Air-Sea Features And Climate Impacts Associated With Two Types Of La Nina

In this dissertation, historical La Nina events during1951-2010are divided into two types (i.e., Eastern Pacific (EP) type and Central Pacific (CP) type), based on the sea surface temperature anomalies (SSTA) distribution pattern during mature phase. Many data sets are used here, which include the monthly sea surface temperature from HadlSST, subsurface sea temperature from SODA, reanalysis data from NCEP/NCAR, high resolution data from CRU and precipitation data of160stations from Chinese National Climate Center. The local air-sea coupled features and extratropical atmospheric responses are identified associated with EP La Nina and CP La Nina during seasonal evolution. In particularly, different climate impacts of two-type La Nina events over North Atlantic-Western Europe and southern China are analyzed using observed data and modelling stimulations. The major conclusions are summarized as follows:(1) The two types of La Nina events show distinct SSTA distribution patterns and significant differences in SSTA intensity, persistence and propagation. For EP La Nina, the SSTA start over the South American coast during developing summer and then propagate westward. The negative SSTA center covers the eastern equatorial Pacific near120°W during the mature phase. Nevertheless, no significant propagation of SSTA is observed for the CP La Nina and its SSTA center maintains at around160°W. The CP La Nina exhibits stronger intensity with larger zonal and meridional SSTA gradients, and persists longer compared to EP La Nina. Cold and warm SSTA in the Pacific form a "horseshoe" pattern during developing autumn and last till next spring.(2) Tropical atmospheric responses are different for two types of La Nina. EP La Nina induces a single anomalous Walker circulation. The anomalous zonal circulation is obvious in developing autumn and winter, but dissipates in next spring. During CP La Nina, anomalous sinking motion moves westward to the equatorial central Pacific, with a branch of rising motion west to the120°W in western Pacific and another one near90°W in eastern Pacific. The anomalous double-Walker circulation is stronger, appears earlier (developing summer) and disappears later (next spring) than EP La Nina. Anomalous precipitation associated with two types of La Nina is mainly different in equatorial Pacific and Pacific rim. CP La Nina is featured by a stronger intensity, a larger extent, and a slightly westward displacement in moist divergence compared to EP La Nina during mature phase.(3) The two types of La Nina can give rise to a significantly different teleconnection in North Atlantic-Western Europe. During boreal winter, North Atlantic-Western European district experiences the atmospheric anomaly resembling a negative North Atlantic Oscillation (NAO) pattern accompanied by a weakening Atlantic jet. It leads to a cooler and drier than normal winter over Western Europe. However, the CP La Nina has a roughly opposing impact on the North Atlantic-Western European climate. A positive NAO-like climate anomaly is observed with a strengthening Atlantic jet, and there appears a warmer and wetter than normal winter over Western Europe. Modeling experiments reveal the effects of tropical eastern-central Pacfic, western Pacific and North Atlantic SSTA forcing. It indicates that the above contrasting atmospheric anomalies are mainly attributed to the different SST cooling patterns for the two types of La Nina. Mixing up their signals would lead to difficulty in seasonal prediction of regional climate. Since the La Nina-related SST anomaly is clearly observed during the developing autumn, the associated winter climate anomalies over Western Europe could be predicted a season in advance.(4) Different precipitation anomaly patterns are shown over Southern China for the EP and CP La Nifna episodes. Southern China experiences a drier than normal autumn when EP La Nina occurs. However, CP La Nina has almost opposing impacts on the precipitation during boreal autumn over Southeast China, where precipitation is increased. Both types of La Nina events seem to lead to a drier than normal winter over Southern China. Southwest China is much drier during CP La Nina winter. The net flux of vertically integrated moisture divergence agrees with composited precipitation anomalies. The flux across the meridional boundaries is more important than zonal. During autumn, CP La Nina induces stronger and more westward anticyclonic circulations and carries larger water vapor to coastal area with anomalous moisture convergence. During winter, the anomalous western Pacific warming SSTA during CP La Nina events induces stronger cyclonic circulations and brings more northeasterly winds. Rising motion dominate112°E-120°Ein southeast coast of China during CP La Nina autumn. It should be attributed to the westward movement of the rising which located in ocean normal years. However, vertical motions during winter have little effects on precipitation pattern.