1. Introduction

This annual climate assessment is the ninth in an ongoing series produced by the Climate Prediction Center. It is designed to provide a timely summary of the global climate system during 1998. 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 and regional climate change, including atmospheric temperatures, ozone, carbon dioxide, and snow cover are addressed in section 2. In section 3 an analysis is presented of the very strong Pacific warm episode (El Niņo) conditions which continued during January–May 1998, and the development of Pacific cold episode (La Niņa) conditions during the second half of the year. The impacts of these extremes in the El Niņo/Southern Oscillation (ENSO) cycle on the wintertime jet streams in both the Northern and Southern Hemispheres are also examined. Regional climate highlights and summaries of the major monsoon systems are discussed in section 4. Topics include severe flooding during June–August in the Yangtze River basin of central and eastern China, a very active North Atlantic hurricane season, a summary of the rainy seasons in southern and western Africa, extreme heat and drought in the south-central and southeastern United States during April–June, and temperature and precipitation highlights over South America. In section 5, 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 set of maps that have appeared in previous annual climate assessments. Although 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 fields for anomaly computations.

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