Daily temperature and precipitation data from Japanese stations are studied to estimate climate noise, an unpredictable part of long-term variability. The climate noise is compared to the interannual variance of monthly mean temperatures, and monthly precipitation totals, to access the potential predictability. Potential predictability is that part of the interannual variance that exceeds climate noise. Primarily during July and January, it is found that potential predictability of temperature can exceed 40% at some stations. Less potential predictability is present during April and October. The results are similar for monthly precipitation totals.
Mesoscale disturbances in the tropical lower troposphere having periods of 1-2 hours were detected in the wind profiler data during the intensive observing period of the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Seven cases were detected with average amplitudes of the vertical velocity greater than 0.1ms-1. One case that was observed at Kapingamarangi (1.1°N, 154.8°E) on January 7, 1993 was examined in detail. This disturbance had a period of 60 minutes and a duration of 6 hours. Assuming that this disturbance was a gravity wave, the vertical and horizontal wavelengths were estimated as 5.4-7.6km and 24-37km, respectively. Using satellite and sounding data, it was inferred that the disturbance was excited by a mesoscale cloud line lying about 400km ESE from Kapingamarangi, and then propagated in a wave duct in the troposphere.
By using data for 18 years, diurnal variation in the frequency of heavy precipitation in Japan was analyzed. Diurnal variation patterns at each station were obtained for three durations (one, three and six hours), and several thresholds ranging from moderate to very high intensities (5-80mm/h, 10-160mm/3h and 20-320mm/6h). Then their regional characteristics were examined by using the fuzzy c-means method (FCM) in a search for representative patterns. The results are summarized as follows: (a) In coastal regions a morning maximum prevails, especially on the western side of land, (b) A broad maximum around midnight is found in many regions, especially on the southern and eastern sides of mountainous regions, as well as a part of the inland region, (c) A sharp maximum in late afternoon is found in part of the inland regions, especially in the eastern part of mountain regions and the adjacent plain, and (d) A maximum around midday is found over the Nansei Islands. The midnight maximum is more conspicuous for heavier precipitation on longer time scales (three- and six-hourly), in agreement with the empirical knowledge among forecasters that disastrous rainfall is more often encountered in the nighttime.
The turnabout and growth mechanisms of the ENSO are diagnostically studied by analyzing the SST budget of the Cane-Zebiak model. The SST change rates, which are directly linked to the phase transition and growth of the ENSO, are attributed to two processes: the anomalous vertical advection of subsurface temperature by the mean upwelling (thermocline feedback), and the zonal advection of the climatological mean SST by the anomalous zonal current (zonal advection feedback). The contributions to the phase transition, and growth of the ENSO, can be systematically separated by decomposing the equatorial thermocline depth anomaly, and zonal current anomaly into their zonal mean and zonal contrast fields. It is found that the thermocline feedback, associated with the zonal mean thermocline depth anomaly, and the zonal advection feedback by the equatorial zonal mean zonal current anomaly are responsible for the phase transition of the ENSO. Those associated with zonal contrast fields are responsible for the growth of the ENSO. The two processes in the SST change contribute to the phase transition and growth of the ENSO in an almost equally significant manner. They are closely related, as a result of the geostrophic balance between the meridional gradient of the thermocline depth and the zonal current. These findings suggest that the conceptual understanding of the ENSO in the Cane-Zebiak model, should include both of these two processes.
To predict the tracer cloud evolution in the European Tracer Experiment, two numerical simulation runs of air tracer dispersion are performed by using different meteorological input data. There are the global objective analyses, by the European Centre for Medium-Range Weather Forecast (ECMWF), and by the Japan Meteorological Agency (JMA). The simulated results are compared with each other, and evaluated statistically based on measured tracer concentration at specific sampling sites. The results of the statistical evaluation show that the predicted tracer cloud evolution, using the JMA analysis data, indicates a slightly better statistical score in the global and space dependent analyses, than the result by the ECMWF. The scores in the time dependent analysis show deferent values at every specific sampling sites, however, the averaged scores are slightly better for results by the ECMWF. It is difficult to discriminate synthetic superiority, and inferiority between both runs. It is inferred that the difference between both wind fields leads to different cloud evolution. The simulated tracer cloud evolution showed relatively good correspondence with measurements. Further references on the appropriate deposition velocity, and inclusion of precipitation process, are needed to get better correspondence with measurements.
A spectral analysis of lower tropospheric winds, observed by the Boundary Layer Radar (BLR) at Serpong (6°S, 107°E) in West Java, Indonesia during 1992/93, 1993/94, and 1994/95 rainy seasons reveals a pronounced spectral peak, at a period near 4 days. Filtered analysis of this quasi 4 day mode (Q4DM) shows almost an in-phase structure in the vertical. The GANAL (Global Objective Analysis by Japan Meteorological Agency) data for 1992/93 rainy seasons shows that in general the meridional wind component, at the altitude 700hPa, has a westward phase propagation.
