Atlantic Ocean Showing Signs Of A Significant Shift To Cold Phase

  • Date: 29/04/16
  • Paul Dorian, Vencore, Inc.

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.

Overview

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 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.

Sea surface temperature anomalies in the Atlantic Ocean: August 2014 (top), April 25, 2016 (bottom); courtesy NOAA

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.

Global monthly heat content anomaly (GJ/m2) in the uppermost 700 m of the North Atlantic since January 1955. The thin line indicates monthly values and the thick line represents the simple running 37 month (c. 3 year) average. Data source: National Oceanographic Data Center (NODC), climate4you.com. Last period shown: October-December 2015.

Global monthly heat content anomaly (GJ/m2) in the uppermost 700 m of the North Atlantic since January 1955. The thin line indicates monthly values and the thick line represents the simple running 37 month (c. 3 year) average. Data source: National Oceanographic Data Center (NODC), climate4you.com. Last period shown: October-December 2015.

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.

Observed AMO index, defined as detrended 10-year low-pass filtered annual mean area-averaged SST anomalies over the North Atlantic basin (0N-65N, 80W-0E), using HadISST dataset (Rayner, et al., 2003) for the period 1870-2015.;  courtesy NCAR: https://climatedataguide.ucar.edu/climate-data/atlantic-multi-decadal-oscillation-amo

Observed AMO index, defined as detrended 10-year low-pass filtered annual mean area-averaged SST anomalies over the North Atlantic basin (0N-65N, 80W-0E), using HadISST dataset (Rayner, et al., 2003) for the period 1870-2015.;  courtesy NCAR: https://climatedataguide.ucar.edu/climate-data/atlantic-multi-decadal-oscillation-amo

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).

JAMSTEC model forecast of sea surface temperature anomalies for winter season 2016-2017; courtesy Japan Agency for Marine-Earth Science and Technology

JAMSTEC model forecast of sea surface temperature anomalies for winter season 2016-2017; 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 (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.

Northern Hemisphere sea ice areal extent from 1979-2015; courtesy University of Illinois "cryosphere", NOAA

Northern Hemisphere sea ice areal extent from 1979-2015; courtesy University of Illinois “cryosphere”, NOAA

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