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Chung-Kyu Park and Siegfried D. Schubert

Abstract

Intraseasonal (20–70 day) variability is examined in the Atlantic region during Northern Hemisphere winter using ECMWF analyses and NOAA outgoing longwave radiation (OLR). It is found that the dominant 200-mb zonal-wind fluctuation over the tropical Atlantic, A1, is related to global-scale circulation anomalies with their origins in the Pacific. Compositing techniques are used to investigate the nature of the Pacific-Atlantic teleconnections and related changes in the tropical OLR and moisture convergence.

The OLR anomalies associated with A1 are characterized by eastward propagation over the Indian Ocean and the western Pacific and a standing oscillation over the tropical Atlantic; the latter extends from Northeast Brazil to West Africa and is the dominant component of the Atlantic OLR variability on these time scales. An analysis of the velocity potential and moisture convergence fields suggests that the fluctuations in convection are coupled between the western Pacific and Atlantic via large-scale (zonal wavenumber 1), equatorially trapped, eastward-propagating waves associated with the Madden-Julian oscillation.

The zonal-wind fluctuation, A1, is also related to extratropical waves propagating into the tropics from both the Northern and Southern hemispheres. The Southern Hemispheric wave train, which makes up the dominant contribution to the A1 circulation pattern, appears to emanate from the western South Pacific and amplifies near the west coast of South America. The Northern Hemispheric wave train resembles the Pacific/North American pattern and emanates from the central North Pacific near the East Asian jet exit region.

These results suggest that a major component of the 20–70-day variability over the Atlantic region is remotely forced. The forcing occurs via the Madden-Julian oscillation, which is strongly coupled with eastward-migrating heating anomalies in the western Pacific and Rossby wave trains, which appear to have their origins in the middle latitudes of the Pacific. The Northern Hemispheric wave train appears to be maintained by energy exchange with the East Asian jet, while the nature of the Southern Hemispheric branch is unclear.

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Chung-Kyu Park and Siegfried D. Schubert

Abstract

East Asian countries experienced record-breaking heat waves and drought conditions during the summer monsoon season of 1994. This study documents the large-scale circulation associated with the drought and suggests a forcing mechanism responsible for the anomalous evolution of the East Asian monsoon. The results, based on Goddard Earth Observing System (GEOS) global assimilated data for 1985–94, indicate that the absence of monsoon rainfall during July 1994 over central China and the southern parts of Korea and Japan is due to the unusually early development of the climatological upper-level anticyclonic flow east of the Tibetan Plateau. The anomalous July anticyclonic circulation over the East Asian–northwestern Pacific region and the cyclonic circulation over the subtropical western Pacific, which are more typical of August, acted to reduce the moisture supply from the western Pacific and the Indian Ocean leading to suppressed rainfall over East Asia. The similarity of the July 1994 East Asian circulation anomalies to the climatological July to August change in these quantities suggests that the anomalies may be viewed as an acceleration of the seasonal cycle in which the circulation transitions to August conditions earlier than normal.

Neither tropical nor middle latitude SST anomalies provide a viable forcing mechanism for the 1994 East Asian circulation anomalies: the tropical anomalies are weak and the middle latitude anomalies, while stronger, appear to be primarily a response to atmospheric forcing, though they may feed back to reinforce the atmospheric anomalies. It is suggested, instead, that the anomalous circulation is primarily the result of an orographic forcing associated with zonal wind changes over Tibet. The zonal wind change, characterized by an anomalous northward shift of the East Asian jet is, in turn, tied to unusually persistent stationary waves extending from northern Europe, which developed prior to the onset of the East Asian anticyclone. Several other occurrences of atmospheric anomalies similar in structure (though weaker in amplitude) to the July 1994 anomalies are found in the previous nine summers, suggesting the operative mechanism is not unique to 1994. Such a mechanism appears to operate both for the climatological development of the ridge and for the occurrences of similar anomalies in previous summers: in the former the northward shift of the jet over Tibet is a reflection of climatological seasonal change in the zonal wind, while in the latter, the shift is the result of anomalies similar in structure to the 1994 European–Asian wave pattern.

