Abstract

Two years of regional analyses based on the Eta Data Assimilation System (EDAS) are used to examine the mesoscale features of the moisture budgets of the Mississippi River basin and its subbasins. Despite the short period, basic aspects of the regional-scale seasonal means, annual cycle, and even diurnal cycle of the atmospheric water cycle are represented. The ability of the Eta Model to resolve mesoscale features of the low-level circulation is an important factor in improving the estimates of moisture flux convergence at regional scales. It appears that the internal consistency of moisture budgets estimated from EDAS analyses for basins of nearly 5 × 10⁵ km² is comparable to that computed from radiosondes for basins of about 2 × 10⁶ km² or larger. In other terms, the spatial scale of basins where consistent moisture budgets can be estimated appears to be reduced by almost one order of magnitude.

Area-averaged evaporation estimates (computed as residuals of the moisture budget equation) for basins of about 5 × 10⁵ km² range from near zero during winter in the northern subbasins to about 5–6 mm day⁻¹ during summer in the southern subbasin. It is suggested that the slightly negative estimates of evaporation in the northern subbasins during winter may partly result from an underestimation of observed precipitation due to the combined effect of wind and solid precipitation. No attempt was made at computing the model’s moisture budget, since changes in the surface parameterizations prevented having a period long enough to achieve stable results. Broad aspects of the diurnal cycle during summer were also examined through nighttime–daytime differences. Consistent with other studies over the central United States, results show that the nighttime development of moisture flux convergence is associated with an increase of intensity of the low-level jet. Interestingly, the nighttime convergence of moisture flux is offset by divergence during daytime and, as a result, overall moisture flux divergence is observed during summer.

A comparative analysis was made of the observed and model forecast precipitation to assess the model’s overall performance during the 2-yr period. It was found that the spatial patterns, intensity, and even the broad aspects of the summertime diurnal cycle of the model forecast precipitation are similar to those observed. Nevertheless, some deficiencies exist: a dry bias was obtained over the central United States during summer and winter; during summer, the southeastern United States had an excess of precipitation similar to that observed in the National Centers for Environmental Prediction global model; during winter, forecast precipitation in the northwestern United States appears to have biases in location and intensity, which can be related to the large-scale component of the model precipitation.

Abstract

The physical mechanisms associated with precipitation in southeastern South America during spring are investigated using short-term integrations with the regional mesoscale Eta Model. An evaluation of the model’s performance using in situ measurements of precipitation as well as satellite estimates reveals that the model performed satisfactorily in the subtropics and extratropics. Deficiencies in tropical Brazil are partly related to the model’s convective adjustment scheme and possibly to surface parameterizations as well. The model forecasts reproduce all observed centers of precipitation south of about 20°S, although in some cases the magnitude is somewhat smaller. Of particular relevance for this study is the finding that spatial correlations between the model forecast and observed precipitation over Cuenca del Plata are almost as high as those obtained for the Mississippi River basin using forecasts of the National Centers for Environmental Prediction operational Eta Model. Cuenca del Plata is a basin in southeastern South America that is the water resource for a largely populated area and is well known for its agricultural production and other factors that sustain the region’s economies.

An important component of the circulation reproduced in the simulations is the low-level jet east of the Andes that feeds moisture from the Amazon basin to higher latitudes. It has a diurnal cycle with a nighttime maximum that favors increased moisture flux convergence in southeastern South America. This convergence, in turn, is associated with generalized nighttime ascent and precipitation. The results are consistent with previous observational studies that show a nighttime maximum of precipitation over the region. A second regime of precipitation is found toward the eastern coast, where maximum daytime precipitation appears to be associated with a convectively unstable atmosphere, with convection being triggered by a sea–land breeze enhanced by the topography of southern Brazil. These diurnal regimes of precipitation have a significant impact in the atmospheric water cycle in Cuenca del Plata.

The basin-averaged vertically integrated moisture flux convergence is about 4 mm day⁻¹ and almost doubles the spring values for the Mississippi River basin. The large values may be related to the particular conditions of the period under analysis and the stronger low-level jet. The results reported here provide a preliminary description of the basin-averaged moisture flux convergence and its diurnal variability, but basin-averaged precipitation is still the component that needs to be improved. It is assumed that a blend of observations and high-resolution satellite estimates will be needed to complete the description of the atmospheric water cycle.

