With a 3D Var assimilation scheme, several types of observations—sea surface temperatures (SST), sea level height anomalies (SLHA), and the upper ocean 400 meter depth-averaged heat content anomalies (HCA)—were assimilated into a hybrid coupled model of the tropical Pacific. The ocean analyses, and prediction skills of the SST anomalies (SSTA) from the assimilation of each type of observation, were presented for 1980-998. SST assimilation, besides improving the simulation of SSTA, also slightly improved the HCA and SLHA simulations in the equatorial Pacific, especially in the east. The ocean analyses with the assimilation of SLHA improved the simulations of SSTA, SLHA and HCA in the equatorial Pacific, while the assimilation of HCA improved the SLHA and HCA simulations. For ENSO predictions, assimilating SST yielded the best prediction skills for the Niño3 region SSTA at lead times of 3 months or shorter, but severely degraded the predictions at longer lead times. The best Niño3 SSTA predictions for lead times longer than 3 months came from the initializations with the assimilation of HCA and SLHA data. Assimilating SLHA yielded prediction skills for the Niño3 SSTA almost as good as assimilating HCA, indicating considerable potential for improving ENSO predictions from altimetry data. In this study, a neural network (NN) approach was used to find the nonlinear statistical relations among model variables for the assimilation of HCA and SLHA. Using NN yielded better prediction skills than using multiple linear regression.
Brute-force implementation of the extended Kalman (EK) filter in realistic ocean models is not possible because of its prohibitive cost. Different degraded forms of the EK filter, which basically reduce the dimension of the system through some kind of projection onto a low dimensional subspace, have been proposed (Cane et al. 1996; Dee 1990; Fukumori and Malanotte-Rizzoli 1995b; Hoang et al. 1997). The goal of this paper is to study the usefulness of the evolution in time of these reduced state spaces. This is based on the comparison, both from the theoretical and practical points of view, of the singular evolutive extended Kalman (SEEK) filter introduced by Pham et al. (1997), and the reduced-order extended Kalman (ROEK) filter introduced by Cane et al. (1996). To reduce the cost of the ROEK filter, we further approximate the nonlinear dynamics of the system by a first order autoregressive stochastic model. The assimilation results of twin experiments, which we have conducted in a realistic setting of the OPA model in the tropical Pacific ocean, seem to indicate that the evolution of the reduced state space is beneficial during the model unstable periods, where the reduced space may not well represent the variability of the model.
Using observed sea surface temperature data from 1871-1998, and observed wind data from 1958-1998, it is confirmed that the recently discovered Indian Ocean Dipole (IOD) is a physical entity. Many IOD events are shown to occur independently of the El Niño. By estimating the contribution from an appropriate El Niño index based on sea surface temperature anomaly in the eastern Pacific, it is shown that the major fraction of the IOD Mode Index is due to the regional processes within the Indian Ocean. Our circulation analysis shows that the Walker circulation during the pure IOD events over the Indian/ Pacific Ocean is distinctly different from that during the El Niño events. Our power spectrum analysis, and wavelet power spectrum analysis show that the periodicities of El Niño and IOD events are different. The results from the wavelet coherence analysis show that, during the periods when strong and frequent IOD events occurred, the Indian Ocean Dipole Mode Index is significantly coherent with the equatorial zonal winds in the central Indian Ocean, suggesting that these events are well coupled. During the periods when there seems to be some relationship between the equatorial zonal winds in the central Indian Ocean and ENSO index, no significant coherence is seen between the Indian Ocean Dipole Mode Index and the equatorial zonal winds in the central Indian Ocean, except after 1995, suggesting that most of the IOD events are not related to ENSO.
