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Charles Jones and Leila M. V. Carvalho

Abstract

The South American monsoon system (SAMS) refers to the austral summer season features of deep convective activity and large-scale circulation. This study examines intraseasonal variations in the low-level wind circulation in the Amazon and their modulating effects on active and “break” phases in SAMS. Daily averages of outgoing longwave radiation (OLR), NCEP–NCAR reanalysis, and gridded rainfall station data in Brazil are used from 1 November to 28 February 1980–99. The direction of wind anomalies (10–70 days) in the Rondônia State, Brazil, is used to classify periods of westerly (W) and easterly (E) low-level wind regimes. Composites of W regime show low-level wind anomalies crossing the equator southward and closing in a cyclonic anomalous circulation off the coast of Argentina and Uruguay. Broad areas of enhanced convection and rainfall are observed in central and southeast Brazil. Suppressed convection is observed over the Bolivian Altiplano and in northern South America. In contrast, in the E regime, opposite patterns are observed in the low-level circulation, convection, and rainfall anomalies. The duration of active (W regimes) and break (E regimes) periods are quite similar, with median values of 4 and 5 days, respectively. Further investigation showed that the region of convection and rainfall anomalies over Venezuela and northwest Brazil is observed only in the 10–30-day band. Comparison of the results shown here with previous studies indicates the importance of intraseasonal variations in the activity of SAMS.

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Charles Jones and Leila M. V. Carvalho

Abstract

The spatial–intensity variability of extreme precipitation over the contiguous United States (CONUS) during boreal winter and relationships with the Madden–Julian oscillation (MJO) are investigated. Daily gridded precipitation is used to define two types of contiguous regions of extreme precipitation (CREPs): the intensity and spatial extent exceeding the 75th and 90th percentiles of frequency distributions. Extreme precipitation occurs twice as frequently when the MJO is active than inactive. Joint probabilities of the fractional area of CONUS sectors when the MJO is active are 2.0–2.5 higher than probabilities during inactive days for both 75th and 90th percentile CREPs (similar for the intensity of CREPs). Probabilities of the fractional area of 75th percentile CREPs when the MJO is active in neutral ENSO are higher than during warm or cold ENSO. Joint probabilities of the fractional area during MJO and warm ENSO are higher than MJO and cold ENSO and statistically significant over southern sectors. Results are similar for joint probabilities of intensity exceedance and MJO activity in warm and cold ENSO phases. Proportions of 75th and 90th percentile CREPs for each sector and phase of the MJO are predominantly large when MJO convective signals are over the central Indian Ocean or western Pacific. Probabilities of the fractional area of 90th percentile CREPs conditioned on MJO phases, however, do not show clear predominance. This indicates that the MJO is not the sole player in the occurrences of CREPs. Last, this study concludes that probabilities of the fractional area and intensity of 75th and 90th percentile CREPs in the CONUS do not depend on the amplitude of the MJO.

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Charles Jones and Leila M. V. Carvalho

Abstract

The South American monsoon system (SAMS) is the most important climatic feature in South America. This study focuses on the large-scale characteristics of the SAMS: seasonal amplitudes, onset and demise dates, and durations. Changes in the SAMS are investigated with the gridded precipitation, Climate Forecast System Reanalysis (CFSR), and the fifth phase of the Coupled Model Intercomparison Project (CMIP5) simulations for two scenarios [“historical” and high-emission representative concentration pathways (rcp8.5)]. Qualitative comparisons with a previous study indicate that some CMIP5 models have significantly improved their representation of the SAMS relative to their CMIP3 versions. Some models exhibit persistent deficiencies in simulating the SAMS. CMIP5 model simulations for the historical experiment show signals of climate change in South America. While the observational data show trends, the period used is too short for final conclusions concerning climate change. Future changes in the SAMS are analyzed with six CMIP5 model simulations of the rcp8.5 high-emission scenario. Most of the simulations show significant increases in seasonal amplitudes, early onsets, late demises, and durations of the SAMS. The simulations for this scenario project a 30% increase in the amplitude from the current level by 2045–50. In addition, the rcp8.5 scenario projects an ensemble mean decrease of 14 days in the onset and 17-day increase in the demise date of the SAMS by 2045–50. The results additionally indicate lack of spatial agreement in model projections of changes in total wet-season precipitation over South America during 2070–2100. The most consistent CMIP5 projections analyzed here are the increase in the total monsoon precipitation over southern Brazil, Uruguay, and northern Argentina.

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Charles Jones and Leila M. V. Carvalho

Abstract

The Madden–Julian oscillation (MJO) is the most prominent mode of tropical intraseasonal variability. This study investigates the following questions. Is there statistical evidence of linear trends in MJO activity since the mid-1970s? Does the MJO exhibit changes in regimes of high and low activity? Are there significant seasonal differences in the activity of the MJO on time scales longer than interannual?

