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H. Zhang and C. S. Frederiksen

Abstract

Using a version of the Australian Bureau of Meteorology Research Centre (BMRC) atmospheric general circulation model, this study investigates the model's sensitivity to different soil moisture initial conditions in its dynamically extended seasonal forecasts of June–August 1998 climate anomalies, with focus on the south and northeast China regions where severe floods occurred. The authors' primary aim is to understand the model's responses to different soil moisture initial conditions in terms of the physical and dynamical processes involved. Due to a lack of observed global soil moisture data, the efficacy of using soil moisture anomalies derived from the NCEP–NCAR reanalysis is assessed. Results show that by imposing soil moisture percentile anomalies derived from the reanalysis data into the BMRC model initial condition, the regional features of the model's simulation of seasonal precipitation and temperature anomalies are modulated. Further analyses reveal that the impacts of soil moisture conditions on the model's surface temperature forecasts are mainly from localized interactions between land surface and the overlying atmosphere. In contrast, the model's sensitivity in its forecasts of rainfall anomalies is mainly due to the nonlocal impacts of the soil moisture conditions. Over the monsoon-dominated east Asian region, the contribution from local water recycling, through surface evaporation, to the model simulation of precipitation is limited. Rather, it is the horizontal moisture transport by the regional atmospheric circulation that is the dominant factor in controlling the model rainfall. The influence of different soil moisture conditions on the model forecasts of rainfall anomalies is the result of the response of regional circulation to the anomalous soil moisture condition imposed. Results from the BMRC model sensitivity study support similar findings from other model studies that have appeared in recent years and emphasize the importance of improving the land surface data assimilation and soil hydrological processes in dynamically extended GCM seasonal forecasts.

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Gregory C. Johnson and Dongxiao Zhang

Abstract

The equatorial deep jets in the Atlantic Ocean are described using vertical strain, ξ z, estimated from all available deep CTD stations in the region. Wavelet analysis reveals a distinct energy peak around 661-sdbar vertical wavelength, 1232-dbar pressure, and ±1.5° latitude from the equator. This high-vertical-wavenumber and off-equatorial maximum, coupled with previously published velocity data that show nodes in zonal velocity near ±1.5°, is grossly consistent with the structure of first-meridional-mode equatorial Rossby waves. However, the meridional scale obtained from the observations exceeds, by about 1.5, the theoretical meridional scale for these waves. The jets are strong, with zonal velocities similar in magnitude to the Kelvin wave phase speed for their vertical wavelength. Harmonics of ξ z at vertical wavelengths of 1/2, 1/4, and perhaps 1/8 that of the primary peak provide evidence of a large-amplitude structure. Although sparse, available phase data at the 661-sdbar vertical wavelength suggest downward and westward phase propagation. Assuming sinusoidal character in time and longitude gives estimates of a 5- (±1) yr period and a 70° (±60°) zonal wavelength. These vertical, temporal, and zonal scales are roughly consistent with first-meridional-mode equatorial Rossby wave dynamics. However, although vertical and zonal phase propagation are discernible, there is no obvious signature of upward energy propagation in the variance vertical maxima, which is problematic for a simple linear Rossby wave interpretation.

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E. C. Hunke and Y. Zhang

Abstract

An elastic-viscous-plastic (EVP) model for sea ice dynamics has recently been proposed as a computationally efficient alternative to the viscous-plastic (VP) model widely in use. The EVP model features a fully explicit discretization that improves the model’s efficiency, particularly on high-resolution grids, and adapts easily to parallel computation. Comparison of two high-resolution Arctic sea ice simulations, identical except for the ice dynamics, indicates that the EVP model reproduces the VP model behavior on timescales relevant to climate studies. The ice concentration and thickness distributions over a 1-yr integration period are remarkably similar in the two models, although the EVP model responds more rapidly and accurately to strong synoptic weather systems than does the VP model, compared to drifting Arctic buoys. A close look at rates of strain shows that elastic waves in the EVP model do not significantly alter the ice behavior in highly compact areas, where the waves most benefit numerical efficiency. Internal stress of the ice is also similar in the two models; both deviate from viscoplasticity in regions of nearly rigid ice and in regions of low concentration undergoing approximately free drift motion.

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John C. Wyngaard and Shi-Feng Zhang

Abstract

We show that the horizontal turbulent velocity components measured by the common sonic anemometer array can suffer attenuation and crosstalk as a result of the flow blockage caused by the acoustic transducer assemblies. Using an analytical model of this “transducer-shadow effect”, flow-blockage data from test arrays, and a simple linear model of the fluctuating response, we show the nature of the distortion in the measured velocity spectra. We suggest that rather than correct for the shadow effect, which ran be quite significant for horizontal velocity spectra and stress cospectra, it would be preferable to minimize it through design. There is encouraging evidence that the Kaijo-Denki transducer design produces much less shadow effect than the conventional (right circular cylinder) shape.

