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Chi-Yung Tam and Tim Li

Abstract

The origin, initiation, and dispersion behavior of the observed summertime synoptic-scale disturbances in the tropical western Pacific are studied. These westward-propagating disturbances have the strongest growth rate over the region of ∼130°–160°E off the equator. The three-dimensional wave activity flux associated with a wave packet in the vicinity of this region is computed. In general, wave activity is directed westward. There is accumulation of activity flux, which gives rise to the amplification of waves. In the low levels, such accumulation can be attributed to the convergence of both the mean flow and the intrinsic group velocity. Diabatic forcing also contributes to the growth of disturbances and is most important in the 500–600-hPa layer. Along the east–west-oriented “storm tracks” of the synoptic-scale disturbances, there are two different dynamical regimes. West of ∼150°E, enhanced convection is associated with increased specific humidity at the top of the planetary boundary layer and is in phase with positive low-level vorticity anomalies. To the east of 150°E the vorticity leads the convection by about one-quarter of a wavelength. This phase relationship can be explained by adiabatic dynamics and is related to the positive vertical shear of the mean zonal flow in the latter region.

Near and to the east of the date line where disturbances are initiated in the low levels, the heat flux associated with the synoptic-scale eddies is negative (i.e., υT ′ < 0) from about 300 to 700 hPa. This implies downward-directed wave activity. In the upper troposphere at the same geographical location, there is southward wave activity from the extratropics penetrating into the Tropics. These findings suggest that summertime synoptic-scale disturbances may originate from extratropical forcing. This hypothesis is supported by a case study. Intrusion of high potential vorticity into the Tropics was seen to be followed by downward development, resulting in low-level disturbances that subsequently moved westward in the western Pacific and grew.

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Guanghua Chen and Chi-Yung Tam

Abstract

This study investigates the synoptic-scale equatorial response to Rossby wave energy dispersion associated with off-equatorial wave activity sources and proposes a new mechanism for triggering low-level mixed Rossby–gravity (MRG) waves. A case study based on observations in boreal summer 2002 reveals that a vortex related to tropical cyclogenesis generated a coherent wave train through southeastward energy dispersion. The southeastward-propagating energy packet gave rise to the equatorial atmospheric response with a temporal scale similar to the wave train and with a structure consistent with the equatorially trapped MRG wave. A baroclinic multilevel anomaly model is employed to verify the excitation of MRG associated with the energy dispersion originating outside of the equatorial region and to explore the discrepancy in the equatorial responses under the different background flows corresponding to El Niño and La Niña. The results show that the prevalence of the low-level westerly flow, the associated zonal wind convergence, and the easterly vertical wind shear can be more favorable for the enhancement of southeastward-propagating energy dispersion and equatorial MRG response in the low troposphere during El Niño than those during La Niña. In addition, the strength of the mean flow can strongly affect the extent of equatorial wave response and modulate its phase and group velocity due to the Doppler shift effect.

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Chi-Yung Tam and Ngar-Cheung Lau

Abstract

The impact of the El Niño–Southern Oscillation (ENSO) on the atmospheric intraseasonal variability in the North Pacific is assessed, with emphasis on how ENSO modulates midlatitude circulation anomalies associated with the Madden–Julian oscillation (MJO) in the Tropics and the westward-traveling patterns (WTP) in high latitudes. The database for this study consists of the output of a general circulation model (GCM) experiment subjected to temporally varying sea surface temperature (SST) forcing in the tropical Pacific, and observational reanalysis products.

Diagnosis of the GCM experiment indicates a key region in the North Pacific over which the year-to-year variation of intraseasonal activity is sensitive to the SST conditions in the Tropics. In both the simulated and observed atmospheres, the development phase of the dominant circulation anomaly in this region is characterized by incoming wave activity from northeast Asia and the subtropical western Pacific. Southeastward dispersion from the North Pacific to North America can be found in later phases of the life cycle of the anomaly. The spatial pattern of this recurrent extratropical anomaly contains regional features that are similar to those appearing in composite charts for prominent episodes of the MJO and the WTP.

Both the GCM and reanalysis data indicate that the amplitude of intraseasonal variability near the key region, as well as incoming wave activity in the western Pacific and dispersion to the western United States, are enhanced in cold ENSO events as compared to warm events. Similar modulations of the MJO-related circulation patterns in the extratropics by ENSO forcing are discernible in the model simulation. It is inferred from these findings that ENSO can influence the North Pacific intraseasonal activity through its effects on the evolution of convective anomalies in the tropical western Pacific. On the other hand, there is little modification by ENSO of the circulation features associated with the WTP.

The combined effect of the MJO and WTP on the intraseasonal circulation in the North Pacific is studied. Based on multiple regression analysis, it is found that the MJO and WTP make comparable contributions to the variability in the midlatitude North Pacific. These contributions may be treated as a linear combination of the anomalies attributed to the MJO and WTP separately.

