Ocean Modeling and Data Assimilation Division

The interannual variability in the tropical Indian Ocean, and in particular the Indian Ocean dipole mode (IODM), is investigated using both observations and a multi-decadal simulations performed by the coupled atmosphere-ocean general circulation model SINTEX. Overall, the characteristics of the simulated IODM are close to the features of the observed mode. Evidence of significant correlations between sea level pressure anomalies in the southeastern Indian Ocean and sea surface temperature anomalies in the tropical Indian and Pacific Oceans have been found both in observations and a multi-decadal simulation. In particular, a positive SLP anomaly in the southeastern part of the basin seems to produce favorable conditions for the development of an IODM event. The role played by the ocean dynamics both in the developing and closing phases of the IODM events is also investigated. Our results suggest that, during the developing phase, the heat content and SST variability associated with the IODM are influenced by a local response of the ocean to the winds, and a remote response with the excitation of Kelvin and Rossby waves. Ocean wave dynamics appear to be important also during the dying phase of the IODM, when equatorial downwelling Kelvin waves transport positive heat content anomalies from the western to the eastern part of the basin, suppressing the zonal heat content anomaly gradient. The results obtained from the model suggest a mechanism for the IODM. This mechanism is generally consistent with the characteristics of the observed IODM. Furthermore, it might give some clue in understanding the correlation between IODM and ENSO activity found both in the model and in the observations.

The first version of the coupled GCM SINTEX-FRSGC (SINTEX-F1.0: ECHAM4.0 + 2˚ × 1.5˚~0.5˚ OPA 8.2) developed under the EU-Japan collaborative framework was successfully integrated using the Earth Simulator. Results from the last 200 years of 220 years model run are analyzed. As reported in SINTEX (Gualdi et al., 2003), the SINTEX-F1 showed remarkable skill in simulating ocean-atmosphere conditions related to the IOD and ENSO: model statistics compare very well with that of the observed data available for the last 50-years. The eastern pole of the SST anomaly in the model IOD is found to intrude into the central part of the basin. Therefore, the western box (40˚-60˚E, 10˚S-10˚N) used in deriving the model dipole mode index (DMI) is slightly different from that for the observation (50˚-70˚E, 10˚S-10˚N) (Saji et al., 1999). This model bias will be In this project, a reasonably high resolution coupled GCM SINTEX-FRSGC (SINTEX-F1.0: ECHAM4.0/5 AGCM + OPA 8.2 OGCM + OASIS/PRISM Coupler) will be developed under the EU-Japan collaboration. Long model integration will be conducted to understand tropical phenomena IOD and ENSO, and their teleconnections. The coupled model also will be used in an intercomparison study with CFES to understand model biases, and in seasonal prediction experiments. Several model versions with variety of model resolutions will be tested before realization of a target resolution (SINTEX-F2.0: T255L191 ECHAM5 and 0.5˚ × 0.5˚ with 300 levels OPA). This will help us to investigate the role of model resolution in realistic simulation of global climate phenomena, intraseasonal oscillations, diurnal cycles in upper ocean, and in particular the derivative phenomena like ocean domes, eddies etc. investigated through intercomparison of various available CGCMs. The standard deviation of the model DMI is 0.5˚C that is slightly higher compared to the observed DMI, whereas the model Niño-3 standard deviation is 0.8˚C that is similar to the observation. As in the observation, more than 70% of IOD model events are not accompanied by ENSO events in the Pacific. Composite of pure IOD/ENSO events and partial correlation analyses of the model results confirmed the observed independent nature of the IOD in the Indian Ocean. Analysis of observed and model results showed that the subsurface equatorial long Rossby waves play a major role in strengthening SST anomalies in the central and western parts of the basin during IOD events. The SINTEX-F1 model results support the observational finding ) that these equatorial Rossby waves are coupled to the surface wind forcing associated with

Global Ocean Biogeochemistry General Circulation Models are useful tools to study biogeochemical processes at global and large scales under current climate and future scenario conditions. The credibility of future estimates is however dependent on the model skill in capturing the observed multi-annual variability of firstly the mean bulk biogeochemical properties, and secondly the rates at which organic matter is processed within the food web. For this double purpose, the results of a multi-annual simulation of the global ocean biogeochemical model PELAGOS have been objectively compared with multi-variate observations from the last 20 years of the 20th century, both considering bulk variables and carbon production/consumption rates. Simulated net primary production (NPP) is comparable with satellite-derived estimates at the global scale and when compared with an independent data-set of in situ observations in the equatorial Pacific. The usage of objective skill indicators allowed us to demonstrate the importance of comparing like with like when considering carbon transformation processes. NPP scores improve substantially when in situ data are compared with modeled NPP which takes into account the excretion of freshly-produced dissolved organic carbon (DOC). It is thus recommended that DOC measurements be performed during in situ NPP measurements to quantify the actual production of organic carbon in the surface ocean. The chlorophyll bias in the Southern Ocean that affects this model as well as several others is linked to the inadequate representation of the mixed layer seasonal cycle in the region. A sensitivity experiment confirms that the artificial increase of mixed layer depths towards the observed values substantially reduces the bias. Our assessment results