A hydrologic excitation of ten-yearly polar motion appearing in the combined five-daily polar motion data of International Polar Motion Service (IPMS) and SPACE94 during 1962-1995 is discussed. The discussion is based on time-dependent exponential decay models of land water storage supplied by precipitation, in which the National Oceanic and Atmospheric Administration (NOAA) monthly gridded precipitation anomaly data for 5°×5° is used. The decay parameter of the land water storage τd is determined when square of the difference between the observed ten-yearly polar motion and calculated one becomes minimum. Amplitude in the calculated polar motion can explain about 36% of that in observed ten-yearly polar motion when τd takes 3.7 months. The behavior of the calculated polar motion is similar to the observed one though the phase of the former leads that of the latter by about one year. When we do not consider the small phase difference, about 52% of amplitude in the observed polar motion can be explained when τd takes 5.1 months, suggesting a globally averaged decay time of the land water storage. The seesaw-like changes of precipitation that excite efficiently the ten-yearly polar motion are also confirmed between the North American continent and the western region of the Eurasian continent. These facts suggest a conceptual model of the feedback system on decadal hydrologic cycle centered in the North Atlantic Ocean, in which the system consists of atmospheric variations connected with the North Atlantic Oscillation, seesaw-like sea level changes, and precipitation changes in the North American and Eurasian continents.
Cumulus convection occurs over the Kanto plain during the afternoons of fair summer days. By investigating cloud amount data, it was revealed that the afternoon cloud amount on a given day, and that on the following day are dependent on each other. If the cloud amount is small on a given day, it tends to be greater on the following day, and vice versa. It can be concluded that fair weather does not tend to persist from day to day, when the Kanto plain is covered by the subtropical anticyclone and the weather conditions are stable. The interdiurnal variation was then simulated in two-dimensional numerical experiments. Days with strong and weak convection appeared quasi-periodically, even though the complete diurnal boundary conditions were given corresponding to the sea and land breezes. By examining the results of the numerical experiments, it was shown that the interdiurnal variation resulted from differences in vertical profiles of potential temperature and water vapor mixing ratio in the lower troposphere. The ‘over-adjustment’ effect that the condition will become less favorable for cumulus convection on the following day as a result of strong convection, is essentially important for the interdiurnal variation. Furthermore, the differences in the vertical profiles were compared with those from aerological data, in order to examine the above mentioned results of the interdiurnal variation. The differences simulated in the numerical experiments were qualitatively similar to those found in the aerological data, although the differences in potential temperature were not statistically significant.
Climatological features and interannual variability of the Asian summer monsoon and its relationship with equatorial Pacific sea surface temperature (SST) anomalies simulated by a global coupled atmosphere-ocean GCM (CGCM) are investigated. The coupled model results are compared with the observation as well as the simulations by an atmospheric general circulation model (AGCM). Overall features of the model climatology and variability of the Asian summer monsoon in the CGCM are as close to the observed one as in the ALCM. The simulated SST and its variability in the CGCM shows some bias compared to the observation. The monsoon region in the models, as defined by the seasonal change of wind direction and convection proxy, agrees with the observed. The models are less successful in simulating the summertime wind system in the western Pacific region. The CGCM reasonably well reproduces the observed ENSO-related interannual variability of the tropical circulation system. Its main deficit is associated with a westward displacement of simulated SST variability. There is an underestimation of precipitation around the Philippines. Differences are found between the CGCM and the AGCM in the variability over the Indian monsoon region. The AGCM responds well to the prescribed SST anomaly in the Pacific. It behaves erroneously over the Indian Ocean. This may be related to the fact that the AGCM is only responding to the prescribed SST fields, while the CGCM includes two-way atmosphere-ocean interactions. The CGCM results show that the simulated Indian summer monsoon rainfall anomalies are negatively correlated with the equatorial Pacific SST anomalies. It is consistent with the observed one, i, e., good monsoon is associated with La Niña. As a precursory signal, ground temperature is significantly warmer in spring in central Asia preceding a good monsoon. It is noted that the snow cover anomalies are negative in the above region, but its significance is marginal.
