OS1.8

The equatorial Atlantic is subject to interannual variability that is centered in the eastern cold tongue region and is known as the Atlantic Zonal Mode (AZM). Previous studies have indicated that AZM variability has declined over the recent decades and this tendency is projected to continue based on climate change simulations. The period 2000 to mid-2019 was arguably most conspicuous in this regard, as it did not contain any major AZM event. In late 2019, however, the strongest event in more than 40 years developed. This was followed, in 2021, by an equally warm event. In the present work we examine the mechanisms behind these recent events. We show that while the accompanying wind stress forcing was strong, it cannot account for the exceptional strength of the two events. Analysis suggests that Ekman pumping north of the equator contributed to the strength of the events by generating downwelling Rossby waves that were reflected into downwelling Kelvin waves at the equator. In addition, an examination of observed sea-surface height and ocean temperature from reanalysis and PIRATA buoys suggests that there was a steady buildup of heat in the eastern equatorial region (20W-10E, 10S-5N) since about 2015. This excessive heat content was discharged during the 2019 and 2021 events and may have contributed to their exceptional strength. Our results highlight the need for a close monitoring of oceanic conditions in the region. This will not only have implications for seasonal prediction but also for the long-term development of AZM variability.

How to cite: Richter, I., Tokinaga, H., Okumura, Y., and Keenlyside, N.: Is equatorial Atlantic variability resurging?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11006, https://doi.org/10.5194/egusphere-egu22-11006, 2022.

In 2021 there was an exceptionally strong Atlantic Niño—stronger than the last major event in 1996. Positive SST anomalies developed in May and peaked in June-August. There was a build up of heat content in the spring in the western north Atlantic that could be related to local wind stress curl anomalies.  The event appears to have been triggered by zonal wind anomalies in April and May in the western equatorial Atlantic, when strong rainfall anomalies were also observed along the equator. The event terminated with rainfall anomalies shifting northward in late summer. Interestingly, there was also a strong Benguela Niño that developed already in April and has persisted into boreal summer. Furthermore, the event may have contributed to the La Niña event that developed later in the year in the Pacific.

How to cite: Keenlyside, N.: The Super Atlantic Niño of 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13546, https://doi.org/10.5194/egusphere-egu22-13546, 2022.

Marine ecosystems are largely impacted by marine heat waves (MHWs). That includes coral reefs which are experiencing coral bleaching and subsequently loss of marine biodiversity because of MHWs. Such reefs are crucial habitat of fish stocks feeding the world’s population. As ocean temperatures increase, the occurrences of MHWs become more frequent. A further solid mechanistic understanding is therefore urgently required for adaptation and mitigation of future changes in MHWs. Importantly, this knowledge is needed on a local-scale.

Here we use a coupled ocean-atmosphere regional modelling system (COAWST), consisting of the atmospheric model WRF and the ocean model ROMS, to dynamically downscale an area over the Caribbean Sea and the Gulf of Mexico. Compared to a global model with coarser horizontal resolution, our 12 km grid spacing resolves smaller scale phenomena and ensures a skilled representation of the air-sea interactions which are important for a correct representation of MHWs. We show the results of a 20-year regional climate simulation and compare the output with two global climate model simulations (NorESM2-MM and NorESM2-MH) to address the added value of the regional simulation. Our high-resolution simulation represents the temporal distribution (frequency and duration) of MHWs well compared to the coarser global models which produce too few, but too long heatwaves in the area.

How to cite: Pontoppidan, M., De Falco, C., Mooney, P. A., and Tjiputra, J.: Marine heat waves: The added value of a high resolution, coupled atmosphere-ocean regional climate model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4629, https://doi.org/10.5194/egusphere-egu22-4629, 2022.

The scarcity of in-situ measurements and the variability among individual events has limited our understanding of the drivers and impacts of the tropical Atlantic Ocean circulation. Here we investigate the response of the surface and subsurface ocean circulation to the two main modes of tropical Atlantic Variability (TAV): the Meridional Mode (MM) and Equatorial Mode (EM). For this purpose, we use three oceanic reanalyses and an interannual forced-ocean simulation covering the period 1982-2018. The developing phase of the MM is associated with a spring intensification of the North Equatorial Countercurrent (NECC), the Equatorial Undercurrent (EUC) and the north South Equatorial Current (nSEC) in the eastern equatorial margin. It also triggers Rossby waves that reach the western boundary and are reflected as equatorial Kelvin waves that weaken the ocean surface and subsurface transports and cause anomalous warm equatorial conditions in boreal summer. During the developing spring-summer phase of the EM, the westward surface zonal transport is considerably reduced with no clear impact at subsurface levels. During the fall EM decaying phase, the reflected Kelvin wave reverses the zonal pressure-gradients at the equator and the westward equatorial nSEC is reinforced. This is accompanied by a weakening of the EUC that suggests an additional off-equatorial forcing. Our results reveal that the ocean circulation responds to both MM and EM, endorsing the key role played by the propagating zonal waves in connecting the tropical and equatorial ocean transports.

How to cite: Martín-Rey, M., Vallès-Casanova, I., and Pelegri, J. L.: Response of the upper ocean circulation to tropical Atlantic interannual modes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6900, https://doi.org/10.5194/egusphere-egu22-6900, 2022.

