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An intrinsic mode of self-sustained, interannual variability is identified in a coarse-resolution ocean model forced by an annually repeating atmospheric state. The variability has maximum loading in the Indian Ocean, with a significant projection into the South Atlantic Ocean. It is argued that this intrinsic mode is caused by baroclinic instability of the model’s Leeuwin Current, which radiates out to the tropical Indian and South Atlantic Oceans as long Rossby waves at a period of 4 yr. This previously undescribed mode has a remarkably narrowband time series. However, the variability is not synchronized with the annual cycle; the phase of the oscillation varies chaotically on decadal time scales. The presence of this internal mode reduces the predictability of the ocean circulation by obscuring the response to forcing or initial condition perturbations. The signature of this mode can be seen in higher-resolution global ocean models driven by high-frequency atmospheric forcing, but altimeter and assimilation analyses do not show obvious signatures of such a mode, perhaps because of insufficient duration.

Polar temperature is often inferred from water isotopes in ice cores. However, non-temperature effects on δ 18 O are important during the abrupt events of the last glacial period, such as changes in the seasonality of precipitation, the northward movement of the storm track, and the increase in accumulation. These effects complicate the interpretation of δ 18 O as a temperature proxy. Here, we present an independent surface temperature reconstruction, which allows us to test the relationship between δ 18 O ice and temperature, during Dansgaard–Oeschger event 8, 38.2 thousand yrs ago using new δ 15 N and δ 40 Ar data from the GISP2 ice core in Greenland. This temperature reconstruction relies on a new inversion of inert gas isotope data using generalized least-squares, and includes a robust uncertainty estimation. We find that both temperature and δ 18 O increased in two steps of 20 and 140 yrs, with an overall amplitude of 11.80 ± 1.8 °C between the stadial and interstadial centennial-mean temperature. The coefficient α = d δ O 18 / d T changes with each time-segment, which shows that non-temperature sources of fractionation have a significant contribution to the δ 18 O signal. When measured on century-averaged values, we find that α = d δ O 18 / d T = 0.32 ± 0.06 ‰ / ° C , which is similar to the glacial/Holocene value of 0.328‰/°C.

A variational data assimilation system has been implemented for the tropical Pacific Ocean using an eddy-permitting regional implementation of the MITgcm. The adjoint assimilation system was developed by the Estimation of the Circulation and the Climate of the Ocean consortium, and has been extended to deal with open boundaries. This system is used to adjust the model to match observations in the tropical Pacific region using control parameters which include initial conditions, open boundaries and time-dependent surface fluxes. This paper focuses on problems related to strong adjoint sensitivities that may impede the model fit to the observations. A decomposition of the velocities at the open boundaries into barotropic and baroclinic modes is introduced to deal with very strong sensitivities of the model sea surface height to the barotropic component of the inflow. Increased viscosity and diffusivity terms are used in the adjoint model to reduce exponentially growing sensitivities in the backward run associated with nonlinearity of the forward model. Simplified experiments in which the model was constrained with Levitus temperature and salinity data, Reynolds sea surface temperature data and TOPEX/POSEIDON altimeter data were performed to demonstrate the controllability of this assimilation system and to study its sensitivity to the starting guesses for forcing and initial conditions.