8:00 AM | *The Arctic appears to be headed into another summer melting season with normal to slightly below-normal temperatures*
Paul Dorian
The daily Arctic temperatures for 2018 (shown in red) as derived by using the ECMWF operational forecast model. The climatological mean temperature (shown in green) is shown for the base period of 1958-2002 using reanalysis data. The freezing point (273.15K, 0°C) is represented by the horizontal blue line and the summer season is typically the only time of year with temperatures averaging at slightly above freezing levels in the Arctic region. Source Danish Meteorological Institute (DMI), ECMWF
Overview
In recent days, Arctic sea ice volume has reached nearly normal levels when compared to the period of 2004-2013 for this time of year and it is noticeably higher than one year ago. Arctic sea ice extent has been relatively stable during the past decade or so albeit consistently at below-normal levels. This recent uptick to near normalcy in sea ice volume and the relative stability over the past decade or so in sea ice extent is related to long-term temperature trends in the Arctic region. Specifically, despite the fact that Arctic temperatures have often run at above-normal levels in the cold season in recent years, they have usually been running at normal to slightly below-normal levels during the all-important summer melting season. Above-normal temperatures during the cold season in the Arctic are usually well below freezing which minimizes the overall impact on the melting of sea ice. The summer season is when temperatures are slightly above the freezing mark from a climatological viewpoint and any sustained warmer-than-normal conditions during those particular months could have an important impact on sea ice; however, this just has not been happening over an extended period of time.
Graph showing daily Arctic sea ice extent since June 2002, courtesy of Japan Aerospace Exploration Agency (JAXA). The IARC-JAXA Information System is a facility for satellite image analysis and computational modelling/visualization in support of international collaboration in arctic and global change research at the International Arctic Research Center (IARC) in corporation with the Japan Aerospace Exploration Agency (JAXA) and the Advanced Earth Science and Technology Organization of Japan (AESTO). The thin blue line indicates the daily Arctic sea ice extent, while the thick line indicates the running 365 day average sea ice extent. Last day shown: 24 May 2018. Courtesy climate4you.com
Arctic sea ice volume as of 04 June 2018 now right near the mean when compared to the period 2004-2013 (gray line) and well above levels of the past two years at this same time of year. Source Danish Meteorological Institute (DMI).
The Atlantic Multidecadal Oscillation (AMO) and its impact on Arctic sea ice
The Arctic sea ice extent has generally been below-normal since the middle 1990’s at which time the northern Atlantic Ocean switched sea surface temperature phases from cold-to-warm and it is likely to return to pre-mid 1990’s levels when the oceanic cycle flips back to a cold phase in coming years. Meteorologists track oceanic temperature cycles in the northern Atlantic Ocean with an index value known as the Atlantic Multidecadal Oscillation (AMO). The AMO flipped from negative-to-positive in the mid 1990's signaling an important long-term sea surface temperature phase shift from cold-to-warm and it has stayed generally positive ever since. Typically, oceanic temperature cycles in the Atlantic Ocean have lasted for about 20 to 30 years.
The observed Atlantic Multidecadal Oscillation (AMO) index is shown here back to the year 1870. There was an important phase shift in the AMO from cold-to-warm in the mid 1990’s (indicated by arrow). The AMO index is depicted here as the de-trended 10-year low-pass filtered annual mean area-averaged SST anomalies over the North Atlantic basin (0°N-65°N, 80°W-0°E) using HadISST dataset (Rayner, et al., 2003) for the period 1870-2015. Map courtesy NOAA/NCAR
Recent temperature trends in the Arctic
Actual temperatures in the Arctic have been running consistently at above-normal levels this year so far, but in recent days they have slipped into below-normal territory as we approach the all-important summer melting season and this follows long-term trends. In fact, the temperatures in the Arctic region have typically been near normal to slightly below-normal in the summer melting season going all the way back to last century despite generally featuring above-normal conditions during the cold months of the year.
Each of the past six years have featured near normal to slightly below normal temperatures in the Arctic region during the summer melting season and generally above-normal temperatures in the other (cold) months of the year. Source Danish Meteorological Institute (DMI), ECMWF
The impact of oceanic cycles and water vapor on Arctic temperatures
The rather consistent temperature trend in the Arctic featuring above-normal temperatures during the Arctic cold (and dry) season and near normal to slightly below normal levels in the summer (more humid) season suggests water vapor may be playing a critical role. Water vapor is a critical greenhouse gas and even a small increase in the atmosphere can have a (positive) impact on temperatures in the very cold and dry air masses of the Arctic winter where moisture content is extremely low. In recent years, oceans around the world including in the nearby North Atlantic have been generally running at warmer-than-normal levels and this can result in the release of water vapor into the atmosphere. This uptick in water vapor, in turn, may be resulting in warmer-than-normal conditions during the Arctic cold (and very dry) season, but it would likely have less of an impact during the normally more humid summer season.
Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com