Abstract

Intraseasonal variation of convection over South America is examined using singular spectrum analysis (SSA). The dipole convection pattern with centers of action over the SACZ and the subtropical plains is modulated by modes of different timescales. Both oscillatory modes with periods of 36–40 days (mode 40) and 22–28 days (mode 22) influence convection over the SACZ with the faster mode (mode 22) leading the variability over the subtropical plains.

Mode 40 is related to the Madden–Julian oscillation. Outgoing longwave radiation anomalies (OLRA) propagate eastward from the western Pacific to the central Pacific with a period of 40–48 days. The 200-hPa streamfunction composites show a wavenumber 1 structure in the Tropics and a wave train propagating downstream from the convective area in the tropical Pacific. The development of the dipole pattern is also contributed by mode 22, which features the meridional propagation of OLRA over South America from midlatitudes to the Tropics. The streamfunction composites for mode 22 show a wave train extending from the central Pacific eastward to about 60°S and curving toward the northeast over South America.

When the SACZ is enhanced, these two modes become meridionally aligned locally. Such episodes are characterized by a wave train propagating northeastward from southern South America toward the Tropics. Composites based on the SSA decomposition indicate that the dominant periods of variability (22 and 40 days) cancel out over the Pacific region and reinforce each other over the SACZ for times close to onset of the enhanced SACZ episodes.

Abstract

The impact of the sea surface temperature (SST) anomalies on predictions in the extratropics has been studied by comparing circulation changes in general circulation model experiments generated with observed and climatological sea surface temperatures for warm and cold Southern Oscillation events. The small samples may be insufficient for drawing firm conclusions, but results suggest that the linkage between tropical and extratropical circulations in the model resembles observed relationships.

As the atmosphere responds to the warm (cold) tropical SSTs, the convection in the Pacific intensifies (diminishes). The enhanced (suppressed) convection in the tropics enhances (suppresses) the local Hadley circulation and changes the position and strength of the divergent outflow. This in turn changes the position, shape, and strength of the upper-level subtropical jet streams. After the jets move to their new positions, synoptic eddies organize themselves at the exit regions of the jets.

The response time for the upper-level streamfunction in the tropics is about 10 days, but the changes in the position of the subtropical jets occur after 15–20 days. The largest impact on predictions is located in the tropics and downstream in the Pacific-North America and the Pacific-South America regions.

Abstract

This study shows that the North American monsoon system’s (NAMS) strength, onset, and retreat over northwestern Mexico exhibit multidecadal variations during the period 1948–2009. Two dry regimes, associated with late onsets, early retreats, and weaker rainfall rates, occurred in 1948–70 and 1991–2005, whereas a strong regime, associated with early onsets, late retreats, and stronger rainfall rates, occurred in 1971–90. A recovery of the monsoon strength was observed after 2005. This multidecadal variation is linked to the sea surface temperature anomalies’ (SSTAs) variability, which is a combination of the Atlantic multidecadal oscillation (AMO) and the warming SST trends. These SST modes appear to cause an anomalous cyclonic circulation and enhanced rainfall over the southeastern United States and the Gulf of Mexico, which in turn increases the atmospheric stability over the monsoon region. However, these SST modes cannot fully explain the circulation and rainfall anomalies observed during the early-retreat monsoons. An expansion of the North Atlantic surface high (NASH) in recent decades also contributes to the anomalous circulation associated with the early retreats of the NAMS. A northwestward expansion of the NASH further enhances the anomalous cyclonic circulation and rainfall over the southeastern United States and the Gulf of Mexico. Its associated northwestward shift of the subtropical jets over the western United States enhances subsidence over the NAMS region. The combined effects of the AMO, the warming trends, and the NASH expansion on atmospheric circulation contribute to a stronger and more persistent earlier retreat during the recent dry regime (1991–2005), while the earlier dry regime (1948–70) appears to be only influenced by the positive phase of the AMO.

Abstract

Composites based on observations and model outputs from the Climate Variability and Predictability (CLIVAR) drought experiments were used to examine the impact of El Niño–Southern Oscillation (ENSO) and the Atlantic multidecadal oscillation (AMO) on drought over the United States. Because drought implies persistent dryness, the 6-month standardized precipitation index, standardized runoff index, and soil moisture anomalies are used to represent drought. The experiments were performed by forcing an AGCM with prescribed sea surface temperature anomalies (SSTAs) superimposed on the monthly mean SST climatology. Four model outputs from the NCEP Global Forecast System (GFS), NASA’s Seasonal-to-Interannual Prediction Project, version 1 (NSIPP1), GFDL’s global atmospheric model, version 2.1 (AM2.1), and the Lamont-Doherty Earth Observatory (LDEO)/NCAR Community Climate System Model, version 3 (CCM3) were analyzed in this study. Each run lasts from 36 to 51 yr.

