This paper proposes a new simple method of multivariable maximum covariance analysis (MMCA), for extracting common variability from multiple (more than two) datasets, that expands the singular value decomposition analysis method. The method is based on iteration of a recurrence equation derived by a dual relationship between pattern vectors and time coefficients. Two approaches of the method are proposed, one using the extreme of a summation of covariances (sum MMCA) and the other using the product of covariances (product MMCA). Both approaches are demonstrated by successfully extracting the variability related to the Arctic Oscillation from three monthly-mean meteorological datasets. The method is useful because it is easily programmed and is computationally inexpensive. The method can be applied to an arbitrary number of datasets, although a complete set of the product MMCA method cannot apply to an even number of datasets.
It has been argued that the Coupled Model Intercomparison Project phase 5 (CMIP5) models underestimate the frequency of atmospheric blocking, while project a decreasing trend of blocking in the 21st century in the Northern Hemisphere. This average trend may not be true for regional blockings. Focusing on three key regions in Eurasia (the Urals, Baikal, and Okhotsk regions) where blocking significantly influences the weather and climate of East Asia, this study first evaluates the performance of the CMIP5 models by comparing historical simulations with NCEP/NCAR reanalysis (NNR). Possible changes in the first half of the 21st century are then analyzed using the RCP4.5 and RCP8.5 experiments.
It is found that instantaneous blocking frequencies are underestimated in the Urals and Baikal regions for the whole year and in the Okhotsk region in summertime, but are overestimated in Okhotsk in wintertime. Blocking episode frequency in the Urals and Baikal regions is underestimated by most of the 13 CMIP5 models, especially the short-duration blocking episodes (4–5 days), and the simulations are better in wintertime than in summertime. However, in the Okhotsk region the modeled frequency of blocking episodes is close to the value from NNR in summertime but overestimated in wintertime. Model projections of instantaneous blocking frequency for the first half of the 21st century (2016–2065) show that both RCP4.5 and RCP8.5 runs yield an increasing frequency except during June–August in the Eurasia. The multi-model ensemble mean frequency of blocking episodes clearly decreases in the whole year in the Urals and Baikal regions (especially blocking episodes with short duration) and increases a little in summertime of the Okhotsk region in the first half of the 21st century. The model ensemble-mean frequency of blocking episodes with long duration (more than 9 days) decreases by ~40% in the Urals region but increases by no more than 5% in Okhotsk.
Persistent severe rainfall (PSR) events during the rainy season (April to July) in southern China were studied in terms of the dynamical features of the large-scale circulation. The aim was to understand the formation mechanism and improve forecasting. The circulation field and spatiotemporal distribution of waves at 500hPa, for different types of PSR were analysed. The results reveal the following: (1) During the pre-flood season (April to June) in South China, troughs have the same phase in the middle latitudes as those in the high latitudes. The East Asia major trough (3–5 wave numbers) in the middle latitudes strengthens southwards and interacts with the 30°N subtropical high (1–2 wave numbers) from 3 days prior to the PSR events. (2) During the post-flood season (June to July) in South China, the weather regime transitions occur on 5 days prior to the PSR events. The 40°N trough (2–4 wave numbers) strengthens southwards and interacts with the subtropical high (1–2 wave numbers). It is also affected by the blocking ridge (3 wave number) in the high latitudes. (3) During the Mei-yu period (June to July) over the Yangtze–Huaihe River basin, the transitions of circulation pattern starts on 3 days prior to the PSR events. With the northwest development of the subtropical high, there is a transfer process from long to short waves in terms of energy for the trough at 50°N.
Polar mesoscale cyclones (PMCs) frequently developed over the Japan Sea. Genesis of PMCs over the East China Sea is rare, but could occur under the certain synoptic-scale conditions. In this observational case study, the feature of a PMC generated over the eastern East China Sea on 20 February 1975 is studied by using observation data including those obtained during Air-mass Transformation Experiment, satellite cloud images and objective-reanalysis data.
The PMC with comma-cloud formed within cyclonic polar-air streams induced by an upper cold trough and a synoptic-scale parent cyclone which developed near Japan. Within 3-hour after generation of the PMC, its central pressure deepened from 1016 hPa to 1012 hPa. Strong surface winds occurred in the trail of the comma-cloud. The large-scale conditions for the generation stage were characterized by the southward intruding of the cold core in the upper cold trough beyond 34°N to the East China Sea, positive vorticity advection at 500 hPa, and the moist-neutral layer formed over the warm Tsushima Current in the eastern East China Sea.
The PMC, after passing over Kyushu, developed as it moved eastward along the Pacific coast of Japan. It developed further in the low-level baroclinic zone over the Northwestern Pacific, into the secondary cyclone comparable to the parent cyclone. The large-scale conditions for the development were characterized by the upper cold trough and the low-level baroclinic zone formed over the zone of maximum sea-surface temperature gradient along north of the Kuroshio extension.
This work investigates development processes of Baiu frontal depressions (BFDs) using a numerical model. To investigate the effects of upper-level disturbances, latent heating, and baroclinicity on the development of BFDs, case-study numerical simulations are performed. In the present study, two typical cases were selected from BFDs that appeared in June and July, 2000-2007: a BFD that developed in the western part of the Baiu frontal zone (W-BFD) from 26 to 27 June 2003 and a BFD that had formed in the eastern part of the Baiu frontal zone (E-BFD) from 1 to 3 July 2003. An available potential energy (APE) diagnosis shows that the effect of latent heating is dominant during the W-BFD development, while baroclinicity as well as latent heating is important to the E-BFD development. A sensitivity experiment excluding upper-level potential vorticity (PV) anomalies shows that upper-level disturbances are important contributors to the development of E-BFDs.