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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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Andie Y. M. Au-Yeung and Chi-Yung Tam

Abstract

An algorithm has been developed to track synoptic-scale, westward-traveling mixed Rossby–gravity (MRG) wave–like disturbances with a cross-equatorial component. Applied to space–time-filtered meridional wind data, this algorithm finds locations with Gaussian-shaped wind structures stated in the solutions of shallow-water equations (SWEs). Based on 850-hPa meridional wind from the global National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) dataset, local and instantaneous wave properties including the occurrence time, wavenumber, intrinsic frequency, and magnitude were examined. It was found that these low-level MRG wave–like disturbances can be classified into a longer-wavelength group and a shorter-wavelength group. While most waves identified in the eastern Pacific give longer wavelengths, disturbances in the western Pacific tend to have a wider range of wavenumbers. Composite analysis revealed that east of ~140°E, low-level disturbances are characterized by cross-equatorial wind anomalies with alternating signs, thus consistent with the MRG wave solution. West of ~140°E, they appear as northeast–southwest-tilted eddies that propagate northwestward. Examination of their energetics suggests that such a tilting structure is favorable to the maintenance of these transients because of the meridional shear of background zonal wind west of ~140°E in the off-equatorial Pacific. Farther east, the confluent nature of the low-level background flow plays a dominant role in maintaining the MRG wave–like disturbances because of barotropic conversion. Finally, there is evidence of downward energy dispersion in the mid- to upper levels, suggesting that the upper-level wave activity might be important in triggering these low-level waves in the Pacific basin.

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Ronald Kwan Kit Li, Chi Yung Tam, Ngar Cheung Lau, Soo Jin Sohn, and Joong Bae Ahn

Abstract

The Silk Road pattern (SR) is a leading mode of atmospheric circulation over midlatitude Eurasia in boreal summer. Its temporal phase is known to be unpredictable in many models. Previous studies have not reached a clear consensus on the role of sea surface temperature (SST) associated with SR. By comparing seasonal hindcasts from the Pusan National University (PNU) coupled general circulation model with reanalysis, we investigate if there are any sources of predictability originating from the SST. It was found that the PNU model cannot predict SR temporally. In fact, SR is associated with El Niño–Southern Oscillation (ENSO) in the model hindcasts, in contrast to reanalysis results in which SR is more associated with North Atlantic SST anomalies. The PNU system, however, shows potential predictability in SR associated with tropical Pacific SST. Bias in stationary Rossby waveguides is proposed as an explanation for the SR–ENSO relationship in hindcast runs. Model upper-level wind bias in the North Atlantic results in a less continuous waveguide connecting the North Atlantic to Asia, and may hinder wave propagations induced by North Atlantic SST to trigger SR. On the other hand, model upper-level wind bias in the subtropical western Pacific may favor westward propagation of zonally elongated waves from the ENSO region to trigger SR. This study implies that the role of SST with regard to SR can be substantially changed depending on the fidelity of model upper-level background winds.

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Kang Xu, Chi-Yung Tam, Congwen Zhu, Boqi Liu, and Weiqiang Wang

Abstract

Future projections of the eastern-Pacific (EP) and central-Pacific (CP) types of El Niño in the twenty-first century, as well as their associated tropical circulation and precipitation variability, are investigated using historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). As inferred from CMIP5 models that best capture both El Niño flavors, EP El Niño sea surface temperature (SST) variability will become weaker in the future climate, while no robust change of CP El Niño SST is found. Models also reach no consensus on the future change of relative frequency from CP to EP El Niño. However, there are robust changes in the tropical overturning circulation and precipitation associated with both types of El Niño. Under a warmer climate, magnitudes of precipitation anomalies during EP El Niño are projected to increase, presenting significant enhancement of the dry (wet) signal over the western (central–eastern) Pacific. This is consistent with an accelerated hydrological cycle in the deep tropics; hence, a “wet get wetter” picture appears under global warming, accompanied by a weakened anomalous Walker circulation. For CP El Niño, drier-than-normal conditions will be intensified over the tropical central–eastern Pacific in the future climate, with stronger anomalous sinking related to the strengthened North Pacific local Hadley cell. These results suggest that, besides the enhanced basic-state hydrological cycle over the tropics, other elements, such as the anomalous overturning circulation, might also play a role in determining the ENSO precipitation response to a warmer background climate.

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Kang Xu, Chi-Yung Tam, Boqi Liu, Sheng Chen, Xiaoyi Yang, Zhuoqi He, Qiang Xie, and Weiqiang Wang

Abstract

There exists a pronounced asymmetry between the amplitudes of central Pacific (CP) and eastern Pacific (EP) El Niño sea surface temperature anomalies (SSTA). The present study examines such an asymmetry and its relationship with the North Pacific SSTA. Results indicate that the weaker CP El Niño amplitude can be attributed to the weaker anomalous zonal wind response to the east–west equatorial SSTA gradient during its growing phase compared with EP El Niño. Furthermore, the occurrence of CP El Niño is closely associated with southwesterly surface wind anomalies in the subtropical North Pacific, as well as ocean warming reminiscent of the North Pacific Gyre Oscillation (NPGO) pattern in its vicinity. Both the observations as well as the pacemaker experiments with a coupled global climate model suggest that the anomalous low-level southwesterlies, induced by the North Pacific Oscillation (NPO)-like atmospheric variability, can enhance anomalously positive SST signals and extend them southwestward to the central equatorial Pacific via the wind–evaporation–SST feedback. This will further attenuate the atmospheric response to zonal SSTA gradient, and hence weaken the amplitude of CP El Niño. Therefore, anomalous low-level southwesterlies over the subtropical North Pacific can effectively act as a conduit for tropical–subtropical air–sea interaction in that region, and can play an important role in limiting the intensity of CP El Niño.

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