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Weather forecasting, detailed weather analysis and climate information

12:30 PM | The Atlantic Ocean is showing signs of a possible significant long-term shift in temperatures from warm-to-cold

Paul Dorian

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[Sea Surface Temperature Anomaly comparison chart between July 2014 and today; courtesy NOAA]

Overview

In addition to solar cycles, temperature cycles in the planet’s oceans play critical roles in climate and on the ever-changing distribution 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. 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 signs of a possible long-term shift back to colder-than-normal sea surface temperatures (SST) and this could have serious implications on the climate and sea ice areal extent in the Northern Hemisphere.

Recent temperature trends in the Atlantic Ocean

Sea surface temperatures have dropped considerably during the past eight months or so in much of the northern Atlantic Ocean. The comparison chart above of SST anomalies between July 2014 (top portion) and today (bottom portion) shows a big drop in temperatures across much of the northern Atlantic Ocean. First, the rather limited colder-than-normal (blue) regions from July 2014 have increased noticeably in areal extent during the past eight months. Second, the well above normal waters (orange) of July 2014 that existed east of Greenland have cooled off dramatically during this time period to only slightly above normal (yellow) and there has even been a switch from well above normal (orange) to below normal (blue) in this area east of Greenland. Notice also that most of warm pockets of water off the Northeast US coastline in July 2014 (orange) have persisted for the most part during this general cool-down in the northern Atlantic Ocean during the past eight months; however, that may be about to change.

Longer-term 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 2006/2007, there has been a definitive downward trend in “monthly heat content anomaly” in the top 700 meters of the northern Atlantic Ocean (below). The heat content in this part of the Atlantic Ocean ramped up rather sharply beginning around the middle 1990’s (first arrow) and seemingly peaked during 2006/2007 (second arrow).

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[Monthly heat content anomaly trend in the top 700 meters of the northern Atlantic Ocean; courtesy National Oceanographic Data Center]

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). The monthly values for the AMO index are shown in the plot below for the period of 1856-2013 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. Since late last summer, the AMO index has actually dropped rather consistently from +0.355 to a now barely positive value of +0.016 (not shown in plot) providing supporting evidence that a cool-down is indeed occurring in recent months in the Atlantic Ocean.

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[Atlantic Multidecadel Oscillation monthly index values where red indicates warm phases and blue cold phases; courtesy NOAA]

Model Forecast for Atlantic Ocean sea surface temperatures

One indication that this downward trend in Atlantic Ocean sea surface temperatures may continue 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 by the fall of 2015 (blue regions) and this includes the noticeable disappearance of those very warm pockets that are currently situated in the western Atlantic near the US coastline.

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[JAMSTEC model forecast of sea surface temperature anomalies for fall season 2015; courtesy Japan Agency for Marine-Earth Science and Technology]

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" (and 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.

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[Northern Hemisphere sea ice areal extent; courtesy University of Illinois "cryosphere", NOAA]

Ramifications on Climate

The impact on climate from a potential Atlantic Ocean temperature shift of warm-to-cold are less clear as the all-important Pacific Ocean would have to be factored into the equation. The temperature cycles in the Pacific Ocean are critically important to weather and climate across the US and elsewhere and will have to be monitored in the years to come. Certainly, the last cold phase of the Atlantic Ocean produced numerous cold winters in the Northeast US such as during the latter part of the 1960's and also in the 1970's. If the eastern Pacific Ocean remains warmer-than-normal for next winter season as it was for this year and the western Atlantic Ocean cools down as predicted, this could very well set the stage for another colder-than-normal winter in the Northeast US for 2015-2016.

Paul Dorian

Vencore, Inc.