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The North Atlantic Subtropical Anticyclone

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The North Atlantic Subtropical Anticyclone 728 JOURNAL OF CLIMATE VOLUME 10 The North Atlantic Subtropical Anticyclone ROBERT E. DAVIS,BRUCE P. H AYDEN,DAVID A. GAY,WILLIAM L. PHILLIPS, AND GREGORY V. J ONES Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia (Manuscript received 28 September 1993, in ®nal form 22 August 1996) ABSTRACT The semipermanent subtropical anticyclone over the North Atlantic basin (the ``Azores high'') has a major in¯uence on the weather and climate of much of North America, western Europe, and northwestern Africa. The authors develop a climatology of the Azores high by examining its spatial and temporal changes since 1899. Using gridded surface pressure values, anticyclones are identi®ed when the daily pressure is $1020 mb and frequencies are tabulated for each half month from 1899 to 1990. Principal components analysis is applied to analyze the anticyclone's spatial variance structure. The Azores high is dominated by two spatial modes: a summer pattern, in which high pressure dominates the Atlantic basin, and a winter pattern, in which anticyclones are present over eastern North America and northwestern Africa. Century-long declines in these two modes indicate that there has been a net removal of atmospheric mass over the subtropical Atlantic. Other modes include a meridional versus zonal circulation pattern and omega blocks. Time series of the mean annual principal component scores indicate that meridional ¯ow has been increasing over the Atlantic and that blocking anticyclones have become more prevalent over west-central Europe and less common over the northeastern Atlantic and the British Isles. 1. Introduction manent circulation features for changes in their seasonal or yearly strength or position that might indicate longer- Atmospheric scientists have long recognized the exis- term climatic variability or climate change. To date no tence of certain quasi-permanent features of the global thorough study has been completed documenting the atmospheric circulation and their in¯uence upon regional size, intensity, and variability of the Azores high on weather and climate. These ``centers of actions'' were ®rst both a seasonal and interannual basis. identi®ed by Teissereng de Bort (1883) from maps of av- The primary goal of our research is to develop a cli- erage monthly sea level pressure. In the Northern Hemi- matology of the frequency of high pressure over the sub- sphere, circulation is closely coupled to daily, seasonal, tropical and midlatitude North Atlantic Ocean and the ad- and interannual changes in four primary centers of action: jacent continents. Speci®cally, we investigate 1) seasonal the Aleutian and Icelandic cyclones, and the subtropical variations in half-month anticyclone frequencies by ana- anticyclones over the Atlantic and Paci®c Oceans. lyzing mean maps and maps of half-month frequency The subtropical anticyclone over the North Atlantic changes, and 2) yearly changes in the frequency of high Ocean basin, commonly referred to as the ``Azores'' or pressure by analyzing time series of principal component the ``Bermuda high,'' has a major in¯uence on weather scores. The results provide a complete description of both and climate over the eastern United States, western Eu- inter- and intraannual changes in the strength and position rope, and northwestern Africa. Because of its in¯uence, of the North Atlantic subtropical high over the last century. it is important to investigate this and other semiper- This type of study is particularly important since it allows for examination of possible climate changes over a long period of record, in this case almost 100 yr. 1 The terms Bermuda and Azores high occasionally refer to the western and eastern extensions of the North Atlantic anticyclone, respectively. However, common usage, especially in North America, 2. Background considers these terms to be interchangeable, however geographically Anticyclone climatologies are frequently developed inexact. In this paper we refer to this semipermanent high pressure feature as the North Atlantic subtropical anticyclone or the Azores by counting the number of occurrences of high pressure high. (usually de®ned by a closed isobar) within grid boxes of various sizes. Such studies have been completed us- ing 58 lat 3 58 long grid cells by Harman (1987) and Corresponding author address: Dr. Robert E. Davis, University of Virginia, Department of Environmental Sciences, Charlottesville, VA Klein (1957) for North America and the Northern Hemi- 22903. sphere, respectively. Zishka and Smith (1980) per- E-mail: [email protected] formed a similar analysis of migratory anticyclones over q1997 American Meteorological Society Unauthenticated | Downloaded 10/03/21 04:32 AM UTC APRIL 1997 DAVIS ET AL. 729 North America on a 28328grid. These analyses proved derived from the U.S. Historical Map Series; after 1939 useful in identifying predominant anticyclone tracks and several sources were used, including both objective and their seasonality, as well as interannual anticyclone fre- hand-drawn analyses by the navy and the National Me- quency changes. The semipermanent anticyclones have teorological Center (currently known as the National also been studied using mean sea level pressure maps. Centers for Environmental Prediction) (Williams and Lamb (1973) computed mean sea level pressure for van Loon 1976; Trenberth and Paolino 1980). 