Abstract

European Centre for Medium-range Weather Forecasts analyses during June 1985 are used to characterize the flow in the South America sector during a typical blocking episode. Numerical experiments are performed using a hemispheric shallow-water model to test whether such blocking episodes can be a result of local resonance between forced Rossby waves generated by the Andes Mountains and by an upstream forcing.

It appears that while blocks generated in the Atlantic Ocean may respond to this mechanism from the beginning, the more frequency ones that develop from a ridge that advances from the Pacific Ocean may also benefit from it in the intensification and maintenance stages.

Abstract

This study employs observations and the model simulations from the U.S. Climate Variability and Predictability (CLIVAR) Drought Working Group to examine extreme precipitation events like drought and wet spells that persist more than one season over South America. These events tend to persist over northeastern Brazil, the Guianas, and the west coast of Colombia, Ecuador, and Peru. They are least likely to persist over southeastern South America, which includes Uruguay, southern Brazil, and northeastern Argentina.

The U.S. CLIVAR simulations, particularly those of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3.5 (CAM3.5), capture satisfactorily the impact of the El Niño–Southern Oscillation (ENSO) and the north tropical Atlantic (NTA) sea surface temperature anomaly (SSTA) signals on persistent extreme events and reproduce the mechanisms inducing the teleconnection patterns. The cold (warm) ENSO favors wetness (dryness) over Venezuela, Colombia, and northeastern Brazil and dryness (wet spells) over southeastern South America and southern Argentina. The NTA SSTAs alone tend to have a more local impact affecting mostly over northern South America in March–May.

The simulations show that when the two modes (ENSO and NTA) act in concert, the effects may become noticeable in different and remote areas of the continent, as they shift the probability of drought and persistent wet spells over different regions of South America. The impact is strong when the ENSO and the NTA are in opposite phases. For the cold (warm) Pacific and warm (cold) Atlantic, droughts (persistent wet spells) are intensified over southeastern South America, while persistent wet spells (droughts) are favored over the northern part of the continent. The changes in the patterns are regional and not as intense when both oceans are warm (or cold).

Abstract

The mechanics of the interaction between tropical beating [estimated from outgoing longwave radiation (OLR)] and Southern Hemisphere (SH) subtropical and extratropical circulations on intraseasonal time scales are discussed. Base points are selected from teleconnectivity and teleconnection maps between OLR and zonal wind, heights, and meridional component of the divergent wind. Then, composites are formed for pentads with OLR anomalies at the base point greater in magnitude than one standard deviation.

Enhanced convection over Indonesia is found to be associated with increases of both the southward component of the meridional divergent wind and of the westerly, zonal wind to the south of the heating region during the SH summer. The increased westerly wind gradients, resulting to a certain extent from strengthened northerly flow, together with increased values of the southward component of the divergent wind, lead to an enhancement of the Rossby wave source in the vorticity equation in the vicinity of Australia. Streamfunction anomalies indicate that a wave train evolves from this region, following the typical ray path expected from linear theory.

Tropical-extratropical connections are less pronounced during SH winter than during summer, though an increase of westerly winds in the SH is found associated with enhanced convective activity in the Northern Hemisphere. The increase of the zonal wind during winter is again explained by meridional overturnings that emanate from the heating regions. Isentropic trajectories are used to show that the zonal accelerations caused by the poleward motion at upper levels are in agreement with observed values. The enhancement of convective activity is also related to a southward increase of the meridional component of the divergent wind that maximizes near the equator. However, since the latitudes of maximum southward component of the meridional divergent wind differ from those with maximum changes in the gradient of absolute vorticity, no increase of the Rossby wave source or excitation of Rossby waves due to tropical heating is found during this season.

Abstract

The structure and evolution of the fluctuations in synoptic scales in the Southern Hemisphere (SH) during winter are discussed using six years of European Centre for Medium-Range Weather Forecasts analyses.