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Tingting Qian, Craig C. Epifanio, and Fuqing Zhang

Abstract

The effect of an inland plateau on the tropical sea breeze is considered in terms of idealized numerical experiments, with a particular emphasis on offshore effects. The sea breeze is modeled as the response to an oscillating interior heat source over land. The parameter space for the calculations is defined by a nondimensional wind speed, a scaled plateau height, and the nondimensional heating amplitude.

The experiments show that the inland plateau tends to significantly strengthen the land-breeze part of the circulation, as compared to the case without terrain. The strengthening of the land breeze is tied to blocking of the sea-breeze density current during the warm phase of the cycle. The blocked sea breeze produces a pool of relatively cold, stagnant air at the base of the plateau, which in turn produces a stronger land-breeze density current the following morning. Experiments show that the strength of the land breeze increases with the terrain height, at least for moderate values of the height. For very large terrain, the sea breeze is apparently blocked entirely, and further increases in terrain height lead to only small changes in land-breeze intensity and propagation.

Details of the dynamics are described in terms of the transition from linear to nonlinear heating amplitudes, as well as for cases with and without background winds. The results show that for the present experiments, significant offshore effects are tied to nonlinear frontal propagation, as opposed to quasi-linear wave features.

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Rachel C. Zelinsky, Chidong Zhang, and Chuntao Liu

Abstract

Understanding convective initiation of the Madden–Julian oscillation (MJO) remains an unmet challenge. MJO initiation has been perceived as a process starting from a convectively suppressed large-scale condition with gradual growth of shallow convection to congestus and to deep convective and stratiform systems that cover a large-scale area. During the DYNAMO field campaign over the Indian Ocean, MJO initiation was observed to start from an existing intertropical convergence zone (ITCZ) south of the equator. This raises a question of what possible role the ITCZ may play in convective initiation of the MJO. This study addresses this question through analysis of satellite observations of precipitation and a global reanalysis product. By setting several criteria, MJO and ITCZ events were objectively identified and grouped according to whether MJO initiation was immediately preceded by an ITCZ. The results demonstrate that an ITCZ is neither a necessary nor sufficient condition for convective initiation of the MJO. Nonetheless, evolution of the large-scale circulation, moisture, and convective characteristics during MJO initiation can be different with and without a preexisting ITCZ. Convective growth begins gradually before and during MJO initiation when there is a preexisting ITCZ whereas it is abrupt and slightly delayed without a preexisting ITCZ. Such differences are presumably related to the existing large-scale moist condition of the ITCZ. The results from this study suggest that there are multiple mechanisms for convective initiation of the MJO, which should be considered in theoretical understanding of the MJO.

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R. Krishnan, C. Zhang, and M. Sugi

Abstract

In this paper the authors present results of diagnostic analysis of observations and complementary experiments with a simple numerical model that enable them to synthesize the morphology and dynamics of “breaks” in the Indian summer monsoon (ISM). Almost one week ahead of the onset of a break spell over India, a monotonically decreasing trend in convective activity is found to occur over the Bay of Bengal in response to a steady eastward spreading of dry convectively stable anomalies from the equatorial Indian Ocean. A major intensification of the convectively stable anomalies over the Bay of Bengal is seen about 2–3 days prior to commencement of a monsoon break. Both observations and modeling experiments reveal that rapid northwest propagating Rossby waves are triggered in response to such a large strengthening of the convectively stable anomalies. It is shown that an abrupt movement of anomalous Rossby waves from the Bay of Bengal into northwest and central India marks the initiation of a break monsoon spell. Typically the Rossby waves are found to traverse from the central Bay of Bengal to northwest India in about 2–3 days’ time. With the establishment of a break phase, the eastward spreading low-latitude anomaly decouples from the rapid northwest propagating anomaly. This decoupling effect paves the way for the emergence of a convectively unstable anomaly over the equatorial Indian Ocean. It is proposed that the dynamics of the rapid northwest propagating anomalous Rossby waves from the central Bay of Bengal toward northwest India and decoupling of the eastward propagating anomaly are two extremely vital elements that determine the transition from an above normal phase to a break phase of the ISM and also help maintain the mutual competition between convection over the Indian subcontinent and that over the equatorial Indian Ocean. Through modeling experiments it is demonstrated that low-latitude Rossby wave dynamics in the presence of a monsoon basic flow, which is driven by a steady north–south differential heating, is a primary physical mechanism that controls the so-called monsoon breaks.

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C-P. Chang, Yongsheng Zhang, and Tim Li

Abstract

The relationship between the interannual variations of the East Asian summer monsoon and that of the tropical SST shows considerable variations. In this study, rainfall in the southeastern coastal area of China (SEC) during 1951–96 is used to composite the tropical SST, 850-hPa wind, and 500-hPa height. The results relative to the May–June rainfall, which represents most of the SEC summer monsoon rainfall, are compared to the Yangtze River Valley (YRV) rainfall composites. It is shown that strong interdecadal changes in the Pacific may account for the observed variations in the meridional structure of the monsoon–SST relationship. The western Pacific 500-hPa subtropical ridge, which is influenced by the equatorial eastern Pacific SST, is crucial to these variations.

