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Michael S. Halpert
and
Gerald D. Bell

The climate of 1996 can be characterized by several phenomena that reflect substantial deviations from the mean state of the atmosphere persisting from months to seasons. First, mature cold-episode conditions persisted across the tropical Pacific from November 1995 through May 1996 and contributed to large-scale anomalies of atmospheric circulation, temperature, and precipitation across the Tropics, the North Pacific and North America. These anomalies were in many respects opposite to those that had prevailed during the past several years in association with a prolonged period of tropical Pacific warm-episode conditions (ENSO). Second, strong tropical intraseasonal (Madden–Julian oscillations) activity was observed during most of the year. The impact of these oscillations on extratropical circulation variability was most evident late in the year in association with strong variations in the eastward extent of the East Asian jet and in the attendant downstream circulation, temperature, and precipitation patterns over the eastern North Pacific and central North America. Third, a return to the strong negative phase of the atmospheric North Atlantic oscillation (NAO) during November 1995–February 1996, following a nearly continuous 15-yr period of positive-phase NAO conditions, played a critical role in affecting temperature and precipitation patterns across the North Atlantic, Eurasia, and northern Africa. The NAO also contributed to a significant decrease in wintertime temperatures across large portions of Siberia and northern Russia from those that had prevailed during much of the 1980s and early 1990s.

Other regional aspects of the short-term climate during 1996 included severe drought across the southwestern United States and southern plains states during October 1995–May 1996, flooding in the Pacific Northwest region of the United States during the 1995/96 and 1996/97 winters, a cold and extremely snowy 1995/96 winter in the eastern United States, a second consecutive year of above-normal North Atlantic hurricane activity, near-normal rains in the African Sahel, above-normal rainfall across southeastern Africa during October 1995–April 1996, above-normal precipitation for most of the year across eastern and southeastern Australia following severe drought in these areas during 1995, and generally nearnormal monsoonal rains in India with significantly below-normal rainfall in Bangladesh and western Burma.

The global annual mean surface temperature for land and marine areas during 1996 averaged 0.21°C above the 1961–90 base period means. This is a decrease of 0.19°C from the record warm year of 1995 but was still among the 10 highest values observed since 1860. The global land-only temperature for 1996 was 0.06°C above normal and was the lowest anomaly observed since 1985 (−0.11°C). Much of this relative decrease in global temperatures occurred in the Northern Hemisphere extratropics, where land-only temperatures dropped from 0.42°C above normal in 1995 to 0.04°C below normal in 1996.

The year also witnessed a continuation of near-record low ozone amounts in the Southern Hemisphere stratosphere, along with an abnormally prolonged appearance of the “ozone hole” into early December. The areal extent of the ozone hole in November and early December exceeded that previously observed for any such period on record. However, its areal extent at peak amplitude during late September–early October was near that observed during the past several years.

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Gerald D. Bell
and
Michael S. Halpert

The global climate during 1997 was affected by both extremes of the El Niño-Southern Oscillation (ENSO), with weak Pacific cold episode conditions prevailing during January and February, and one of the strongest Pacific warm episodes (El Niño) in the historical record prevailing during the remainder of the year. This warm episode contributed to major regional rainfall and temperature anomalies over large portions of the Tropics and extratropics, which were generally consistent with those observed during past warm episodes. In many regions, these anomalies were opposite to those observed during 1996 and early 1997 in association with Pacific cold episode conditions.

Some of the most dramatic El Niño impacts during 1997 were observed in the Tropics, where anomalous convection was evident across the entire Pacific and throughout most major monsoon regions of the world. Tropical regions most affected by excessive El Niño–related rainfall during the year included 1) the eastern half of the tropical Pacific, where extremely heavy rainfall and strong convective activity covered the region from April through December; 2) equatorial eastern Africa, where excessive rainfall during October–December led to widespread flooding and massive property damage; 3) Chile, where a highly amplified and extended South Pacific jet stream brought increased storminess and above-normal rainfall during the winter and spring; 4) southeastern South America, where these same storms produced above-normal rainfall during June–December; and 5) Ecuador and northern Peru, which began receiving excessive rainfall totals in November and December as deep tropical convection spread eastward across the extreme eastern Pacific.

