The surface pressure drives surface winds and Wintertime storms from West-to-East across the North Atlantic, ultimately affecting weather and climate from New England to Western Europe, as far eastward as central Siberia and the Eastern Mediterranean, and southward to Africa. In case you're wondering, we broke the record for the deepest negative NAO for the month of June since 1950. The negative phase of the NAO increases sea surface temperature due to decreased evaporational cooling. This chart was provided by Ralph Fato. Take a look:
So, why is this important? When the NAO Index is in its negative phase, the Tropical Atlantic and Gulf Coast have an increased number of strong hurricanes, northern Europe is drier, and Mediterranean countries have increased precipitation. When the NAO is positive phase, the northeastern United States sees an increase in temperature and a decrease in snow days, the central U. S. has increased precipitation, the North Sea has an increase in storms, and Northern Europe has warmer temperatures and increased precipitation.
The negative NAO typically enhances tropical cyclone activity in the Atlantic due to weaker trade winds, while a positive NAO typically suppresses tropical cyclone activity due to stronger than normal trade winds that prevent the disturbance from attaining a well-defined center. The positive NAO correlates with above average Mean Sea Level Pressure (MSLP), which is unfavorable for tropical cyclogenesis. A negative NAO correlates with below average MSLP, which enhances tropical cyclogenesis.
As far as the long-range forecast is concerned, the negative NAO index phase results in fewer and weaker Winter storms crossing on a more West-to-East pathway. They bring moist air into the Mediterranean and cold air to northern Europe. The East Coast of the United States experiences more cold air outbreaks and snowy weather conditions.
As for the long-term, will there be more tropical activity in the Atlantic this season? Are we in for a Winter similar to the one two years ago? The NAO suggests we may very well be. As for the short-term forecast, another sunny and warm day is ahead with afternoon temperatures in the mid-to-upper 80s. Tonight will be mostly clear and mild with lows in the 60s. The weekend will become quite warm and more humid with showers and thunderstorms Sunday. Daytime highs will reach well into the 80s.
Paul
