An investigation into 2009 summer low temperature in Northeast China and its association with prophase changes of the air-sea system

Under the background of global warming, a summer (JJA) low temperature event, which has not occurred for about 15 years since 1994, happened in 2009 in the central and northern region of northeast China. By using the NCEP/NCAR reanalysis data, the one hundred year observed temperature data from the stations at Changchun and Harbin, and the Hadley Center sea surface temperature (SST) data, this paper tries to reveal the reason of the northeast China summer low temperature (NESLT) event. Some results were found as follows: The study on both horizontal and vertical circulation structures over northeast China during the period when summer low temperature events occurred, reveals that when it occurred both the horizontal and vertical circulation over northeast China were associated with a low-value system and anomalous activities of the Northeast China cold vortex (NECV) were the most direct influential factor to the low temperature events. The decadal variability (≥9 years) of temperature was favorable to the occurrence of summer low temperature in 2009: the decadal oscillation of summer average temperature of Harbin and Changchun in 2009 was still in an obviously downward trend from the late 1990’s to the early 2010’s with its declining change rate from 1999 to 2008 reached 0.8 ℃/10 years, obviously exceeding the linear rising tendency of 0.2 ℃/10 years of the regional warming of northeast China from 1961 to 2000. And the previous winter North Pacific polar vortex (NPPV) area index, which was significantly positively related with the observed summer temperatures of Harbin and Changchun, was also in a significantly declining tendency. In addition, in the summer of 2009, the Arctic Oscillation (AO) appeared a negative phase distribution, and the Northeast Asia, the Aleutian Islands and the North Atlantic were significant negative anomaly regions, which are opposite to the pattern of summer average 500 hPa height anomalies over 1994-2008 when the NESLT event was absent. Furthermore, the negative phase of the North Pacific Oscillation (NPO) in the preceding winter of 2009 was obviously strong, just as it once happened in 1972, when a summer low temperature and cool injury event occurred in northeast China. Meanwhile， the SSTA in the easterncentral equatorial Pacific Ocean showed a La Nina phase in the preceding winter of 2009 but the strength of La Nina phase weakened obviously in the spring of 2009. The abnormally strong activity of the NECV in the June and July of 2009 was related with the fact that the disturbances of stationary wave replaced the original ones of ultralong wave over the North Pacific region in the April and May of 2009.By employing the method of the singular value decomposition (SVD) and harmonic analyzing, it is found that the phase of NPO was not only an important precursor for summer temperature variations over northeast China, but also a stable component on the planetary scale that we can extract from the atmosphere in addition to the chaos component on the synoptic scale.