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Kingtse C. Mo

Abstract

A statistical model based on the combination of singular spectrum analysis (SSA) and the maximum entropy method (MEM) is applied to monitor and forecast outgoing longwave radiation anomalies (OLRAs) in the intraseasonal band over the Indian–Pacific sector and in the pan-American region. SSA is related to empirical orthogonal function analysis (EOF) but is applied to time series. The leading SSA modes (T-EOFs) are orthogonal and they are determined from the training period before filtering. The OLRA time series can be projected onto T-EOFs to obtain the principal components (T-PCs). To obtain fluctuations in any frequency band, one can partially sum up a chosen subset of T-EOFs and the related T-PCs in that band. The filter based on the SSA modes is data adaptive and there is no loss of end points. It is well suited for real-time monitoring of intraseasonal oscillations.

In the Pacific and the pan-American region, there are three leading modes (T-EOFs) of oscillations with periods near 40, 22, and 18 days. The T-PCs associated with these modes are quasiperiodic and they can be modeled by an autoregressive process. To perform forecasts, the MEM is used to determine the autoregressive coefficients from the training period. These coefficients are used to advance T-PCs. The summation of T-EOFs and T-PCs related to three preferred modes gives the predicted OLRAs. For 5-day mean OLRAs, the averaged correlation between the predicted and the observed anomalies is 0.65 at the lead times of four pentads (20 days). The SSA–MEM method is effective for any time series containing large oscillatory components. The deficiency of this method is that the forecasted magnitudes of anomalies are usually weaker than observations.

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Kingtse C. Mo

Abstract

No abstract available.

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Kingtse C. Mo

Abstract

Pattern correlations between daily anomalies have been used to study the persistence of the Southern Hemisphere circulations. The dataset consists of daily Australian analyses of 500 mb heights and sea level pressure for the period from 1972 to 1983. Compared to the Northern Hemisphere, the pattern correlations are much lower and more variable in the Southern Hemisphere. The mean one-day lag autocorrelation is only 0.57, compared to 0.81 in the Northern Hemisphere. The correlations increase significantly for the filtered anomalies, which consist of the planetary wavenumbers from 0 to 4.

Subjective criteria based on the pattern correlations are used to select quasi-stationary events. A series of 5 or more daily maps is defined to be quasi-stationary if the pattern correlations between all pairs of five consecutive maps in this time series are larger than or equal to 0.5. In winter, quasi-stationary events can be classified in terms of wavenumbers. Waves 3 and 4 are by far the dominant waves. More than half of the events have wave 3 amplitude with geographically fixed orientations.

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Kingtse C. Mo

Abstract

The NCEP–NCAR reanalysis together with the outgoing longwave radiation anomalies (OLRAs) and a gridded daily precipitation over the United States were used to analyze precipitation over California on intraseasonal timescales. The intraseasonal (10–90 days) filtered OLRAs were subjected to singular spectrum analysis, which identifies nonlinear oscillations in noisy time series. There are two dominant oscillatory modes associated with California rainfall with periods near 36–40 and 20–25 days.

The 36–40-day mode is related to the Madden–Julian Oscillation (MJO) in the Tropics. Enhanced tropical convection propagates from the western Pacific to the central Pacific. A three-cell pattern with negative OLRAs in California and positive anomalies in the eastern Pacific and the Pacific Northwest starts to develop 4 days later and rainfall starts in California.

Anomalies associated with the 20–25-day mode are responsible for alternating wet and dry episodes over California with periods shorter than the timescales of the MJO. The 20–25-day mode is the leading mode in the 7–30-day band and is related to tropical convection in the Pacific. In the extratropics, cloud bands propagate northward along the west coast of North America from the eastern Pacific just north of the ITCZ through California to the Pacific Northwest. The 200-hPa streamfunction anomaly composites associated with the 20–25-day mode reveal a westward propagating wave train dominated by a zonal wavenumber 2. This mode has a spatial structure similar to the traveling pattern described by Branstator.

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Kingtse C. Mo

Abstract

No abstract available.

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Kingtse C. Mo

Abstract

Two sets of experiments were performed. The first set, denoted SSTA, consisted of 90-day forecasts with sea surface temperature anomalies updated with observed values daily during the entire integration. For the summers 1987 and 1988, three SSTA experiments were made using three different initial conditions centered on 1 June of that year, separated by 1 day. The second set of experiments, denoted CSST, used the same initial conditions as the first set, but the integrations were performed using climatological SSTs. All numerical experiments were done using the NMC T80 spectral model of 1990, which is the same model used in making operational medium-range forecasts.

Simulated seasonal ensemble-mean rainfall was compared with satellite estimates of precipitation and observed station rainfall data. Overall agreement between them is good. Two centers of maximum rainfall, over the Arabian Sea and the Bay of Bengal, are captured by the model, but it fails to capture the movement of the rainfall associated with the Indian monsoon. The model is able to simulate the interannual variability of rain in India and over the Sahel, although the simulated convection in the central Pacific associated with the 1987 warm episode is not realistic.