Positive linear trends are observed in zonal wind anomalies at 200 (U200) and 850 (U850) hPa during summer and winter seasons. Positive trends are also observed in the number of summer MJO events. Resampling statistical tests indicate that positive trends in summer U200 and U850 anomalies are statistically different from random occurrences at a 5% significance level.

A methodology based on the number of events is used to characterize low-frequency (LF) changes in MJO activity. Mean winter LF activity was characterized by nearly uniform variability from the early 1960s until the mid-1990s. In contrast, mean summer LF changes showed a regime of high activity from the mid-1960s until the late 1970s, a low regime from 1980 to 1988, and a regime of high activity from the early 1990s to early 2000. Fourier analysis of the mean summer LF index indicates that regimes of high MJO activity were separated by 18.5 yr. The substantial changes in summer MJO regimes do not appear to be related to increases in observational samplings due to satellite-derived winds assimilated in the NCEP–NCAR reanalysis. Monte Carlo experiments indicate that the observed changes in regimes of MJO activity in summer are statistically different from random occurrences at the 10% significance level only.

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Leila M. V. Carvalho and Charles Jones

Abstract

A simple, fully automated, and efficient method to determine the structural properties and evolution (tracking) of cloud shields of convective systems (CS) is described. The method, which is based on the maximum spatial correlation tracking technique (MASCOTTE), is a new alternative to the existent techniques available for studies that monitor the evolution of CS using satellite images. MASCOTTE provides as CS structural properties the following parameters: mean and variance of brightness temperature, horizontal area, perimeter, minimum brightness temperature, fractional convective area, center of gravity, and fragmentation. The fragmentation parameter has the potential to monitor the evolution of the CS. A new way of estimating the orientation and eccentricity of CS is proposed and is based on the empirical orthogonal function analysis of CS pixel coordinates. The method includes an accurate detection of splitting and merging of convective systems, which is a critical step in the automated satellite CS life cycle determination. Based on the magnitudes of the spatial correlation between consecutive satellite images and the changes in horizontal areas of CS, MASCOTTE provides a simple and skillful technique to track the evolution of CS life cycles. The MASCOTTE methodology is applied to infrared satellite images during seven consecutive days of the Wet-Season Atmospheric Mesoscale Campaign of the Large-Scale Biosphere–Atmosphere Experiment and ground validation experiment of the Tropical Rainfall Measuring Mission in the Brazilian state of Rondônia in the Amazon basin. The results indicate that MASCOTTE is a valuable approach to understanding the variability of CS.

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Leila M. V. Carvalho and Charles Jones

Abstract

Global warming has been linked to systematic changes in North and South America's climates and may severely impact the North American monsoon system (NAMS) and South American monsoon system (SAMS). This study examines interannual-to-decadal variations and changes in the low-troposphere (850 hPa) temperature (T850) and specific humidity (Q850) and relationships with daily precipitation over the tropical Americas using the NCEP–NCAR reanalysis, the Climate Forecast System Reanalysis (CFSR), and phase 5 of the Coupled Model Intercomparison Project (CMIP5) simulations for two scenarios: “historic” and high-emission representative concentration pathway 8.5 (RCP8.5). Trends in the magnitude and area of the 85th percentiles were distinctly examined over North America (NA) and South America (SA) during the peak of the respective monsoon season. The historic simulations (1951–2005) and the two reanalyses agree well and indicate that significant warming has occurred over tropical SA with a remarkable increase in the area and magnitude of the 85th percentile in the last decade (1996–2005). The RCP8.5 CMIP5 ensemble mean projects an increase in the T850 85th percentile of about 2.5°C (2.8°C) by 2050 and 4.8°C (5.5°C) over SA (NA) by 2095 relative to 1955. The area of SA (NA) with T850 ≥ the 85th percentile is projected to increase from ~10% (15%) in 1955 to ~58% (~33%) by 2050 and ~80% (~50%) by 2095. The respective increase in the 85th percentile of Q850 is about 3 g kg−1 over SAMS and NAMS by 2095. CMIP5 models project variable changes in daily precipitation over the tropical Americas. The most consistent is increased rainfall in the intertropical convergence zone in December–February (DJF) and June–August (JJA) and decreased precipitation over NAMS in JJA.

