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  • 1
    Publication Date: 2019-03-28
    Description: Beside its global effects, climate change is manifested in many regionally pronounced features mainly resulting from changes in the oceanic and atmospheric circulation. Here we investigate the influence of the North Atlantic SST on shaping the winter-time response to global warming. Our results are based on a long-term climate projection with the Max Planck Institute Earth System Model (MPI-ESM) to investigate the influence of North Atlantic sea surface temperature pattern changes on shaping the atmospheric climate change signal. In sensitivity experiments with the model’s atmospheric component we decompose the response into components controlled by the local SST structure and components controlled by global/remote changes. MPI-ESM simulates a global warming response in SST similar to other climate models: there is a warming minimum—or ”warming hole”—in the subpolar North Atlantic, and the sharp SST gradients associated with the Gulf Stream and the North Atlantic Current shift northward by a few a degrees. Over the warming hole, global warming causes a relatively weak increase in rainfall. Beyond this, our experiments show more localized effects, likely resulting from future SST gradient changes in the North Atlantic. This includes a significant precipitation decrease to the south of the Gulf Stream despite increased underlying SSTs. Since this region is characterised by a strong band of precipitation in the current climate, this is contrary to the usual case that wet regions become wetter and dry regions become drier in a warmer climate. A moisture budget analysis identifies a complex interplay of various processes in the region of modified SST gradients: reduced surface winds cause a decrease in evaporation; and thermodynamic, modified atmospheric eddy transports, and coastal processes cause a change in the moisture convergence. The changes in the the North Atlantic storm track are mainly controlled by the non-regional changes in the forcing. The impact of the local SST pattern changes on regions outside the North Atlantic is small in our setup.
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2017-02-07
    Description: This study examines the circulation and associated monthly-to-seasonal variability in the Caribbean Sea using a regional ocean circulation model. The model domain covers the region between 99.0 and 54.0°W and between 8.0 and 30.3°N, with a horizontal resolution of 1/6°. The ocean circulation model is driven by 6-hourly atmospheric reanalysis data from the National Center for Environmental Prediction and boundary forcing extracted from 5-day global ocean reanalysis data produced by Smith et al. (Mercator Newsletter 36:39–49, 2010), and integrated for 7 years. A comparison of model results with observations demonstrates that the regional ocean circulation model has skill in simulating circulation and associated variability in the study region. Analysis of the model results, as well as a companion model run that uses steady annual mean forcing, illustrates the role of Caribbean eddies for driving monthly-to-seasonal circulation variability in the model. It is found that vertically integrated transport between Nicaragua and Jamaica is influenced by the interaction between the density perturbations associated with Caribbean eddies and the Nicaraguan Ridge. The impact of Caribbean eddies squeezing through the Yucatan Channel is also discussed.
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2017-12-19
    Description: The North Atlantic cold bias, associated with the misplacement of the North Atlantic Current (NAC) and typically extending from the surface to 1000 m depth, is a common problem in coupled models that compromises model fidelity. We investigate the use of a flow field correction (FFC) to adjust the path of the NAC and alleviate the cold bias. The FFC consists of three steps. First, climatological potential temperature (T) and salinity (S) fields for use with the model are produced using a three-dimensional restoring technique. Second, these T, S fields are used to modify the momentum equations of the ocean model. In the third stage, the correction term is diagnosed to construct a flow-independent correction. Results using the Kiel Climate Model show that the FFC allows the establishment of a northwest corner, substantially alleviating the subsurface cold bias. A cold bias remains at the surface but can be eliminated by additionally correcting the surface freshwater flux, without adjusting the surface heat flux seen by the ocean model. A model version in which only the surface fluxes of heat and freshwater are corrected continues to exhibit the incorrect path of the NAC and a strong subsurface bias. We also show that the bias in the atmospheric circulation is reduced in some corrected model versions. The FFC can be regarded as a way to correct for model error, e.g. associated with the deep water mass pathways and their impact on the large-scale ocean circulation, and unresolved processes such as eddy momentum flux convergence.
