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Abstract Atlantic Niños dominate the equatorial Atlantic variability during boreal summer (June–August). The coupled ocean‐atmosphere processes associated with Atlantic Niños have been extensively documented. However, the role of atmospheric convectively coupled Kelvin waves (CCKWs), which are uncorrelated to those previously identified processes, in triggering Atlantic Niños has been unclear. Here we identify CCKWs using Wheeler‐Kiladis filtering based on 10°S–10°N averaged daily outgoing longwave radiation. CCKWs propagate eastward from South America and induce surface zonal wind anomalies over the equatorial Atlantic Ocean. Strong anomalous CCKWs during spring (March–May) and their associated surface westerly wind anomalies can trigger downwelling oceanic Kelvin waves that change the east–west slope of the thermocline, consequently leading to Atlantic Niño. A causal effect network reveals that interannual sea surface temperature (SST) anomalies in the Atlantic Niño Index area and CCKWs, both in spring, are uncorrelated, but both appear to influence SST anomalies over the Atlantic Niño Index area in summer. The CCKWs are also uncorrelated to other coupled ocean‐atmosphere sources, such as El Niño–Southern Oscillation and Atlantic Meridional Mode. Among a total of 15 Atlantic Niño/Niña events identified for the period of 1980–2017, two‐thirds of the events are linked to CCKWs. In particular, three Atlantic Niña events (1982, 1994, and 2005) are mainly triggered by CCKWs, under unfavorable SST preconditions. Thus, CCKWs in spring, due to atmospheric internal variability, provide another mechanism for triggering Atlantic Niños and probably weaken their predictability.
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Interannual Variability and Trends of Sea Surface Temperature Around Southern South America
The interannual variability and trends of sea surface temperature (SST) around southern South America are studied from 1982 to 2017 using monthly values of the Optimally Interpolation SST version 2 gridded database. Mid-latitude (30°–50°S) regions in the eastern South Pacific and western South Atlantic present moderate to intense warming (~0.4°C decade −1 ), while south of 50°S the region around southern South America presents moderate cooling (~ −0.3°C decade −1 ). Two areas of statistically significant trends of SST anomalies (SSTa) with opposite sign are found on the Patagonian Shelf over the southwest South Atlantic: a warming area delimited between 42 and 45°S (Northern Patagonian Shelf; NPS), and a cooling area between 49 and 52°S (Southern Patagonian Shelf; SPS). Between 1982 and 2017 the warming rate has been 0.15 ± 0.01°C decade −1 representing an increase of 0.52°C at NPS, and the cooling rate has been –0.12 ± 0.01°C decade −1 representing a decrease of 0.42°C at SPS. On both regions, the largest trends are observed during 2008–2017 (0.35 ± 0.02°C decade −1 at NPS and –0.27 ± 0.03°C decade −1 at SPS), while the trends in 1982–2007 are non-significant, indicating the record-length SSTa trends are mostly associated with the variability observed during the past 10 years of the record. The spectra of the records present significant variance at interannual time scales, centered at about 80 months (~6 years). The observed variability of SSTa is studied in connection with atmospheric forcing (zonal and meridional wind components, wind speed, wind stress curl and surface heat fluxes). During 1982–2007, the local meridional wind explains 25–30% of the total variance at NPS and SPS on interannual time scales. During 2008–2017, the SSTa at NPS is significantly anticorrelated with the local zonal wind ( r = –0.85), while at SPS it is significantly anticorrelated with the meridional wind ( r = –0.61). Our results show that a substantial fraction of the interannual variability of SSTa around southern South America can be described by the first three empirical orthogonal function (EOF) modes which explain 28, 16, and 12% of the variance, respectively. The variability of the three EOF principal components time series is associated with the combined variability of El Niño–Southern Oscillation, the Interdecadal Pacific Oscillation and the Southern Annular Mode.
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- Award ID(s):
- 1645887
- PAR ID:
- 10398936
- Date Published:
- Journal Name:
- Frontiers in Marine Science
- Volume:
- 9
- ISSN:
- 2296-7745
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/fmars.2022.829144
