Global Patterns - Interactions | State Climate Office of North Carolina

Interactions

Global climate patterns do not occur independently. In fact, they evolve over various timescales that often overlap, leading to important interactions that are one of the driving forces behind weather across the globe. It is important for meteorologists and climatologists to recognize how interactions amongst the phases of ENSO, PDO, PNA, and AO/NAO affect global temperature and precipitation patterns in order to improve weather and climate predictions. The knowledge of these interactions can be used to increase confidence in predicting significant weather events such as: cold air outbreaks and winter storms, heat waves and droughts, and even increased hurricane activity. Below are several examples of how climate pattern interactions have impacted weather across the southeast.

Southeast U.S. Winter Storm

In early December 2009, climate pattern signals indicated that the atmosphere would become primed to produce cold, and potentially wintry, weather in the eastern U.S later in the month. The ENSO cycle had transitioned into a strengthening El Niño phase, while the PDO was in a weak cold to near neutral phase (which means it probably had little impact on the El Niño episode). Meanwhile, the PNA and NAO were both forecasted to become favorable for cold air to move southeastward. All that was needed was a storm, and this was delivered by the active sub-tropical jet stream compliments of El Niño. With cold air in place, a significant early season winter storm occurred over portions of the southeast including western N.C. and most of Virginia.

500mb Height Anomalies	Sea Surface Temperature Anomalies
Surface Temperatures	NWS Composite Reflectivity

Three important climate indices can be deduced from the 500mb height anomalies (top left): 1) The AO is clearly negative, which is apparent from the well above average heights that exist over the polar region. 2) The PNA exhibits a positive signal, which can be seen in the trough-ridge-trough pattern that extends from the Pacific across the U.S. into the Atlantic. 3) The NAO is in a negative phase, evident by the above normal heights over Iceland, and the below normal heights in the Azores region. The interaction of these three indices allowed cold, Arctic air to push into North Carolina, as seen in the bottom left figure, setting the stage for a winter storm.

By examining the sea surface temperature anomalies (top right), the ENSO and PDO phases can be determined. Forecasters realized that the 2009-2010 winter season would likely feature a moderate El Niño, suggesting increased precipitation would be likely across the southeastern United States. Meanwhile, a weakly cold PDO (evident by below average sea surface temperatures near the western U.S. coastal region and slightly above average temperatures in the central Pacific) meant that it would have little effect on the El Niño episode. The result was numerous strong coastal low pressure systems coming out of the Gulf of Mexico, one of which was the powerful storm system on December 18-19th, 2009.

Drought of Winter 2007-2008

The winter of 2007-2008 featured a strong La Niña episode in the central and eastern Pacific, which led to an exceptionally dry winter in the southeast. The strong La Niña caused the subtropical jet stream, which is responsible for transporting moisture northward into the mid-latitudes, to stay further south than normal; this prevented moisture from being transported into the southeastern United States. As the winter season progressed and the La Niña continued to remain strong, drought conditions continued to intensify which can be seen in the animation of the North Carolina U.S. Drought Monitor below.

US Drought Monitor of North Carolina

Sea Surface Temperature Anomalies

2005 Atlantic Hurricane Season

The 2005 Atlantic hurricane season was the most active hurricane season on record. This was a season that saw not only the most intense hurricane on record (Hurricane Wilma at 882mb), but also the costliest in Hurricane Katrina, which caused an estimated $82 billion in damages across the Gulf Coast region. Although the entire hurricane season was active, the most intense hurricanes formed from August 20th onward, and this correlated strongly with an intensifying La Niña episode in the central and eastern Pacific Ocean.

Pacific SSTs (June to August 2005)	Pacific SSTs (August to January 2005-2006)
2005 Hurricane Season Timeline

Many factors combined to produce the most active Atlantic hurricane season on record in 2005. One contributing factor was the developing La Niña episode which resulted in lighter upper level winds and reduced shear, which is favorable for hurricane development and maintenance. Examining the timeline of hurricanes, it would appear that La Niña conditions began to influence the hurricane season around the third week of August 2005, near the time that powerful Hurricane Katrina was first labeled as a tropical depression. From August 20th, 2005 through January 6th, 2006 three prolonged category 5 hurricanes were observed, with a total of 20 tropical systems forming (out of a season total of 31 named storms).