Closing the sea surface mixed layer temperature budget from in situ observations alone: Operation Advection during BoBBLE

Sea surface temperature (SST) is a fundamental driver of tropical weather systems such as monsoon rainfall and tropical cyclones. However, understanding of the factors that control SST variability is lacking, especially during the monsoons when in situ observations are sparse. Here we use a ground-breaking observational approach to determine the controls on the SST variability in the southern Bay of Bengal. We achieve this through the first full closure of the ocean mixed layer energy budget derived entirely from in situ observations during the Bay of Bengal Boundary Layer Experiment (BoBBLE). Locally measured horizontal advection and entrainment contribute more significantly than expected to SST evolution and thus oceanic variability during the observation period. These processes are poorly resolved by state-of-the-art climate models, which may contribute to poor representation of monsoon rainfall variability. The novel techniques presented here provide a blueprint for future observational experiments to quantify the mixed layer heat budget on longer time scales and to evaluate these processes in models

The separation of the East Australian Current: A Lagrangian approach to potential vorticity and upstream control

The East Australian Current (EAC) is the western boundary current flowing along the east coast of Australia separating from the coast at approximately 34°S. After the separation two main pathways can be distinguished, the eastward flowing Tasman Front and the extension of the EAC flowing southward. The area south of the separation latitude is eddy-rich and the separation latitude of the EAC is variable. Little is known of the properties of the water masses that separate at the bifurcation of the EAC. This paper presents new insights from the Lagrangian perspective, where the water masses that veer east and those that continue south are tracked in an eddy-permitting numerical model. The transport along the two pathways is computed, and a 1:3 ratio between transport in the EAC extension and transport in the Tasman Front is found. The results show that the "fate" of the particles is to first order already determined by the particle distribution within the EAC current upstream of the separation latitude, where 85% of the particles following the EAC extension originate from below 460 m and 90% of the particles following the Tasman Front originate from the top 460 m depth at 28°S. The separation and pathways are controlled by the structure of the isopycnals in this region. Analysis of anomalies in potential vorticity show that in the region where the two water masses overlap, the fate of the water depends on the presence of anticyclonic eddies that push isopycnals down and therefore enable particles to travel further south

The Madagascar Bloom – a serendipitous study

The late austral summer (February-April) phytoplankton bloom that occurs east of Madagascar exhibits significant interannual variability and at its largest extent covers ~1% of the world’s ocean surface area. The bloom raises many intriguing questions about how it begins, is sustained, propagates to the east, exports carbon and ends. It has been observed and studied using satellite ocean color observations, but the lack of in situ data makes it difficult to address these questions. Here we describe observations that were made serendipitously on a cruise in February 2005. These show clearly for the first time the simultaneous existence of a deep chlorophyll maximum at ~70-110 m depths (seen in SeaSoar fluorimeter data) and a surface chlorophyll signature (seen in SeaWiFS satellite ocean color data). The observations also show the modulation of biological signature at the surface by the eddy field, but not of the deep chlorophyll maximum. Trichodesmium dominates the bloom nearer to Madagascar, while the diatom Rhizosolenia clevei (and its symbiont Richelia intracellularis) dominates further from the island. The surface bloom seen in the SeaWiFS data is confined to the shallow (~30 m) mixed layer. It is hypothesized that the interannual variability in bloom intensity may be due to variations in coastal upwelling and thus the supply of iron, which is a micronutrient that can limit diazotroph growth

Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea

Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies