Flow structure and variability in the subtropical Indian Ocean: Instability of the South Indian Ocean Countercurrent

The origin of the eddy variability around the 25°S band in the Indian Ocean is investigated. We have found that the surface circulation east of Madagascar shows an anticyclonic subgyre bounded to the south by eastward flow from southwest Madagascar, and to the north by the westward flowing South Equatorial Current (SEC) between 15° and 20°S. The shallow, eastward flowing South Indian Ocean Countercurrent (SICC) extends above the deep reaching, westward flowing SEC to 95°E around the latitude of the high variability band. Applying a two-layer model reveals that regions of large vertical shear along the SICC-SEC system are baroclinically unstable. Estimates of the frequencies (3.5–6 times/year) and wavelengths (290–470 km) of the unstable modes are close to observations of the mesoscale variability derived from altimetry data. It is likely then that Rossby wave variability locally generated in the subtropical South Indian Ocean by baroclinic instability is the origin of the eddy variability around 25°S as seen, for example, in satellite altimetry.

Use of 16S rRNA-targeted oligonucleotide probes to investigate function and phylogeny of sulphate-reducing bacteria and methanogenic archaea in a UK estuary

Sulphate-reducing bacteria (SRB) and methanogenic archaea (MA) are important anaerobic terminal oxidisers of organic matter. However, we have little knowledge about the distribution and types of SRB and MA in the environment or the functional role they play in situ. Here we have utilised sediment slurry microcosms amended with ecologically significant substrates, including acetate and hydrogen, and specific functional inhibitors, to identify the important SRB and MA groups in two contrasting sites on a UK estuary. Substrate and inhibitor additions had significant effects on methane production and on acetate and sulphate consumption in the slurries. By using specific 16S-targeted oligonucleotide probes we were able to link specific SRB and MA groups to the use of the added substrates. Acetate consumption in the freshwater-dominated sediments was mediated by Methanosarcinales under low-sulphate conditions and Desulfobacter under the high-sulphate conditions that simulated a tidal incursion. In the marine-dominated sediments, acetate consumption was linked to Desulfobacter. Addition of trimethylamine, a non-competitive substrate for methanogenesis, led to a large increase in Methanosarcinales signal in marine slurries. Desulfobulbus was linked to non-sulphate-dependent H-2 consumption in the freshwater sediments. The addition of sulphate to freshwater sediments inhibited methane production and reduced signal from probes targeted to Methanosarcinales and Methanomicrobiales, while the addition of molybdate to marine sediments inhibited Desulfobulbus and Desulfobacterium. These data complement our understanding of the ecophysiology of the organisms detected and make a firm connection between the capabilities of species, as observed in the laboratory, to their roles in the environment. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.

The relationship between the North American summer monsoon, the Rocky Mountains and the North Pacific subtropical anticyclone in HadAM3

In this study the relationship between the North American monsoon, the Californian sea surface temperature (SST) cold pool, the Rocky Mountains and the North Pacific subtropical anticyclone is investigated using the Hadley Centre's atmospheric climate model, HadAM3. In 1996 Hoskins hypothesized that heating in the North American monsoon might be important for the maintenance of the summertime North Pacific subtropical anticyclone, since the monsoon heating may induce descent to the north-west of the monsoon in the descending eastern flank of the subtropical anticyclone. This descent is further enhanced by radiative cooling and is associated with equatorward surface winds parallel to the western coast of North America. These equatorward winds induce oceanic upwelling of cold water and contribute to the formation of the Californian SST cold pool, which may feed back on the anticyclone by further suppressing convection and inducing descent. More recently, Rodwell and Hoskins also investigated the global summer monsoon–subtropical anticyclone relationship. They examined the role that mountains play in impeding the progress of the low-level mid-latitude westerlies, either deflecting the westerlies northwards where they ascend along the sloping mid-latitude isentropes or deflecting them southwards forcing them to descend along the isentropes. In particular, the introduction of the Rockies into a primitive-equation model adiabatically induces descent in the eastern descending flank of the North Pacific subtropical anticyclone. These hypothesized mechanisms have been investigated using HadAM3, focusing on the possible suppression of convection by the Californian SST cold pool, the response of the North Pacific subtropical anticyclone to the strength of the North American monsoon and the ‘blocking’ of the mid-latitude westerlies by the Rocky Mountains. The role of the Rockies is examined by integrating the model with modified orography for the Rocky Mountains. Changing the height of the Rockies alters the circulation in a way consistent with the mechanism outlined above. Higher Rocky mountains force the westerlies southwards, inducing descent in the eastern flank of the subtropical anticyclone as the air descends along the sloping isentropes. The relationship between the North American monsoon and the North Pacific subtropical anticyclone is investigated by suppressing the monsoon in HadAM3. The suppression of the monsoon is accomplished by increasing the surface albedo over Mexico, which induces anomalous ascent on the eastward flank of the subtropical anticyclone and anomalous polewards surface winds along the western coast of the North American continent, also providing support for the above hypothesis. The removal of the Californian SST cold pool, however, has a statistically insignificant effect on the model, suggesting that in this model the feedback of the SST cold pool on the eastern flank of the anticyclone is weak.

A robust mechanism for strengthening of the Brewer–Dobson circulation in response to climate change: critical-layer control of subtropical wave breaking

Climate models consistently predict a strengthened Brewer–Dobson circulation in response to greenhouse gas (GHG)-induced climate change. Although the predicted circulation changes are clearly the result of changes in stratospheric wave drag, the mechanism behind the wave-drag changes remains unclear. Here, simulations from a chemistry–climate model are analyzed to show that the changes in resolved wave drag are largely explainable in terms of a simple and robust dynamical mechanism, namely changes in the location of critical layers within the subtropical lower stratosphere, which are known from observations to control the spatial distribution of Rossby wave breaking. In particular, the strengthening of the upper flanks of the subtropical jets that is robustly expected from GHG-induced tropospheric warming pushes the critical layers (and the associated regions of wave drag) upward, allowing more wave activity to penetrate into the subtropical lower stratosphere. Because the subtropics represent the critical region for wave driving of the Brewer–Dobson circulation, the circulation is thereby strengthened. Transient planetary-scale waves and synoptic-scale waves generated by baroclinic instability are both found to play a crucial role in this process. Changes in stationary planetary wave drag are not so important because they largely occur away from subtropical latitudes.

Explosive development of winter storm Xynthia over the subtropical North Atlantic Ocean

In winter of 2009–2010 south-western Europe was hit by several destructive windstorms. The most important was Xynthia (26–28 February 2010), which caused 64 reported casualties and was classified as the 2nd most expensive natural hazard event for 2010 in terms of economic losses. In this work we assess the synoptic evolution, dynamical characteristics and the main impacts of storm Xynthia, whose genesis, development and path were very uncommon. Wind speed gusts observed at more than 500 stations across Europe are evaluated as well as the wind gust field obtained with a regional climate model simulation for the entire North Atlantic and European area. Storm Xynthia was first identified on 25 February around 30° N, 50° W over the subtropical North Atlantic Ocean. Its genesis occurred on a region characterized by warm and moist air under the influence of a strong upper level wave embedded in the westerlies. Xynthia followed an unusual SW–NE path towards Iberia, France and central Europe. The role of moist air masses on the explosive development of Xynthia is analysed by considering the evaporative sources. A lagrangian model is used to identify the moisture sources, sinks and moisture transport associated with the cyclone during its development phase. The main supply of moisture is located over an elongated region of the subtropical North Atlantic Ocean with anomalously high SST, confirming that the explosive development of storm Xynthia had a significant contribution from the subtropics.