It is shown that patterns from unfiltered meridional wind series in the SH display all the features needed to represent the synoptic-scale waves. Typical periods and wavelengths are similar to those observed in the Northern Hemisphere (4 days, 4000 km), although over the Pacific Ocean they can be as high as 7–8 days and 4700 km, respectively. As in the Northern Hemisphere, tilts are not geographically fixed but change with the stage of the evolution of the wave. The phase speed of the waves agrees with the low-level winds in extensive areas of the middle latitudes and ranges from 12 m s ⁻¹ in the Indian Ocean to 6 m s⁻¹ in the Pacific Ocean. The estimated group velocities achieve maximum values of about 38 m s ⁻¹, also in the Indian Ocean, and agree with the upper-level maximum winds, in accord with reported model results for the leading fringe of the wave packets.

The wave packets show a decay of upstream centers as new ones grow downstream, suggesting that down-stream development contributes to the evolution of the synoptic-scale waves in the SH storm track. This process is observed both in the subpolar and subtropical jets, but the sequence of centers developing downstream is more coherent in the latter, probably due to the weaker baroclinicity.

Abstract

A dynamically oriented description of the North American summer monsoon system, which encompasses the Mexican monsoon and the associated large-scale circulation over the continental United States, is provided by developing an evolution climatology of the precipitation, tropospheric circulation, moisture fluxes, diabatic heating, convective environment, and the adjoining basin SSTs.

A distinguishing aspect of this study is the amount of independent data analyzed, such as the newly available European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses, the National Centers for Environmental Prediction (NCEP) reanalyses, both satellite-derived and station data–based precipitation estimates, and the heating diagnosed from both reanalyses. This also provides a preliminary evaluation and comparison of the newly available NCEP and ECMWF reanalyses at the regional level, including the model-generated precipitation and heating distributions. The principal findings are the following.

• The accompaniment of the Mexican monsoon onset by decreased precipitation to the east is shown to be a robust climatological feature. This striking linkage is also evident in the associated tropospheric circulation and, notably, in the upper-level heating fields. The climatological phasing of the precipitation between the two areas is coherent even at the pentad timescale.

• While the Mexican monsoon onset is closely associated with thermodynamic favorability, the linkage to the central United States, as reflected in the vertical velocity and the low-level height fields, appears to be consistent with several possible forcings: the monsoon deep heating, the elevated heating of the North American cordillera and plateau, and orographic forcing associated with the seasonal movement of the easterlies encroaching on the North American cordillera.

• Although both reanalyses yield a tropical-type deep tropospheric heating distribution in the Mexican monsoon region and, therefore, a potentially prominent role for the monsoon in the regional circulation, the considerable differences in the diagnosed heating vertical structure, thermodynamic balance, and the overall heating magnitude between the two reanalyses, and even between the NCEP reanalysis-consistent heating and the NCEP model-produced heating, suggest potentially significant differences in the implied dynamics of the North American monsoon system.

Abstract

The circulation features associated with intense precipitation events over the La Plata Basin (LPB) during the austral summers of 2001/02 and 2002/03 are investigated using the Eta Model runs generated at the University of Maryland. Based on the main mode of variability over LPB, two regions were selected: (i) the region of Brazil that is at the core of the South American summer monsoon system (SAMS) and (ii) the central region of LPB in southeastern South America (SESA). First, a comparison between the 24-h total precipitation in the Eta Model and the 24-h observed precipitation was made. Results show that the Eta Model captures well the temporal variability of precipitation events in both regions, although a positive bias is noticed over SAMS. Likewise, the model reproduces the distribution of precipitation rate over SESA, but not over SAMS. Nevertheless, the distribution of the moisture flux convergence intensity, which represents the dynamical forcing, is closer in shape to the observed precipitation distribution, suggesting that the model can be a useful tool in identifying the forcing for heavy precipitation events over both regions.

