This annual climate assessment is the tenth in an ongoing series produced by the Climate Prediction Center. It is designed to provide a timely summary of the global climate system during 1999. Specific components of the assessment include a documentation of global climate variations, an examination of oceanic and atmospheric anomalies in the global Tropics and extratropics, and an analysis of selected significant regional climate highlights.

Issues related to global temperatures are addressed in section 2. In section 3 an analysis is presented of the Pacific cold episode (La Niņa) which persisted throughout the year. The La Niņa impacts on global precipitation and atmospheric circulation patterns are also examined. Regional climate highlights and summaries of the major monsoon systems are discussed in section 4. Topics include heavy wintertime precipitation in the Pacific Northwest United States, an active North Atlantic hurricane season, above-average monsoon rains over the southwestern United States, severe drought in the northeastern and south-central United States, above-average rains in southern Africa, the African Sahel, and northern/ northwestern Australia, dryness in southeastern Australia, near-average monsoon rains in India, above-average rains in central China, excessive January–February snowfall in the Alps, and December wind storms in Europe.

In section 5, an assessment of various trace gas concentrations and their potential impacts on climate is presented. This analysis includes a description of the near-record Antarctic ozone hole during 1999, along with trends in such gases as carbon dioxide, carbon monoxide, methane, and several chlorofluorocarbon species. In section 6, seasonal maps of temperature anomalies, precipitation percentiles, and 500-hPa heights and anomalies are presented. These maps are included for reference, and to continue the analyses that have appeared in previous annual climate assessments. While each section is essentially self-contained, there is liberal cross-referencing between sections to aid navigation through the document.

A variety of data sources were used in the compilation of this assessment, including 1) gridded analyses from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/ NCAR) Climate Data Assimilation System/Reanalysis Project (Kalnay et al. 1996), 2) surface data obtained from the operational Global Telecommunications System (GTS), 3) satellites, 4) radiosondes, and 5) ship reports. Selected analyses were also obtained from international climate data centers. It should be noted that due to the variety of different data sources used in this assessment, it is not possible to maintain a consistent base period among all anomaly fields.