Search Results

You are looking at 1 - 10 of 69 items for

  • Author or Editor: Mingfang Ting x
  • Refine by Access: All Content x
Clear All Modify Search
Mingfang Ting

Abstract

The atmospheric response to tropical heating is examined using both the linear, multilevel baroclinic model with an imposed tropical heat source, and the one-level barotropic model with a tropical divergence forcing. The divergent component of the response in the baroclinic model is characterized by a tropical divergence confined to the heated region, plus convergence and divergence centers away from the tropical heated region at the outflow level. The rotational component of the response is depicted by a local baroclinic response in the Tropics and a remote equivalent barotropic wave train in the extratropics.

The barotropic model responses to a fixed tropical divergence are highly sensitive to the strength of the zonal mena zonal flow at different vertical levels in the upper troposphere. The sensitivity is induced by the dependence of the propagation speed of the stationary Rossby wave rays on the strength of the zonal mean zonal flow. The barotropic response to a tropical divergence when linearized about the zonal mean state at the outflow level differs significantly from the equivalent barotropic wave train in the baroclinic model. However, when the barotropic model is linearized about the zonal mean flow at the equivalent barotropic level, around 350 mb in winter and 500 mb in summer, its response to tropical divergence forcing is very similar to the baroclinic model result. The similarity confirms that the nature of the remote atmospheric response is indeed equivalent barotropic, but it is important to apply the barotropic model at the appropriate upper-tropospheric level. The barotropic Rossby wave energy dispersion can be applied to the baroclinic atmosphere when the equivalent barotropic level is chosen.

Full access
Mingfang Ting

Abstract

The maintenance of northern summer stationary waves is studied with data from a 15-year integration of the general circulation model (GCM) experiment performed at the Geophysical Fluid Dynamics Laboratory. The model has relatively high resolution (rhomboidal 30 wavenumbers, 9 vertical levels) and simulates the summertime stationary waves reasonably well.

A steady, linear, baroclinic model is used to understand the various forcing mechanisms for the northern summer stationary waves. The linear model response to global diabatic heating is found to play a dominant role in maintaining the summertime stationary waves in the GCM, especially in the subtropics. This response to diabatic heating shows a baroclinic structure in the vertical with a node at about σ = 0.5. On the other hand, stationary nonlinear interaction terms are found to be largely responsible for the extratropical, equivalent barotropic stationary wave features. It is hypothesized that this nonlinear interaction is a result of the thermally induced stationary waves interacting with the local orography. The direct linear response to orography is found to be rather insignificant, however. Transient vorticity and heat fluxes also tend to play a negligible role in explaining the summer stationary wave patterns.

Further decomposition of the linear model response to global diabatic heating indicates that the response to the Indian monsoon and the western Pacific heat source is of primary importance in determining the global stationary wave pattern. This large heat source not only determines the stationary flow features locally, but also remotely controls the flow structure over the whole Pacific, North America, and the Atlantic region. Thus, variabilities in the Indian monsoon and the western Pacific heating may exert a strong influence on the global climate variability.

Full access
Mingfang Ting

Abstract

The atmospheric stationary wave response to a midlatitude sea surface temperature (SST) anomaly is examined with an idealized general circulation model (GCM) as well as steady linear model, in a similar way as Ting and Held, for a tropical SST anomaly. The control climate of the GCM is zonally symmetric; this symmetric climate is then perturbed by a monopole SST anomaly centered at 40°N.

Two experiments, with SST anomalies of opposite sign, have been conducted. The stationary response is roughly linear in the sign of the SST anomaly, despite the fact that precipitation shows strong nonlinearity. The linear model, which is an exact linearization of the GCM equations in use, when forced by anomalous heating and transients, reproduces the GCM's stationary response excellently. The low-level transient eddy heat fluxes act to damp the lower level temperature signal. When this damping effect is mimicked by a horizontal thermal diffusion in the linear model, the response to the diabatic heating alone gives a reasonably good simulation of the GCM's anomaly; the effect of the anomalous transient momentum fluxes is relatively small.

A crude latent heat parameterization scheme, using an evaporation anomaly that is proportional to the mean air–sea surface moisture difference and including the effects of mean moisture advection, is developed. When the perturbation mixing ratio is approximated by assuming fixed relative humidity and by linearizing the Clausius–Clapeyron equation, the linear model's response, utilizing this latent heat parameterization scheme, gives a useful fit to the GCM's anomalous flow.

Full access
Mingfang Ting and Hui Wang

Abstract

The year-to-year fluctuations in summertime precipitation over the U.S. Great Plains are examined in this study using data from 1950 to 1990. There are large interannual variabilities in precipitation amounts over the Great Plains during the period considered. A long-term trend in Great Plains precipitation from relatively wet conditions in the 1950s to relatively dry conditions in the 1980s is also identified. The spatial scale of the anomalous precipitation covers a large portion of the United States on seasonal mean timescales.

