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.

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.

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.

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.

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).

Abstract

The variability of winter average U.S. precipitation displays strong geographical dependence with large variability in the southeastern and northwestern United States. The covariance of the U.S. winter mean precipitation with Pacific sea surface temperature (SST) is examined in this study using the singular value decomposition (SVD) method. The first SVD mode indicates the U.S. precipitation pattern that is associated with the tropical El Niño/La Niña SST variation, while the second and third SVD modes relate the precipitation variability in the Pacific Northwest and southeast that is associated with the North Pacific SST variation. About 45% of the U.S. precipitation variabilities is related to the Pacific SST anomalies, among which, 35% is related to the North Pacific SST and 10% is related to the tropical Pacific SST. Each SVD precipitation pattern is associated with well-organized 500-mb height and zonal mean zonal wind anomalies. It is shown that the North Pacific SST anomalies associated with the U.S. precipitation are primarily driven by extratropical atmospheric circulation anomalies.

Abstract

Summer precipitation over the central United States depends strongly on the strength of the Great Plains low-level jet (LLJ). The Geophysical Fluid Dynamics Laboratory’s new generation of the atmospheric general circulation model (GCM) and the linear and nonlinear stationary wave models are used in this study to examine the role of North American topography in maintaining the Great Plains summer mean LLJ and precipitation. Atmospheric GCM experiments were first performed with and without the North American topography and with prescribed climatological sea surface temperatures. Results show that the Great Plains LLJ disappears completely in the experiment when the North American topography is removed, while the summer seasonal mean LLJ is well simulated in the experiment with full earth topography. In the absence of the North American topography, the summer precipitation is significantly reduced over the central United States and increased along the Gulf States and northeast Mexico.

Linear and nonlinear stationary wave models are used to determine the physical mechanisms through which the North American topography maintains the Great Plains time mean LLJ. Possible mechanisms include the physical blocking of the topography and the induced flow over and around the mountains, the thermal effect due to the elevation of the topography, and the transient thermal and vorticity forcing due to the modification of transient eddy activities in the presence of the topography. The linear and nonlinear model results indicate that the dominant mechanism for maintaining the time mean Great Plains LLJ is through the nonlinear effect of the trade wind along the southern flank of the North Atlantic subtropical high encountering the east slope of the Sierra Oriental and causing the flow to turn northward. As the flow turns north along the east slope of the North American topography, it obtains anticyclonic shear vorticity and thus the LLJ. The effect of the thermal forcing is negligible, while the effect of transient forcing is only important in extending the jet farther northward and eastward. The results suggest that variations in the strength of the North Atlantic subtropical anticyclone and the associated trade wind over the Caribbean Sea and the Gulf of Mexico may be important for understanding the interannual variation of the Great Plains LLJ and U.S. precipitation.

Abstract

Intraseasonal variability of rainfall over the Indian subcontinent (IS) and the Tibetan Plateau (TP) has been discussed widely but often separately. In this study, we investigate the covariability of rainfall across the IS and the TP on intraseasonal time scales and its impact on interannual variability of regional rainfall. The most dominant mode of rainfall intraseasonal variability across the region features a dipole pattern with significant out-of-phase rainfall anomalies between the southeastern TP and the central and northern IS. This dipole rainfall pattern is associated with intraseasonal oscillations (ISOs) of 10–20 days and 30–60 days, especially the latter. An active spell of rainfall in the central and northern IS (southeastern TP) is associated with the strengthening (northward shift) of water vapor transport of the Indian summer monsoon, resulting in more water vapor entering into the central and northern IS (southeastern TP) and thus more rainfall. The 10–20-day ISO of the dipole rainfall pattern is caused by the 10–20-day atmospheric ISO in both the tropics and the extratropics, whereas the 30–60-day ISO of the dipole rainfall pattern is only associated with atmospheric ISO in the tropics. The dipole rainfall pattern resembles the most dominant mode of interannual variability of July–August mean rainfall. The 30–60-day ISO of the dipole rainfall pattern has an important contribution to the dipole pattern of July–August mean rainfall anomalies on an interannual time scale due to the different frequencies of occurrence of the active and break phases.

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.