    Type: Article , PeerReviewed
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  • 4
    Publication Date: 2016-11-03
    Description: The distribution of the mean oceanic oxygen concentration results from a balance between ventilation and consumption. In the eastern tropical Pacific and Atlantic, this balance creates extended oxygen minimum zones (OMZ) at intermediate depth. Here, we analyze hydrographic and velocity data from shipboard and moored observations, which were taken along the 23°W meridian cutting through the Tropical North East Atlantic (TNEA) OMZ, to study the distribution and generation of oxygen variability. By applying the extended Osborn–Cox model, the respective role of mesoscale stirring and diapycnal mixing in producing enhanced oxygen variability, found at the southern and upper boundary of the OMZ, is quantified. From the well-ventilated equatorial region toward the OMZ core a northward eddy-driven oxygen flux is observed whose divergence corresponds to an oxygen supply of about 2.4 μmol kg−1 year−1 at the OMZ core depth. Above the OMZ core, mesoscale eddies act to redistribute low- and high-oxygen waters associated with westward and eastward currents, respectively. Here, absolute values of the local oxygen supply 〉10 μmol kg−1 year−1 are found, likely balanced by mean zonal advection. Combining our results with recent studies, a refined oxygen budget for the TNEA OMZ is derived. Eddy-driven meridional oxygen supply contributes more than 50 % of the supply required to balance the estimated oxygen consumption. The oxygen tendency in the OMZ, as given by the multidecadal oxygen decline, is maximum slightly above the OMZ core and represents a substantial imbalance of the oxygen budget reaching about 20 % of the magnitude of the eddy-driven oxygen supply.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2017-04-13
    Description: Tropical influence on the austral summer Southern Annular Mode (SAM) over the ERA-40 period 1960/1961–2001/2002 is investigated using (1) a partially coupled climate model (PCM) driven by observed wind stress and (2) a version of the ECMWF atmospheric model by means of a relaxation technique. We show that the tropical influence in the PCM is dominated by El Niño Southern Oscillation (ENSO) whereas the relaxation experiments suggest an additional influence independent of ENSO. In the observations, we find that the simultaneous influence of ENSO on the summer SAM was much stronger after 1979 than before, with the consequence that the ensemble mean of the PCM captures around 50 % of the interannual variance of the SAM after 1979 and less than 10 % before. Nevertheless, in the ensemble mean of the PCM, the relationship between ENSO and the summer SAM is stable throughout the whole period 1960/1961–2001/2002, and it is the individual ensemble members that exhibit a non-stationary relationship like that found in the observations. It follows that variability not related to the observed wind forcing used to drive the PCM is important for obscuring the ENSO/SAM relationship. The experiments using relaxation show that tropical forcing was important for both the interannual variability and the trend of the summer SAM, even before 1979. Adding the observed extratropical sea surface temperature and sea-ice (SSTSI) to the tropical relaxation runs improves the model performance, indicative of a positive feedback from extratropical SSTSI onto the SAM.
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2019-02-01
    Description: Low prediction skill in the tropical Pacific is a common problem in decadal prediction systems, especially for lead years 2–5 which, in many systems, is lower than in uninitialized experiments. On the other hand, the tropical Pacific is of almost worldwide climate relevance through its teleconnections with other tropical and extratropical regions and also of importance for global mean temperature. Understanding the causes of the reduced prediction skill is thus of major interest for decadal climate predictions. We look into the problem of reduced prediction skill by analyzing the Max Planck Institute Earth System Model (MPI-ESM) decadal hindcasts for the fifth phase of the Climate Model Intercomparison Project and performing a sensitivity experiment in which hindcasts are initialized from a model run forced only by surface wind stress. In both systems, sea surface temperature variability in the tropical Pacific is successfully initialized, but most skill is lost at lead years 2–5. Utilizing the sensitivity experiment enables us to pin down the reason for the reduced prediction skill in MPI-ESM to errors in wind stress used for the initialization. A spurious trend in the wind stress forcing displaces the equatorial thermocline in MPI-ESM unrealistically. When the climate model is then switched into its forecast mode, the recovery process triggers artificial El Niño and La Niña events at the surface. Our results demonstrate the importance of realistic wind stress products for the initialization of decadal predictions
    Type: Article , PeerReviewed
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  • 7
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    Springer
    In:  Journal of Ocean University of China, 12 (3). pp. 345-354.
