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David S. Gutzler, Sharon M. Sullivan, and Deirdre M. Kann

Abstract

The wettest year, by a huge margin, in the instrumental history for the state of New Mexico was 1941. The authors describe the extraordinary magnitude and persistence of above-average precipitation across the seasonal cycle during this year and consider possible climatic causes of this exceptional annual anomaly through examination of a wide variety of historical records and modern analysis tools. Indices of the Pacific decadal oscillation and the El Niño–Southern Oscillation were both extremely positive in 1941, consistent with the historical tendency for above-average precipitation across the southern United States under such conditions. However, the largest precipitation anomalies occurred in transition season months that do not fit the typical seasonality associated with strong ENSO- or PDO-related continental climate anomalies in the more recent historical record. The difficulty in attributing this extreme annual anomaly to any specific climatic cause is a reminder that the radiosonde era provides only a limited sample of natural climatic variability. The number and quality of data sources available for preradiosonde years allows for surprisingly in-depth observational analysis of early twentieth-century climatic anomalies.

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Deirdre M. Kann, Geoffrey J. DiMego, and Ralph A. Petersen

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No abstract available.

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David S. Gutzler, Deirdre M. Kann, and Casey Thornbrugh

Abstract

Seasonal predictability of winter precipitation anomalies across the U.S. Southwest derived from knowledge of antecedent, late-summer Pacific Ocean surface temperatures is examined empirically. Previous studies have shown that equatorial Pacific SST anomalies associated with the El Niño–Southern Oscillation (ENSO) cycle, which are persistent from late summer through winter, exhibit a strong relationship with winter precipitation in Arizona and New Mexico. Here the degree to which seasonal predictability in this region is modulated by longer-term oceanic fluctuations associated with the Pacific decadal oscillation (PDO) is assessed. When all years from 1950 through 1997 are considered as a single dataset, inclusion of the PDO signal adds only slightly to the ENSO-based statistical predictability of Southwest winter precipitation anomalies. However, when the dataset is split into two subperiods delineated by a major shift in the PDO (before and after 1977), the ENSO-based predictability and, to a lesser extent, PDO-based predictability are substantially modified. Before 1977, negative winter precipitation anomalies are strongly tied to ENSO cold years but warm years do not systematically lead to positive precipitation anomalies. After 1977, this asymmetry is reversed and positive precipitation anomalies predictably follow warm ENSO years but cold years yield no precipitation predictability. Within each subperiod, interannual PDO fluctuations yield less predictability than ENSO fluctuations. Thus ENSO-based predictability seems to undergo a profound decadal modification that might be associated statistically with the PDO, but the physical link to North Pacific Ocean temperatures is problematic.

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David A. Salstein, Deirdre M. Kann, Alvin J. Miller, and Richard D. Rosen

By exchanging angular momentum with the solid portion of the earth, the atmosphere plays a vital role in exciting small but measurable changes in the rotation of our planet. Recognizing this relationship, the International Earth Rotation Service invited the U.S. National Meteorological Center to organize a Sub-bureau for Atmospheric Angular Momentum (SBAAM) for the purpose of collecting, distributing, archiving, and analyzing atmospheric parameters relevant to earth rotation/polar motion. These functions of wind and surface pressure are being computed with data from several of the world's weather services, and they are being widely applied to the research and operations of the geodetic community. The SBAAM began operating formally in October 1989, and this article highlights its development, operations, and significance.

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Eric Rogers, Geoffrey J. DiMego, Joseph P. Gerrity, Ralph A. Petersen, Brian D. Schmidt, and Deirdre M. Kann

Analyses and forecasts for the first 2 weeks of the Genesis of Atlantic Lows Experiment (GALE) are described. These fields were produced using the National Meteorological Center (NMC) Regional Analysis and Forecast System (RAFS). Two sets of analyses and forecasts were constructed: one using the NMC operational database only (Level IIIa), and one using the NMC data merged with high-density observations taken during GALE (Level IIIb).

During the first 14 days of GALE, supplemental data were collected throughout two Intensive Observing Periods (IOPs). Comparisons of the Level IIIa and IIIb analyses over the GALE observing region in the southeastern United States indicated a worsening of the geopotential height analysis at operational NWS rawinsonde sites using the supplemental IIIb data. This was caused by inconsistencies in the height measurements at the high-density GALE rawinsonde sites. Such patterns were not observed in the wind and temperature analyses.

During IOP No. 1, the Level IIIa and IIIb Nested Grid Model (NGM) forecasts were nearly identical. For IOP No. 2, one forecast cycle saw an improvement in the Level IIIb forecasts due to offshore GALE dropwindsonde data, while another was improved by the inclusion of late-arriving rawinsonde data in the IIIb analysis. The inland, high-density GALE soundings, however, had a negligible impact on NGM forecasts during the entire 12-day period.

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