Using atmospheric variability to understand the wintertime regional warming and cooling patterns in the North Atlantic Sector

Abstract

<p>The 20th century “early warming” (1910-1940) and cooling (1940-1970) of the Northern Hemisphere offer an interesting contrast of periods with opposite temperature trends, similar hemispheric temperature anomalies, yet very different temperature anomaly patterns. These contrasts are particularly clear in the North Atlantic sector, which exhibits large climate variability over a range of time scales, from short (weather regimes) to long (Atlantic Multidecadal Variability). In this study, we explore the role of the atmospheric circulation (North Atlantic jet stream) in determining the temperature anomaly patterns over the 20<sup>th</sup> century. While different jet configurations are associated with distinct synoptic temperature patterns in the North Atlantic sector, only some are found to contribute substantially to longer term temperature trends. Notably, the southern jet configuration has the strongest temperature anomalies, with a dipole signal that is opposite from the one under the tilted jet configuration. At the same time, these two jet configurations exhibit relatively large decadal variations in frequency (days of occurrence in given winter seasons), with trends that are almost the opposite. In fact, changes in the frequency of southern and tilted jet “days” alone account for much of the North Atlantic and Arctic temperature variability on decadal time scales, including the differences between the early warming and cooling periods (e.g., the flipped warming versus cooling patterns are associated with fewer southern jet days and more tilted jet days). However, the reconstruction skill of the 30-year mean temperature anomaly in the North Atlantic sector using jet frequency exhibits decadal variability, with high skill scores interestingly coinciding with the positive phases of the Atlantic Multidecadal Variability. The lower reconstruction skill especially during the global warming period from the1980s onwards is likely due to the impact from the warming hole in the North Atlantic, which dominates the temperature patterns in the North Atlantic. Overall, the evolution of Northern Hemisphere surface temperature over the 20<sup>th</sup> century is found to be influenced by North Atlantic jet variability, with lower frequency ocean effects contributing more in recent decades.</p>