Seasonal mean features as well as subseasonal variations of the Indian summer monsoon are investigated on the basis of two simulations with the ECHAM4 atmospheric general circulation model at a high (T106) horizontal resolution. In the first one (period 1979-1993; AMI) observed monthly mean values of sea surface temperatures (SSTs) and sea-ice extent have been prescribed as lower boundary forcing to the model. The second one (period 1970-1999; TSL) is part of a time-slice experiment, where the atmospheric model has been forced by monthly mean values of SSTs, sea-ice extent and sea-ice thickness originating from a transient simulation with the ECHAM4/OPYC coupled atmosphere-ocean model at a low (T42) horizontal resolution. In TSL and the coupled simulation the concentrations of the important greenhouse gases have been prescribed according to observations until 1990, and after 1990 according to the IPCC scenario IS92a. In AMI the concentrations of the important greenhouse gases have been kept constant at a level close to the observed values for the period 1978-1988. The comparison between AMI and observations indicates a quite realistic simulation of the seasonal mean characteristics of the Indian summer monsoon, as well as of its subseasonal variability, though some deficiencies remain. These are a warm bias of the seasonal mean temperature over land, leading to a too strong temperature difference between the Indian Ocean and the land areas to the north; an overestimation of the low-level monsoon flow, an underestimation of the upper-level monsoon flow; and, an underestimation of precipitation over India, in particular for the rainfall maximum near the west coast of the Indian peninsula. Other shortcomings are a northward shift and deformation of the ITCZ over the Indian Ocean and over the western Pacific. The latter is associated with an unrealistic simulation of the large-scale circulation over the western Pacific, including the East Asian monsoon. The underestimation of precipitation near the west coast of the Indian peninsula affects the patterns describing the day-to day-variability of rainfall during the monsoon season. Though the most prominent patterns of variability obtained from the simulations have structures similar to those obtained for observations, the strength of the center of action on the west coast of the Indian peninsula is considerably underestimated. As for the seasonal mean features, most of the model deficiencies are improved in TSL. These improvements can be explained by the following mechanisms: (a) The initial warm temperature bias over the Indian Ocean acts to diminish the underestimation of precipitation over India, the warm temperature bias over India and Pakistan, and the too strong temperature difference between the Indian Ocean and the land areas to the north. (b) The El Niño-like warming pattern in the tropical Pacific helps to reverse the too strong low-level monsoon flow. (c) The erroneous SSTs in the western Pacific generated by the coupled model improve the simulation of the ITCZ and the large-scale circulation over the western Pacific, in particular the East Asian monsoon, and the ITCZ over the Indian Ocean via the Walker circulation. Little improvement is seen, however, in the simulation of the subseasonal variability.
Probabilistic forecast skill of the atmospheric seasonal predictability experiments is evaluated using the Japan Meteorological Agency (JMA) Atmospheric General Circulation Model (AGCM), which is a global spectral model of T63 resolution. Four-month ensemble integrations were carried out with nine consecutive days of initial condition preceding the target season. All four seasons in a 15-year period from 1979 to 1993 were chosen as target seasons. The model was forced with observed sea surface temperature (SST) during the time integrations. Probabilistic forecasts of 500 hPa height, 850 hPa temperature and precipitation are verified by four skill measures; the Brier skill score and its decomposition to reliability and resolution, relative operating characteristics (ROC), ranked probability score (RPS) and rank histogram. It is revealed that probabilistic forecast bears some similarity in the seasonality and regionality of skill, with deterministic forecast such as relatively higher skill in winter of the Northern Hemisphere, and over East Asia and North America. Skill of precipitation is found generally lower than that of 500 hPa height and 850 hPa temperature, as is also recognized for deterministic forecast skill.
The features of the Baiu front are examined using an AGCM with high resolution (T106L60). The Arakawa-Schubert scheme is used for the cumulus parameterization. The model clearly simulates the eastward-moving band of rain concentrated south of Japan during June, which can be considered the Baiu front. The characteristics of the Baiu front, such as the low level jet stream, a moist neutral stratification and a strong meridional gradient of moisture, are well simulated. Meso-a-scale disturbances appearing in the model are also comparable to the observational results. Experiments with varying horizontal resolutions keeping the same vertical resolution are also performed to examine resolution dependences. The distribution of precipitation is improved using higher resolution. In the T106L20 experiment, the Baiu-like frontal zone is formed by using a strong gravity wave drag parameterization. The strength of the subtropical jet is important over the Baiu front for a good simulation. Experiments with two different cumulus parameterizations (the moist convective adjustment, and Kuo schemes) are performed at T106L20 resolution to examine the effect of the parameterization on the simulation of the Baiu front. The moist convective adjustment scheme leads to high rainfall in the extra tropical region. The patterns of the Indian monsoon, and the North Pacific subtropical anticyclone, are quite different from those favorable to the formation of the Baiu front. On the other hand, a better distribution of precipitation is found using the Kuo scheme. However, an earlier disappearance of the Baiu front is seen in each cumulus parameterization experiment.
Employing a mass spectrometry method, a high precision measurement system was developed for analysis of the atmospheric O2/N2 ratio. Sample air and reference air were introduced into the mass spectrometer through thermally-insulated thin fused silica capillaries from an inlet system. Interference by CO generated in the ion source of the mass spectrometer from CO2 in the sample air, and the O2/N2 ratio biased due to pressure imbalance between the sample air and the reference air during their introduction into the mass spectrometer were experimentally corrected. Deterioration of sampled air during storage in flasks, as well as air sampling procedures, was also examined. The precision of our measurement system was estimated to be 5.4 per meg for the O2/N2 ratio, which corresponds to 1.1 ppmv of the atmospheric O2 concentration. Our standard air with 6 different O2/N2 ratios were prepared by drying the atmosphere and then stored in 47 L high-pressure cylinders; their O2/N2 ratios were confirmed to be stable within 20.0 per meg over the last 2 years. This system has been used for actual measurements of the atmospheric O2/N2 ratio since May 1999. Preliminary results of the measurements made in the suburbs of Sendai, Japan showed clear evidence for the seasonal cycle and the secular trend of the atmospheric O2/N2 ratio, which are opposite in phase with those of the CO2 concentration.