When the model is able to simulate the convection associated with the SSTAs, then the updated SSTs have a large positive impact on tropical impact seasonal forecasts. The impact on the extratropical forecasts is, in general, positive but small.

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Kingtse C. Mo

Abstract

Drought indices derived from the North American Land Data Assimilation System (NLDAS) Variable Infiltration Capacity (VIC) and Noah models from 1950 to 2000 are intercompared and evaluated for their ability to classify drought across the United States. For meteorological drought, the standardized precipitation index (SPI) is used to measure precipitation deficits. The standardized runoff index (SRI), which is similar to the SPI, is used to classify hydrological drought. Agricultural drought is measured by monthly-mean soil moisture (SM) anomaly percentiles based on probability distributions (PDs). The PDs for total SM are regionally dependent and influenced by the seasonal cycle, but the PDs for SM monthly-mean anomalies are unimodal and Gaussian.

Across the eastern United States (east of 95°W), the indices derived from VIC and Noah are similar, and they are able to detect the same drought events. Indices are also well correlated. For river forecast centers (RFCs) across the eastern United States, different drought indices are likely to detect the same drought events.

The monthly-mean soil moisture (SM) percentiles and runoff indices between VIC and Noah have large differences across the western interior of the United States. For small areas with a horizontal resolution of 0.5° on the time scales of one to three months, the differences of SM percentiles and SRI between VIC and Noah are larger than the thresholds used to classify drought. For the western RFCs, drought events selected according to SM percentiles or SRI derived from different NLDAS systems do not always overlap.

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Kingtse C. Mo

Abstract

Tropical intraseasonal variations in the Pacific are related to the tropical storm activity in the Atlantic basin using outgoing longwave radiation anomalies (OLRAs) and circulation anomalies from the NCEP–NCAR reanalysis. Tropical storms are most likely to develop and maintain in the Atlantic, when enhanced convection associated with the tropical intraseasonal oscillations (TIOs) is located over the Indian Ocean and convection in the Pacific is suppressed. Tropical storm activity decreases when the TIO shifts to the opposite phase.

The dominant signal associated with the TIO is the Madden–Julian oscillation. The atmospheric response in the Tropics is a dipole pattern in the 200-hPa streamfunction anomalies just north of the equator. Positive OLRA propagates eastward from the Indian Ocean to the central Pacific. The dipole moves eastward in concert with OLRAs. When enhanced convection is located in the Indian Ocean and convection in the Pacific is suppressed, positive 200-hPa streamfunction anomalies as a part of the dipole extend from Central America to the central Atlantic. There are more upper-tropospheric easterly wind anomalies over the Caribbeans and the tropical Atlantic. The vertical wind shear decreases. These conditions are favorable for tropical storms to development and enhance. When the TIO shifts to the opposite phase with enhanced convection in the Pacific, the wind shear in the tropical Atlantic increases and the occurrence of tropical storms decreases.

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Kingtse C. Mo

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 T air 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 T air 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) maximum in the eastern Pacific. Enhanced convection extends from the date line to the eastern Pacific, with negative anomalies in the western Pacific. The atmospheric responses resemble a tropical Northern Hemisphere pattern. The wave train is consistent with the north–south T air contrast over North America during the EPAC ENSO winters. The CPAC ENSO has enhanced convection in the central Pacific. The atmospheric responses show a Pacific–North American pattern. It is consistent with west–east contrast in T air and more rainfall over the Southwest during the CPAC ENSO winters.

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Kingtse C. Mo

Abstract

The intraseasonal rainfall variability over North America is examined using singular spectrum analysis (SSA) and composites of outgoing longwave radiation anomalies (OLRAs), 200-hPa divergence and a gridded rainfall dataset over the United States. The evolution of the Arizona and New Mexico (AZNM) monsoon based on composites indicates that rainfall anomalies propagate eastward from the North Pacific through AZNM, the Great Plains, to the eastern United States. During summer, the wet and dry periods of the AZNM monsoon are modulated by an oscillatory mode with a period of 22–25 days (22-day mode). This is also the dominant mode associated with rainfall events over the Great Plains. The influence of the Madden–Julian Oscillation (MJO) on the AZNM monsoon is secondary. The strongest impact of the MJO is on precipitation over Mexico. SSA performed on the 200-hPa divergence and OLRAs averaged over Mexico show only one oscillatory mode with a period of about 36–40 days.

The 22–25-day mode also exists in the vertically integrated moisture fluxes over the Great Plains. During the wet periods of the AZNM monsoon, more moisture is transported from both the Gulf of Mexico and the Gulf of California to AZNM. The situation reverses when the oscillation reaches the other phase. The 22-day mode is linked to tropical convection. When rainfall associated with the 22-day mode travels eastward from AZNM to the Great Plains, the OLRA composites show westward propagating waves just north of the equator. When enhanced convection reaches the western Pacific, rainfall diminishes over AZNM. When convection in the western Pacific is suppressed and enhanced convection is located in the central Pacific, rainfall intensifies over AZNM.

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