This means that air travels from the North Pole toward lower pressure in the mid- and low- latitudes. Wind patterns relax, and rather than maintaining a west to east orientation, the polar vortex circulation meanders north and south allowing cold air from the Arctic to overrun North America, Europe, and Asia.

 

 

  A large difference in the pressure at the two stations (a high index year, denoted NAO+) leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if the index is low (NAO-), westerlies are suppressed, these areas suffer cold winters and storms track southerly toward the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa.

  Although having a less direct influence than for Western Europe, the NAO is also believed to have an impact on the weather over much of eastern North America.

 

• Positive Phase (NAO+)

During a positive NAO there is a strengthening of the Icelandic low and Azores high. This strengthening results in an increased pressure gradient over the North Atlantic, which cause the westerlies to increase in strength. The increased westerlies allow cold air to drain off the North American continent rather than letting it build up and move south.

1. Above average geopotential heights are observed over the eastern U.S., which correlates to above average temperatures.
2. The eastern U.S. often sees a wetter pattern with stronger storms during the winter season in this phase due to increased upper level winds.
3. Recent studies at the SCO indicate a decreased potential for wintry weather in NC due to the lack of cold air availability and above average temperatures associated with a positive NAO in this region.

 

• Negative Phase (NAO-) 

A negative NAO indicates weakening of both the Icelandic low and Azores high, which decreases the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies. The decrease in the westerlies allows cold air to build up over Canada, and this combined with below average heights (troughing) over the eastern U.S. gives the cold air a greater chance to move south and affect the eastern United States.

1. Below average geopotential heights are often observed over the eastern U.S. during the negative phase of the NAO, which correlates to below average temperatures.
2. The eastern U.S. typically receives colder, drier air masses during the winter season in this phase.
3. Recent studies at the SCO indicate an increased potential for wintry weather in NC due to the position and availability of cold air, and a more favorable upper level pattern conducive to coastal storm tracks