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Ryuichi Kawamura

Abstract

Spatiotemporal variability of preferred global-scale sea surface temperature anomaly patterns is documented, applying a varimax-rotated empirical orthogonal function (R-EOF) analysis to monthly mean SST anomalies. The present study focuses especially on the interdecadal variability of leading R-EOF modes. It is first found that temporal variability of R-EOF1 has a quasi periodicity of 2–5 years and coincides quite well with the occurrence of the ENSO event; hence this mode can be identified with the ENSO mode and distinguished from the other modes dominated by interdecadal variability. The authors find that R-EOF2 typically shows the dominance of interdecadal variability and signals of the ENSO phenomenon are removed. This mode is characterized by increasing Indian Ocean SST and decreasing central North Pacific SST around 40°–50°N in the recent ten or more years. A further indication is that both R-EOF3 and R-EOF4, which show the dominance of interdecadal variability, are fundamentally regarded as preferred localized modes confined only to the Atlantic Ocean. Therefore, we find the existence of two kinds of prevailing SST modes; one is a mode fluctuating between oceans, corresponding well to R-EOF2, and the other is an oscillatory mode isolated only in a specific ocean (e.g., R-EOF4).

Analysis of implications for low-frequency modes in the atmosphere showed that the Pacific/North American (PNA) mode, which prevails in the Northern Hemisphere winter, is closely related to the large-scale SST variability with interdecadal time scale rather than the ENSO time scale. Additionally, in the Northern Hemisphere summer, the Subtropical Zonal (SZ) mode, which has a north-south dipole structure over the North Pacific, corresponds fairly well to R-EOF2. It is suggested that the increasing tropical ocean SST in the recent ten or more years, especially the Indian Ocean SST, is responsible for the increase of the zonal, summertime 500-hPa geopotential heights in the low-latitude regions of the North Pacific.

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Ryuichi Kawamura, Masato Sugi, and Nobuo Sato

Abstract

A set of three climate experiments is performed using a T42 GCM version of the Japan Meteorological Agency global model to examine extratropical interdecadal and interannual variations over the North Pacific region associated with the anomalous SST forcing in the Tropics. Three independent 34-yr integrations from January 1955 to December 1988 are forced by the same SST boundary condition observed on the global scale.

The set of these integrations provides clear evidence that the tropical SST impact upon the wintertime extratropical model atmosphere in the North Pacific is very significant. It is also concluded that the abrupt change of midlatitude circulation regime that occurred in the winter of 1976/77 was primarily caused by very localized tropical heating in the central Pacific. This anomalous SST forcing was most likely responsible for persistent negative height anomalies over the central North Pacific during at least the period from 1977 to 1983, which formed a part of the extratropical wave train traversing the North Pacific and North America, which produced warm temperature anomalies along the west coast of North America, as well as western Canada. However, an increase in observed wintertime surface temperature over northern Eurasia at almost the same period can little be explained by anomalous SST forcing from the Tropics. The internal variability of the extratropical atmosphere itself is suggested to contribute much more to the circulation regime over the Eurasian continent.

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Ryuichi Kawamura, Masato Sugi, and Nobuo Sato

Abstract

Interdecadal and interannual) variations of a model atmosphere in the northern extratropics is examined using a T42 GCM forced with observed near-global SSTs from January 1955 to December 1988.

The leading mode of summertime 500-hPa height field deduced from the real SST experiment is found to he dominated by interdecadal variability. This mode shows a zonally elongated pattern with prominent loadings in low-latitude regions and accounts for an increase of the zonal, summertime 500-hPa heights in subtropical regions from the 1970s to the 1980s. Simulated springtime leading mode, which is dominated by interdecadal variability, exhibits a mixed pattern with the wintertime PNA mode and the summertime zonally elongated mode, whereas the zonally elongated pattern like the summertime EOFI cannot be found in northern fall.

From an investigation based on the seasonality of tropical response of the model atmosphere, it is found that the summertime and springtime leading modes with a pronounced zonally symmetric component depend largely upon the tropical SST anomalies of interdecadal variability. The weakness of tropical response in fall contributes largely to the absence of the zonally elongated mode with definite interdecadal variability in this season.

The regional and tempers features of the observed decadal surface air temperature anomalies are well simulated by the real SST experiment. The time sequence of the above summertime EOFI, which accounts for a strong dependence of tropical atmosphere to SST anomaliess, is found to coincide well with the summertime mean hemispheric land surface air temperature. It is inferred, therefore, that the tropical SSTs of interdecadal variability contribute a great deal to the decrease and increase in the Northern Hemispheric land surface temperature observed in recent decades.

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Ryuichi Kawamura, Masato Sugi, and Nobuo Sato

Abstract

Interdecadal and interannual atmospheric variability in the extratropical Northern Hemisphere is investigated using an atmospheric GCM. The model used for this research is a T42 GCM version of the Japan Meteorological Agency (JMA-GSM89) global model. The 34-yr integration from January 1955 to December 1988 has been performed employing the real observed near-global SST condition. To estimate internal variability of the tropical and extratropical atmospheres, another 34-yr integration was conducted using the seasonally varying, climatological SST without interannual variability.

