Significant Role of Internal Climate Variability for the Global Warming Hot-spots in the Northern Hemisphere

The origin of characteristic increase of continental-scale summer surface temperature in decades over Northern hemisphere was investigated. July-temperature in Europe, East Asia, the North Pacific, and western North America have been undergoing more increase than the global average of about 1ºC increase, especially in recent years. On the contrary, MME of 37 CMIP5 models do not show the focused regional warming hot-spots but exhibits hemispheric surface temperature increase. From the comparison between GISTEMP observation record and CMIP5 MME historical data, we show an evidence that the observed characteristic increase of regional temperature for the recent 43 years is dominated by the Internal Climate Variability (ICV) of decadal time-scale. Performing EOF analysis on the ICV, the four dominant modes are identified as Pacific Decadal Oscillation (PDO), North Pacific Oscillation (NPO), Pacific Meridional Mode (PMM), and Atlantic Multi-decadal Oscillation (AMO). It is also shown that, with only two dominant modes among those modes, large portion of continental-scale temperature increase can be explained: We show that PDO and PMM are dominant modes in Europe and East Asia, NPO and AMO in the North Pacific, and PDO and NP in western North America. The observed sub-trend is nicely reproduced with only these two modes. Quantitatively, the observed sub-trend from ICV explains 73%, 60%, 55%, and 18% of the total variability in North Pacific, East Asia, western North America and Europe, respectively. Note that, despite 82% of temperature fluctuation in Europe is attributed by the external forcing, 18% of the internal variability is still important to explain the increasing number of extreme heat events in Europe in recent several decades.