In contrast, El Niño-–elated rainfall deficits during 1997 included 1) Indonesia, where significantly below-normal rainfall from June through December resulted in extreme drought and contributed to uncontrolled wildfires; 2) New Guinea, where drought contributed to large-scale food shortages leading to an outbreak of malnutrition; 3) the Amazon Basin, which received below-normal rainfall during June–December in association with substantially reduced tropical convection throughout the region; 4) the tropical Atlantic, which experienced drier than normal conditions during July–December; and 5) central America and the Caribbean Sea, which experienced below-normal rainfall during March–December.

The El Niño also contributed to a decrease in tropical storm and hurricane activity over the North Atlantic during August–November, and to an expanded area of conditions favorable for tropical cyclone and hurricane formation over the eastern North Pacific. These conditions are in marked contrast to both the 1995 and 1996 hurricane seasons, in which significantly above-normal tropical cyclone activity was observed over the North Atlantic and suppressed activity prevailed across the eastern North Pacific.

Other regional aspects of the short-term climate during 1997 included 1) wetter than average 1996/97 rainy seasons in both northeastern Australia and southern Africa in association with a continuation of weak cold episode conditions into early 1997; 2) below-normal rainfall and drought in southeastern Australia from October 1996 to December 1997 following very wet conditions in this region during most of 1996; 3) widespread flooding in the Red River Valley of the north-central United States during April following an abnormally cold and snowy winter; 4) floods in central Europe during July following several consecutive months of above-normal rainfall; 5) near-record to record rainfall in southeastern Asia during June–August in association with an abnormally weak upper-level monsoon ridge; and 6) near-normal rainfall across India during the Indian monsoon season (June–September) despite the weakened monsoon ridge.

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Gerald D. Bell
and
John E. Janowiak

This paper presents an observational analysis of the large-scale atmospheric circulation prior to and during the Midwest floods of June–July 1993. The floods developed and persisted in association with three major circulation features, none of which alone would likely have produced such intense and prolonged flooding. First, a persistent, positive phase of the North Pacific teleconnection pattern was observed throughout the Pacific sector for four months prior to the onset of the floods. This anomalous circulation was associated with much above-normal cyclone activity over the middle latitudes of the North Pacific and with below-normal cyclone activity over the western and central United States. Second, a major change in this pattern occurred over the western United States in late May, which established very strong zonal flow from the western Pacific to the eastern United States. This flow provided a “duct” for the intense cyclones to propagate directly into the Midwest throughout the month of June. These storms triggered a series of intense convective complexes over the Midwest, resulting in major flooding. Third, during July a persistent wave pattern with highly amplified southwesterly flow became established over the western and central United States. This circulation, in conjunction with a quasi-stationary frontal boundary and sustained moisture transport into the central United States, was associated with a continuation of excessive rainfall and flooding in the Midwest.

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Michael S. Halpert
,
Gerald D. Bell
,
Vernon E. Kousky
, and
Chester F. Ropelewski

The El Niño-Southern Oscillation (ENSO) phenomenon is a major contributor to the observed year-to-year variability in the Pacific Ocean and in the global atmospheric circulation. The short-term climate system witnessed the return to the mature phase of warm ENSO conditions (commonly referred to as the El Nino) during early 1995 for the third time in four years. This frequency of occurrence is unprecedented in the last 50 years and is comparable to that observed during the prolonged 1911–15 ENSO episode.

These warm ENSO conditions contributed to a large-scale disruption of the normal patterns of wind, rainfall, and temperature over much of the tropics and middle latitudes, particularly during the December 1994–February 1995 period. This period was followed by a dramatic decrease in sea surface temperatures in the tropical Pacific, resulting in a complete disappearance of all warm episode conditions during June–August and in the development of weak coldepisode conditions during September–November.

Changes in the tropical Pacific were accompanied by pronounced, large-scale changes in the atmospheric circulation patterns from those that had prevailed during much of the early 1990s. Particular examples of these changes include 1) a dramatic return to a very active hurricane season over the North Atlantic, following four consecutive years of significantly below-normal hurricane activity; 2) the return to above-normal rainfall throughout Indonesia, northern Australia, and southern Africa, following a prolonged period of below-normal rainfall and periodic drought; and 3) a northward shift of the jet stream and storm track position over the eastern half of the North Pacific during the latter part of the year, following several winter seasons (three in the last four) characterized by a significant strengthening, southward shift, and eastward extension of these features toward the southwestern United States.