The impact of ENSO on drought over the United States is concentrated over the Southwest, the Great Plains, and the lower Colorado River basin, with cold (warm) ENSO events favoring drought (wet spells). Over the East Coast and the Southeast, the impact of ENSO is small because the precipitation responses to ENSO are opposite in sign for winter and summer. For these areas, a prolonged ENSO does not always favor either drought or wet spells.

The direct influence of the AMO on drought is small. The major influence of the AMO is to modulate the impact of ENSO on drought. The influence is large when the SSTAs in the tropical Pacific and in the North Atlantic are opposite in phase. A cold (warm) event in a positive (negative) AMO phase amplifies the impact of the cold (warm) ENSO on drought. The ENSO influence on drought is much weaker when the SSTAs in the tropical Pacific and in the North Atlantic are in phase.

Abstract

Atmosphere–land–ocean coupled model simulations are examined to diagnose the ability of models to simulate drought and persistent wet spells over the United States. A total of seven models are selected for this study. They are three versions of the NCEP Climate Forecast System (CFS) coupled general circulation model (CGCM) with a T382, T126, and T62 horizontal resolution; GFDL Climate Model version 2.0 (CM2.0); GFDL CM2.1; Max Planck Institute (MPI) ECHAM5; and third climate configuration of the Met Office Unified Model (HadCM3) simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) experiments.

Over the United States, drought and persistent wet spells are more likely to occur over the western interior region, while extreme events are less likely to persist over the eastern United States and the West Coast. For meteorological drought, which is defined by precipitation (P) deficit, the east–west contrast is well simulated by the CFS T382 and the T126 models. The HadCM3 captures the pattern but not the magnitudes of the frequency of occurrence of persistent extreme events. For agricultural drought, which is defined by soil moisture (SM) deficit, the CFS T382, CFS T126, MPI ECHAM5, and HadCM3 models capture the east–west contrast.

The models that capture the west–east contrast also have a realistic P climatology and seasonal cycle. ENSO is the dominant mode that modulates P over the United States. A model needs to have the ENSO mode and capture the mean P responses to ENSO in order to simulate realistic drought. To simulate realistic agricultural drought, the model needs to capture the persistence of SM anomalies over the western region.

Abstract

Persistent wet and dry events over the central United States are examined during summer. Composites based on selected persistent wet and dry events reveal common atmospheric processes and circulation features. During summer, heavy precipitation in the central United States is accompanied by less precipitation to the south, in a band that extends from the tropical eastern Pacific through the Gulf of Mexico into the western Atlantic. Dry conditions also occur along the western coasts of Canada and Mexico during persistent wet episodes in the central United States. This rainfall pattern is supported by an inverse temperature–rainfall relationship over North America. During dry events, high pressure extends throughout a vertical column in a pattern that covers North America from 30° to 60°N. In contrast, during wet events, the high pressure is confined to the eastern half of North America, with low pressure prevailing in the western half. Increased northward meridional winds are found between this cyclonic–anticyclonic dipole, leading to increased moisture flux from the Gulf of Mexico at low levels.

A significant precursor to wet events is the enhancement of westerlies over the eastern Pacific and western North America from 30° to 40°N. Synoptic-scale eddies intensify prior to onset and accelerate this westerly flow as revealed by Eliassen–Palm flux diagnostics. One pentad before onset, rainfall begins in Texas, and the low level jet (LLJ) in the Great Plains strengthens. The intensified LLJ transports moisture into the central United States and the moisture convergence downwind from the LLJ maintains rainfall. For dry events, heating occurs in the tropical eastern Pacific associated with the northward shift of the ITCZ roughly one pentad prior to onset. The prevailing easterly flow over subtropical portions of North America is not conducive to moisture transport into the United States, and without the support of moisture influx from the Gulf of Mexico, dry conditions prevail.

Abstract

The tropical intraseasonal oscillation is examined using projections of 5-day-averaged wind and height data onto the normal modes of a primitive equation model linearized about a basic state at rest and prescribed global temperature. The dataset utilized in this study is from the Dynamical Extended-Range Forecast (DERF) Experiment, conducted from January 1986 through March 1987.

