In addition to solar cycles, temperature cycles in the planet’s oceans play critical roles in our ever changing climate and also on the extent of global sea ice. Oceanic temperature cycles are often quite long-lasting and a warm or cold phase can persist for two or three decades at a time. In general, the Atlantic Ocean experienced a cold phase from the early 1960’s to the mid 1990’s at which time it flipped to a warm phase and that has continued for the most part ever since. The current warm phase; however, is now showing early signs of a possible long-term shift back to colder-than-normal sea surface temperatures (SST) and this could have serious implications on US climate and sea ice areal extent in the Northern Hemisphere.
Recent temperature trends in the Atlantic Ocean
The comparison chart (above) of SST anomalies between August 2014 (top) and today (bottom) shows a big drop in temperatures across much of the northern Atlantic Ocean. The rather limited colder-than-normal (blue) patches from August 2014 have increased noticeably in areal extent when compared to the most recent measurements. Also, the well above-normal waters (orange) of August 2014 that existed east and south of Greenland have cooled off dramatically during this time period and there has even been a switch from well above-normal (orange) to below-normal (blue) in sections.
Long-term temperature trends in the Atlantic Ocean
On a longer time scale, there is supporting evidence from the National Oceanographic Data Center that something significant is indeed occurring in the Atlantic Ocean. Since around early 2007, there has been a definitive downward trend in “monthly heat content anomaly” in the top 700 meters of the northern Atlantic Ocean (arrow region). The heat content in this part of the Atlantic Ocean ramped up rather sharply beginning around the middle 1990’s and seemingly peaked during early 2007.
Atlantic Multidecadal Oscillation (AMO)
One way meteorologists can monitor sea surface temperature patterns in the North Atlantic Ocean is through an index value known as the Atlantic Multidecadal Oscillation (AMO). Positive (negative) phases of the AMO coincide with warmer (colder) North Atlantic sea surface temperatures. The AMO is linked with decadal climate fluctuations such as European summer precipitation, Atlantic hurricanes and variations in global temperatures. The monthly values for the AMO index are shown in the plot below for the period of 1870-2015 where positive values (reds) represent warmer-than-normal time periods and negative values (blues) indicate cold phases. Sea surface temperature cycles have tended to last for two or three decades at a time before phase changes take place. The last major phase change (cold-to-warm) took place right around the mid-point of the 1990’s (arrow). The AMO index appears to have reached a peak in this current (warm) phase during the latter part of last decade and it has generally been descending during the past few years providing support to the notion that a cool-down is indeed occurring in the North Atlantic.
Computer model forecast for Atlantic Ocean sea surface temperatures
One indication that this apparent downward trend in Atlantic Ocean sea surface temperatures will indeed have some staying power comes from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). JAMSTEC’s global seasonal forecasting of sea surface temperature anomalies has a pretty good track record and its most recent long-range forecast (below) suggests there will be a fairly widespread area of colder-than-normal water in the northern Atlantic Ocean during the winter season of 2016-2017 (circled blue region).
Ramifications on Northern Hemisphere sea ice areal extent
If the Atlantic Ocean is indeed slipping back into a colder-than-normal phase (i.e., negative AMO) then this would quite likely have a significant impact on Northern Hemisphere (NH) sea ice areal extent. The NH sea ice areal extent was generally at above-normal levels before the middle 1990’s (arrow in plot below) which is when the Atlantic Ocean temperature phase change took place from cold-to-warm. Once the warm phase of the Atlantic Ocean became established in the late 1990's, the NH sea ice areal extent trended sharply downward from positive levels into well below-normal territory. In recent years, there has been a jagged, but generally sideways trend in NH sea ice areal extent at those well below-normal levels. However, if these recent signs of a possible long-term Atlantic Ocean temperature phase change from warm-to-cold are real and sustained (sometimes there are false starts), then the NH sea ice areal extent will very likely return to above-normal levels in the not too distant future - just as it was during the last cold phase pre-mid 1990’s.
Potential ramifications of an Atlantic Ocean temperature shift on US Climate
The largest body of water in the world by far is the Pacific Ocean and it is well understood that its temperature cycles play a critically important role in weather and climate not only in the US, but throughout the world. While the Atlantic Ocean is far smaller than the Pacific Ocean, its temperature cycles can also play an important role in the US for weather and climate. In an effort to investigate the possible impact of Atlantic Ocean temperature cycles on US climate, wintertime (DJF) temperature anomaly maps were generated for two distinct twenty-year periods. The first period is 1975-1995 which generally featured a cold phase in the Atlantic Ocean and the second period encompasses the current warm phase period of 1995-2015. The differences in wintertime US temperature anomalies are quite dramatic for these two twenty-year periods. The cold phase from 1975-1995 featured colder-than-normal winters on average throughout the US whereas in the recent 20-year warm phase period of 1995-2015, the opposite took place with virtually nationwide warmer-than-normal winters. No doubt many other important factors played a role in the weather conditions during the past forty winter seasons (e.g., El Nino/La Nino), but these dramatic differences in US temperatures that took place during different phases of the Atlantic Ocean suggest it is definitely something to monitor closely over the next several years.
Meteorologist Paul Dorian