Abstract
In 2010, the Northern Hemisphere, in particular Russia and Japan, experienced an abnormally hot summer characterized by record-breaking warm temperatures and associated with a strongly positive Arctic Oscillation (AO), that is, low pressure in the Arctic and high pressure in the midlatitudes. In contrast, the AO index the previous winter and spring (2009/2010) was record-breaking negative. The AO polarity reversal that began in summer 2010 can explain the abnormally hot summer. The winter sea surface temperatures (SST) in the North Atlantic Ocean showed a tripolar anomaly pattern—warm SST anomalies over the tropics and high latitudes and cold SST anomalies over the midlatitudes—under the influence of the negative AO. The warm SST anomalies continued into summer 2010 because of the large oceanic heat capacity. A model simulation strongly suggested that the AO-related summertime North Atlantic oceanic warm temperature anomalies remotely caused blocking highs to form over Europe, which amplified the positive summertime AO. Thus, a possible cause of the AO polarity reversal might be the “memory” of the negative winter AO in the North Atlantic Ocean, suggesting an interseasonal linkage of the AO in which the oceanic memory of a wintertime negative AO induces a positive AO in the following summer. Understanding of this interseasonal linkage may aid in the long-term prediction of such abnormal summer events.
Highlights
In Japan, summer 2010 was the warmest in about 100 years of countrywide measurement records
A possible cause of the Arctic Oscillation (AO) polarity downward latent and sensible heat flux anomaly over the high latitudes and the tropical Atlantic (Fig. 3) in winter and spring indicates that anomalous heating of the ocean by the atmosphere occurred from winter to spring during the strongly negative phase of the AO in winter 2009/2010
The area of the upward heat flux anomaly coincided with the area of the warm sea surface temperatures (SST) anomaly from May to August
Summary
In Japan, summer 2010 was the warmest in about 100 years of countrywide measurement records. In winter 2009/2010, just a half-year earlier, the continent suffered from anomalously cold weather associated with a record-breaking negative Arctic Oscillation (AO), which is characterized by positive sea level pressure anomalies over the Arctic and negative pressure anomalies over the midlatitudes (Thompson and Wallace 2000). Negative AO index of the past 30 years was observed in December 2009 (Wang and Chen 2010) This drastic reversal from a record-breaking cold winter to a recordbreaking hot summer is preserved in our memory. The abrupt change of the AO index from strongly negative to strongly positive in 2010 corresponded to the change from the abnormally cold winter of 2009/2010 to the abnormally hot summer of 2010, which shows that the AO index is a good indicator of abnormal weather on a planetary-scale, and that extra-seasonal prediction of the AO is a key to long-term forecasting. We aimed to examine the cause of the 2010 change in the AO from strongly negative to strongly positive
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