The indirect role of the Eurasian waves in the development of the East Asian circulation anomalies suggests that useful monthly and longer predictions of the monsoon rests, not only on our ability to predict the occurrence of these waves, but also on our ability to properly model their interaction with orography.

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Siegfried D. Schubert and Chung-Kyu Park

Abstract

Low-frequency (20–70 day) variability is examined during Northern Hemisphere (NH) winter based on seven Years (1981–87) of European Centre for Medium Range Weather Forecasts initialized analyses. The dominant 200 mb zonal wind fluctuations in the Pacific sector, determined from an empirical orthogonal function (EOF) analysis, provide the baseline modes of atmospheric variability, which are related to fluctuations in other circulation parameters and outgoing longwave radiation (OLR). The composite circulation associated with the extreme phases of the zonal wind modes are examined for differences in forcing, wave propagation characteristics and stability.

The dominant upper level zonal wind fluctuation (EOF Z1) is associated with an expanded (contracted) region of easterlies in the tropical western Pacific and changes in the shape and intensity of the subtropical jets. The anomalous (difference between composites) eddy streamfunction at 200 mb shows an enhanced pair of anticyclones (cyclones) straddling the equator. These fluctuations are strongly coupled with eastward traveling tropical convection in the western Pacific with a time scale of about 40 days. The composite circulations show marked differences in the propagation of wave activity in the NH at 200 mb. The low phase (reduced easterlies) shows strong propagation away from the dominant source region over East Asia into the tropical western Pacific in conjunction with what appears to be significant reflection from the equatorward flank of the subtropical jet. In contrast, the high composite (enhanced easterlies) shows much weaker equatorward propagation together with reduced vertical propagation over East Asia and the western North Pacific.

The second zonal wind EOF (Z2) displays a more asymmetric structure with respect to the equator, describing a simultaneous decrease (increase) in the easterly extent of the East Asian jet and increase (decrease) in the strength of the jet over southern Australia. The anomalous eddy stream function at 200 mb shows wave trains apparently emanating from the tropical central Pacific extending into both hemispheres: in the winter hemisphere this resembles the Pacific-North American (PNA) pattern. These fluctuations show some coupling with preceding tropical convection anomalies in the western and central Pacific. Stability calculations show that the PNA pattern is maintained through barotropic energy exchanges with the mean flow. For the low composite, an enhanced source of stationary wave activity in the Gulf of Alaska is associated with an increase in synoptic-scale eddy activity.

These results suggest that tropical convection in the western Pacific has a strong modifying influence on (extratropically forced) middle latitude low-frequency variability. The influence is primarily indirect via zonal wind changes which influence the propagation of waves originating in middle latitudes. The zonal wind changes include those associated with the strength and extent of the tropical easterlies as well as more subtle (but important) changes which effect the curvature of the East Asian jet leading in some instances to turning points for middle latitude waves. The PNA also appears to have its main energy source in middle latitudes and in this case the link with the tropics appears to be more tied to phase locking with anomalies forced by tropical convection in the western and central Pacific.

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Eun-Jeong Lee, Jong-Ghap Jhun, and Chung-Kyu Park

Abstract

A new northeast Asian summer monsoon index is introduced to investigate the characteristics of the northeast Asian summer rainfall variation, including Korea, Japan, and northeast China, and its possible connection to the tropical and midlatitude circulations. The summer precipitation over northeast Asia is separated into two components associated with tropical forcing and midlatitude dynamics using this monsoon index. The connection between the northeast Asian summer rainfall and ENSO is clearly identified by separating the Tropics-related component from the northeast Asian summer rainfall. That is, the Tropics-related precipitation over northeast Asia tends to be enhanced after the mature phase of El Niño. On the other hand, it is revealed that the extratropics-related component of summer precipitation is connected to the Eurasian wave pattern with no significant lag correlation.

