Abstract. Arctic amplification causes the
meridional temperature gradient between middle and high latitudes to
decrease. Through this decrease the large-scale circulation in the
midlatitudes may change and therefore the meridional transport of heat and
moisture increases. This in turn may increase Arctic warming even further. To
investigate patterns of Arctic temperature, horizontal transports and their
changes in time, we analysed ERA-Interim daily winter data of vertically
integrated horizontal moist static energy transport using self-organizing
maps (SOMs). Three general transport pathways have been identified: the North
Atlantic pathway with transport mainly over the northern Atlantic, the North
Pacific pathway with transport from the Pacific region, and the Siberian
pathway with transport towards the Arctic over the eastern Siberian region.
Transports that originate from the North Pacific are connected to negative
temperature anomalies over the central Arctic. These North Pacific pathways
have been becoming less frequent during the last decades. Patterns with
origin of transport in Siberia are found to have no trend and show cold
temperature anomalies north of Svalbard. It was found that transport patterns
that favour transport through the North Atlantic into the central Arctic are
connected to positive temperature anomalies over large regions of the Arctic.
These temperature anomalies resemble the warm Arctic–cold continents
pattern. Further, it could be shown that transport through the North Atlantic
has been becoming more frequent during the last decades.
Observations from 1979 to 2014 show a positive trend in the summer sea ice melt rate with an acceleration particularly in June and August. This is associated with atmospheric circulation changes such as a tendency toward a dipole pattern in the mean sea level pressure (SLP) trend with an increase over the Arctic Ocean and a decrease over Siberia. Consistent with previous studies, we here show the statistical relationship between the summer sea ice melt rate and SLP and that more than one SLP pattern is associated with anomalously high melt rates. Most high melt rates occur during high pressure over the Arctic Ocean accompanied by low pressure over Siberia, but a strong Beaufort High and advection of warm air associated with a cyclone located over the Taymyr Peninsula can also trigger anomalous high ice melt. We evaluate 10‐member ensemble simulations with the coupled atmosphere‐ice‐ocean Arctic regional climate model HIRHAM‐NAOSIM. The simulations have systematically low acceleration of sea ice melt rate in August, related to shortcomings in representing the strengthening pressure gradient from the Barents/Kara Sea toward Northern Greenland in recent decades. In general, the model shows the same classification of SLP patterns related to anomalous melt rates as the observations. However, the evolution of sea ice melt‐related cloud‐radiation feedback over the summer reveals contrary effects from low‐level clouds in the reanalysis and in the simulations.
Abstract. Mesosphere/lower thermosphere (MLT) zonal winds continuously measured by a VHF meteor radar at Collm, Germany (51.3 • N, 13.0 • E) in the height range 82 -97 km from 2004 to date are analyzed with respect to the signature of El Niño. The comparison of Niño3 equatorial SST index and MLT wind time series shows that in January and especially in February zonal winds are positively correlated with the Niño3 index. We note a delay of about one month of the MLT zonal wind effect with respect to equatorial sea surface temperature variability. The signal is strong for the upper altitudes (above 90 km) accessible to the radar observations, but weakens with decreasing height. This reflects the fact that during El Niño years the westerly winter middle atmosphere wind jet is weaker, and this is also the case with the easterly lower thermospheric jet. Owing to the reversal of the absolute El Niño signal from negative to positive with altitude, at the height of the maximum meteor flux, which is around 90 km, the El Niño signal is weak. The experimental results can be qualitatively reproduced by numerical experiments using a mechanistic global circulation model with prescribed tropospheric temperatures and latent heat release for El Niño and La Niña conditions.
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