Other regional climate anomalies during 1995 included extreme warmth throughout western and central Asia during January–May and colder than normal conditions in this region during November–December, severe flooding in the midwestern United States (April–May), abnormally wet conditions in California and the southwestern United States (December–February) combined with near-record warmth over eastern North America, deadly heat waves in the central United States (mid-July) and India (first three weeks of June), drought in the northeastern United States (August), a drier-than-normal rainy season in central Brazil (September–December), and an intensification of drier-than-normal conditions over southern Brazil, Uruguay, and northeastern Argentina at the end of the year.

The global annual mean surface temperature for land and marine areas during 1995 averaged 0.40°C above the 1961–90 mean. This value exceeds the previous warmest year in the record (1990) by 0.04°C. The Northern Hemisphere also recorded its warmest year on record during 1995, with a mean departure from normal of 0.55°C. The global annual mean surface temperature for land areas only during 1995 was the second warmest since 1951.

The year also witnessed near-record low ozone amounts in the Southern Hemisphere stratosphere, with minimum values only slightly higher than the record low values observed in 1993. The areal extent of very low ozone values during 1995 was as widespread over Antarctica as in the record low year of 1993.

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Gerald D. Bell
,
Michael S. Halpert
,
Chester F. Ropelewski
,
Vernon E. Kousky
,
Arthur V. Douglas
,
Russell C. Schnell
, and
Melvyn E. Gelman

The global climate during 1998 was affected by opposite extremes of the ENSO cycle, with one of the strongest Pacific warm episodes (El Niño) in the historical record continuing during January–early May and Pacific cold episode (La Niña) conditions occurring from JulyñDecember. In both periods, regional temperature, rainfall, and atmospheric circulation patterns across the Pacific Ocean and the Americas were generally consistent with those observed during past warm and cold episodes.

Some of the most dramatic impacts from both episodes were observed in the Tropics, where anomalous convection was evident across the entire tropical Pacific and in most major monsoon regions of the world. Over the Americas, many of the El Niño– (La Niña–) related rainfall anomalies in the subtropical and extratropical latitudes were linked to an extension (retraction) of the jet streams and their attendant circulation features typically located over the subtropical latitudes of both the North Pacific and South Pacific.

The regions most affected by excessive El Niño–related rainfall included 1) the eastern half of the tropical Pacific, including western Ecuador and northwestern Peru, which experienced significant flooding and mudslides; 2) southeastern South America, where substantial flooding was also observed; and 3) California and much of the central and southern United States during January–March, and the central United States during April–June.

El Niño–related rainfall deficits during 1998 included 1) Indonesia and portions of northern Australia; 2) the Amazon Basin, in association with a substantially weaker-than-normal South American monsoon circulation; 3) Mexico, which experienced extreme drought throughout the El Niño episode; and 4) the Gulf Coast states of the United States, which experienced extreme drought during April–June 1998. The El Niño also contributed to extreme warmth across North America during January–May.

The primary La Niña–related precipitation anomalies included 1) increased rainfall across Indonesia, and a nearly complete disappearance of rainfall across the east-central equatorial Pacific; 2) above-normal rains across northwestern, eastern, and northern Australia; 3) increased monsoon rains across central America and Mexico during October–December; and 4) dryness across equatorial eastern Africa.

The active 1998 North Atlantic hurricane season featured 14 named storms (9 of which became hurricanes) and the strongest October hurricane (Mitch) in the historical record. In Honduras and Nicaragua extreme flooding and mudslides associated with Hurricane Mitch claimed more than 11 000 lives. During the peak of activity in August–September, the vertical wind shear across the western Atlantic, along with both the structure and location of the African easterly jet, were typical of other active seasons.

Other regional aspects of the short-term climate included 1) record rainfall and massive flooding in the Yangtze River Basin of central China during June–July; 2) a drier and shorter-than-normal 1997/98 rainy season in southern Africa; 3) above-normal rains across the northern section of the African Sahel during June–September 1998; and 4) a continuation of record warmth across Canada during June–November.

Global annual mean surface temperatures during 1998 for land and marine areas were 0.56°C above the 1961–90 base period means. This record warmth surpasses the previous highest anomaly of +0.43°C set in 1997. Record warmth was also observed in the global Tropics and Northern Hemisphere extratropics during the year, and is partly linked to the strong El Nino conditions during January–early May.

