A coupled climate model is used to study the response of interannual variability of Asian summer monsoon, and its relation with tropical climate variability over the Pacific and Indian Oceans. Two simulations are performed, one for the present and the other for the Last Glacial Maximum. It is found that at glacial times, the weakened Asian monsoon climatology results in a significant shallowing of mean tropical thermocline in the central-western Indian Ocean. This shallower thermocline intensifies the positive ocean-atmosphere feedback in the western Indian Ocean, and therefore contributes to an enhanced Indian Ocean Dipole (IOP) variability. The increased IOD variability in turn exerts a stronger affect on the variability of South Asian monsoon rainfall, and therefore may have contributed to a significant reduction of correlation between the monsoon and the Pacific El Niño/Southern Oscillation (ENSO) variability.
Using ERA-40 and NCEP/NCAR reanalysis data, the characteristics of major teleconnection patterns were examined, particularly the West Asia-Japan (WJ) and Pacific-Japan (PJ) patterns, with a high-frequency (HF) component (from half a week to two or three weeks), and a low-frequency (LF) component (greater than about one month), and discussed the combined effects of the teleconnection patterns on the anomalous summer weather in Japan and the surrounding regions. Both patterns of the HF-WJ and LF-WJ, which propagate eastward along the upper-level Asian jet, induce an anomalous barotropic anticyclone centered on the Japan Sea. The HF-WJ pattern has no close link with the Asian summer monsoon activity, but, in contrast, the LF-WJ pattern is significantly correlated with anomalous monsoonal heating over the summer monsoon region. The HF-PJ and LF-PJ patterns, which can be identified with stationary waves stimulated by intense convection around the Philippine Sea, generate a nearly barotropic anticyclone anomaly to the east of Japan. A combination of the HF-WJ and HF-PJ patterns establishes a zonally elongated anticyclonic anomaly over northern Japan, resulting in anomalous high surface temperatures in northern Japan. Such a coupling was found to lead to a larger temperature increase in that region than a single teleconnection pattern alone. A typical case of the LF-WJ and LF-PJ combination also indicates a zonally elongated anticyclonic anomaly over northern Japan, which is similar to the combined pattern of HF-WJ and HF-PJ. The dynamic impact of LF-WJ on the surface temperature field around Japan differs significantly from that of LF-PJ. The dominance of the LF-WJ causes enhanced subsidence over Japan, which can bring about adiabatic heating and increased incoming solar radiation. On the other hand, PJ-induced anomalous anticyclone in the lower troposphere facilitates northward warm advection to the east of northern Japan, where the north-south temperature gradient is large. As for the extreme hot summer of 2004, no combined patterns of the LF-WJ and LF-PJ were found during the summer. Alternatively, a tripole structure appeared in the lower geopotential height field in mid-June. Such a tripole pattern may be established by a combination of the LF-PJ and a barotropic Rossby wave train propagating southeastward from high latitudes.
Global Water Cycle Experiment (GEWEX)-Asian Monsoon Experiment (GAME)/Huaihe River Basin Energy Water Cycle Experiments (GAME/HUBEX) were conducted during the Meiyu period in 1998 and 1999. Using infrared brightness temperature (TBB) data of the Geostationary Meteorological Satellite (GMS)-5, we investigated the diurnal variation of 61 long-lasting cloud clusters that developed during GAME/HUBEX Intensive Observation Periods (IOPs). More than two-thirds of the cloud clusters, named nocturnal-type clusters, attained a convective peak between midnight and early morning, with most peaking between 00 and 02 LST. Almost all of these nocturnal-type cloud clusters developed in, or south of the Meiyu frontal zone. The other clusters, named evening-type cloud clusters, peaked from late afternoon to evening, and were less intense than the nocturnal clusters. GAME-Reanalysis (Version 1.5) data showed that the ageostrophic wind component over a large domain to the south of the Meiyu front shifted from easterly to southerly, and increased the velocity of the southwesterly airflow largely at low levels at nighttime (02 LST). This southwesterly ageostrophic synoptic-scale low-level jet (S-LLJ) transported large amounts of water vapor to the Meiyu front, forcing large moisture convergence within, and immediately south of the Meiyu frontal zone. The low-level moisture convergence, and the S-LLJ itself are proposed to support the nocturnal evolution or redevelopment of cloud clusters.
Aerosol physical and chemical properties and relative humidity (RH) in the marine atmosphere undergo their modifications under the influence of continentally polluted air mass. Effects of continentally polluted air mass on aerosol optical properties over the East China Sea were studied by using data of aerosol size and number, aerosol composition, and RH. These data were obtained under Atmospheric Particulate Environmental Change Experiment 2/Asian Pacific Regional Aerosol Characterization Experiment (APEX-E2/ACE-Asia). Data collected on 18, 21, 23, and 26 April 2001 were used in this study. In order to evaluate contributions of such modifications to aerosol optical thickness (AOT) and single scattering albedo (SSA), AOT and SSA of the continentally polluted marine atmospheres were compared with values calculated by replacing each parameter (aerosol composition, aerosol size and number, and RH) of the continentally polluted marine atmospheres with that of the marine atmosphere. Further, by not separating aerosol composition and aerosol size and number parameters, investigations were performed by replacing these parameters of the continentally polluted marine atmospheres with those of the marine atmosphere. Modification of aerosol size and number was found to have a tendency to increase AOT significantly, whereas modification of aerosol composition by only a few percentages. Since continental air had relatively low RHs, modification of RH was found to have a tendency to decrease the value. The result also suggested that the decrease of aerosol hygroscopicity by continentally polluted air mass tended to decrease AOT. On the other hand, not only aerosol composition and RH modifications, but also aerosol size and number modification had a tendency to decrease SSA The study pointed out an important role ofcontinentally polluted air mass on aerosol radiative forcing, due to the increase of AOT and the decrease of SSA over the East China Sea.
Using satellite ice water path (IWP) retrieval as an example, two types ofsystematic errors when using a Bayesian algorithm are documented. These errors arise from the discontinuous and heavily skewed data distributions of IWP in the supporting database. Discontinuity in the data distribution occurs at IWP = 0 since there are no negative IWPs, which causes a positive bias in the IWP retrievals near ice-free conditions. On the other hand, since clouds with small IWPs occur more often than clouds with large IWPs in nature, data points in the database are heavily populated in the low IWP range if we build the supporting database based on observed data. This skewed data distribution leads to a negative bias. A number of remedies, while some are ad hoc, to reduce these errors are also suggested.