[1] Intraseasonal variability of SST in the SCS is examined. The results demonstrate that characteristics of intraseasonal SST variations in summer are remarkably different from those in winter. In summer, the spatial correlation pattern of intraseasonal SST perturbations is zonally distributed in the monsoon region. They are characterized by the northeastward propagation. In contrast, it appears to be localized in the SCS region without any propagating signal in winter. In addition, intraseasonal SST fluctuations are highly related to zonal winds and Outgoing Long-wave Radiation (OLR) in summer. However, they are primarily associated with meridional winds in winter. It has potentially important implication that the monsoonal air-sea interaction in summer is somewhat different from that in winter.

Using the observations from ICOADS datasets and contemporaneous NCEP/NCAR reanalysis datasets during 1960-2002, the study classifies the airflows in favor of sea fog over the Huanghai (Yellow) Sea in boreal spring (April-May) with the method of trajectory analysis, and analyzes the changes of proportions of warm and cold sea fogs along different paths of airflow. According to the heat balance equation, we investigate the relationships between the marine meteorological conditions and the proportion of warm and cold sea fog along different airflow paths. The major results are summarized as follows.(1) Sea fogs over the Huanghai Sea in spring are not only warm fog but also cold fog. The proportion of warm fog only accounts for 44% in April, while increases as high as 57% in May. (2) Four primary airflow paths leading to spring sea fog are identified. They are originated from the northwest, east, southeast and southwest of the Huanghai Sea, respectively. The occurrence ratios of the warm sea fog along the east and southeast airflow paths are high of 55% and 70%, while these along the southwest and northwest airflow paths are merely 17.9% and 50%. (3) The key physical processes governing the warm/cold sea fog are heat advection transport, longwave radiation cooling at fog top, solar shortwave warming and latent heat flux between airsea interfaces. (4) The characteristics of sea fog along the four airflow paths relate closely to the conditions of water vapor advection, and the vertical distribution of relative humidity.

The lower-frequency variations in the winter monsoon system have been examined using a l0-day low-pass filter. Results indicate that the lower tropospheric north-south thermal gradient over eastern Asia, the Siberian high pressure system, and the eastern Asia subtropical jet stream varied coherently with a time scale of 10-20 days. Composite study reveals that the 200-mb streamfunction variations on the 10-20 day time scale are longsynoptic-scale disturbances propagating slowly eastward. The cyclonic disturbances weaken as they move eastward south of the Tibetan Plateau and begin to strengthen when the lower tropospheric thermal troughs from Siberia move southward. The irrotational circulations on the 10-20 day time scale are closely related to the streamfunction variation with a quarter-wave phase shift. The tropical-midlatitude interactions in the winter monsoon system are seen to be the interactions between these long-synoptic-scale waves and the local monsoon circulations based on the phase relationships among various elements studied. The divergent circulations of the propagating waves trigger the cold air outbreak over eastern Asia when the wave trough is situated south of the Tibetan Plateau. The propagating waves are seen to intensify over the ocean suggesting a transfer of energy from the monsoon system to the waves.

Atmospheric circulation associated with anomalous variation of North Pacific blocking during the northern winter (December to February) is described and examined using the National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP-NCAR) reanalysis data from 1948/49 to 1999/2000. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. There are several atmospheric cells over the North Pacific associated with an anomalous blocking situation during winter. They are the zonal Walker cell along the equator (ZWC), the regional Hadley cell in the western Pacific (WHC), the regional Hadley cell in the eastern Pacific (EHC), the regional Ferrel cell over the eastern Pacific in the midlatitudes (EFC), and the midlatitude zonal cell (MZC) over the Pacific. During a strong blocking winter (SBW), the ZWC is strengthened and the anomalous EHC is opposite to the anomalous WHC and the thermally driven Hadley cell. The anomalous MZC is characterized by air rising in the west part of the North Pacific, flowing eastward in the upper troposphere, descending in the eastern North Pacific, then returning back to the east coast of Asia in the lower troposphere, modulating the mean MZC. It is also found that the anomalous regional Ferrel cell at midlatitudes exists in the eastern Pacific (EFC), whereas it is not apparent in the western Pacific. All anomalous atmospheric cells almost completely reverse during a weak blocking winter (WBW). Evolutions of each cell are also investigated. The atmospheric cells over the tropical and subtropical regions (ZWC, EHC, and WHC) always emerge ahead of the anomalous blocking winter, and then lower troposphere signals propagate upward after the anomalous blocking winter. This may suggest a mid-to-low-latitude interaction of the response of the midlatitude atmospheric blocking to tropical SST variations and a feedback via the atmospheric cells to the Tropics. In contrast, the midlatitude cells (MZC, EFC) evolve very locally, with a simultaneous response to the blocking event and no propagation of signal.

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