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Gerald D. Bell
,
Michael S. Halpert
,
Russell C. Schnell
,
R. Wayne Higgins
,
Jay Lawrimore
,
Vernon E. Kousky
,
Richard Tinker
,
Wasila Thiaw
,
Muthuvel Chelliah
, and
Anthony Artusa

The global climate during 1999 was impacted by Pacific cold episode (La Niña) conditions throughout the year, which resulted in regional precipitation and atmospheric circulation patterns across the Pacific Ocean and the Americas that are generally consistent with those observed during past cold episodes. The primary La Niña-related precipitation anomalies included 1) increased rainfall across Indonesia, and a nearly complete disappearance of rainfall across the east-central and eastern equatorial Pacific; 2) above-normal rains across northwestern and northern Australia; 3) increased monsoon rains across the Sahel region of western Africa; 4) above-average rains over southeastern Africa, 5) above-average rains over the Caribbean Sea and portions of Central America, and 6) below-average rains in southeastern South America.

The La Niña also contributed to persistent cyclonic circulation anomalies in the subtropics of both hemispheres, which flanked the area of suppressed convective activity over the eastern half of the equatorial Pacific. In the Northern Hemisphere this anomaly feature contributed to a pronounced westward retraction of the wintertime East Asian jet stream, which subsequently impacted precipitation and storm patterns across the eastern North Pacific and western North America. The La Niña-related pattern of tropical rainfall also contributed to a very persistent pattern of anticyclonic circulation anomalies in the middle latitude of both hemispheres, extending from the eastern Pacific across the Atlantic and Africa eastward to Australasia. This anomaly pattern was associated with an active Atlantic hurricane season, an inactive eastern North Pacific hurricane season, above-average rains in the African Sahel, and an overall amplification of the entire southeast Asian summer monsoon complex.

The active 1999 North Atlantic hurricane season featured 12 named storms, 8 of which became hurricanes, and 5 of which became intense hurricanes. The peak of activity during mid-August–October was accompanied by low vertical wind shear across the central and western Atlantic, along with both a favorable structure and location of the African easterly jet. In contrast, only 9 tropical storms formed over the eastern North Pacific during the year, making it one of the most inactive years for that region in the historical record. This relative inactivity was linked to a persistent pattern of high vertical wind shear that covered much of the main development region of the eastern North Pacific.

Other regional aspects of the short-term climate included: 1) above-average wintertime precipitation and increased storminess in the Pacific Northwest, United States; 2) above-average monsoonal rainfall across the southwestern United States; 3) drought over the northeastern quadrant of the United States during April–mid-August; 4) hurricane-related flooding in the Carolinas during September; 5) drought over the south-central United States during July–November; 6) below-average rainfall in the Hawaiian Islands throughout the year, with long-term dryness affecting some parts of the islands since October 1997; 7) a continuation of long-term drought conditions in southeastern Australia, with most of Victoria experiencing below-average rainfall since late 1996; and 8) above-average rainfall in central China during April–August.

Global annual mean surface temperatures during 1999 for land and marine areas were 0.41°C above the 1880–1998 long-term mean, making it the fifth warmest year in the record. However, significant cooling was evident in the Tropics during 1999 in association with a continuation of La Niña conditions. In contrast, temperatures in both the Northern Hemisphere and Southern Hemisphere extratropics were the second warmest in the historical record during 1999, and only slightly below the record 1998 anomalies.

The areal extent of the Antarctic ozone hole remained near record levels during 1999. The ozone hole also lasted longer than has been observed in past years.

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Jay H. Lawrimore
,
Michael S. Halpert
,
Gerald D. Bell
,
Matthew J. Menne
,
Bradfield Lyon
,
Russell C. Schnell
,
Karin L. Gleason
,
David R. Easterling
,
Wasila Thiaw
,
William J. Wrightand
,
Richard R. Heim Jr.
,
David A. Robinson
, and
Lisa Alexander

The global climate in 2000 was again influenced by the long-running Pacific cold episode (La Niña) that began in mid-1998. Consistent with past cold episodes, enhanced convection occurred across the climatologically convective regions of Indonesia and the western equatorial Pacific, while convection was suppressed in the central Pacific. The La Niña was also associated with a well-defined African easterly jet located north of its climatological mean position and low vertical wind shear in the tropical Atlantic and Caribbean, both of which contributed to an active North Atlantic hurricane season. Precipitation patterns influenced by typical La Niña conditions included 1) above-average rainfall in southeastern Africa, 2) unusually heavy rainfall in northern and central regions of Australia, 3) enhanced precipitation in the tropical Indian Ocean and western tropical Pacific, 4) little rainfall in the central tropical Pacific, 5) below-normal precipitation over equatorial east Africa, and 6) drier-than-normal conditions along the Gulf coast of the United States.