    Publication Date: 2017-10-11
    Description: The distribution of the suspended sediment concentration (SSC) in the Bohai Sea, Yellow Sea and East China Sea (BYECS) is studied based on the observed turbidity data and model simulation results. The observed turbidity results show that (i) the highest SSC is found in the coastal areas while in the outer shelf sea areas turbid water is much more difficult to observe, (ii) the surface layer SSC is much lower than the bottom layer SSC and (iii) the winter SSC is higher than the summer SSC. The Regional Ocean Modeling System (ROMS) is used to simulate the SSC distribution in the BYECS. A comparison between the modeled SSC and the observed SSC in the BYECS shows that the modeled SSC can reproduce the principal features of the SSC distribution in the BYECS. The dynamic mechanisms of the sediment erosion and transport processes are studied based on the modeled results. The horizontal distribution of the SSC in the BYECS is mainly determined by the current-wave induced bottom stress and the fine-grain sediment distribution. The current-induced bottom stress is much higher than the wave-induced bottom stress, which means the tidal currents play a more significant role in the sediment resuspension than the wind waves. The vertical mixing strength is studied based on the mixed layer depth and the turbulent kinetic energy distribution in the BYECS. The strong winter time vertical mixing, which is mainly caused by the strong wind stress and surface cooling, leads to high surface layer SSC in winter. High surface layer SSC in summer is restricted in the coastal areas.
    Type: Article , PeerReviewed
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  • 8
    Publication Date: 2017-02-06
    Description: The 5th International Workshop on Modeling the Ocean (IWMO http://www.uib.no/en/IWMO2013/-58927/iwmo-2013-bergen-norway) was held in June 17–20, in Bergen, Norway. The historic city of Bergen is the gateway to the fjords and a center for oceanic research. The workshop was hosted by the University of Bergen and also sponsored by the Research Council of Norway. Approximately 80 researchers worldwide participated in the workshop. Professor Mellor, Princeton University, gave the keynote lecture. The 5th IWMO meeting in Bergen was the first IWMO held in Europe, followed on the footsteps of previous meetings, IWMO-2009 in Taipei, Taiwan (Oey et al. 2010a, b), IWMO-2010 in Norfolk, USA (Ezer et al. 2011), IWMO-2011 in Qingdao, China (Oey et al. 2013a), and IWMO-2012 in Yokohama, Japan (Oey et al. 2013b). The participants presented approximately 60 oral talks and 20 posters, covering a wide range of ocean modeling and data analysis topics, as described below. In the spirit of promoting young s ...
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  • 9
    Publication Date: 2017-04-12
    Description: Near-inertial oscillations are ubiquitous in the ocean and are believed to play an important role in the global climate system. Studies on wind power input to near-inertial motions (WPI) have so far focused primarily on estimating the time-mean WPI, with little attention being paid to its temporal variability. In this study, a combination of atmospheric reanalysis products, a high-resolution ocean model and linear regression models are used to investigate for the first time the relationship between interannual variability of WPI in the North Atlantic and the North Atlantic Oscillation (NAO), motivated by the idea that the NAO serves as a good indicator for storminess over the North Atlantic and that storms account for the majority of WPI. It is found that WPI at low and high latitudes of the North Atlantic is significantly correlated to the NAO, owing to its influence on the configuration of the storm track. Positive (negative) NAO conditions are associated with increased WPI in the subpolar (subtropical) ocean. Basin-wide WPI is found to be significantly enhanced under negative NAO conditions, but is not significantly different from the climatological average under positive NAO conditions. This indicates a weak inverse relationship between basin-wide WPI and the NAO, contradicting intuitive expectations. The asymmetric impact of the NAO on basin-wide WPI results from greater sensitivity of WPI to near-inertial wind forcing at lower latitudes due to the variation of the Coriolis parameter with latitude.
    Type: Article , PeerReviewed
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  • 10
    Publication Date: 2017-12-19
    Description: An ocean circulation model is run using two different idealized equatorial basin configurations under steady wind forcing. Both model versions produce bands of vertically alternating zonal flow at depth, similar to observed Equatorial Deep Jets (EDJs) and with a time scale corresponding to that of the gravest equatorial basin mode for the dominant baroclinic vertical normal mode. Both model runs show evidence for enhanced variability in the surface signature of the North Equatorial Counter Current (NECC) with the same time scale. We also find the same link between the observed NECC and the EDJs in the Atlantic by comparing the signature of the EDJ in moored zonal velocity data at 23° W on the equator with the signature of the NECC in geostrophic velocities from altimeter data. We argue that the presence of a peak in variability in the NECC associated with the EDJ basin mode period is evidence that the influenceatthis time scale is upward, from the EDJ to the NECC
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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