Using the rotated EOF analysis, the authors made an intercomparison of the Pacific/North American (PNA) wintertime teleconnection patterns prevailing in the observed and simulated extratropical atmospheres in the two experiments. The polarity of PNA derived from the real SST experiment is indicative of definite interdecal variability. particularly an abrupt change of the midlatitude circulation regime over the North Pacific in the 1976/77 winter. By contrast, this mode, deduced from the climatological SST control run, has intermonthly and short-term interannual variability but no pronounced interdecadal variability.

It is strongly suggested that the anomalous SST forcing exerts strong influence on the PNA mode and modulates its amplitude, and as a consequence, longer-tem variability, such as interdecadal variability, has appeared in the time sequence of this mode. It is confirmed from the T42 GCM experiment that the interdecadal variations of the wintertime extratropical atmosphere over the North Pacific are substantially controlled by the anomalous SST forcing in the Tropics, and that, in particular, the tropical forcing is primarily responsible for the abrupt change of the midlatitude circulation regime in the 1976/77 winter.

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Hidetaka Hirata, Ryuichi Kawamura, Masaya Kato, and Taro Shinoda

Abstract

The active roles of sensible heat supply from the Kuroshio/Kuroshio Extension in the rapid development of an extratropical cyclone, which occurred in the middle of January 2013, were examined by using a regional cloud-resolving model. In this study, a control experiment and three sensitivity experiments without sensible and latent heat fluxes from the warm currents were conducted. When the cyclone intensified, sensible heat fluxes from these currents become prominent around the cold conveyor belt (CCB) in the control run. Comparisons among the four runs revealed that the sensible heat supply facilitates deepening of the cyclone’s central pressure, CCB development, and enhanced latent heating over the bent-back front. The sensible heat supply enhances convectively unstable conditions within the atmospheric boundary layer along the CCB. The increased convective instability is released by the forced ascent associated with frontogenesis around the bent-back front, eventually promoting updraft and resultant latent heating. Additionally, the sensible heating leads to an increase in the water vapor content of the saturated air related to the CCB through an increase in the saturation mixing ratio. This increased water vapor content reinforces the moisture flux convergence at the bent-back front, contributing to the activation of latent heating. Previous research has proposed a positive feedback process between the CCB and latent heating over the bent-back front in terms of moisture supply from warm currents. Considering the above two effects of the sensible heat supply, this study revises the positive feedback process.

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Xiaoyang Li, Ryuichi Kawamura, Atsuko Sugimoto, and Kei Yoshimura

Abstract

Moisture sources and their corresponding temperature and humidity are important for explosive extratropical cyclones’ development regarding latent heating. To clarify the water origins and moisture transport processes within an explosive cyclone, we simulated an explosive cyclone migrating poleward across the Sea of Japan on 30 November 2014, by using an isotopic regional spectral model. In the cyclone’s center area, a replacement of water origins occurred during the cyclone’s development. During the early stage, the warm conveyor belt (WCB) transported large amounts of moisture from the East China Sea and Kuroshio into the cyclone’s inner region. While in the deepening stage, the cold conveyor belt (CCB) and dry intrusion (DI) conveyed more moisture from the northwest Pacific Ocean and the Sea of Japan, respectively. Compared with the contribution of local moisture, that of remote moisture was dominant in the cyclone’s center area. Regarding the water origins of condensation within the frontal system in the deepening stage, the northwest Pacific Ocean vapor, principally transported by the CCB, contributed 35.5% of the condensation in the western warm front. The East China Sea and Kuroshio moisture, conveyed by the WCB, accounted for 32.4% of the condensation in the cold and eastern warm fronts. In addition, condensation from the Sea of Japan, which was mainly triggered by the DI and induced by the topography, occurred on the west coast of the mainland of Japan and near the cyclone center. The spatial distribution of the isotopic composition in condensation and water vapor also supports the water-origin results.

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Hidetaka Hirata, Ryuichi Kawamura, Masaya Kato, and Taro Shinoda

Abstract

This study focused on an explosive cyclone migrating along the southern periphery of the Kuroshio/Kuroshio Extension in the middle of January 2013 and examined how those warm currents played an active role in the rapid development of the cyclone using a high-resolution coupled atmosphere–ocean regional model. The evolutions of surface fronts of the simulated cyclone resemble the Shapiro–Keyser model. At the time of the maximum deepening rate, strong mesoscale diabatic heating areas appear over the bent-back front and the warm front east of the cyclone center. Diabatic heating over the bent-back front and the eastern warm front is mainly induced by the condensation of moisture imported by the cold conveyor belt (CCB) and the warm conveyor belt (WCB), respectively. The dry air parcels transported by the CCB can receive large amounts of moisture from the warm currents, whereas the very humid air parcels transported by the WCB can hardly be modified by those currents. The well-organized nature of the CCB plays a key role not only in enhancing surface evaporation from the warm currents but also in importing the evaporated vapor into the bent-back front. The imported vapor converges at the bent-back front, leading to latent heat release. The latent heating facilitates the cyclone’s development through the production of positive potential vorticity in the lower troposphere. Its deepening can, in turn, reinforce the CCB. In the presence of a favorable synoptic-scale environment, such a positive feedback process can lead to the rapid intensification of a cyclone over warm currents.

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