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Gershunov, A.  1998.  ENSO influence on intraseasonal extreme rainfall and temperature frequencies in the contiguous United States: Implications for long-range predictability. Journal of Climate. 11:3192-3203.   10.1175/1520-0442(1998)011<3192:eioier>;2   AbstractWebsite

Potential ENSO-related predictability of wintertime daily extreme precipitation and temperature frequencies is investigated. This is done empirically using six decades of daily data at 168 stations distributed over the contiguous United States. ENSO sensitivity in the extreme ranges of intraseasonal precipitation and temperature probability density functions is demonstrated via a compositing technique. Potential predictability of extremes is then investigated with a simple statistical model. Given a perfect forecast of ENSO, the frequency of intraseasonal extremes is specified as the average frequency of occurrence during similar-phased ENSO winters on record. Specification skill is assessed as the cross-validated proportion of local variance explained by this method. The skill depends on varying ENSO sensitivity in different geographic regions and quantile ranges and on consistency or variability from one like-phased ENSO event to another. ENSO sensitivity also varies according to the intensity of the tropical forcing; however, not always in the expected sense. Good predictability is likely for variables and in regions displaying a strong and consistent ENSO signal. This is found in some coherent regions of the United States for various combinations of frequency variable and ENSO phase. ENSO-based predictability of heavy and extreme precipitation frequency is potentially good along the Gulf Coast, central plains, Southwest, and in the Ohio River valley for El Nino winters and in the Southwest and Florida for La Nina winters. Not all large magnitude signals translate into significant specification skill. Extreme precipitation frequency in the Southwest is a good example of this. Extreme warm temperature frequency (EWF) is potentially predictable in the southern and eastern United States during Fl Nino winters and in the Midwest during the strongest events. La Nina winters exhibit potentially very good EWF predictability in a Large area of the southern United States centered on Texas. Despite showing coherent ENSO patterns, extreme cold temperature frequency (ECF) signals are mostly weak and inconsistent, especially during strong ENSO events. Curiously, specification skill improves in the northern United States, along the West Coast and in the southeast during weaker El Nino winters. An improvement in potential ECF predictability is also observed in the Midwest during weaker La Nina winters.

Gershunov, A, Barnett TP.  1998.  Interdecadal modulation of ENSO teleconnections. Bulletin of the American Meteorological Society. 79:2715-2725.   10.1175/1520-0477(1998)079<2715:imoet>;2   AbstractWebsite

Seasonal climate anomalies over North America exhibit rather large variability between years characterized by the same ENSO phase. This lack of consistency reduces potential statistically based ENSO-related climate predictability. The authors show that the North Pacific oscillation (NPO) exerts a modulating effect on ENSO teleconnections. Sea lever pressure (SLP) data over the North Pacific, North America, and the North Atlantic and daily rainfall records in the contiguous United States are used to demonstrate that typical ENSO signals tend to be stronger and more stable during preferred phases of the NPO. Typical El Nino patterns (e.g., low pressure over the northeastern Pacific, dry northwest, and wet southwest, etc.) are strong and consistent only during the high phase of the NPO, which is associated with an anomalously cold northwestern Pacific. The generally reversed SLP and precipitation patterns during La Nina winters are consistent only during the low NPO phase. Climatic anomalies tend to be weak and spatially incoherent during low NPO-El Nino and high NPO-La Nina winters. These results suggest that confidence in ENSO-based long-range climate forecasts for North America should reflect interdecadal climatic anomalies in the North Pacific.

Gershunov, A, Michaelsen J.  1996.  Climatic-scale space-time variability of tropical precipitation. Journal of Geophysical Research-Atmospheres. 101:26297-26307.   10.1029/96jd01382   AbstractWebsite

More than 15 years of monthly microwave sounding unit rainfall data over the tropical oceans are analyzed to illustrate rainfall variability on various timescales and delineate its spatial patterns. The annual and semiannual components of the seasonal cycle are modeled with first and second annual harmonics at every 2.5 degrees x 2.5 degrees grid square. Regions of highest rainfall variability tend to be characterized by a powerful annual cycle. The semiannual cycle is generally a trivial component of the seasonal cycle, except in some regions where either the mean climatological precipitation is low or where the total seasonal cycle is weak. An interesting exception, in this respect, is a band of the southeastern tropical Pacific extending immediately to the south of the eastern equatorial Pacific cold tongue. Regions of highest climatological mean rainfall are characterized by weak seasonality but strong nonseasonal variability. After seasonality is described and removed from the data, nonseasonal variability is considered via principal component analysis in the time domain. The two dominant modes together describe precipitation variability associated with the El Nino-Southern Oscillation: they outline the evolution of warm- and cold-event precipitation anomalies and contrast the intense 1982-1983 warm event with the moderate events of 1986-1987 and 1992-1993. These two modes display oscillations with predominantly quasi-biennial and similar to 5-year periods. Another coherent mode summarizes intraseasonal variability which, although inadequately resolved by the monthly average rainfall data, displays typical signs of the 40- to 50-day oscillation. All coherent modes, despite having much of their energy concentrated around rather different frequencies, show signs of interaction.

Gershunov, A, Michaelsen J.  1996.  Vertical variability of water vapor in the midlatitude upper troposphere. Contributions to Atmospheric Physics [Beitraege zur Physik der Atmosphaere.], Wiesbaden, Germany. 69:205-214. AbstractWebsite

Radiative, dynamical and phase-change considerations of tropospheric moisture, all point to moisture in the upper troposphere as a major determinant of the global climate. Hemispheric-scale vertical variability of upper-tropospheric moisture is observed through a multivariate statistical analysis of three years of monthly mean SAGE-II data for 1986-88. Midlatitude zonally averaged vertical variability of moisture in the upper troposphere is separated into coherent modes using principal components analysis. Bulk vertical variations in the upper troposphere are separated from a dynamical mode of variability representing vertical moisture gradient and horizontal advection. The procedure is repeated for the northern and southern midlatitudes. We discuss the vertical structure and temporal variability of the meaningful modes and observe a north-south hemispheric asymmetry in the characteristics of the vertical moisture variability. It is hypothesized that midlatitude wave cyclones are responsible for the poleward and vertical transport of water vapor to and in the midlatitude upper troposphere.