During 1951–77 the SEC wet phase is produced by an anomalous anticyclone in the northern South China Sea, which tends to make the monsoon pre-Mei-yu and Mei-yu fronts quasi-stationary in the general area of both SEC and YRV, and also helps to warm the SST in the northern South China Sea. In this case the monsoon rainfalls in the two regions are in phase.

During 1978–96 the mean equatorial eastern Pacific SST is higher, leading to a stronger and more expansive mean western Pacific subtropical ridge. Its proximity to the SEC region causes the latter to experience a strong interdecadal change, with less mean rainfall than 1951–77. Within the 1978–96 period, the anomalous anticyclone sustaining the YRV wet phase is situated near SEC, suppressing the SEC rainfall. Therefore the SEC and YRV rainfalls become out of phase.

The SEC wet phase in 1978–96 depends on an anomalous 850-hPa cyclone in the East China Sea. This anomalous cyclone, which transports moist air onshore from the east resulting in maximum moisture convergence in SEC, develops when the western Pacific subtropical ridge is weak and displaced equatorward. The flow is more baroclinic and the monsoon fronts are active in the southeast coastal area. In this case the SEC and YRV rainfalls are uncorrelated.

The July and August SEC wet phases show opposite characteristics. The wet July phase depends on anomalous 850-hPa cyclonic circulation in the northern South China Sea (and the East China Sea during 1951–77), which requires a retreat of the western edge of the western Pacific subtropical ridge. The anomalous South China Sea cyclone may be due to more frequent tropical cyclone activity. This is in contrast to the wet August phase, which is associated with anomalous anticyclones in the northern South China Sea and a greater westward extension of the subtropical ridge.

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Tingting Qian, Craig C. Epifanio, and Fuqing Zhang

Abstract

The equatorial coastal circulation is modeled in terms of the linear wave response to a diurnally oscillating heat source gradient in a background wind. A diurnal scaling shows that the solution depends on two parameters: a nondimensional coastal width L and a nondimensional wind speed U. The solutions are interpreted by comparing to the U = 0 theory of Rotunno.

For U ≠ 0 the Fourier integral solution consists of three distinct wave branches. Two of these branches correspond to the prior no-wind solution of Rotunno, except with Doppler shifting and associated wave dispersion. The third branch exists only for U ≠ 0 and is shown to be broadly similar to flow past a steady heat source or a topographic obstacle. The relative importance of this third branch is determined largely by the parameter combination U/L. For sufficiently large U/L the third branch becomes the dominant part of the solution.

The spatial structures of the three branches are described in terms of group velocity arguments combined with a desingularized quadrature method.

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Y-C. Zhang and W. B. Rossow

Abstract

The annual-mean meridional energy transport in the atmosphere–ocean system (total transport) is estimated using 4-yr mean net radiative fluxes at the top of the atmosphere (TOA) calculated from the International Satellite Cloud Climatology Project cloud datasets. In addition, the net atmospheric and surface radiative fluxes are calculated. When supplemented by a climatology of the surface latent and sensible heat fluxes, these radiative fluxes are used to derive the separate atmospheric and oceanic energy transports using a surface and planetary energy-balance method. Most previous results are based on direct calculations of the atmospheric energy transport from in situ measurements of horizontal wind velocity, temperature, and humidity in the atmosphere and on inference of oceanic heat transports as the difference between the atmospheric transports and the total energy transport (the planetary energy-balance method). Total, atmospheric, and oceanic energy transports from this study are in good agreement with more recent results (within mutual uncertainties). A detailed assessment is made of the uncertainties in the atmospheric and ocean energy transports that arise from uncertainties in the TOA and surface energy fluxes: the largest uncertainties are associated with the surface radiative and latent heat fluxes. Since the errors in the present method are from different sources and have different geographic distributions, the results of this study complement previous estimates of the atmospheric and oceanic energy transports. Assessment of error sources also suggests that improvement of this type of result is more likely in the near future than for the other methods. Because the radiative fluxes are calculated from physical quantities, the authors can characterize the mean effects of clouds on the atmospheric and oceanic energy transports: 1) cloud effects on the TOA radiation budget reduce hemispheric differences introduced by hemispheric differences of surface properties, 2) the cloud effects on the atmospheric and surface radiation budgets induce hemispheric differences in the heating/cooling of the atmosphere and ocean that require cross-equatorial transports in opposite directions by the atmosphere and ocean, and 3) all other factors held constant, clouds tend to reduce oceanic energy transports and increase atmospheric energy transports.

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