The weather has been making a lot of headlines lately. According to NOAA scientists and our partners: the past twelve months have been the warmest on record for the globe; wintertime Arctic sea ice measured an all-time low; and the hurricane season is expected to be near-average for the Atlantic and Gulf coasts.
Scientists like Tom Delworth at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, work to understand our climate and future conditions as the world continues to warm. The journal Nature Geoscience published a paper by Delworth and his colleagues examining how a natural atmospheric force--the North Atlantic Oscillation--may be changing ocean currents in the North Atlantic. Among other impacts, the stronger ocean currents increase the amount of heat flowing toward polar areas, which could speed up Arctic ice melt and affect how hurricanes form.
We asked Delworth a few questions about his study:
Q: What exactly is the North Atlantic Oscillation and how does it affect the U.S.?
A: The North Atlantic Oscillation (NAO) is a measure of the strength and position of the westerly winds over the North Atlantic Ocean. A positive value of the NAO means that the winds over portions of the North Atlantic are stronger than normal. One measure of this is the difference in sea level pressure between Lisbon, Portugal and Iceland. The larger this difference, the stronger the winds. Stronger westerly winds bring more cold, dry air off the North American continent, and increase the heat loss from the ocean to the atmosphere.
Q: If ocean heat is moving northward, does that mean fewer hurricanes in the U.S.?
A: A positive phase of the North Atlantic Oscillation leads, after a few years, to stronger northward ocean currents extending from the South Atlantic all the way to the Arctic. These unusually strong currents bring more heat from the South Atlantic and tropics into the North Atlantic and Arctic. This can lead to widespread warming in the North Atlantic (oftentimes called a warm phase of the Atlantic Multidecadal Oscillation, or AMO). This warming tends to be associated with above average hurricane activity in the North Atlantic, and can lead to more hurricanes in the U.S.
Q: Aren’t there other natural climate cycles that influence hurricane activity?
Effects of North Atlantic Oscillation
This figure shows how a positive phase of the North Atlantic Oscillation (NAO) results in stronger westerly winds that extract heat from the Labrador Sea and subpolar gyre. Heat is transported in the North Atlantic toward the pole, reducing Arctic sea ice and warming the Northern Hemisphere. Credit: NOAA Geophysical Fluid Dynamics Laboratory
A: The El Nino Southern Oscillation (ENSO) has a very strong impact on North Atlantic hurricanes. A warm phase of ENSO – when the tropical eastern Pacific is unusually warm – generally means fewer hurricanes in the North Atlantic, and more hurricanes in the eastern tropical Pacific. Emerging research is examining linkages between ENSO and other types of climate variability, such as the AMO or Pacific Decadal Oscillation.
Q: Isn’t Arctic melt happening already because of global warming?
A: It is very clear that the dominant cause of the long-term decrease in Arctic sea ice is warming due to increasing greenhouse gas concentration in the atmosphere, which is a result of human activity. Our work shows that the ocean contributes to decadal scale variations in the rate of ice melt that are superimposed on top of this long-term decline. There is a strong background decline in Arctic sea ice caused by human activity, with natural decadal variability in sea ice changes superimposed on top of that.
Q: Does more research on the North Atlantic Oscillation need to happen?
A: The climate changes that we will experience over the next decade or two are a combination of natural decadal variability and human-induced climate change from increasing greenhouse gases and other pollutants, such as small particles (called "aerosols") that are the byproduct of fossil fuel burning. We need to better understand both aspects in order to better predict our future climate. The world ocean is an important contributor to these decadal variations, and scientists are working to better understand these through observational programs and theoretical studies, including computer climate modeling. These studies focus not only on the Atlantic Ocean, but on the Pacific, Arctic, and Southern Oceans as well. Each plays a vital role in the global climate system.
More details about this study can be found in a research highlight on NOAA's Geophysical Fluid Dynamics Laboratory website. The paper's abstract can be read online at Nature Geoscience.
For more information, please contact Monica Allen, director of public affairs for NOAA Research, at 301-734-1123 or by email at email@example.com