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Abstract South American climate is influenced by both Atlantic multidecadal variability (AMV) and Pacific multidecadal variability (PMV). But how they jointly affect South American precipitation and surface air temperature is not well understood. Here we analyze composite anomalies to quantify their combined impacts using observations and reanalysis data. During an AMV warm (cold) phase, PMV-induced JJA precipitation anomalies are more positive (negative) over 0°-10°S and southeastern South America, but more negative (positive) over the northern Amazon and central Brazil. PMV-induced precipitation anomalies in DJF are more positive (negative) over Northeast Brazil and southeastern South America during the warm (cold) AMV phase, but more negative (positive) over the central Amazon Basin and central-eastern Brazil. PMV’s impact on AMV-induced precipitation anomalies shows similar dipole patterns. The precipitation changes result from perturbations of the local Hadley and Walker Circulations. In JJA, PMV- and AMV-induced temperature anomalies are more positive (negative) over entire South America when the other basin is in a warm (cold) phase, but in DJF temperature anomalies are more positive (negative) only over the central Andes and central-eastern Brazil and more negative (positive) over southeastern South America and Patagonia. Over central Brazil in JJA and southern Bolivia and northern Argentina in DJF, the temperature and precipitation anomalies are negatively correlated. Our results show that the influence of Pacific and Atlantic multidecadal variability need to be considered jointly, as significant departures from the mean AMV or PMV fingerprint can occur during a cold or warm phase of the other basin’s mode.
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Impacts of Atlantic and Pacific Multidecadal Variability on South American Precipitation and Temperature in the CESM Simulations and Observations
Abstract The Community Earth System Model version 1 (CESM1) and version 2 (CESM2)'s abilities to simulate the impacts of Atlantic multidecadal variability (AMV) and Pacific multidecadal variability (PMV) on South American precipitation and temperature have not been assessed, and how the AMV and PMV modulate each other's influences on South American climate is not well understood. Here we use observations, reanalyses, and CESM1 and CESM2 simulations from 1920 to 2015 to study those problems. The models can reproduce the observed precipitation and temperature responses to AMV well, but can only roughly reproduce such responses to PMV. The precipitation response over the South Atlantic convergence zone (SACZ) is better simulated by CESM2 compared to CESM1, which is associated with an improved horizontal moisture flux over this region. However, the models cannot accurately simulate the observed differences between the influences of Pacific interannual and multidecadal variability on South American precipitation and temperature. The impacts of AMV and PMV on South American precipitation are modulated by the other mode via changes in horizontal moisture flux over the SACZ and River Plate basin in summer, as well as changes in vertical motion over the equatorial regions in winter. Similarly, the impacts of AMV and PMV on South American temperature are also modulated by the other mode. Over water‐limited regions, such as northeastern Brazil and southern Argentina, the precipitation and temperature responses are anti‐correlated, possibly via surface evaporation.
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- PAR ID:
- 10517795
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 129
- Issue:
- 12
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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