Effects of ocean biology on the penetrative radiation in a coupled climate model

The influence of phytoplankton on the seasonal cycle and the mean global climate is investigated in a fully coupled climate model. The control experiment uses a fixed attenuation depth for shortwave radiation, while the attenuation depth in the experiment with biology is derived from phytoplankton concentrations simulated with a marine biogeochemical model coupled online to the ocean model. Some of the changes in the upper ocean are similar to the results from previous studies that did not use interactive atmospheres, for example, amplification of the seasonal cycle; warming in upwelling regions, such as the equatorial Pacific and the Arabian Sea; and reduction in sea ice cover in the high latitudes. In addition, positive feedbacks within the climate system cause a global shift of the seasonal cycle. The onset of spring is about 2 weeks earlier, which results in a more realistic representation of the seasons. Feedback mechanisms, such as increased wind stress and changes in the shortwave radiation, lead to significant warming in the midlatitudes in summer and to seasonal modifications of the overall warming in the equatorial Pacific. Temperature changes also occur over land where they are sometimes even larger than over the ocean. In the equatorial Pacific, the strength of interannual SST variability is reduced by about 10%–15% and phase locking to the annual cycle is improved. The ENSO spectral peak is broader than in the experiment without biology and the dominant ENSO period is increased to around 5 yr. Also the skewness of ENSO variability is slightly improved. All of these changes lead to the conclusion that the influence of marine biology on the radiative budget of the upper ocean should be considered in detailed simulations of the earth’s climate

Correction to “Using altimetry to help explain patchy changes in hydrographic carbon measurements”

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 114 (2009): C12099, doi:10.1029/2009JC005835

Decadal variability of the North Atlantic in an ocean general circulation model

Climatic fluctuations on a decadal timescale in the North Atlantic in a global ocean general circulation model were considered. The analysis was carried out for the 3800-year stochastic forcing simulation of Mikolajewicz and Maier-Reimer in which the Hamburg LargeScale Geostrophic ocean model was driven by monthly climatologies of wind stress, air temperature, and freshwater flux with superimposed white noise freshwater fluxes with an amplitude of about 16 mm/month. We applied a Principal Oscillation Pattern analysis to the vector time series of the upper level salinity fields, so that the examined fluctuations appear as estimated eigenmodes of the system. In addition to an oscillation with a period of 320 years as already described by Mikolajewicz and Maier-Reimer, we found a broadband Principal Oscillation Pattern with a timescale of the order of 10 to 40 years. It describes the generation of salinity anomalies in the Labrador Sea and the following discharge into the North Atlantic. In sensitivity experiments we clarified that the source of the variability lies in the Labrador Sea and showed that the generation of the salinity anomalies is mainly due to an undisturbed local integration of the white noise freshwater fluxes

Simulations of magnetic fields generated by the Antarctic Circumpolar Current at satellite altitude: Can geomagnetic measurements be used to monitor the flow?

With a volume transport of similar to134 x 10(6) m(3)/s at the Drake Passage, the Antarctic Circumpolar Current (ACC) is the strongest ocean current. In the interest of estimating the secondary magnetic fields generated by the magnetohydrodynamic interaction of this flow with Earth's main field, we compare numerical results for the magnetic fields obtained using flow from three different ocean general circulation models. These simulations all expect detectable ocean signals in the magnetic records at ground and satellite altitude ( 400 km). The variability of this contribution is highly correlated with the ACC transport, a very important variable for climate studies. Observed magnetic fields could then be used, in principle, to derive an index of variability of the ACC. However given its small amplitude compared with other magnetic contributions, extracting the ocean's signal from observations remains a challenge at this tim

Tracing the conveyor belt in the Hamburg large-scale geostrophic ocean general circulation model

The flow which constitutes the conveyor belt in the Hamburg large-scale geostrophic ocean general circulation model has been investigated with the help of a particle tracking method. In the region of North Atlantic Deep Water formation a thousand trajectories were calculated backward in time to the point where they upwell from the deep ocean. Both the three-dimensional velocity field and convective overturning have been used for this calculation. Together, the trajectories form a representative picture of the upper branch of the conveyor belt in the model. In the Atlantic Ocean the path and strength (17 Sv) of the conveyor belt in the model are found to be consistent with observations. All trajectories enter the South Atlantic via Drake Passage, as the model does not simulate any Agulhas leakage. Large changes in water masses occur in the South Atlantic midlatitudes and subtropical North Atlantic. Along its path in the Atlantic the water in the conveyor belt is transformed from Antarctic Intermediate Water to Central North Atlantic Water. On the average the timescale on which the water mass characteristics are approximately conserved is only a few years compared to the timescale of 70 years for the conveyor belt to cross the Atlantic. The ventilation of thermocline waters in the South Atlantic midlatitudes is overestimated in the model due to too much convective deepening of the winter mixed layer. As a result the fraction of the conveyor belt water flowing in the surface layer is also overestimated, along with integrated effects of atmospheric forcing. The abnormally strong water mass transformation in the South Atlantic might be related to the absence of Agulhas leakage in the model