It is shown that the Great Plains precipitation fluctuations are significantly correlated with the tropical, as well as North Pacific, sea surface temperature (SST) variations. Two leading modes of covariation between Pacific SST and the U.S. precipitation are identified, with the first mode having spatial and temporal characteristics of the El Niño–La Niña SST variation, while the second mode is confined to the North Pacific and contains the decadal trend. The relationship of both the SST and the precipitation variation with the atmospheric circulation is established through 500-mb height, as well as the sea level pressure fields. A well-defined wave train over the Pacific and North American region is found to be associated with the two leading modes. A southward-shifted jet stream over the central United States brings more synoptic storms into the region and causes excessive precipitation during wet events. The tropical SST and the U.S. precipitation may be connected through the anomalous tropical convection and its effects on the circulation. The relation between North Pacific SST and the U.S. precipitation is consistent with a strong atmospheric forcing on the North Pacific SST at a 1-month lead. It is also hypothesized that North Pacific SST feeds back onto the circulation through an enhanced (reduced) Pacific jet due to the increase (decrease) of the meridional SST gradient during dry (wet) summers. This appears to be consistent with the enhanced convection along the Pacific storm track and the intensified Pacific jet stream in the two recent dry summers (1983 and 1988).

Full access
Xingwen Jiang and Mingfang Ting

Abstract

The Tibetan Plateau (TP) has long been regarded as a key driver for the formation and variations of the Indian summer monsoon (ISM). Recent studies, however, have indicated that the ISM also exerts a considerable impact on rainfall variations in the TP, suggesting that the ISM and the TP should be considered as an interactive system. From this perspective, the covariability of the July–August mean rainfall across the Indian subcontinent (IS) and the TP is investigated. It is found that the interannual variation of IS and TP rainfall exhibits a dipole pattern in which rainfall in the central and northern IS tends to be out of phase with that in the southeastern TP. This dipole pattern is associated with significant anomalies in rainfall, atmospheric circulation, and water vapor transport over the Asian continent and nearby oceans. Rainfall anomalies and the associated latent heating in the central and northern IS tend to induce changes in regional circulation that suppress rainfall in the southeastern TP and vice versa. Furthermore, the sea surface temperature anomalies in the tropical southeastern Indian Ocean can trigger the dipole rainfall pattern by suppressing convection over the central IS and the northern Bay of Bengal, which further induces anomalous anticyclonic circulation to the south of TP that favors more rainfall in the southeastern TP by transporting more water vapor to the region. The dipole pattern is also linked to the Silk Road wave train via its link to rainfall over the northwestern IS.

Full access
Yujia You and Mingfang Ting

Abstract

Understanding the physical mechanisms behind the secular trends of summer rainfall extremes over the heavily populated Southeast and East Asian monsoon regions is not only of scientific importance but also of considerable socioeconomic implications. In this study, the relevance of the excessive-rain-producing low pressure systems (LPSs) to extreme rainfall is quantified. Using an objective feature-tracking algorithm, the synoptic-scale LPSs are identified and tracked in the 40-yr ECMWF interim reanalysis. The region experiences approximately 16 terrestrial and 18 marine LPSs each summer. The terrestrial LPSs form near the downwind side of the Tibetan Plateau and travel northeastward toward jet latitudes. The marine LPSs form over the western North Pacific Ocean and migrate along the western periphery of the subtropical high. While both types of LPSs account for a large portion of upper-tail rainfall, the terrestrial LPSs predominantly impact the extreme rainfall over inland areas, and the marine LPSs primarily affect the coastal regions where they frequently make landfall. The historical extreme rainfall trend during 1979–2018 aligns with the changes in LPS tracks. The decreasing number of northeastward-moving terrestrial LPSs leads to an extreme rainfall dipole with negative trends in north-central China and positive trends in southern China, while the increasing number of northward-recurving marine LPSs enhances the extreme rainfall in the eastern China coast but suppresses it over the South China Sea. These trends are driven dynamically by the weakening of the monsoonal southwesterlies and the eastward retreat of the subtropical high, which might be attributable to anthropogenic forcings.

Full access
Mingfang Ting and Shiling Peng

Abstract

The differences between early and middle winter atmospheric responses to the sea surface temperature anomalies (SSTA) in the northwest Atlantic are examined using a linear baroclinic model. Using a global spectral model, Peng et al. found a positive height anomaly in the perpetual November and a negative height anomaly in the perpetual January experiments in response to a warm SSTA over the northwest Atlantic. These height anomalies are found to be associated with the reduced Atlantic jet stream in November and enhanced jet in January. Linear model diagnostics suggest that the difference in jet stream response may induce anomalous storm track eddy vorticity fluxes, which in turn maintain the different atmospheric responses under the early and middle winter conditions.

