Kingtse C. Mo

Alternating Wet and Dry Conditions over South America during Summer

Abstract Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin. This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry cond...

The Pacific–South American Modes and Tropical Convection during the Southern Hemisphere Winter

Abstract Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation. A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase...

Statistics and dynamics of persistent anomalies

Abstract Persistent anomalies with recurrent spatial patterns play an important role in the atmospheres low-frequency variability. We establish a connection between statistical and dynamical methods of description and prediction of persistent anomalies. This is done by computing and analyzing the empirical orthogonal functions (EOFs) in a simple deterministic model, on the one hand, and in Southern Hemisphere geopotential heights, on the other. The dynamical model is governed by the fully nonlinear, equivalent-barotropic vorticity equation on the sphere, with simplified forcing, dissipation and topography. Model solutions exhibit persistent anomalies identifiable with blocked, zonal and wave-train anomalies in Northern Hemisphere atmospheric data. Flow structures similar to the patterns above occur as high-variance EOFs of this nonlinear model. The Southern Hemisphere data we analyze consist in gridded daily maps of 500 mb heights from June 1972 to July 1983. Two types of persistent anomalies appear in t...

Teleconnections in the Southern Hemisphere

Abstract Teleconnections are calculated from monthly mean anomalies of sea level pressure and 500 mb geopotential height for the Southern Hemisphere (10–90°S) for five-month winter and summer seasons. The monthly means were calculated from Australian analyses for the period from June 1972 to November 1980. Zonally averaged anomalies at both sea level and 500 mb display an out-of-phase relation between low and high latitudes and in midlatitudes are negatively correlated with anomalies in the subtropics and polar regions. In winter a striking zonal wavenumber 3 pattern is found over the Southern Ocean. Anomalies in 500 mb geopotential heights at (50°S, 95°E), (58°S, 150°W) and (38°S, 15°W) exhibit strong positive correlations while showing weaker negative correlations with heights over Antarctica and in low latitudes. A similar pattern appears in sea level pressure. In summer anomalies in 500 mb geopotential height over the three subtropical continents appear to occur out of phase with anomalies over the su...

Intraseasonal Modulation of South American Summer Precipitation

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 streamfunctio...

Intraseasonal oscillations in the global atmosphere. I - Northern Hemisphere and tropics

Abstract We have examined systematically oscillatory modes in the Northern Hemisphere and in the tropics. The 700 mb heights were used to analyze extratropical oscillations, and the outgoing longwave radiation to study tropical oscillations in convection. All datasets were band-pass filtered to focus on the intraseasonal (IS) band of 10–120 days. Leading spatial patterns of variability were obtained by applying EOF analysis to these IS data. The leading principal components (PCs) were subjected to singular spectrum analysis (SSA). SSA is a statistical technique related to EOF analysis, but in the time domain, rather than the spatial domain. It helps identify nonlinear oscillations in short and noisy time series. In the Northern Hemisphere, there are two important modes of oscillation with periods near 48 and 23 days, respectively. The 48-day mode is the most important of the two. It has both traveling and standing components, and is dominated by a zonal wavenumber two. The 23-day mode has the spatial stru...

Tropical-extratropical geopotential height teleconnections during the Northern Hemisphere winter

Abstract Simultaneous and lagged correlation statistics have been calculated between time series of seasonal height anomalies at selected stations and extratropical grid-point anomalies in both hemispheres. The tropical stations in two major tropical precipitation zones, the Indo-China maritime continent and Africa, are well correlated with each other. These stations are also correlated with stations in the North Pacific and Australia, but the coefficients are smaller. The correlations between height anomalies at any of these stations and Northern Hemisphere height anomalies show a well-defined global pattern. Depending upon the location of the stations, the pattern is either a Pacific North American (PNA), a Tropical Northern Hemisphere (TNH) pattern or a mixed pattern having both elements. All three patterns, PNA, TNH and WPO (Western Pacific Oscillation), have been linked to tropical variations. The correlations between height anomalies at these well-correlated stations and the Southern Hemisphere heig...

Linkages between Summer Rainfall Variability over South America and Sea Surface Temperature Anomalies

Abstract A reconstructed rainfall dataset, and satellite estimates are used to analyze interannual to decadal variability of austral summer precipitation over South America. Rotated empirical orthogonal function (REOF) analysis is applied to isolate dominant patterns of rainfall. Links of these patterns to sea surface temperature anomalies (SSTAs) are examined. The leading mode is related to El Nino–Southern Oscillation (ENSO), which explains 12% of the total variance. During warm ENSO events, the positive phase of this mode shows dry conditions over northern South America and wet conditions over the subtropical plains between 25° and 35°S. The situation reverses during cold events. The second REOF 2, which explains about 10.8% of the total variance, consists of positive loadings over northeast Brazil centered at 50°W near the equator and negative loadings over Colombia and the subtropical plains. For December–January–February (DJF), REOF 2 is influenced by tropical South Atlantic SSTAs through displaceme...

Interdecadal Modulation of the Impact of ENSO on Precipitation and Temperature over the United States

Abstract Data from observations and the Intergovernmental Panel on Climate Change (IPCC) twentieth-century climate change model [phase 3 of the Coupled Model Intercomparison Project (CMIP3)] simulations were analyzed to examine the decadal changes of the impact of ENSO on air temperature Tair and precipitation P over the United States. The comparison of composites for the early period (1915–60) and the recent period (1962–2006) indicates that cooling (warming) over the south and warming (cooling) over the north during ENSO warm (cold) winters have been weakening. The ENSO influence on winter P over the Southwest is strengthening, while the impact on P over the Ohio Valley is weakening for the recent decades. These differences are not due to the long-term trends in Tair or P; they are attributed to the occurrence of the central Pacific (CPAC) ENSO events in the recent years. The CPAC ENSO differs from the canonical eastern Pacific (EPAC) ENSO. The EPAC ENSO has a sea surface temperature anomaly (SSTA) maxi...

Intraseasonal Oscillations in the Global Atmosphere. Part II: Southern Hemisphere

Abstract In Part II of this two-part article, we complete the systematic examination of oscillatory modes in the global atmosphere by studying 12 years of 500 mb geopotential heights in the Southern Hemisphere. As in Part I, for the tropics and Northern Hemisphere extratropics, the data were band-pass filtered to focus on intraseasonal (IS) phenomena, and spatial EOFs were obtained. The leading principal components were subjected to singular spectrum analysis (SSA), in order to identify nonlinear IS oscillations with high statistical confidence. In the Southern Hemisphere, the dominant mode has a period of 23 days, with spatial patterns carried by the second and third winter EOF of the IS band. It has a zonal wavenumber-four structure. The 40-day mode is second, and dominated by wavenumbers three and four, while a 16-day mode is too weak to separate its spatial behavior from the previous two. The IS dynamics in the Southern Hemisphere is more complex and dominated by shorter wavenumbers than the Northern ...