Abstract

Interannual variability of seasonal rainfall in the Brazilian Amazon basin is examined in context of its relationship to sea surface temperatures in the tropical Pacific and Atlantic Oceans. Linear correlations reveal strong relationships, but rainfall patterns are of regional scale. Areas of rainfall exhibiting strong relationships with SST are confined to the equatorial region of the Brazilian Amazon. The best relationships are found either during the season of transition between wet and dry regimes, or entirely within the dry season. It is hypothesized, and results are shown in support, that during the transition seasons, an important contributor to the SST control on seasonal totals is its influence on the timing on the rainy season onset or end. That influence appears to be stronger than the SST influence on the rainy season rain rate.

Abstract

Any discussion of intraseasonal and interannual variability in the atmosphere must presume a reliable assessment of the observed variability. In spite of continued improvements in observing systems, quality control techniques, and data analysis schemes, however, and also because of them, this assessment remains difficult in the tropics.

In this paper the authors examine the mean tropical circulation during two Januarys, 1988 and 1989, as described by the circulation analyses produced at two weather prediction centers, the National Meteorological Center (NMC) in Washington, D.C., and the European Center for Medium-Range Weather Forecast (ECMWF) in Reading, England. In particular, the authors’ focus is on the change in the circulation between 1988 and 1989 as estimated by these two sets of analyses, especially the change in the 200-mb wind divergence associated with organized deep convection. The authors find that in many regions the discrepancy between thew estimates is of the order of the change itself. A comparison with maps of the outgoing longwave radiation (OLR) is not quantitatively useful in this regard.

One way out of this dilemma is to derive divergence fields that are consistent with the 200-mb vorticity balance. The authors do so by solving the “chi problem” of Sardeshmukh and Hoskins. Because the large-scale vorticity fields generated by NMC and ECMWF are highly correlated (∼98%), the divergence fields derived in this manner are also better correlated than the analyzed fields and enable a more reliable assessment of the observed change between these two periods.

Abstract

The characteristics of intensity, geographical location, and persistence of the South Atlantic convergence zone (SACZ) during the austral summer are investigated. Intensity and spatial features of the SACZ are identified by performing a factor analysis of structural properties of outgoing longwave radiation (OLR) data. The first two leading factors explain 65% of the total variance of structural properties and characterize the SACZ according to intensity and location (oceanic versus continental). An index is constructed based on the magnitude of the factor scores to identify intense (weak) and oceanic (continental) SACZ. The intense SACZ category is associated with negative OLR anomalies over a large area of tropical South America, extending from the western Amazon to the Atlantic Ocean. The weak SACZ category is observed with positive OLR anomalies over tropical South America and negative OLR anomalies over southeastern South America. Oceanic and continental aspects of the SACZ are related to a midlatitude wave train pattern. The Madden–Julian oscillation (MJO) modulates intense SACZ events with persistence longer than 3 days. Interannual variability of persistent events indicates that the ratio of oceanic to continental SACZ as well as their frequency depends on the phase of El Niño–Southern Oscillation (ENSO). Occurrence of extreme rainfall in Brazil is discussed in the context of variations in the SACZ and MJO. Intense (weak) SACZ increases (decreases) the 95th daily rainfall percentile over central-eastern Brazil compared to the climatology. Oceanic (continental) SACZ increases (decreases) the 95th daily rainfall percentile over southeastern Brazil. The MJO phase characterized by suppression of convective activity over Indonesia and enhancement over the central Pacific increases the 95th daily rainfall percentile over north-northeastern Brazil, whereas opposite features are observed for the phase of the MJO characterized by the enhancement of convection over Indonesia and suppression over the central Pacific.

Abstract

The climatology and interannual variability of heavy, or “extreme,” precipitation events are studied, using station data from the state of São Paulo, Brazil. An extreme event is defined at each station when daily rainfall exceeds a certain percent of its seasonal or annual mean. It is found that these events occur mainly from November to March and that there is a distinct interannual variation in their number. The count of extreme events is not well correlated with mean precipitation. The relationship between extreme events and activity in the South Atlantic convergence zone (which, when active, is associated with increased precipitation) is therefore not obvious. From October to March, the interannual count of extreme events in the entire state is correlated positively with SST anomalies in the equatorial Pacific from near the date line to the west coast of South America. The interannual count at stations near the Atlantic coast from November to February is correlated positively with SST anomalies in the Atlantic Ocean near the latitude of São Paulo. In both cases the relationship between SST and mean precipitation is weak. The associations are confirmed with composites and rank correlations. The relationships described are apparent in the period 1976–77 to 1994–95. There is no correspondence evident between extreme events and SST if data beginning in 1948 are included in the analysis.

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.

Abstract

The South American monsoon system (SAMS) is the most important climatic feature in South America and is characterized by pronounced seasonality in precipitation. This study uses the National Centers for Environmental Prediction Climate Forecast System, reforecasts version 2 (CFSRv2), to investigate the skill of probabilistic forecasts of onset and demise dates, duration, and amplitude of SAMS during 1982–2009. A simple index based on the empirical orthogonal function of precipitation anomalies is employed to characterize onsets, demises, durations, and amplitudes of SAMS. The CFSv2 model has useful skill to forecast seasonal changes in SAMS. Probabilistic forecasts of onset and demise dates have 16.5% and 43.3% improvements, respectively, over climatological forecasts. Verification of hindcasts of durations and amplitudes of SAMS shows relatively small biases and root-mean-square errors.

Abstract

Two theories for observed East Africa drying trends during March–May 1979–2013 are reconciled. Both hypothesize that variations in tropical sea surface temperatures (SSTs) caused East Africa drying. The first invokes a mainly human cause resulting from sensitivity to secular warming of Indo–western Pacific SSTs. The second invokes a mainly natural cause resulting from sensitivity to a strong articulation of ENSO-like Pacific decadal variability involving warming of the western Pacific and cooling of the central Pacific. Historical atmospheric model simulations indicate that observed SST variations contributed significantly to the East Africa drying trend during March–May 1979–2013. By contrast, historical coupled model simulations suggest that external radiative forcing alone, including the ocean’s response to that forcing, did not contribute significantly to East Africa drying. Recognizing that the observed SST variations involved a commingling of natural and anthropogenic effects, this study diagnosed how East African rainfall sensitivity was conditionally dependent on the interplay of those factors. East African rainfall trends in historical coupled models were intercompared between two composites of ENSO-like decadal variability, one operating in the early twentieth century before appreciable global warming and the other in the early twenty-first century of strong global warming. The authors find the coaction of global warming with ENSO-like decadal variability can significantly enhance 35-yr East Africa drying trends relative to when the natural mode of ocean variability acts alone. A human-induced change via its interplay with an extreme articulation of natural variability may thus have been key to Africa drying; however, these results are speculative owing to differences among two independent suites of coupled model ensembles.

Abstract

The date of Australian summer monsoon onset (ASMO) is found to be well correlated with the monsoon rainfall of India during the preceding June to September. Years of below (above) normal Indian summer monsoon rainfall (ISMR) are followed by delayed (early) ASMO. Sea surface temperature (SST) anomalies during the September to November season over the tropical Indian Ocean, the equatorial eastern Pacific Ocean, and the ocean north of Australia also correlate significantly with the date of the following ASMO. Delays in ASMO are associated with cold SST north of Australia and warm SST in the tropical Indian and equatorial cast Pacific oceans.

Previous studies have shown that a warm SST is created over the tropical Indian Ocean in years of poor ISMR. We hypothesize that a warm SST anomaly over the Indian Ocean delays the seasonal southward and eastward migration of the cloudiness maximum. A delay in the southeastward movement of cloudiness results in a delayed ASMO. A similar hypothesis already has been suggested to explain the variability of the date of monsoon onset over India.

Weak ISMR often is associated with the contemporaneous presence of El Niño, although many weak monsoons occur without El Niño. Thus warm SSTs in the eastern equatorial Pacific are related to a delayed ASMO through the Indian monsoon. Another signature of El Niño is the presence of negative SST anomalies north of Australia, adding to the delay in ASMO. Warm SSTs in the central and eastern Pacific may also act directly to delay ASMO by causing convection near and east of the date line and subsidence near Australia.

Abstract

Observed rainfall, outgoing longwave radiation (OLR), divergence, and precipitation from the reanalysis project of the National Centers for Environmental Prediction and the National Center for Atmospheric Research are compared over the Amazon Basin. The spatial pattern of the mean and the phase of the annual cycle generally compare well, except that the amplitude of the annual cycle of model precipitation is much smaller than observed. On 10–30-day timescales, it is shown that averaging stations within a 5° radius is approximately equivalent to total wavenumber 20 (T20) spatial scale, although it is more important to have a high density of stations than an exact match of spatial scales. Ideally, there should be one station per 20 000 km². On 10–30-day scales, observed rainfall is best correlated with OLR. Correlations between OLR and 150-mb divergence are larger than between observed rainfall and divergence or between rainfall and model precipitation. For example, if 10–30-day filtered OLR and divergence are truncated at T20 and rainfall is averaged to include stations within a 5° radius, OLR is correlated with rainfall at about −0.6, OLR is correlated with divergence at about −0.35, and rainfall is correlated with divergence at about 0.2. At least part of the lack of correlation is due to inadequate spatial sampling of rainfall. Correlations improve with larger spatial scale. The major seasonal transitions from dry to rainy regimes are captured well by OLR but not by the model quantities. The mean diurnal cycle is represented reasonably by 150-mb divergence.

Abstract

The core region of the North American summer monsoon is examined using spatially averaged daily rainfall observations obtained from gauges, with the objective of improving understanding of its climatology and variability. At most grid points, composite and interannual variations of the onset and end of the wet season are well defined, although, among individual stations that make up a grid average, variability is large. The trigger for monsoon onset in southern and eastern Mexico appears to be related to a change in vertical velocity, while for northwestern Mexico, Arizona, and New Mexico it is related to a reduction in stability, as indicated by a decrease in the lifted index. The wet-season rain rate is a combination of the wet-day rain rate, which decreases with distance from the coast, and the wet-day frequency, which is largest over the Sierra Madre Occidental. Thus the maximum total rate lies slightly to the west of the highest orography. As has been previously noted, onset is not always well correlated with total seasonal precipitation, so in these areas, variations of wet-day frequency and wet-day rain rate must be important. Correlations are small between the wet-day frequency and the wet-day rate, and the former is better correlated than the latter with the seasonal rain rate. Summer rainfall in central to southern Mexico exhibits moderate negative correlations with the leading pattern of sea surface temperature (SST) anomalies in the equatorial Pacific, which projects strongly onto El Niño. The influence of equatorial SSTs on southern Mexico rainfall seems to operate mainly through variability of the wet-day frequency, rather than through variations of the wet-day rain rate.