Madagascar: A pacemaker for the Agulhas Current system?

[1] Western boundary currents are driven by zonally integrated wind-stress curl over the width of subtropical basins. This cross-basin integration is interrupted in the South Indian Ocean where Madagascar presents a formidable barrier. Nevertheless, a western boundary current has been thought to exist in the Mozambique Channel, the Mozambique Current. Recent observations have however shown that no such current exists and that the flow in the channel instead consists of a train of eddies. Is this western boundary anomaly due to the presence of Madagascar? We have used a primitive equations model to investigate the flow in the South West Indian Ocean as if there were no Madagascar. We show that a normal, continuous western boundary current is then formed that constitutes a continuum with the Agulhas Current. The presence of Madagascar is shown to affect the frequency of inter-ocean exchange events south of Africa

On the Tropical Atlantic SST warm bias in the Kiel Climate Model

Most of the current coupled general circulation models show a strong warm bias in the eastern Tropical Atlantic. In this paper, various sensitivity experiments with the Kiel Climate Model (KCM) are described. A largely reduced warm bias and an improved seasonal cycle in the eastern Tropical Atlantic are simulated in one particular version of KCM. By comparing the stable and well-tested standard version with the sensitivity experiments and the modified version, mechanisms contributing to the reduction of the eastern Atlantic warm bias are identified and compared to what has been proposed in literature. The error in the spring and early summer zonal winds associated with erroneous zonal precipitation seems to be the key mechanism, and large-scale coupled ocean-atmosphere feedbacks play an important role in reducing the warm bias. Improved winds in boreal spring cause the summer cooling in the eastern Tropical Atlantic (ETA) via shoaling of the thermocline and increased upwelling, and hence reduced sea surface temperature (SST). Reduced SSTs in the summer suppress convection and favor the development of low-level cloud cover in the ETA region. Subsurface ocean structure is shown to be improved, and potentially influences the development of the bias. The strong warm bias along the southeastern coastline is related to underestimation of low-level cloud cover and the associated overestimation of surface shortwave radiation in the same region. Therefore, in addition to the primarily wind forced response at the equator both changes in surface shortwave radiation and outgoing longwave radiation contribute significantly to reduction of the warm bias from summer to fall

Climate fluctuations of tropical coupled system: The role of ocean dynamics

The tropical oceans have long been recognized as the most important region for large-scale ocean–atmosphere interactions, giving rise to coupled climate variations on several time scales. During the Tropical Ocean Global Atmosphere (TOGA) decade, the focus of much tropical ocean research was on understanding El Niño–related processes and on development of tropical ocean models capable of simulating and predicting El Niño. These studies led to an appreciation of the vital role the ocean plays in providing the memory for predicting El Niño and thus making seasonal climate prediction feasible. With the end of TOGA and the beginning of Climate Variability and Prediction (CLIVAR), the scope of climate variability and predictability studies has expanded from the tropical Pacific and ENSO-centric basis to the global domain. In this paper the progress that has been made in tropical ocean climate studies during the early years of CLIVAR is discussed. The discussion is divided geographically into three tropical ocean basins with an emphasis on the dynamical processes that are most relevant to the coupling between the atmosphere and oceans. For the tropical Pacific, the continuing effort to improve understanding of large- and small-scale dynamics for the purpose of extending the skill of ENSO prediction is assessed. This paper then goes beyond the time and space scales of El Niño and discusses recent research activities on the fundamental issue of the processes maintaining the tropical thermocline. This includes the study of subtropical cells (STCs) and ventilated thermocline processes, which are potentially important to the understanding of the low-frequency modulation of El Niño. For the tropical Atlantic, the dominant oceanic processes that interact with regional atmospheric feedbacks are examined as well as the remote influence from both the Pacific El Niño and extratropical climate fluctuations giving rise to multiple patterns of variability distinguished by season and location. The potential impact of Atlantic thermohaline circulation on tropical Atlantic variability (TAV) is also discussed. For the tropical Indian Ocean, local and remote mechanisms governing low-frequency sea surface temperature variations are examined. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, this study focuses on the active role of ocean dynamics in a seasonally locked east–west internal mode of variability, known as the Indian Ocean dipole (IOD). Influences of the IOD on climatic conditions in Asia, Australia, East Africa, and Europe are discussed. While the attempt throughout is to give a comprehensive overview of what is known about the role of the tropical oceans in climate, the fact of the matter is that much remains to be understood and explained. The complex nature of the tropical coupled phenomena and the interaction among them argue strongly for coordinated and sustained observations, as well as additional careful modeling investigations in order to further advance the current understanding of the role of tropical oceans in climate

South-east tropical atlantic warm events and southern african rainfall

Intrusions of warm equatorial water in the South East Atlantic Ocean off Angola and Namibia may be linked with above average rainfall along the coast of those countries but sometimes also with inland areas of southern Africa e.g. Zambia. During the 1984, 1986, 1995 and 2001 warm events, above average rainfall occurred near the sea surface temperature (SST) anomalies and extended inland from the coast to an extent that appeared to depend on the intensity of the regional moisture convergence and atmospheric circulation anomalies. Rainfall over western Angola/Namibia is greatest for those events for which the local circulation anomalies act to strengthen the climatological westwards flux of Indian Ocean sourced moisture across low latitude southern Africa and which flow anticyclonically over the warmest SST off the coast thereby weakening the mean southeasterly moisture flux away from Africa over the SE Atlantic. The significance of the warm events occurring during the February to April period is that this is the time when SST reaches its maximum in the annual cycle (up to 28°C off northern Angola) and this favours more intense local evaporation and convection and a greater impact on late austral summer rainfall. Better understanding of these warm events is necessary for assessing impacts on regional rainfall, agriculture and fisheries and for improving seasonal forecasting in this region

Interannual hypoxia variability in a coastal upwelling system : ocean-shelf exchange, climate and ecosystem-state implications

In this study we use multi-year time series to examine the dynamic characteristics of coupled physical-biogeochemical processes that modulate interannual coastal hypoxia in the Benguela upwelling system in the southeast Atlantic. The results confirmed earlier findings on the role of advection to explain much of the seasonal-decadal variability. These results challenge the predominantly biogeochemical basis, namely benthic-pelagic coupling, to understand the variability of hypoxia and its ecosystem implications. Unexpectedly, the results showed that the variability was insensitive to changes in the electron-donating capacity (carbon export fluxes) but strongly dependent on the advected oxygen fluxes. The dynamics of the interaction of equatorial and polar boundary conditions (ocean-shelf exchange) and seasonally phased shelf advection were the key forcing functions that explained hypoxia variability in seasonal-decadal time scales. The vulnerability of the system to climate change lies in the long-term response of the equatorial system that governs seasonal and interannual warming at the Angola-Benguela front as well as wind stress in the Luderitz southern boundary that governs ventilation. The proposed model was able to explain most of the decadal scale variability of two different ecosystem-state indicators. The model predicts a long-term decline of present ecosystem function with climate change

Climate Science: Tropical Atlantic warm events

Sea surface temperatures in the eastern equatorial Atlantic Ocean are subject to year-to-year variations. Reanalysis data and model simulations suggest that advection of warm water from north of the Equator can drive some of the warm event