Meridional migration of ENSO impact on tropical Atlantic precipitation controlled by the seasonal cycle

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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Seasonal Cycle of the Mixed Layer Heat Budget in the Northeastern Tropical Atlantic Ocean

Journal of Climate ◽

10.1175/jcli-d-13-00037.1 ◽

2013 ◽

Vol 26 (20) ◽

pp. 8169-8188 ◽

Cited By ~ 19

Author(s):

Gregory R. Foltz ◽

Claudia Schmid ◽

Rick Lumpkin

Keyword(s):

Mixed Layer ◽

Annual Cycle ◽

Turbulent Mixing ◽

Seasonal Cycle ◽

Heat Budget ◽

Heat Content ◽

Boreal Summer ◽

Tropical Atlantic ◽

Mixed Layer Heat Budget ◽

Vertical Turbulent Mixing

Abstract The seasonal cycle of the mixed layer heat budget in the northeastern tropical Atlantic (0°–25°N, 18°–28°W) is quantified using in situ and satellite measurements together with atmospheric reanalysis products. This region is characterized by pronounced latitudinal movements of the intertropical convergence zone (ITCZ) and strong meridional variations of the terms in the heat budget. Three distinct regimes within the northeastern tropical Atlantic are identified. The trade wind region (15°–25°N) experiences a strong annual cycle of mixed layer heat content that is driven by approximately out-of-phase annual cycles of surface shortwave radiation (SWR), which peaks in boreal summer, and evaporative cooling, which reaches a minimum in boreal summer. The surface heat-flux-induced changes in the mixed layer heat content are damped by a strong annual cycle of cooling from vertical turbulent mixing, estimated from the residual in the heat balance. In the ITCZ core region (3°–8°N) a weak seasonal cycle of mixed layer heat content is driven by a semiannual cycle of SWR and damped by evaporative cooling and vertical turbulent mixing. On the equator the seasonal cycle of mixed layer heat content is balanced by an annual cycle of SWR that reaches a maximum in October and a semiannual cycle of turbulent mixing that cools the mixed layer most strongly during May–July and November. These results emphasize the importance of the surface heat flux and vertical turbulent mixing for the seasonal cycle of mixed layer heat content in the northeastern tropical Atlantic.

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An assessment of marine biogeochemical processes in the tropical Atlantic in NorESMs

10.5194/egusphere-egu21-9592 ◽

2021 ◽

Author(s):

Shunya Koseki ◽

Lander Rodriguez Crespo ◽

Noel Keenlyside

Keyword(s):

Seasonal Cycle ◽

Earth System Model ◽

System Model ◽

Tropical Atlantic ◽

Biogeochemical Processes ◽

Earth System ◽

Equatorial Atlantic ◽

Subsurface Ocean ◽

Mean Sea Surface ◽

Summer Bloom

Most state-of-the-art earth system model still exhibit large biases in the tropical Atlantic. This study aims to investigate how the physical bias influences the marine biogeochemical processes in the tropical Atlantic using Norwegian Earth System Model (NorESM). We assess four different configurations of NorESM: NorESM version 1is taken as benchmark (NorESM-CTL), a version of this model with a physical bias correction using anomaly coupling (NorESM-AC), and NorESM version 2 with low and medium atmospheric resolution (NorESM-LM/NorESM-MM) is also utilized.&#160;With respect to NorESM-CTL, the annual-mean sea surface temperature (SST) bias is improved largely in NorESM-AC and NorESM-MM in the equatorial Atlantic and southeast Atlantic. On the other hand, the improvement of seasonal cycle of SST can be seen in NorESM-AC and the two versions of NorESM2; development of Atlantic Cold Tongue (ACT) is realistic in terms of location and timing. Corresponding to the ACT seasonal cycle, the primary production in the equatorial Atlantic is also improved and in particular, the Atlantic summer bloom is well represented in NorESM-AC and NorESM-MM even though the amount of production is still much smaller than satellite observations. This realistic summer bloom can be related to the well-represented shallow thermocline and associated nitrate supply from the subsurface ocean at the equator.

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A NEMO-based model of Sargassum distribution in the Tropical Atlantic: description of the model and sensitivity analysis (NEMO-Sarg1.0)

10.5194/gmd-2020-383 ◽

2020 ◽

Author(s):

Julien Jouanno ◽

Rachid Benshila ◽

Léo Berline ◽

Antonin Soulié ◽

Marie-Hélène Radenac ◽

...

Keyword(s):

Seasonal Cycle ◽

Large Scale ◽

Seasonal Forecast ◽

Ocean Model ◽

Model Parameters ◽

Tropical Atlantic ◽

Socioeconomic Impacts ◽

Modelling Framework ◽

Basin Scale ◽

And Performance

Abstract. The Tropical Atlantic is facing a massive proliferation of Sargassum since 2011, with severe environmental and socioeconomic impacts. The development of Sargassum modelling is essential to clarify the link between Sargassum distribution and environmental conditions, and to lay the groundwork for a seasonal forecast on the scale of the Tropical Atlantic basin. We developed a modelling framework based on the NEMO ocean model, which integrates transport by currents and waves, physiology of Sargassum with varying internal nutrients quota, and considers stranding at the coast. The model is initialized from basin scale satellite observations and performance was assessed over the Sargassum year 2017. Model parameters are calibrated through the analysis of a large ensemble of simulations, and the sensitivity to forcing fields like riverine nutrients inputs, atmospheric deposition, and waves is discussed. Overall, results demonstrate the ability of the model to reproduce and forecast the seasonal cycle and large-scale distribution of Sargassum biomass.

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