5-day periods for two 20-yr time periods characterized In our research, an anticyclone was deemed to be by different circulation types. He noted that certain cir- present at a grid node on a given day when the sea level culation features (including the Azores high) exhibit a pressure equalled or exceeded 1020 mb. This threshold ``pulse'' phenomenon, in which their mean position and was selected based upon observations that values above intensity change dramatically between consecutive 1020 mb tend to isolate regions of local pressure max- 5-day periods. Makrogiannis (1988) used mean sea level ima over subtropical and midlatitude regions of the At- pressure values to calculate the total atmospheric mass lantic Ocean and adjacent land areas. Sahsamanoglou over seven 58358grids over Europe and the eastern (1990a) determined that the average central pressure of Atlantic Ocean. the Azores high is 1023.5 mb and the standard deviation In the study most closely related to ours, Sahsaman- is 1.2 mb. Thus, 75% of all central pressures fall be- oglou (1990a) focused on spatial and temporal changes tween 1022 and 1025 mb, and values less than 1021 of Atlantic centers of action based upon monthly and mb occur less than 4% of the time. For each half month, annual mean sea level pressure values from 1873 to the number of days in which the pressure exceeded 1020 1980 at a 58 lat 3 108 long resolution. His analysis was mb was counted at each grid node. Therefore, our ®nal primarily concerned with changes in the magnitude and dataset consists of frequency counts of high pressure location of the highest central pressure of the North occurrence within 58358cells for 24 half months yr21 Atlantic subtropical high. His general results showed over 91 yr. that the anticyclone migrates in a somewhat elliptical The NCAR gridded dataset is commonly used in at- pattern from month to month within an area bounded mospheric research and thus has been carefully scruti- by 258±408N and 208±508W. Based upon mean central nized by several researchers. Williams and van Loon pressures, he also detected three Azores high epochs: (1976) identi®ed numerous anomalies in which obser- from 1903 to 1930 when the pressure was extremely vations deviated by more than three standard deviations high, from 1968 to 1980 when it was somewhat above from the nodal mean and did not correspond with values normal, and from 1931 to 1967 when pressures were at nearby stations. However, they detected almost no generally below normal. Angell and Korshover (1974) outliers over the Atlantic basinÐthe area used in our detected an eastward shift in the mean position of the study. Trenberth and Paolino (1980) noted a substantial Azores high from 1899 to 1967, as well as a signi®cant number of errors in the dataset, as well as several dis- decline in central pressure between 1920 and 1965. continuities between 1899 and 1977. The primary dis- Our work differs from previous research on the continuities occurred in the summer of 1939, in Decem- Azores high for the following reasons. ber 1945, and during 1956. In comparison to other regions of the globe, particularly Asia, errors and sys- 1) The data collection method incorporates the entire tematic biases are not prevalent over the subtropical regional domain occupied by the anticyclone, yet is Atlantic, where substantial ship traf®c has allowed for still responsive to intensity changes. Thus, even the compilation of a fairly robust dataset. The apparent though our analysis focuses on changes in the size bias of primary concern to our research is in elevated and position of the system, variations in the anti- pressure in the early part of the record. T-tests of mean cyclone's strength are implied. pressure between two subperiods (1899±1939 and 2) This is the ®rst multivariate analysis of the spatial 1956±77) indicate that pressures are signi®cantly higher dimensions of the Azores high. in the earlier period over the western Atlantic south of 3) The combination of the half-month time interval em- 308±358N. The question of whether these early readings ployed, the long period of record (1899±1990), and are real or ®ctitious is addressed in our conclusions and the 58358grid provides the most detailed spatial implications section. and temporal analysis of the Azores high yet un- Prior to 1940 data are frequently missing. In our anal- dertaken. ysis, a grid node is coded as ``missing'' for a half month if any single daily value is missing over that 15-day period. This conservative criterion assures that all of 3. Data collection and analysis the semimonthly frequency counts are directly compa- rable.
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