French SO-Argo contribution to ICES 2016 Report
A summary of recent conditions in the North Atlantic is updated annually. It has been established using the ARGO global observing system based on profiling floats, complemented by CTDs and mooring data available at Coriolis datacenter.
During winter 2015, the near surface waters were anomalously cold and fresh in the middle of subpolare gyre. The cold temperatures in the Labrador Sea were associated with a strengthening of North-easterly winds (Fig. 1). Further South, waters were extremely warm and salty in the western basin south of 40°N, indicating a northward shift of the Gulf Stream. A warmer than normal subtropcial gyre is also observed.
This subpolar cold anomaly persistes and increases troughout the year 2015 (Fig. 1). Summer 2015 has been anomalously cold over most of the subpolar basin, north of 40°N. South of 40°N, a strong warm anomaly is persistent over the whole subtropical gyre.
During summer fresh salinity anomalies north of 40°N is also more intense than during the winter period and correlated with temperature anomalies. Waters were very salty in the Greenland Sea/Norvegian Sea and along the East Greenland coast. They were fresh along the western boundary: starting from the West Greenland coast, following the North American coast and from there, extending toward the west.
Figure 1: Near surface (10 meter) temperature (upper) and salinity (lower) averaged over Winter (JFM), Spring (AMJ), Summer (JAS) and Autumn (OND) 2015. The anomalies are shown relative to the World Ocean Atlas (WOA-05).
Figure 2: Seasonal cycle for temperature at 4 points in the North Atlantic basin (see the map below). In heavy red the year 2015, in dashed black the WOA05 climatology, other curves show the years 2002-2014.
The year 2015 appears as an extremum in the 2002-2015 decade for the cold winter observed in the Labrador Sea and the Irminger Sea, (Fig. 2ab) where temperatures went well below the climatological mean (nearly 2° lower in the Irminger Sea) and the warm temperature in early winter in the south-west part of the basin (Fig. 2c). North of 40°N, the cold temperatures persists during the summer especially in the Irminger Sea and in the Eastern basin off European coasts (Fig. 2bd). In contrast, the warm summer that extends south of 40°N shows extrem values in the Eastern basin (Fig. 2c).
Winter surface conditions determine the mixed layer properties. In order to compare all areas over the decade, we adopt a simple definition for the mixed layer depth, using the level at which temperature changes by more than 0.5°C with respect to the 10 meter depth. The
month of February is selected as the common period for maximum mixed layer depth. This is not perfectly true since the time of the deepest mixed layer may vary from year to year at a single location and does not occur at the same time over the whole basin.
During the year 2015 the area covered by a deep mixed layer (deeper than 900 m) is more extended than usual in the North of the basin (even more than during the 5 previous winters since 2010), extending from the Labrador Sea to the Irminger Sea (Fig. 3). This deeper than usual mixed layer may reflect strong winter convection in both Labrador and Irminger basin. Unusual deep mixed layer is also observed in the eastern side of the basin off Scotland and Ireland coasts. In the South-East of the basin, the deep mixed layer extension stops around 48/50°N such that only moderate
mixed layer depths are observed along the shelf in the Bay of Biscay contrary to the 2009, 2010 and 2011 winters. Note that the deep mixed layer anomaly centered at 45°N-20°W in 2014 is likely an artifact due to Near Real Time data in use of this period. Additional quality control and analysis is needed for this period.
Figure 3: North Atlantic mixed layer depth in February from 2010 to 2015. The mixed layer is defined as the depth at which the temperature has decreased by more than 0.5° from the temperature at 10 m. This criterion is not suitable for areas of salinity compensation or very week stratification.
The most salient feature of the 2015 annual mean temperature is an intense cold anomaly (persistant and increasing since 2013) over the subpolar basin from the tip of Greenland to 40°N and the persistence of a moderately warm anomaly over the Greenland Sea and along the East Greenland coast (Fig. 4left). Since 2013, the structure of the salinity anomaly appears correlated with the temperature anomalies with cold/fresh anomaly in the subpolare gyreand the Labrador Sea (Fig. 4right); and warm/salty anomaly over the Greenland Sea. In 2015, the most remarkable feature is the large cold/fresh anomaly, 2°C/0.2 pss below the ‘normal’ WOA05 conditions, is observed in the subpolar gyre and Labrador Sea.
Figure 4: Annual average temperature (upper) and salinity (lower) anomalies at 10 m during 2010-2015
2. Deep layers
At 1000 m (Fig. 5) :
The Labrador Sea and the Irminger Sea are warmer than normal, but the warming tendency observed since 2002 is interrupted since 2012 as seen in the time series (Fig. 6).
However, deep Greenland Sea keeps on warming (Fig. 6).
The Mediterranean Outflow water seems warmer and saltier south of 40°N and off Gibraltar straight.
The salt increase seems to extend over the basin. A cold and fresh anomaly stands from the South of Iceland down to Rockall Trough.
A warm and salty anomaly is observed south of the Gulf-Stream and Azores
current (subtropical gyre).
Figure 5: Annual average temperature (upper) and salinity (lower) anomalies at 1000 m during 2010-2015.
Figure 6: Time series of temperature anomalies averaged over the 800-1200m layer over 2002-2015 period.
Gaillard, F., 2012. ISAS-Tool Version 6: Method and configuration. Rapport LPO-12-02, http://archimer.ifremer.fr/doc/00115/22583/
Gaillard, F., T. Reynaud, V. Thierry, N. Kolodziejczyk and K. von Schukmann , 2016 : In Situ–Based Reanalysis of the Global Ocean Temperature and Salinity with ISAS: Variability of the Heat Content and Steric Height, J. Clim., 29, 1305-1323.