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Leila M. V. Carvalho, Charles Jones, and Brant Liebmann

Abstract

The occurrence of daily extreme precipitation events in southeast South America (São Paulo, Brazil) and the spatial features of convective activity in the South Atlantic convergence zone (SACZ) are investigated. Precipitation data from surface stations in São Paulo state from 1979 to 1996 are used to determine the frequency of occurrence of extremely heavy daily precipitation events. Daily averages of outgoing longwave radiation (OLR) are examined to characterize convective activity in the SACZ. OLR features are identified with factor analysis. Two factors explain ∼65% of the total variance of the convective activity patterns in tropical South America and characterize events according to the intensity and extent of the OLR features over the Atlantic Ocean. The combination of factors indicates that 35% of extreme precipitation events occurred when convective activity in the SACZ was intense over large parts of tropical South America, which includes São Paulo, but with less extent toward the Atlantic Ocean. Warm SST episodes (El Niño) seem to modulate the occurrence of extremes associated with intense convection in the SACZ displaced northward of São Paulo and toward the Atlantic Ocean. The remaining events associated with weak convective activity in the SACZ suggest the role of transient systems producing extreme precipitation in São Paulo. The important contribution of the present work is the documentation of the role of orographic features for the regional distribution of extreme precipitation in São Paulo. It is shown that the regional distribution of extreme precipitation depends on both the intensity and form of the convection in the SACZ.

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Charles Jones, Jon Gottschalck, Leila M. V. Carvalho, and Wayne Higgins

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Extreme precipitation events are among the most devastating weather phenomena since they are frequently accompanied by loss of life and property. This study uses reforecasts of the NCEP Climate Forecast System (CFS.v1) to evaluate the skill of nonprobabilistic and probabilistic forecasts of extreme precipitation in the contiguous United States (CONUS) during boreal winter for lead times up to two weeks.

The CFS model realistically simulates the spatial patterns of extreme precipitation events over the CONUS, although the magnitudes of the extremes in the model are much larger than in the observations. Heidke skill scores (HSS) for forecasts of extreme precipitation at the 75th and 90th percentiles showed that the CFS model has good skill at week 1 and modest skill at week 2. Forecast skill is usually higher when the Madden–Julian oscillation (MJO) is active and has enhanced convection occurring over the Western Hemisphere, Africa, and/or the western Indian Ocean than in quiescent periods. HSS greater than 0.1 extends to lead times of up to two weeks in these situations. Approximately 10%–30% of the CONUS has HSS greater than 0.1 at lead times of 1–14 days when the MJO is active.

Probabilistic forecasts for extreme precipitation events at the 75th percentile show improvements over climatology of 0%–40% at 1-day lead and 0%–5% at 7-day leads. The CFS has better skill in forecasting severe extremes (i.e., events exceeding the 90th percentile) at longer leads than moderate extremes (75th percentile). Improvements over climatology between 10% and 30% at leads of 3 days are observed over several areas across the CONUS—especially in California and in the Midwest.

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Charles Jones, Leila M. V. Carvalho, Jon Gottschalck, and Wayne Higgins

Abstract

The Madden–Julian oscillation (MJO) is the most prominent form of tropical intraseasonal variability that impacts weather and climate. Forecast skill of extreme precipitation in the contiguous United States (CONUS) during winter is higher when the MJO is active and has enhanced convection over the Western Hemisphere, Africa, and/or the western Indian Ocean. This study applies a simple decision model to examine the relationships between the MJO and the relative value of deterministic forecasts of extreme precipitation. Value in the forecasts is significantly higher and extends to longer leads (2 weeks) during active MJO.

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Charles Jones, Francis Fujioka, and Leila M. V. Carvalho

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Santa Ana winds (SAW) are synoptically driven mesoscale winds observed in Southern California usually during late fall and winter. Because of the complex topography of the region, SAW episodes can sometimes be extremely intense and pose significant environmental hazards, especially during wildfire incidents. A simple set of criteria was used to identify synoptic-scale conditions associated with SAW events in the NCEP–Department of Energy (DOE) reanalysis. SAW events start in late summer and early fall, peak in December–January, and decrease by early spring. The typical duration of SAW conditions is 1–3 days, although extreme cases can last more than 5 days. SAW events exhibit large interannual variations and possible mechanisms responsible for trends and low-frequency variations need further study. A climate run of the NCEP Climate Forecast System (CFS) model showed good agreement and generally small differences with the observed climatological characteristics of SAW conditions.

Nonprobabilistic and probabilistic forecasts of synoptic-scale conditions associated with SAW were derived from NCEP CFS reforecasts. The CFS model exhibits small systematic biases in sea level pressure and surface winds in the range of a 1–4-week lead time. Several skill measures indicate that nonprobabilistic forecasts of SAW conditions are typically skillful to about a 6–7-day lead time and large interannual variations are observed. NCEP CFS reforecasts were also applied to derive probabilistic forecasts of synoptic conditions during SAW events and indicate skills to about a 6-day lead time.

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