The intensity of the western North Pacific anticyclone modulated by ENSO is a key factor in the variation of the northeast Asian summer precipitation. It is found that the warm SST over the tropical eastern Pacific plays an important role in establishing the western North Pacific anticyclone during the preceding winter of strong northeast Asian summer monsoon years, whereas convective activities over the Bay of Bengal are contributed to the modulation of the anticyclonic circulation in the summer. The warming over the Indian Ocean in the summer of strong monsoon years induces the development of the anticyclone over the western North Pacific and the suppressed convection over the western Pacific tends to enhance the northeast Asian summer rainfall through the Pacific–Japan or East Asia–Pacific teleconnections.

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In-Sik Kang, June-Yi Lee, and Chung-Kyu Park

Abstract

Potential predictability of summer mean precipitation over the globe is investigated using data obtained from seasonal prediction experiments for 21 yr from 1979 to 1999 using the Korea Meteorological Administration–Seoul National University (KMA–SNU) seasonal prediction system. This experiment is a part of the Climate Variability and Predictability Program (CLIVAR) Seasonal Model Intercomparison Project II (SMIP II). The observed SSTs are used for the external boundary condition of the model integration; thus, the present study assesses the upper limit of predictability of the seasonal prediction system. The analysis shows that the tropical precipitation is largely controlled by the given SST condition and is thus predictable, particularly in the ENSO region. But the extratropical precipitation is less predictable due to the large contribution of the internal atmospheric processes to the seasonal mean. The systematic error of the ensemble mean prediction is particularly large in the subtropical western Pacific, where the air–sea interaction is active and thus the two-tier approach of the present prediction experiment is not appropriate for correct predictions in the region.

The statistical postprocessing method based on singular value decomposition corrects a large part of the systematic errors over the globe. In particular, about two-thirds of the total errors in the western Pacific are corrected by the postprocessing method. As a result, the potential predictability of the summer-mean precipitation is greatly enhanced over most of the globe by the statistical correction method; the 21-yr-averaged pattern-correlation value between the predictions and their observed counterparts is changed from 0.31 before the correction to 0.48 after the correction for the global domain and from 0.04 before the correction to 0.26 after the correction for the Asian monsoon and the western Pacific region.

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Chung-Kyu Park, Max J. Suarez, and Siegfried D. Schubert

Abstract

An atmospheric general circulation model is used to study the impact of idealized zonally propagating tropical heating anomalies on the low-frequency variability in the North Pacific region. The propagating heating is designed to mimic the thermal forcing associated with the Madden–Julian oscillation (MJO). Results are examined by separating the forced response from other variability and by comparing with runs employing fixed-phase (stationary) heating anomalies.

For both the forced and free circulations, the main modes of variability consist of a zonal expansion and retraction of the East Asian jet. The effective Rossby wave forcing associated with the heating is dominated by the advection term and located in the subtropics in the regions of strong absolute vorticity gradients.

Compared with cases using stationary forcing, the response to the propagating forcing is weaker and of different phase, indicating that the 40-day period used for the propagating anomalies is too short to allow the development of the steady-state response in the extratropics.

The model's total low-frequency variability in the North Pacific sector is dominated by the free oscillations that are the result of local processes uncorrelated with tropical variability. The relatively small forced response appear to be partly the result of the simplicity of the propagating heating anomaly that propagates at a constant phase speed and the simplification introduced into the GCM that do not allow transient feedback in the diabatic heating.

It is suggested that the lack of a significant Rossby wave stretching term in the subtropics is a distinguishing feature of the east–west dipole heating anomalies of the MJO and may contribute to the weakness of the response compared to interannual tropical heating anomalies.

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Kyong-Hee An, Chi-Yung Tam, and Chung-Kyu Park

Abstract

This study investigates the role of model tropical diabatic heating error on the boreal summer northeast Asian monsoon (NEAM) simulation given by a general circulation model (GCM). A numerical experiment is carried out in which the GCM diabatic heating is adjusted toward more realistic values in the tropics. It is found that the seasonal mean NEAM circulation and rainfall are improved in the GCM. This can be attributed to the reduced positive heating bias in the western Pacific Ocean around 10°–15°N in the model, which in turn leads to better-simulated low-level southerly winds over eastern Asia and more moisture supply to the NEAM region. The GCM’s ability in capturing the year-to-year variation of NEAM rainfall is also markedly improved in the experiment. These results show that the diabatic heating error over the western Pacific can be one reason for poor NEAM simulations in GCMs. The authors also suggest a simple method to reduce model heating biases that can be readily applied to dynamical seasonal prediction systems.

