Seasonal predictability of the tropical Atlantic variability: northern tropical Atlantic pattern

Abstract The present study investigates relative contributions of interannual, intraseasonal, and synoptic variations of environmental factors to tropical cyclone (TC) genesis over the northern tropical Atlantic (NTA) during July–October. Analysis shows that convection, lower-level vorticity, and midlevel specific humidity contribute to TC genesis through intraseasonal and synoptic variations with a larger contribution of the latter. The relative contribution of three components of vertical wind shear depends largely on its magnitude. The contribution of sea surface temperature (SST) to TC genesis is mainly due to the interannual component when total SST is above 27.5°C. The barotropic energy for the development of synoptic-scale disturbances comes mainly from climatological mean flows and intraseasonal wind variations. The proportion of contribution between synoptic and intraseasonal variations of convection, relative vorticity, and specific humidity is larger over the eastern NTA than over the western NTA. The barotropic energy conversion has a larger part related to climatological mean flows and intraseasonal wind variations over the eastern and western NTA, respectively. There are notable differences between the NTA and the western North Pacific (WNP). One is that the relative contribution of synoptic variations of convection, relative vorticity, and specific humidity is larger over the NTA, whereas that of intraseasonal variations is larger over the WNP. The other is that the barotropic energy conversion related to climatological mean flows and intraseasonal wind variations is comparable over the NTA, whereas that related to climatological mean flows is larger over the WNP.

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Aerosol concentration statistics for the Northern Tropical Atlantic

Journal of Geophysical Research Atmospheres ◽

10.1029/jc082i037p05954 ◽

1977 ◽

Vol 82 (37) ◽

pp. 5954-5964 ◽

Cited By ~ 88

Author(s):

D. L. Savoie ◽

J. M. Prospero

Keyword(s):

Aerosol Concentration ◽

Tropical Atlantic ◽

Concentration Statistics ◽

Northern Tropical Atlantic

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On the Inconsistent Relationship between Pacific and Atlantic Niños*

Journal of Climate ◽

10.1175/jcli-d-11-00553.1 ◽

2012 ◽

Vol 25 (12) ◽

pp. 4294-4303 ◽

Cited By ~ 42

Author(s):

Joke F. Lübbecke ◽

Michael J. McPhaden

Keyword(s):

Wind Stress ◽

El Niño ◽

El Nino ◽

Boreal Summer ◽

Western Boundary ◽

Tropical Atlantic ◽

Cold Tongue ◽

Trade Winds ◽

Boreal Spring ◽

Northern Tropical Atlantic

Abstract The tropical Atlantic wind response to El Niño forcing is robust, with weakened northeast trade winds north of the equator and strengthened southeast trade winds along and south of the equator. However, the relationship between sea surface temperature (SST) anomalies in the eastern equatorial Pacific and Atlantic is inconsistent, with El Niño events followed sometimes by warm and other times by cold boreal summer anomalies in the Atlantic cold tongue region. Using observational data and a hindcast simulation of the Nucleus for European Modeling of the Ocean (NEMO) global model at 0.5° resolution (NEMO-ORCA05), this inconsistent SST relationship is shown to be at least partly attributable to a delayed negative feedback in the tropical Atlantic that is active in years with a warm or neutral response in the eastern equatorial Atlantic. In these years, the boreal spring warming in the northern tropical Atlantic that is a typical response to El Niño is pronounced, setting up a strong meridional SST gradient. This leads to a negative wind stress curl anomaly to the north of the equator that generates downwelling Rossby waves. When these waves reach the western boundary, they are reflected into downwelling equatorial Kelvin waves that reach the cold tongue region in late boreal summer to counteract the initial cooling that is due to the boreal winter wind stress response to El Niño. In contrast, this initial cooling persists or is amplified in years in which the boreal spring northern tropical Atlantic warming is weak or absent either because of a positive North Atlantic Oscillation (NAO) phase or an early termination of the Pacific El Niño event.

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Mechanisms of Northern Tropical Atlantic Variability and Response to CO2 Doubling

Journal of Climate ◽

10.1175/jcli4137.1 ◽

2007 ◽

Vol 20 (11) ◽

pp. 2691-2705 ◽

Cited By ~ 15

Author(s):

Wim-Paul Breugem ◽

Wilco Hazeleger ◽

Reindert J. Haarsma

Keyword(s):

Moisture Transport ◽

Seasonal Cycle ◽

General Circulation ◽

Circulation Model ◽

Atmospheric Co2 ◽

Boreal Winter ◽

Tropical Atlantic ◽

Wes Feedback ◽

Meridional Mode ◽

Northern Tropical Atlantic

Abstract A model study has been made of the mechanisms of the meridional mode in the northern tropical Atlantic (NTA) and the response to a doubling of atmospheric CO2. The numerical model consists of an atmospheric general circulation model (GCM) coupled to a passive mixed layer model for the ocean. Results from two simulations are shown: a control run with present-day atmospheric CO2 and a run with a doubled CO2 concentration. The results from the control run show that the wind–evaporation–SST (WES) feedback is confined to the deep NTA. Furthermore, the temporal evolution of the meridional mode is phase locked with the seasonal cycle of the climatological intertropical convergence zone (CITCZ). The WES feedback is positive in boreal winter and spring when the CITCZ is located close to the equator but negative in summer and fall when the CITCZ shifts toward the north of the deep NTA. Similarly, the damping of the SST anomalies in the deep NTA by moisture-induced evaporation anomalies is much stronger in summer and fall than in winter and spring, related to a change in anomalous moisture transport. The results from the double-CO2 run show a substantial northward shift of the CITCZ in boreal winter and spring but little change in summer and fall. The change in the CITCZ can be explained by strong warming at the high northern latitudes in combination with a seasonally dependent WES feedback with accompanying changes in moisture transport in the deep NTA. The latter indicates that the change in the CITCZ is subject to phase locking with the seasonal cycle of the CITCZ itself. The meridional mode in the double-CO2 run weakens by 10%–20%. This originates from the weakening of the positive WES feedback in the deep NTA, which in turn is attributed to the northward shift of the CITCZ; because in the double-CO2 run the CITCZ stays south of the deep NTA for a shorter time period, the positive WES feedback in the deep NTA acts less long, and damping by moisture-induced evaporation anomalies starts earlier than in the control run.

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