Composites of atmospheric and surface variables were constructed for intense precipitation events during austral summer over both regions. Intense rainfall over the central La Plata Basin (SESA) is linked to an amplified upper-tropospheric midlatitude wave pattern in which rainfall occurs just east of an enhanced cyclonic circulation. Accompanying this circulation pattern, an enhanced low-level jet (LLJ) transports warm, moist air from the Amazon toward the region, contributing to an increase in the thermal contrast over SESA. The combined patterns of thermal and dynamical variables suggest that large-scale systems, like frontal systems, are important in producing intense rainfall events. The SAMS region events have a similar upper-level structure as in SESA, but they are longer lived. In this case, the moisture fluxes are determined by an eastward shift of the LLJ, but also directly from the Amazon Basin to the north. As expected, precipitation events produce large increases of simulated runoff. The largest impact is on the SESA region, affecting the streamflow of the Paraná, Paraguay, and Uruguay, the three main rivers of the LPB.

Abstract

Weather forecasting and monitoring systems based on regional models are becoming increasingly relevant for decision support in agriculture and water management. This work evaluates the predictive and monitoring capabilities of a system based on WRF Model simulations at 15-km grid spacing over the La Plata basin (LPB) in southern South America, where agriculture and water resources are essential. The model’s skill up to a lead time of 7 days is evaluated with daily precipitation and 2-m temperature in situ observations for the 2-yr period from 1 August 2012 to 31 July 2014. Results show high prediction performance with 7-day lead time throughout the domain and particularly over LPB, where about 70% of rain and no-rain days are correctly predicted. Also, the probability of detection of rain days is above 80% in humid regions. Temperature observations and forecasts are highly correlated (r > 0.80) while mean absolute errors, even at the maximum lead time, remain below 2.7°C for minimum and mean temperatures and below 3.7°C for maximum temperatures. The usefulness of WRF products for hydroclimate monitoring was tested for an unprecedented drought in southern Brazil and for a slightly above normal precipitation season in northeastern Argentina. In both cases the model products reproduce the observed precipitation conditions with consistent impacts on soil moisture, evapotranspiration, and runoff. This evaluation validates the model’s usefulness for forecasting weather up to 1 week in advance and for monitoring climate conditions in real time. The scores suggest that the forecast lead time can be extended into a second week, while bias correction methods can reduce some of the systematic errors.

Abstract

The dynamical basis of intraseasonal oscillations of the Southern Hemisphere summer and winter seasons is studied with a combination of observed diagnostics and simplified prognostic models. High-frequency oscillations, zonal mean variations, and seasonal and interannual variabilities are removed from the six-year dataset in an effort to reduce the effect of high-frequency dynamical instabilities and long-period forced fluctuations. The diagnoses focus upon those processes that have most frequently been explained in terms of Rossby-wave propagation through atmospheres with variable refractive indices. It is useful to study both winter and summer seasons simultaneously because of the large changes in the seasonally averaged state and large consequent changes in atmospheric waveguides between these seasons. A nonlinear shallow-water model slowly relaxed toward the time-averaged winter and summer observed mean fields is used to describe the characteristics of wave propagation in a horizontally varying basic state. Perturbations are introduced in four different regions corresponding to points where observed atmospheric teleconnectivities are relatively large, and the signal propagation is analyzed using averaging procedures similar to those employed for the observational study. Furthermore, differences between stationary and nonstationary patterns are also discussed.

The four general regions selected for the observational study are Australia, New Zealand, South America, and the Atlantic Ocean. Differences from winter to summer are related to concomitant changes of the background latitudinal gradient of absolute vorticity. During winter and summer meridional propagation is toward the tropics. Winter wave patterns have mainly zonal paths and show a slow phase velocity on the order of 3 m s⁻¹, while during summer, patterns tend to be geographically fixed. During winter, regions of imaginary refractive index flank the subtropical and polar jet streams. These jet streams seem to act as waveguides for disturbances emanating from the southern Indian Ocean and western Australia, where two wave trains exist. Wave activity flux vectors suggest that these disturbances originate in the subtropical southern Indian Ocean and that equatorward propagation prevails at the exit region of the subpolar jet stream and over South America and the Atlantic Ocean. During summer, observed wave patterns tend to have a more meridional component, again in agreement with the background latitudinal gradient of absolute vorticity.