The low-level PV and its production associated with latent heating suggest that the W-BFD has a development mechanism driven by latent heating. In the early developmental stage, PV near the W-BFD center is enhanced. This feature is consistent with the nonlinear conditional instability of the second kind mechanism. In the later developmental stage, PV is produced in front of the W-BFD center, in which low-level baroclinicity is large. This process is consistent with a diabatic Rossby vortex. In contrast, the E-BFD develops through a baroclinic instability-like mechanism in the moist atmosphere.
The present study has investigated how the impacts of the inclusion of radiosonde observations conducted locally in early summer of 2012 over the Kuroshio and Kuroshio Extension (KE) can spread over time across the North Pacific basin to influence the predictability of synoptic and large-scale tropospheric circulation. For that purpose, observing system experiments (OSEs) has been performed where each of two extra sets of radiosonde data, one obtained over the East China Sea in mid-May and the other over the KE in early July, is added to an atmospheric ensemble data assimilation system for comparison with the corresponding analyses without those data. The experiments show that the impact of the extra data assimilated propagates eastward mainly due to the advection by the subtropical jet (STJ) in May and July. The strong STJ in May allows the upper-tropospheric impact to travel across the basin only within two days. Under the weaker STJ, the corresponding impact in July tends to remain within the western Pacific, until it eventually reaches the eastern portion of the basin. The assimilation of the extra radiosonde data over the Kuroshio or KE can lead to decrease of pressure over the Gulf of Alaska both in May and July.
Additional forecast experiments based on the OSEs for May has revealed that the pressure decrease over the Gulf of Alaska can be traced back to the west of the Alaska Peninsula and to east of Japan over three days. The impacts that originate on different dates via different paths merge over the central North Pacific, acting to reinforcing the cyclone over the Gulf of Alaska. This study presents examples where the impacts of atmospheric observations over the western boundary current can propagate across the ocean basin through the westerlies to influence the forecast skill in distant regions.
This study examined the relationship between the El Niño Southern Oscillation (ENSO) and atmospheric water isotopes during the wet season over the Maritime Continent. The model data used were obtained by incorporating stable isotopes into atmospheric general circulation and analytical moisture transport models. These models were used to analyze the climatological variables and rainout processes from various water sources that control isotopic variation. The correlation between the simulated stable isotope ratios and ENSO varied between –0.31 and 0.75 with stronger correlation over most of the Maritime Continent (|r|› 0.36, corresponding to the 95 % significance level) except Java. Generally, during La Niña years, the isotopic ratio in water vapor and precipitation is lighter than during El Niño years by about 2 ‰. It was suggested that anomalous water vapor flux, precipitable water, and precipitation, but not evaporation, are responsible for isotopic variation. Furthermore, it was revealed that water vapor flux is convergent (divergent) during La Niña (El Niño) years, which suggests that the strengthened (weakened) Walker Circulation increases (reduces) precipitation and results in lighter (heavier) atmospheric water isotopes. The relationship between isotopes and precipitation, or the so-called "amount effect," is evident over the most of the Maritime Continent. Analysis of moisture transport suggested that rainout processes control isotopic variation. The increase in the quantity water source, expressed in precipitable water, transported from the north and south Maritime Continent during El Niño years does not result in isotopic depletion attributable to the lack of condensation processes. Moreover, the decrease in the quantity of both water source during La Niña years does not result in isotopic enrichment attributable to intensive rainout. An asymmetric ENSO feature was found in this study, evidenced by the similar contributions of water source from the northern Maritime Continent and the Pacific Ocean during both ENSO phases.
This paper provides an overview of the Asian monsoon and its change as simulated by atmosphere–ocean coupled general circulation models (AOGCMs) and high-resolution atmospheric general circulation models (AGCMs), focusing on the seasonal mean circulation and precipitation climatology. After reviewing the drivers of, and the elements that affect, the monsoon, the ability of those climate models to reproduce the Asian monsoon is assessed. The Asian monsoon is better reproduced in the Coupled Modeling Intercomparison Project phase 5 (CMIP5) models than in the CMIP3 models, although biases remain. Projected future changes in the Asian monsoon at the end of the 21st century are then reviewed. Overall projections are similar for both CMIP3 and CMIP5 models, with increases in precipitation, albeit with weakened circulation in the South Asian summer, enhanced circulation and increased precipitation in the East Asian summer, and latitude-dependent changes in the winter monsoon circulation in East Asia. However, differences exist in the projected local changes, leading to uncertainty in projections.
The environmental conditions for tropical cyclone genesis is examined by numerical experiment. We focus on the case of a non-developing disturbance that had features for tropical cyclone genesis in the Pacific Area Long-term Atmospheric observation for Understanding climate change in 2010 (PALAU2010) observation campaign over the western North Pacific. We clarify the importance of abundant moisture around the disturbance for continuous convection and demonstrate that the collocation of a mid-level vortex and a low-level vortex, i.e., the persistence of an upright structure of vortices, is important in tropical cyclone genesis. We conduct two numerical experiments by using the Weather Research and Forecasting Model-Advanced Research model in the double nested domains with the horizontal grid space 27 km and 9 km for the outer domain and the inner domain, respectively. The first is based on reanalysis data (a control experiment), and the second includes increased water vapor content over the northwestern dry area of the disturbance. In the control experiment, the disturbance did not develop into a tropical cyclone in spite of the existence of the mid-level and low-level vortices. In contrast, the sensitivity experiment shows that a tropical cyclone was formed from the disturbance with increased water vapor content. The presence of persistent upright vortices was supported by continuous convection until genesis of tropical cyclone.