Abstract
In this paper circulation and precipitation climate patterns over the South America domain obtained from NCEP-NCAR reanalysis I, JRA-55 and ERA-40 reanalysis datasets were intercompared within the period 1970-2001 in order to verify their climatic association with the Pacific Decadal Oscillation, PDO. Although all three datasets show similar climatic patterns during each PDO phase, they show distinct intensities for circulation and precipitation variables. The mean low frequency flow propagation over the South Pacific during negative and positive PDO phases reaches central-eastern South America (SA) with opposite signals, contributing to modulate the local climate. During the negative PDO phases, the northerly flow over South America is intensified, while during the positive phase it is weakened. NCEP-NCAR reanalysis I, in comparison with JRA-55 and ERA-40 values, always indicates the strongest signal for circulation and precipitation variables. NCEP-NCAR reanalysis I meridional wind at the low-level jet area is about 5% and 12% larger than the respective JRA-55 and the ERA-40 values. For the positive and negative PDO phases, only NCEP-NCAR and ERA-40 high level zonal wind anomalies show, respectively, divergent (positive PDO) and convergent (negative PDO) patterns close to 140o W at equatorial latitudes of the Pacific Ocean. This one indicates distinct representations of the positioning of Walker cells in the three reanalysis datasets. GPCC dataset, NCEP-NCAR, JRA-55 and ERA-40 reanalysis, show, in general, the dipole precipitation pattern over eastern of South America, with signs of changes between the central-eastern and southeastern areas during PDO reversal phases. Over the central-eastern South American region, NCEP-NCAR precipitation is, on average, 57 % larger than GPCC values, and, respectively, 35% and 46% larger than JRA-55 and ERA-40 values. Observed patterns of anomalies for different PDO phases appear in all reanalysis data sets, leading us to conclude that the physical meaning is independent from the datasets. Even considering the JRA-55 climate signal over the SA associated with the PDO influence as the weakest one, the ability of JRA-55 to calculate monthly precipitation provided the highest linear correlation coefficient values in tropical areas, leading us to consider this data set as the most reliable for the studied area.
Highlights
Oceanic changes over the Pacific Ocean are potential inductors of the South American climate
During the positive Pacific Decadal Oscillation (PDO) phase (2nd subperiod, 1977– 1996), an opposite pattern was observed in these three datasets, characterized by positive 250 hPa-geopotential height anomalies over the central equatorial Pacific and negative anomalies over centraleastern South America
The circulation and precipitation climate patterns over the South American domain obtained from NCEPNCAR, JRA-55, and ERA-40 reanalysis datasets were intercompared during the period 1970–2001 in order to verify their climatic association with the Pacific Decadal Oscillation
Summary
Oceanic changes over the Pacific Ocean are potential inductors of the South American climate. One of the first studies considering low-frequency circulation patterns between the Southern Hemispheric middle and high latitudes was originally developed by Mo and White (1985) Later on, this pattern was well-described in Mo and Ghil (1987), and it was associated with quasi-stationary modes in the Southern Hemisphere, and characterized by wavetrains extending from central equatorial Pacific areas to the South American continent, known as the Pacific–South America (PSA) pattern. This pattern was well-described in Mo and Ghil (1987), and it was associated with quasi-stationary modes in the Southern Hemisphere, and characterized by wavetrains extending from central equatorial Pacific areas to the South American continent, known as the Pacific–South America (PSA) pattern Before their contribution, other studies on the patterns of the low-frequency circulation characterization over the Northern Hemisphere (PNA patterns) were developed (Horel, 1981; Hoskins and Karoly, 1981; Wallace and Gutzler, 1981). The role of heat sources on South American low-frequency circulation patterns was shown by Karoly (1989), completing the results of Mo and Ghil (1987), which showed the characteristic quasi-stationary pattern over South America
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