The stability of three-dimensional thermally driven ocean flows in a single hemispheric sector basin is investigated using techniques of numerical bifurcation theory. Under restoring conditions for the temperature, the flow is stable. However, when forced with the associated heat flux, an interdecadal oscillatory time scale instability appears. This occurs as a Hopf bifurcation when the horizontal mixing coefficient of heat is decreased. The physical mechanism of the oscillation is described by analyzing the potential energy changes of the perturbation flow near the Hopf bifurcation. In the relatively slow phase of the oscillation, a temperature anomaly propagates westwards near the northern boundary on a background temperature gradient, thereby changing the perturbation zonal temperature gradient, with corresponding changes in meridional overturning. This is followed by a relatively fast phase in which the zonal overturning reacts to a change in sign of the perturbation meridional temperature gradient. The different responses of zonal and meridional overturning cause a phase difference between the effect of temperature and vertical velocity anomalies on the buoyancy work anomaly, the latter dominating the changes in potential energy. This phase difference eventually controls the time scale of the oscillation.

Abstract. Using the ECHAM5/MPI-OM model, we study the relation between the variations in the Atlantic meridional overturning circulation (AMOC) and both the Pacific sea surface temperature (SST) and the El Niño-Southern Oscillation (ENSO) amplitude. In a 17-member 20C3M/SRES-A1b ensemble for 1950-2100 the Pacific response to AMOC variations on different time scales and amplitudes is considered. The Pacific response to AMOC variations associated with the Atlantic Multidecadal Oscillation (AMO) is very small. In a 5-member hosing ensemble where the AMOC collapses due to a large freshwater anomaly, the Pacific SST response is large and in agreement with previous work. Our results show that the modelled connection between AMOC and ENSO depends very strongly on the frequency and/or the modelled amplitude of the AMOC variations. Interannual AMOC variations, decadal AMOC variations and an AMOC collapse lead to entirely different responses in the Pacific Ocean.

Systems capable of forecasting ocean properties and acoustic performance in the littoral ocean are becoming a useful capability for scientific and operational exercises. The coupling of a data-assimilative nested ocean modeling system with an acoustic propagation modeling system was carried out at sea for the first time, within the scope of Battlespace Preparation 2007 (BP07) that was part of Marine Rapid Environmental Assessment (MREA07) exercises. The littoral region for our studies was southeast of the island of Elba ( Italy) in the Tyrrhenian basin east of Corsica and Sardinia. During BP07, several vessels collected in situ ocean data, based in part on recommendations from oceanographic forecasts. The data were assimilated into a fourdimensional high-resolution ocean modeling system. Sound-speed forecasts were then used as inputs for bearing-and range-dependent acoustic propagation forecasts. Data analyses are carried out and the set-up of the coupled oceanographic-acoustic system as well as the results of its real-time use are described. A significant finding is that oceanographic variability can considerably influence acoustic propagation properties, including the probability of detection, even in this apparently quiet region around Elba. This strengthens the importance of coupling at-sea acoustic modeling to real-time ocean forecasting. Other findings include the challenges involved in downscaling basin-scale modeling systems to high-resolution littoral models, especially in the Mediterranean Sea. Due to natural changes, global human activities and present model resolutions, the assimilation of synoptic regional ocean data is recommended in the region.

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