This paper analyzes the relationship between deep sedimentary fluxes and ocean current vertical velocities in an offshore area of the Ionian Sea, the deepest basin of the Eastern Mediterranean Sea. Sediment trap data are collected at 500 m and 2800 m depth in two successive moorings covering the period September 1999-May 2001. A tight coupling is observed between the upper and deep traps and the estimated particle sinking rates are more than 200 m day −1 . The current vertical velocity field is computed from a 1/16 • ×1/16 • Ocean General Circulation Model simulation and from the wind stress curl. Current vertical velocities are larger and more variable than Ekman vertical velocities, yet the general patterns are alike. Current vertical velocities are generally smaller than 1 m day −1 : we therefore exclude a direct effect of downward velocities in determining high sedimentation rates. However we find that upward velocities in the subsurface layers of the water column are positively correlated with deep particle fluxes. We thus hypothesize that upwelling would produce an increase in upper ocean nutrient levels -thus stimulating primary production and grazing -a few weeks before an enhanced vertical flux is found in the sediment traps. High particle sedimentation rates may be attained by means of rapidly sinking fecal pellets produced by gelatinous macro-zooplankton. Other sedimentation mechanisms, such as dust deposition, are also considered in explaining large pulses of deep particle fluxes. The fast sinking rates estimated in this study might be an evi-Correspondence to: L. Patara ([email protected]) dence of the efficiency of the biological pump in sequestering organic carbon from the surface layers of the deep Eastern Mediterranean basins.

This work focuses on the Late Saalian (140 ka) Eurasian ice sheets' surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21 ka) SST reconstructions, is used to compute climate at 140 and 21 ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140 ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140 ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.

A coupled general circulation model has been used to perform a set of experiments with high CO 2 concentration (2, 4, 16 times the present day mean value). The experiments have been analyzed to study the response of the climate system to strong radiative forcing in terms of the processes involved in the adjustment at the oceanatmosphere interface. The analysis of the experiments revealed a non-linear response of the mean state of the atmosphere and ocean to the increase in the carbon dioxide concentration. In the 16xCO 2 experiment the equilibrium at the ocean-atmosphere interface is characterized by an atmosphere with a shut off of the convective precipitation in the tropical Pacific sector, associated with air warmer than the ocean below.

Assessing the skill of the Atlantic meridional overturning circulation (AMOC) in decadal hindcasts (i.e. retrospective predictions) is hampered by a lack of observations for verification. Models are therefore needed to reconstruct the historical AMOC variability. Here we show that ten recent oceanic syntheses provide a common signal of AMOC variability at 45°N, with an increase from the 1960s to the mid-1990s and a decrease thereafter although they disagree on the exact magnitude. This signal correlates with observed key processes such as the North Atlantic Oscillation, sub-polar gyre strength, Atlantic sea surface temperature dipole, and Labrador Sea convection that are thought to be related to the AMOC. Furthermore, we find potential predictability of the mid-latitude AMOC for the first 3-6 year means when we validate decadal hindcasts for the past 50 years against the multi-model signal. However, this predictability is not found in models driven only by external radiative changes, demonstrating the need for initialization of decadal climate predictions.

In this paper we describe a mesoscale data assimilation experiment in the Middle Adriatic Sea. In order to perform dynamical forecasts we provide a quasigeostrophic numerical model with a set of initial fields regularly gridded via an objective analysis technique. Maps of this initial condition show a surface intensified jet meandering around a cyclonic eddy at the thermocline and deep levels. We dynamically forecasted the flow evolution for 30 days after initialization. The time scale of the variability is of the order of a few weeks and the cyclonic vortex seems to be locked to the topography. A set of numerical experiments with different initial bottom boundary conditions, with and without topography, are made to explore the influence of the topographic constraint on the mesoscale flow evolution. As expected the influence of the topography on the dynamical evolution of the flow is very strong and it confines the jet-cyclone along the bathymetric contours, strengthening the flow. On the other hand the flow is not sensitive to changes in the density bottom initial condition.

estimates. The average variation is about 3.6 Sv, which is largely due to the increase of ITF transport from 1993 to 2000. However, the three-year average during the 2004-2006 INSTANT Program period is within 0.5 Sv of the long-term mean for the past few decades.

Keywords: Plio-Pleistocene climate transition Mediterranean surface water high-low-latitude interplay spectral analysis sapropels The high-low latitude climate interplay during the Plio-Pleistocene global cooling is not yet well understood. Insight on the Mediterranean region can provide some clues about past significant climate changes since the basin reflects the climate dynamics of both high-latitude and low-latitude regions, being connected to the North Atlantic and subjected to monsoon influence. Here we shade light on this connection problem by performing a spectral analysis on an Eastern Mediterranean stack of planktonic records spanning the last 5 Ma and by further comparing it to North Atlantic and Pacific deep-and surface-water records. Our main conclusion is that the Mediterranean detected the main global climate transitions over the last 5 Myr although sapropel depositions indicate that it remained influenced by the African summer monsoon during the whole interval. Our analysis reveals that until 2.2 Ma the Mediterranean planktonic record is driven by regional processes dominated by precession. The progressive emergence of the 41-kyr frequency in the Mediterranean records around 2.8 Ma suggests that, since this date, the Mediterranean was more and more affected by the high-latitude climate dynamics forcing than by the low-latitude one. Moreover, during the ongoing Plio-Pleistocene cooling, the 41-kyr frequency signal in the Mediterranean records anticipated high-latitude deep-water response to the intensification of the Northern Hemisphere Glaciations (NHG) and lagged the signal in tropical latitudes. Finally, toward 1.2 Ma the results suggest that the progressive shift from the 41-kyr to the 100-kyr frequency was led by the northern high latitudes. Overall, our results confirm that the Mediterranean is an ideal site to study the interplay between high and low latitude climates.