The mechanism of the tropospheric biennial oscillation (TBO) of the ENSO-monsoon system is investigated by an MRI coupled atmosphere-ocean general circulation model. In this mechanism, biennial variability of the South Asian monsoon affects the global scale climate variability through interactions with the air-sea coupled system over the Pacific and/or the extratropical circulation. In the strong phase of the TBO, the area of relatively strong monsoon convective maximum over South Asia in the spring to summer season moves southeastward to Indonesia in the autumn to winter season. This movement superimposes on its climatological seasonal cycle. It suggests that the northern winter monsoon convection tends to be strong around Indonesia to northern Australia when the summer Asian monsoon is strong. The anomalous state of the air-sea coupled system in the Pacific sector which forms in the summer season, seems to dissipate from its eastern edge. This occurs by a local atmosphere-ocean coupling process through a large scale Walker circulation. The convection anomalies persist during the entire monsoon season over Indonesia and northern Australia. As a response to this equatorial monsoon convection anomaly, a Matsuno-Gill type stationary Rossby wave is established over the South Asian region. The appearance of upper level anticyclonic circulation and lower level cyclonic circulation anomaly in a strong monsoon year is a favorable condition for bringing the cold air advection over the Eurasian continent. Cold air advection after the strong monsoon persists through the whole winter to spring season to form the cold tropospheric temperature around Central to South Asia. Then reduced land-sea, or north-south temperature contrasts sets up the following weak South Asian summer monsoon. The simulated TBO of the South Asian monsoon is tightly phase locked with a seasonal cycle. The phase of the TBO changes in northern spring, which suggests that the extratropical-tropical interaction be realized mainly during winter to spring through the onset of South Asian monsoon. Our results imply that the TBO is an inherent feature in the land-monsoon-ocean coupled system, and emphasize a more active role of monsoon-extratropical interaction in the Indian sector in winter to spring season for regulating the TBO cycle.
The relationship between the intensity of the Tibetan High and the amplitude of the quasi-stationary Rossby wave trapped in the subtropical jet over the Asian Continent is investigated for the 1983 summer. Three major warming and cooling events of the Tibetan High were seen during July and August with about 20- to 30-day intervals. These events coincided with the increasing and decreasing events of the air mass amount within the layer between 350K and 360K isentropic surfaces. All the cooling events occurred when the quasi-stationary Rossby wave was amplified along the subtropical jet just north of the Tibetan High. These Rossby wave packets played important role in cooling events by transporting the air mass with 350K to 360K potential temperatures northward out of the Tibetan High. Stratosphere-troposphere exchange events were observed at the north-eastern periphery of the Tibetan High several days after the amplifications of the quasi-stationary Rossby wave.
The annular mode in the Northern Hemisphere (NH) extratropical circulation, which has an equivalent barotoropic structure from the surface to the lower stratosphere, is called the Arctic Oscillation (AO), by Thompson and Wallace (1998). The AO is the leading empirical orthogonal function (EOF) mode of the sea level pressure field, and it can be seen throughout the year. It is more dominant in winter. It is characterized as a seesaw pattern of mass, between the Arctic region and the midlatitude belt. It is also a seesaw of the mean zonal wind, between the region poleward of 40°N and that equatorward of it. In this study, an analysis of the simulated AO is made, to confirm that the AO is an atmospheric internal mode, and for studying the transition mechanism to the high/low polarity of the AO. The seasonal and perpetual February runs were made by using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (AGCM). In both runs, the AO signature is dominant. Its magnitude is similar to the observed one. Thus, it is confirmed that the AO is an internal mode of the atmosphere. Using data from a perpetual February integration, a composite analysis of the transition to the high/low polarity of the AO is made. Based upon the transformed Eulerian mean equation for the mean zonal wind, it is investigated which term is responsible for the transition. The results indicate that the wave forcing term contributes to the transition and the term of the residual meridional circulation acts to restore the transition. A decomposition of the wave forcing to zonal wave component indicates that planetary-scale wavenumbers 2 and 3 contribute most to the transition. The synoptic-scale waves contribute partly to the low-latitude wind change. In the perpetual February run, a slowly propagating AO-related stratosphere-troposphere coupled mode, is detected. Its oscillation period is 4-6 months. The anomaly of the zonal-mean, zonal wind first appears in the subtropical stratosphere and propagates poleward. Once the anomaly reaches high latitude, it becomes large and propagates into the troposphere.
The transient response of a coupled ocean-atmosphere model to increasing concentrations of greenhouse gases and sulfate aerosols, is investigated with an explicit representation of aerosol scattering. Experiments with an implicit representation of aerosol scattering, through the modification of surface albedo with the same parameter values as used in previous climate projections, are also conducted for comparison. The indirect effect of aerosol is yet to be included. As suggested by previous radiation computation studies, the estimated radiative forcing due to the direct effect of sulfate aerosol is significantly smaller with the explicit representation, than with the implicit one. A principal source of the overestimation by the implicit method is the neglect of the dependence of aerosol scattering on near-surface humidity. The projected surface air temperature change due to the addition of sulfate aerosols is considerably smaller in magnitude especially over dry regions with the explicit method, than with the implicit one. It is also suggested that the change in the Asian summer monsoon precipitation due to an increase in sulfate aerosols is particularly sensitive to the representation of sulfate aerosol scattering.