The different jet stream responses in November and January are further traced to the initial atmospheric response to a local heat source accompanying the warm SSTA. Under both the January and November conditions, the atmospheric response is dominated by an anticyclone downstream from the heat source at the jet stream level. The anticyclone is shifted northward in November, however, from its position in January. Combined with a northeast–southwest tilted jet stream in January and an east–west oriented, southward shifted November jet stream in the Atlantic, the above difference in the atmospheric responses to the initial heat source may lead to a reduced jet in November and an enhanced jet in January. The feedback between the anomalous storm track eddy vorticity fluxes and the anomaly flow induced by the heat source may further enhance the different equilibrium responses in the global spectral model.

Full access
Mingfang Ting and Linhai Yu

Abstract

The linear, steady-state, baroclinic model response to a tropical heating superimposed on a three-dimensional basic state is examined in this study. The emphasis is on the relevance of the linear model solution as compared to a fully nonlinear baroclinic model. The direct response to heating in the fully nonlinear, time-dependent model is obtained as the day-30 model response, following the Jin and Hoskins approach. When a 15-day linear damping is included in addition to Rayleigh friction, Newtonian cooling, and a scale-selective biharmonic diffusion, the comparison of the linear and the nonlinear model responses to a 2°C/day tropical heating reveals a striking similarity in both the spatial distribution and amplitude. Thus nonlinearity appears to be a secondary effect and may be crudely represented by the 15-day linear damping, and the linear steady-state model can be a useful tool in diagnostic studies.

Both the linear and the nonlinear model responses show an insensitivity to heating longitudes, especially when heating is located between 30°E and 120°W. This insensitivity is characterized by a geographically fixed response that consists of a streamfunction center over the central North Pacific and a weak wave train over the Pacific–North American region. The spatial structure of the preferred pattern does not depend on the dissipation or the amplitude of the tropical heating in the nonlinear model. The geographically fixed response is found to be prominent in the Northern Hemisphere for both the northern winter and summer climatological basic states.

Full access
Hailan Wang and Mingfang Ting

Abstract

The maintenance mechanisms of the climatological stationary waves and their seasonal cycle are investigated with a linear stationary wave model and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data from 1985 to 1993. The stationary wave model is linearized about the zonal-mean flow and subjected to the zonally asymmetric stationary wave forcings. It has rhomboidal wavenumber 30 truncation and 14 vertical sigma levels. The forcings for the linear model include diabatic heating, orography, stationary nonlinearity, and transient vorticity and heat flux convergences. The NCEP–NCAR reanalysis provides a high quality global dataset for this study.

When the linear model is subjected to all forcings, it reproduces reasonably well the climatological stationary wave seasonal cycle. The linear stationary wave theory is quantitatively valid at the upper-tropospheric levels for all months and the lower-tropospheric levels for the northern summer months (with pattern correlation greater than 0.8). At the middle- and lower-tropospheric levels for most of the months, the stationary wave theory is qualitatively valid (with pattern correlation greater than 0.5). The effect and relative importance of each individual forcing mechanism in maintaining the stationary waves and their seasonal cycle are determined by the linear model. Within the linear model framework, the global diabatic heating is found to be the most dominant forcing mechanism for the climatological stationary waves throughout the seasonal cycle. Subsequently, the seasonal cycle of the stationary waves is largely caused by the seasonal fluctuations of the atmospheric heating field. By comparison, the linear effect of orography is of less importance in both the Tropics and the extratropics. The effect of stationary nonlinearity is to modify the spatial structure of the stationary waves, particularly over extratropical North America. Comparatively, transient forcing has little contribution. By separating the tropical and the extratropical heatings in the linear model, it is found that the local thermal forcing has the dominant contribution to the local stationary wave seasonal cycle.

The relative contribution of the seasonally varying zonal-mean basic state and the seasonally varying forcing fields is also examined using the linear model. The seasonally varying zonal-mean basic state can account for the zonal-mean amplitude fluctuation of the stationary waves in the Tropics, as well as the seasonal change of the stationary wave spatial structure from September to May. It fails to capture the amplitude fluctuation of the Northern Hemisphere extratropical stationary waves and the northern summer stationary wave spatial structure. On the other hand, the effect of the seasonally varying forcing accounts largely for the zonal-mean amplitude fluctuation of the stationary waves in the Northern Hemisphere extratropics, as well as the transition to the northern summer stationary wave regime.

Full access
Timothy DelSole and Mingfang Ting
Open access