Tropical Instability Waves (TIWs) at the equatorial Atlantic Ocean lead to SST cooling due to enhanced mixing and heat fluxes above the EUC core. This phenomenon has been studied predominantly at the equator and to the north, where TIWs are most pronounced. However, a recent study has shown the presence of subsurface TIWs in the Atlantic Ocean, which frequently occur to the south of the equator. As TIW induced subsurface mixing leads to SST cooling at the equator, we suspect a similar cooling may occur in the Southern Hemisphere due to the presence of subsurface TIWs. Using one decade of high-resolution ICON ocean simulations, we investigate such effect of subsurface TIWs in the southern hemisphere on SST in the tropical Atlantic Ocean. The analysis of all terms of the mixed layer heat budget allows for the investigation and quantification of the processes involved in subsurface TIW induced SST changes.

How to cite: Specht, M. S., Jungclaus, J., and Bader, J.: Influence of subsurface tropical instability waves on sea surface temperature in the tropical Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12325, https://doi.org/10.5194/egusphere-egu22-12325, 2022.

The Azores High shows a strong intra-seasonal variability that, transmitted to the whole North Atlantic by the Trade Wind, generates a multi-factor variability of the Canary Islands eastern edge upwelling system. In this work, we study the cold season (March to April), using satellite observations and numerical simulation, and how the variability of the wind at the equator, the Kelvin and coastal waves, and the local wind along the North-West African coast combine
to force upwelling variability. Composite analyses show how, in 80% of the cases, the pulsations of the anticyclone at 40 d excite equatorial waves that arrive in the Senegalese upwelling 15 d later, precisely at the time of the phase change of coastal wind anomalies.  These waves trapped at the coast, from upwelling or downwelling, reinforce the local wind anomaly. The intra-seasonal variability of the SST is thus the result of a double local and remote effect whose respective contributions we quantify 

How to cite: Sané, B., Lazar, A., Wade, M., and Gaye, A. T.: Pulsations of the Azores anticyclone at intra-seasonal scale: how oceanic waves and coastal wind anomalies combine constructively to force the variability of the north-eastern boundary upwelling system in winter-spring., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12733, https://doi.org/10.5194/egusphere-egu22-12733, 2022.

Tropical interbasin teleconnections at inter-annual time scales are receiving much attention in the last years. However, their controlling factors and long-term changes are still under debate. In this work, we investigate whether selected features in the climatology, the position of the ITCZ and strong tropical convection, can influence the teleconnections between the tropical Atlantic and Pacific basins at inter-annual timescales.

For investigation, we contrast a CGCM control simulation with an experiment in which the climatological position of the ITCZ is shifted in latitude by artificially reducing the shortwave radiation incident in a region of the south Atlantic sector. The perturbation magnitude and sign are such that the local model’s biases in Atlantic SST are reduced. The experiment shows stronger interannual variability over the tropical Atlantic and Pacific oceans, a westward extension of the Atlantic Niño pattern, and enhanced interannual teleconnections between equatorial Atlantic and Pacific.

We examine the mechanisms at work for these changes. We find several factors as major contributors to enhance the tropical interbasin teleconnections. One is the modified Walker circulation resulting from the westward extension of SST anomalies during the Atlantic Niño and concurrent westward displacement of convection. The other factors are the enhancement of the precipitation at the equator and the shallowing of thermocline in the Pacific, which make the latter basin more sensitive to both local and remote perturbations.

On the contrary, the North Tropical Atlantic – equatorial Pacific teleconnection is weakened in the experiment, despite the strongest impact of the NTA anomalies in the north tropical Pacific winds. due to the opposite effect on divergence exerted by the off equatorial winds related to NTA and the equatorial winds related to the concomitant warming in the eastern and central equatorial Pacific.

How to cite: Losada, T., Rodríguez-Fonseca, B., Mechoso, C. R., Mohino, E., and Castaño-Tierno, A.: Interhemispheric asymmetries, ITCZ location and interannual tropical Atlantic-Pacific interactions produced by South Atlantic cooling., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12308, https://doi.org/10.5194/egusphere-egu22-12308, 2022.

Observational studies have reported that tropical Atlantic interannual variability impacts
on ENSO in different seasons and periods: Atlantic Ni ̃nos (AN) in boreal summer during
negative phases of the Atlantic Multidecadal Variability (AMV); and tropical north Atlantic
(TNA) in boreal spring during positive AMV. Nevertheless, this relation is not clear for the
whole observational record. This paper is an step forward towards understanding of tropical
Atlantic impacts on ENSO: how and when do they occur? Using observations and a pool of
preindustrial control simulations from the CMIP5 initiative we investigate the background
ocean and atmospheric conditions promoting these tropical interbasin connections.Periods
with a negative AN-ENSO connection appear characterized by a shallower thermocline
over the western Pacific and deeper in the east, together with an increase in interannual
SST variability over the tropics. Periods with a negative TNA-ENSO connection appear
characterized by a steeper thermocline in the Pacific and positive interhemispheric SST
gradient in the the Atlantic. A decrease in tropical Pacific atmospheric and ocean variability
characterizes these periods.

How to cite: Rodriguez-Fonseca, B., Polo Sanchez, I., Mohino Harris, E., Losada Doval, T., Martin del Rey, M., Keenlyside, N., and Mechoso, C. R.: Multidecadal Modulations of Tropical Atlantic impact on ENSO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12602, https://doi.org/10.5194/egusphere-egu22-12602, 2022.