Beschreibung: Despite the efforts of the modelling community to improve the representation of the sea surface temperature (SST) over the South Eastern Tropical Atlantic, warm biases still persist. In this work we use four different configurations of the fully-coupled AWI Climate Model (AWI-CM) which allow us to gain physics-based insight into the role of the oceanic and atmospheric resolutions of the model in the regional distribution of the SST. Our results show that a sole refinement of the oceanic resolution reduces warm biases further than a single increase of the atmospheric component. An increased oceanic resolution is required (i) to simulate properly the Agulhas Current and its associated rings; (ii) to reinforce the northward-flowing Benguela Current and (iii) to intensify coastal upwelling. The best results are obtained when both resolutions are refined. However, even in that case, warm biases persist, reflecting that some processes and feedbacks are still not optimally resolved. Our results indicate that overheating is not due to insufficient upwelling, but rather due to upwelling of waters which are warmer than observations as a result of an erroneous representation of the vertical distribution of temperature. Errors in the representation of the vertical temperature profile are the consequence of a warm bias in the simulated climate state.

Beschreibung: We investigate how the atmosphere is affected by the cold sea surface temperature (SST) anomalies of the Benguela Niñas using reanalysis data and a high-resolution atmospheric model. A composite analysis of reanalysis data based on five Benguela Niña events (5 years out of 39 years for 1979–2017) reveals that the rainfall along the Angolan coast is reduced significantly and other anomalies of precipitation are detected over the African continent and equatorial Atlantic. Those anomalies can be explained by the anomalies of vertically-integrated moisture flux (VIMF) and its divergence and convergence. Additionally, the Namibian low-level cloud and sea level pressure (SLP) are enhanced around the ABFZ by the Benguela Niñas. A simulation of the model forced by the cold SST anomalies of the Benguela Niñas reproduces the reduction (enhancement) of rainfall and VIMF divergent (convergent) anomaly over the Angolan coastal region (Gulf of Guinea). However, the other anomalies over the African continent are not significant. It is suggested that the effects of the Benguela Niñas are limited along southwestern coast of Africa. Composite analysis of reanalyses of rainfall anomalies associated with Benguela Niña events shows pattern over the other regions which might be induced by larger-scale atmospheric anomaly. This large-scale atmospheric anomaly can be linked with the South Atlantic Anticyclone and/or forced by the teleconnection from the tropical Pacific, for instance. The increment of the Namibian low-level cloud and SLP is simulated by the numerical experiments consistently. The low-level cloud becomes more frequent between 950 and 900 hPa and the radiative cooling by longwave radiation is reinforced during the Benguela Niñas events. In contrast, the cloud formation around 850 hPa is reduced and radiative cooling is weakened. It is indicated that this change in the cooling rate possibly induces the strengthening in the inversion layer over the cold SST anomalies.

Beschreibung: The Agulhas Current (AC) creates a sharp temperature gradient with the surrounding ocean, leading to a large turbulent flux of moisture from ocean to atmosphere. We use two simulations of the Weather Research and Forecasting (WRF) model to show the seasonal impact of the warm core of the AC on southern Africa precipitation. In one simulation the sea surface temperature (SST) of the AC is similar to satellite observations, while the second uses satellite SST observations spatially smoothed to reduce the temperature of the core of the AC by ~1.5°C. We show that decreasing the SST of the AC reduces the precipitation of the wettest seasons (austral summer and autumn) inland. Over the ocean, reducing the SST reduces precipitation, low-level wind convergence, SST and SLP Laplacian above the AC in all seasons, consistent with the pressure adjustment mechanism. Moreover, winter precipitation above the Current may be also related to increased latent flux. In summer and autumn, the AC SST reduction is also associated with decreased precipitation further inland (more than 1.5 mm/day), caused by an atmospheric circulation that decreases the horizontal moisture flux from the AC to South Africa. The reduction is also associated with higher geopotential height extending from the surface east and over the AC to the mid-troposphere over southeastern Africa. The westward tilted geopotential height is consistent with the linear response to shallow diabatic heating in midlatitudes. An identical mechanism occurs in spring but is weaker. Winter rainfall response is confined above the AC.

Beschreibung: The Atlantic Nino is one of the most important patterns of interannual tropical climate variability, but how climate change will influence this pattern is not well known due to large climate model biases. Here we show that state-of-the-art climate models robustly predict a weakening of Atlantic Ninos in response to global warming, mainly due to a decoupling of subsurface and surface temperature variations as the upper equatorial Atlantic Ocean warms. This weakening is predicted by most (〉80%) models in the Coupled Model Intercomparison Project Phases 5 and 6 under the highest emission scenarios. Our results indicate a reduction in variability by the end of the century by 14%, and as much as 24-48% when accounting for model errors using a simple emergent constraint analysis. Such a weakening of Atlantic Nino variability will potentially impact climate conditions and the skill of seasonal predictions in many regions. The Atlantic Nino is an important mode of tropical climate variability, but how it reacts to climate change is not well known due to model biases. Here the authors show a robust weakening of the Atlantic Nino of up to 24-48% under high emissions until the end of the century.