Although no hurricanes made landfall in the United States in 2000, another active North Atlantic hurricane season featured 14 named storms, 8 of which became hurricanes, with 3 growing to major hurricane strength. All of the named storms over the North Atlantic formed during the August–October period with the first hurricane of the season, Hurricane Alberto, notable as the third-longest-lived tropical system since reliable records began in 1945. The primary human loss during the 2000 season occurred in Central America, where Hurricane Gordon killed 19 in Guatemala, and Hurricane Keith killed 19 in Belize and caused $200 million dollars of damage.

Other regional events included 1) record warm January–October temperatures followed by record cold November–December temperatures in the United States, 2) extreme drought and widespread wildfires in the southern and western Unites States, 3) continued long-term drought in the Hawaiian Islands throughout the year with record 24-h rainfall totals in November, 4) deadly storms and flooding in western Europe in October, 5) a summer heat wave and drought in southern Europe, 6) monsoon flooding in parts of Southeast Asia and India, 7) extreme winter conditions in Mongolia, 8) extreme long-term drought in the Middle East and Southwest Asia, and 9) severe flooding in southern Africa.

Global mean temperatures remained much above average in 2000. The average land and ocean temperature was 0.39°C above the 1880–1999 long-term mean, continuing a trend to warmer-than-average temperatures that made the 1990s the warmest decade on record. While the persistence of La Niña conditions in 2000 was associated with somewhat cooler temperatures in the Tropics, temperatures in the extratropics remained near record levels. Land surface temperatures in the high latitudes of the Northern Hemisphere were notably warmer than normal, with annually averaged anomalies greater than 2°C in parts of Alaska, Canada, Asia, and northern Europe.

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Stephen Baxter
,
Gerald D Bell
,
Eric S Blake
,
Francis G Bringas
,
Suzana J Camargo
,
Lin Chen
,
Caio A. S Coelho
,
Ricardo Domingues
,
Stanley B Goldenberg
,
Gustavo Goni
,
Nicolas Fauchereau
,
Michael S Halpert
,
Qiong He
,
Philip J Klotzbach
,
John A Knaff
,
Michelle L'Heureux
,
Chris W Landsea
,
I.-I Lin
,
Andrew M Lorrey
,
Jing-Jia Luo
,
Andrew D Magee
,
Richard J Pasch
,
Petra R Pearce
,
Alexandre B Pezza
,
Matthew Rosencrans
,
Blair C Trewin
,
Ryan E Truchelut
,
Bin Wang
,
H Wang
,
Kimberly M Wood
, and
John-Mark Woolley
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Howard J. Diamond
,
Carl J. Schreck III
,
Emily J. Becker
,
Gerald D. Bell
,
Eric S. Blake
,
Stephanie Bond
,
Francis G. Bringas
,
Suzana J. Camargo
,
Lin Chen
,
Caio A. S. Coelho
,
Ricardo Domingues
,
Stanley B. Goldenberg
,
Gustavo Goni
,
Nicolas Fauchereau
,
Michael S. Halpert
,
Qiong He
,
Philip J. Klotzbach
,
John A. Knaff
,
Michelle L'Heureux
,
Chris W. Landsea
,
I.-I. Lin
,
Andrew M. Lorrey
,
Jing-Jia Luo
,
Kyle MacRitchie
,
Andrew D. Magee
,
Ben Noll
,
Richard J. Pasch
,
Alexandre B. Pezza
,
Matthew Rosencrans
,
Michael K. Tippet
,
Blair C. Trewin
,
Ryan E. Truchelut
,
Bin Wang
,
Hui Wang
,
Kimberly M. Wood
,
John-Mark Woolley
, and
Steven H. Young
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