8:00 AM | *The Arctic appears to be headed for another summer melting season with near normal temperatures*

There has been a relatively neutral trend in Arctic sea ice extent during the past twelve years with the lowest point coming during the melting season of 2012. Data source NOAA’s National Snow and Ice Data Center, Multisensor Analyzed Sea Ice Extent (MASIE) 

Overview
While consistently at below-normal levels since the mid 1990’s, Arctic sea ice extent has stayed relatively stable during the past decade or so.  This recent stability in Arctic sea ice extent is likely due mainly to the fact that temperatures have been regularly near-normal during the all-important summer season and peak melting time of the year.  Temperatures have often run at above-normal levels during the cold season in the Arctic, but during those particular months, conditions are usually well below freezing which minimizes the impact on the melting of sea ice.  In the summer season, Arctic temperatures are slightly above the freezing mark from a climatological viewpoint and this is the time of year when sustained warmer-than-normal conditions could have an important impact on sea ice extent, but it just hasn’t been happening that way in an extended period of time.   
 

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

The 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 daily Arctic temperatures for 2018 (in red) as derived by using the ECMWF operational forecast model.  The climatological mean temperature (in green) is shown for the base period of 1958-2002 using reanalysis data.  The freezing point (273.15K, 0°C) is shown by the horizontal blue line and the summer season is typically the only time of year with temperatures averaging above freezing in the Arctic region.  Source Danish Meteorological Institute, ECMWF

Temperature trends in the Arctic
While actual temperatures in the Arctic have been running at well above-normal levels virtually the entire year so far, they are quite likely to be near normal during the all-important summer melting season according to recent model forecasts and also long-term trends.  In fact, the temperatures in the Arctic region have typically been near normal in the summer melting season going all the way back to last century despite having above-normal conditions during the cold months of the year.  
 

Arctic region (circled) is predicted by NOAA’s climate model (CVSv2) to have near normal temperatures during the 2018 summer season (June, July, August); map courtesy NOAA/EMC, tropicaltidbits.com

The most recent forecast by NOAA’s climate model (CVSv2) indicates 2-meter temperatures should once again be near normal in the Arctic region during the summer season (June, July, August).  Above-normal temperatures during the cold months do little to impact the overall Arctic sea ice extent as temperatures are generally still well below the melting point of ice (273.15K, 0°C which is represented by the horizontal blue line in the yearly Arctic temperature plots). 

Each of the past six years have featured near 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, ECMWF

The possible impact of oceanic cycles and water vapor on Arctic temperatures
The temperature trend in the Arctic of above-normal temperatures during the Arctic cold (and dry) season and near normal temperatures in the summer (milder, more humid) season suggests water vapor may be playing a critical role. Water vapor is a critical greenhouse gas and an increase can have an impact on temperatures; especially, in the very cold and dry air masses of the Arctic winter where moisture content is very low.  When the atmosphere is already moist such as in tropical regions, an increase of water vapor will have less of an impact on temperatures compared with those areas featuring very cold and dry air masses.  In recent years, oceans around the world including in the nearby North Atlantic Ocean have generally been running at warmer-than-normal levels and this can result in the release of water vapor into the atmosphere.  

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com