Editorial Type:
Article
Article Type:
Research Article

Structure of Interannual-to-Decadal Climate Variability in the Tropical Atlantic Sector

Alfredo Ruiz-Barradas Department of Meteorology, University of Maryland at College Park, College Park, Maryland

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James A. Carton Department of Meteorology, University of Maryland at College Park, College Park, Maryland

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Sumant Nigam Department of Meteorology, University of Maryland at College Park, College Park, Maryland

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Print Publication:
15 Sep 2000
DOI:
https://doi.org/10.1175/1520-0442(2000)013<3285:SOITDC>2.0.CO;2
Page(s):
3285–3297
Restricted access

Abstract

A search for coupled modes of atmosphere–ocean interaction in the tropical Atlantic sector is presented. Previous studies have provided conflicting indications of the existence of coupled modes in this region. The subject is revisited through a rotated principal component analysis performed on datasets spanning the 36-yr period 1958–93. The analysis includes four variables, sea surface temperature, oceanic heat content, wind stress, and atmospheric diabatic heating. The authors find that the first rotated principal component is associated with fluctuations in the subtropical wind system and correlates with the North Atlantic oscillation (NAO), while the second and third modes, which are the focus of interest, are related to tropical variability.

The second mode is the Atlantic Niño mode with anomalous sea surface temperature and anomalous heat content in the eastern equatorial basin. Wind stress weakens to the west of anomalously warm water, while convection is shifted south and eastward. Surface and upper-level wind anomalies of this mode resemble those of El Niño–Southern Oscillation (ENSO) events. When the analysis is limited to boreal summer, the season of maximum amplitude, the Atlantic Niño mode explains 7.5% of the variance of the five variables. Thermodynamic air–sea interactions do not seem to play a role for this mode.

The third mode is associated with an interhemispheric gradient of anomalous sea surface temperature and a dipole pattern of atmospheric heating. In its positive phase anomalous heating occurs over the warmer Northern Hemisphere with divergence aloft shifting convection to the north and west of the equator and intensifying the subtropical jet stream, while descending motion occurs on the western side of the Southern Hemisphere. Surface and subsurface structures in the ocean are controlled by surface winds. This interhemispheric mode is strongest in boreal spring when it explains 9.1% of the combined variance of the five variables. Thermodynamic air–sea interactions do seem to control the associated sea surface temperature anomalies, although equatorial dynamics may play a role as well.

The authors also examine the connection of the tropical Atlantic to other basins. ENSO events cause patterns of winds, heating, and sea surface temperatures resembling the interhemispheric mode described above. The lag between changes in the Atlantic and Pacific is 4–5 months for the interhemispheric mode. In contrast, no significant impact of ENSO is found on the Atlantic Niño mode. Likewise, no impact of the midlatitude North Atlantic (the NAO) is found on the Tropics, but some impact of the Tropics is found on the midlatitude North Atlantic.

Corresponding author address: Dr. James A. Carton, Department of Meteorology, University of Maryland at College Park, College Park, MD 20742-2425.

Abstract

A search for coupled modes of atmosphere–ocean interaction in the tropical Atlantic sector is presented. Previous studies have provided conflicting indications of the existence of coupled modes in this region. The subject is revisited through a rotated principal component analysis performed on datasets spanning the 36-yr period 1958–93. The analysis includes four variables, sea surface temperature, oceanic heat content, wind stress, and atmospheric diabatic heating. The authors find that the first rotated principal component is associated with fluctuations in the subtropical wind system and correlates with the North Atlantic oscillation (NAO), while the second and third modes, which are the focus of interest, are related to tropical variability.

The second mode is the Atlantic Niño mode with anomalous sea surface temperature and anomalous heat content in the eastern equatorial basin. Wind stress weakens to the west of anomalously warm water, while convection is shifted south and eastward. Surface and upper-level wind anomalies of this mode resemble those of El Niño–Southern Oscillation (ENSO) events. When the analysis is limited to boreal summer, the season of maximum amplitude, the Atlantic Niño mode explains 7.5% of the variance of the five variables. Thermodynamic air–sea interactions do not seem to play a role for this mode.

The third mode is associated with an interhemispheric gradient of anomalous sea surface temperature and a dipole pattern of atmospheric heating. In its positive phase anomalous heating occurs over the warmer Northern Hemisphere with divergence aloft shifting convection to the north and west of the equator and intensifying the subtropical jet stream, while descending motion occurs on the western side of the Southern Hemisphere. Surface and subsurface structures in the ocean are controlled by surface winds. This interhemispheric mode is strongest in boreal spring when it explains 9.1% of the combined variance of the five variables. Thermodynamic air–sea interactions do seem to control the associated sea surface temperature anomalies, although equatorial dynamics may play a role as well.

The authors also examine the connection of the tropical Atlantic to other basins. ENSO events cause patterns of winds, heating, and sea surface temperatures resembling the interhemispheric mode described above. The lag between changes in the Atlantic and Pacific is 4–5 months for the interhemispheric mode. In contrast, no significant impact of ENSO is found on the Atlantic Niño mode. Likewise, no impact of the midlatitude North Atlantic (the NAO) is found on the Tropics, but some impact of the Tropics is found on the midlatitude North Atlantic.

Corresponding author address: Dr. James A. Carton, Department of Meteorology, University of Maryland at College Park, College Park, MD 20742-2425.

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