The Interaction Of The Kuroshio,PDO And ENSO And Their Impacts On Climate

Kuroshio, El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) are three important climate systems in the Pacific with connection and mutual influence. They play important roles on the atmospheric circulation in the Pacific section and the world and are the key factors for climate prediction. This paper mainly explore the variation feature of the sea surface temperature anomalies and net heat flux (Qnet) over the Kuroshio, its relationship with PDO and ENSO, and how it interacts with the atmospheric circulation and impacts the climate. In addition, what impacts of the PDO on the revised dry-wet trends of the semi-arid areas over North America and East Asia, and the combined effects of the PDO and ENSO on global land dry-wet changes are also investigated in this paper. The main conclusions are summarized as follows:(1) A Kuroshio index (KI) is defined and reflects the SST variation over the Kuroshio east of Tokara strait very well. The results show that KI is characterized by multiple time scales:it has experienced a climate shift from cold to warm phase around 1975, that is, it has the interdecadal variation with period of 20yrs; in addition, KI has the characteristics of interannual variation with period of quasi-3yr and-7yr, especially in the 1970s and ’80s and before 1965, respectively; moreover, it also has the obvious season interlock characteristics. There is a high lagged relationship between the KI, PDO and ENSO. The SST anomaly over Kuroshio lagged the PDO by 2 months and ENSO by 4 months. When El Nino occurs, the eastern equatorial Pacific is warmer abnormally and Hadley circulation is enhanced, thereby resulting in the Aleutian Low deepened (PDO warm phase); the deepened Aleutian Low drivers more cold water from the pole to the mid-latitude along its west side, thereby cooling the SST over the Kuroshio extension region and the central North Pacific; meanwhile, the wind stress on the south side of the enhanced Aleutian Low results in the Kuroshio extension strengthened. In turn, the variation over the Kuroshio further modulates PDO and ENSO via the Aleutian low and the teleconnection process. It is a slow process for the Kuroshio and PDO responding El Nino events. Therefore, ENSO and PDO can be both used as predictor for the SST variation over Kuroshio and then improve the climate predictions.(2) The first mode of the Qnet over the north Pacific is defined as the Kuroshio Qnet intensity index. The results reveal that except for a steady enhancement in the period 1984-2009 with trend of 2.6 W·m-2/yr, the boreal winter Qnet over the KS is characterized by obvious interannual variation of the period quasi-5year as well as decadal variability shifting from negative to positive anomalies in the mid-1990s. In the wintertime, the increasing KS Qnet is primarily responsible for the intensification of the East Asia winter monsoon (EAWM) via a deepening of the Aleutian low and an enhancement of the Siberia high. The enhanced-EAWM induces colder winters in Northeast China and higher Qnet over the KS by carrying significantly greater amounts of cold air mass. During the low Qnet winter, the EAWM is weakened, and the southwesterly wind that contains abundant water vapour enhances and pushes toward southern China, thereby bringing heavier rainfall. Moreover, the anomalous Qnet over the KS in the wintertime lasts until the following spring with weaker relative intensity and then impacts the climate over most of China. Such as, in the spring after the low KS Qnet winter, the temperature is colder and precipitation is more abnormally in the central and eastern region of China, especially in the Yangtze-Huaihe region.(3) PDO is significantly negative related with the precipitation over the semi-arid area in East Asia while highly positive correlative with the precipitation over the semi-arid area in North America. When the PDO is in warm phase, the Aleutian low is deepened, especially in winter. On the one hand, the westerly along the southern side is accelerated and then transport large amount of water to the semi arid regions of western North America; on the other hand, the anomalous southeasterly winds block the cold air from high latitude, thereby favoring wetting over the semi-arid regions of North America. There is an abnormal high pressure center over East Aisa, especially in summer and autumn, the anomalous northerly along its east flank results in the East Asia summer monsoon weakened significantly, thereby the water vapor can’t reach the marginal area of the semi-arid area in East Asia. In recent 60 years, PDO has experienced a shift form cold to warm phase around 1976/77 and reappeared cold phase around 2001/02, but it still shows a clear upward trend on the whole. This rising trend of PDO explained 32.6% of the drying trend in the semi-arid area in East Asia and 58.4% of the wetting trend over the semi-arid regions of North America.(4) The global land distribution of the dry-wet changes associated with the combination of ENSO and the PDO is first investigated and result shows that when in phase with the PDO, ENSO-induced dry-wet changes are magnified with respect to the canonical pattern. When out of phase, these dry-wet variations weaken or even disappear. The El Nino-induced drying when in phase with the PDO covers approximately 18% of the global land area, which is nearly double the coverage during the out-of-phase condition. When La Nina occurs during the cold PDO phase, approximately 16% of the global land area suffers abnormal wetness or flooding, and only 13% in the warm PDO phase. Overall, when in phase with the PDO, the ENSO-induced dry-wet changes are magnified and approximately 28% of the global land area is affected by the significantly abnormal dryness and wetness. This remarkable contrast in ENSO’s influence between the two phases of the PDO highlights exciting new avenues for obtaining improved global climate predictions. During the warm phase of the PDO, El Nino induces much broader and more severe droughts over land compared with the cold PDO phase, which explains the dryland expansion after 1970’s. Around 2000, the PDO entered a cold phase with an increasing frequency of La Nina events, resulting in more rain over land. It implies that the American drought is not likely to ease and China has also experienced a dry-wet pattern turnaround - the pattern of "southern flood and northern drought" has become "southern drought and northern flood" in the coming decades.