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Hongwen Kang, Chung-Kyu Park, Saji N. Hameed, and Karumuri Ashok

Abstract

A pattern projection downscaling method is applied to predict summer precipitation at 60 stations over Korea. The predictors are multiple variables from the output of six operational dynamical models. The hindcast datasets span a period of 21 yr from 1983 to 2003. A downscaled prediction was made for each model separately within a leave-one-out cross-validation framework. The pattern projection method uses a moving window, which scans globally, in order to seek the most optimal predictor for each station. The final forecast is the average of six model downscaled precipitation forecasts using the best predictors and will be referred to as “DMME.” It is found that DMME significantly improves the prediction skill by correcting the erroneous signs of the rainfall anomalies in coarse-resolution predictions of general circulation models. Although Korea’s precipitation is strongly influenced by local mountainous terrain, DMME performs well at 59 stations with correlation skill significant at the 95% confidence level. The improvement of the prediction skill is attributed to three steps: coupled pattern selection, optimal predictor selection, and the multimodel downscaled precipitation ensemble. This study indicates that the large-scale circulation variables, which are predicted by the current operational dynamical models, if selected, can be used to make skillful predictions of the local precipitation by using appropriate statistical downscaling methods.

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Young-Mi Min, Vladimir N. Kryjov, and Chung-Kyu Park

Abstract

A probabilistic multimodel ensemble prediction system (PMME) has been developed to provide operational seasonal forecasts at the Asia–Pacific Economic Cooperation (APEC) Climate Center (APCC). This system is based on an uncalibrated multimodel ensemble, with model weights inversely proportional to the errors in forecast probability associated with the model sampling errors, and a parametric Gaussian fitting method for the estimate of tercile-based categorical probabilities.

It is shown that the suggested method is the most appropriate for use in an operational global prediction system that combines a large number of models, with individual model ensembles essentially differing in size and model weights in the forecast and hindcast datasets being inconsistent. Justification for the use of a Gaussian approximation of the precipitation probability distribution function for global forecasts is also provided.

PMME retrospective and real-time forecasts are assessed. For above normal and below normal categories, temperature forecasts outperform climatology for a large part of the globe. Precipitation forecasts are definitely more skillful than random guessing for the extratropics and climatological forecasts for the tropics. The skill of real-time forecasts lies within the range of the interannual variability of the historical forecasts.

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Siegfried Schubert, Max Suarez, Chung-Kyu Park, and Shrinivas Moorthi

Abstract

General circulation model (GCM) simulations of low-frequency variability with time scales of 20 to 70 days are analyzed for the Pacific sector during boreal winter. The GCM's leading mode in the upper-tropospheric zonal wind is associated with fluctuations of the East Asian jet; this mode resembles, in both structure and amplitude, the Pacific/North American (PNA) pattern found in the observations on these time scales.

In both the model and observations the PNA anomaly is characterized by 1) a linear balance in the upper-tropospheric vorticity budget with no significant Rossby wave source in the tropics, 2) a barotropic conversion of kinetic energy from the time mean Pacific jet, and 3) a north/south displacement of the Pacific storm track. In the GCM, the latter is associated with synoptic eddy heat flux and latent heat anomalies that appear to contribute to a strong lower-tropospheric source of wave activity over the North Pacific. This is in contrast to the observations, which show only a weak source of wave activity in this region.

The GCM produces 60% of the total observed Pacific sector low-frequency zonal wind variance. About one-third of the missing variability appears to be due to unrealistic simulations of the Madden-Julian oscillation; the remainder is characterized in the variance spectrum as a deficit in the overall level of “background” variability. The nature of this missing background variability is unclear.

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