Examination of zonal-wind anomalies in the tropics shows that the forecast model is able to predict propagation of intraseasonal variations more accurately for slow propagation rates. The forecasted amplitude is generally weaker than the analyzed amplitude. Analyzed kinetic energy and error fields exhibit similar horizontal scales for external and internal modes. External Rossby-mode components maximize in the extratropics while Rossby internal modes exhibit patterns that extend over the entire globe.

The total energy error of zonally asymmetric flow exhibits dominant contributions from the first and third vertical modes. Midlatitude zonally averaged zonal-wind error is mostly due to internal (baroclinic) Rossby mode structures. Baroclinic error components are shown to be related to the tropical convective rainfall precipitation error using results obtained from a linearized shallow-water equation model. There is some evidence of internal Rossby-mode errors propagating from low into high latitudes, while the reverse is true for external modes. Tropical forecasts of the divergent wind are found to be less dependent on the extratropics than those for the rotational wind, with tropical forcing and dynamics playing a dominant role in the specification of the tropical circulation.

Abstract

June 1988 has been classified as one of the hottest and driest months on record in the United States. This study used the NMC Medium-Range Forecast(MRF) T40 model to simulate circulation features of June 1988 and to investigate the relationship between sea surface temperature anomalies (SSTA) and circulation patterns in the Northern Hemisphere. Three control experiments have been performed using three different initial conditions, separated by one day (21, 22, and 23 May 1988) and using SSTA fixed at the starting date. The three forecasts, and their average, are remarkably skillful in the Northern Hemisphere. The observed anomaly of June 1988, a wave train with a persistent ridge in the north-central United States and a northward shifting of the jet stream in the Pacific–North America area, is very well simulated in each of the integrations. All three experiments were repeated using the same initial conditions, but with climatological SST. The wave train generated is similar to that in the control experiments, but it is not as robust. The simulated jet streams are also similar to those in the control experiments. Two experiments with the 1988 SSTA, but with initial conditions of 22 May 1987 and 22 May 1989 were also run. The circulation patterns generated by these runs are very different from those of 1988, indicating that the persistence of the anomalous ridge in the north-central United States after late May 1998 was not due to the SSTA of the May 1988 alone.

A barotropic analysis was done to obtain the normal modes associated with the 300-mb streamfuncton of the June climatology. The analysis indicates the existence of a slowly growing mode with structure similar to the anomalies of 1988. This result, as well as the numerical experiments, suggests that the persistence of the June 1988 wave train may be associated with initial conditions, which were in a rather stable regime. The SSTA may have helped to strengthen the pattern, but the wave train associated with the 1988 drought could not have been generated by SSTA alone.

Abstract

During the 2004 North American Monsoon Experiment (NAME) field campaign, an extensive set of enhanced atmospheric soundings was gathered over the southwest United States and Mexico. Most of these soundings were assimilated into the NCEP operational global and regional data assimilation systems in real time. This presents a unique opportunity to carry out a series of data assimilation experiments to examine their influence on the NCEP analyses and short-range forecasts. To quantify these impacts, several data-withholding experiments were carried out using the global Climate Data Assimilation System (CDAS), the Regional Climate Data Assimilation System (RCDAS), and the three-dimensional variational data assimilation (3DVAR) Eta Model Data Assimilation System (EDAS) for the NAME 2004 enhanced observation period (EOP).

The impacts of soundings vary between the assimilation systems examined in this study. Overall, the influence of the enhanced soundings is concentrated over the core monsoon area. While differences at upper levels are small, the differences at lower levels are more substantial. The coarse-resolution CDAS does not properly resolve the Gulf of California (GoC), so the assimilation system is not able to exploit the additional soundings to improve characteristics of the Gulf of California low-level jet (GCLLJ) and the associated moisture transport in the GoC region. In contrast, the GCLLJ produced by RCDAS is conspicuously stronger than the observations, though the problem is somewhat alleviated with additional special NAME soundings. For EDAS, soundings improve the intensity and position of the Great Plains low-level jet (GPLLJ). The soundings in general improve the analyses over the areas where the assimilation system has the largest uncertainties and errors. However, the differences in regional analyses owing to the soundings are smaller than the differences between the two regional data assimilation systems.

Abstract

We present a new empirical orthogonal function (EOF) analysis of winter 500 mb geopotential height anomalies in the Southern Hemisphere. An earlier EOF analysis by two of the present authors prefiltered the anomalies to exclude wavenumbers 5 and higher; we do not. The different preprocessing of data affects the results. All three distinct planetary flow regimes identified in the winter circulation of the Southern Hemisphere by a pattern correlation method are captured by the new set of EOFs; only two of those regimes were captured by the earlier set. The new results, therefore, lend further support to the idea that EOFs point to distinct planetary