The urban heat-island effect of the Tokyo metropolitan area is believed to cause widespread warming in the daytime of the warm season, resulting in intensified convergence over the central Kanto plain. In order to detect this change, a comparative analysis of surface wind fields was made for the periods 1923-1928 and 1991-2001 for sunny days (at least 8 hours’ sunshine) from May to August. It was found that the magnitude of daytime pressure fall (at 1400 LT relative to 0600 and 2200 LT) had increased by 0.2-0.3 hPa in the central and northwestern part of the plain, with a slight change in surface winds in such a way as to converge toward the central Kanto plain.
Shipboard aerosol measurements in the marine boundary layer (MBL) were carried out over the western Pacific Ocean through the eastern Indian Ocean during a cruise of the Antarctic research vessel Shirase from Tokyo, Japan to Fremantle, Australia in November 2000, as part of the 42nd Japanese Antarctic Research Expedition (JARE42) activities. The latitudinal variation of aerosol optical properties is investigated with the origin of aerosols. For elucidation of aerosol properties, a method for retrieving the complex refractive index from combined measurements is proposed using an optical particle counter, an integrating nephelometer and a particle soot absorption photometer. Backward trajectory analyses indicate three regions that are characterized by three distinct aerosol types: anthropogenic /continental aerosols (30°-33.5°N), maritime aerosols (3°-28°N), and biomass-burning aerosols (12°-2°S). Retrieval of the imaginary part of the complex refractive index is successfully achieved by the proposed method, resulting in mean values of 0.0056, 0.0003, and 0.0036 for anthropogenic /continental, maritime, and biomass-burning aerosols, respectively.
The relationship between eddy heat flux and temperature gradient is studied. Correlations between the vertically-integrated total eddy heat flux and the temperature gradient are calculated for both the hemispheres using NCEP/NCAR reanalysis data. The results showed that, differently from the Northern Hemisphere (NH), in the Southern Hemisphere (SH) there is a large difference whether the verticallyintegrated eddy heat flux is correlated with the temperature gradient at 1000 hPa, or with the verticallyintegrated temperature gradient. The correlations are high and positive in both cases in the NH, as noted in earlier studies. However, in the SH when the vertically-integrated total eddy heat flux is correlated with the temperature gradient at 1000 hPa, there is a region in the middle latitudes where the correlations are highly negative. When the vertically-integrated temperature gradient is used, rather than the temperature gradient at 1000 hPa, the correlations are positive in both hemispheres. This indicates that it is necessary to use vertically-integrated temperature for obtaining the relation between eddy heat flux and temperature gradient in the case of the SH.
The influence of the Indian Ocean Dipole (IOD) on the interannual atmospheric pressure variability of the Indo-Pacific sector is investigated. Statistical correlation between the IOD index and the global sea level pressure anomalies demonstrates that loadings of opposite polarity occupy the western and the eastern parts of the Indian Ocean. The area of positive correlation coefficient in the eastern part even extends to the Australian region, and the IOD index has a peak correlation coefficient of about 0.4 with the Darwin pressure index, i.e. the western pole of the Southern Oscillation, when the former leads the latter by one month. The correlation analysis with seasonally stratified data further confirms the lead role of the IOD. The IOD-Darwin relation has undergone interdecadal changes; in the last 50 years the correlation is highest during the most recent decade of 1990-99, and weakest during 1980-89.
The areal extent of the sea ice cover in the Arctic Ocean has declined in the last 40 years with increased decadal variability. The trend is clearly influenced by the radiation balance over all seasons. A cloudiness increase in the fall, winter and spring contributes to a reduction in the absolute amount of net longwave radiation at the sea surface. In the summer, the reduced cloud cover has led to an increase in shortwave radiation, permitting more net outgoing radiation, and yielding a small increase in the total incoming radiation. All of these trends promote ice reduction, and may suggest the importance of clouds during a possible global warming in the near future. The effects of clouds and radiation are comparable with the albedo reduction associated with more open water, which absorbs more solar radiation in the summer. Analyses of the decadal variabilities reveal qualitatively the same effects as those of the radiation on the ice cover.