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  • Other Sources  (6)
  • Articles (OceanRep)  (6)
  • Springer  (6)
  • 2015-2019  (6)
  • 2015  (6)
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  • Other Sources  (6)
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  • Articles (OceanRep)  (6)
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  • 2015-2019  (6)
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  • 1
    Publication Date: 2017-04-13
    Description: The variability of the East Asian summer monsoon (EASM) is studied using a pacemaker technique driven by ENSO in an atmospheric general circulation model (AGCM) coupled to a slab mixed layer model. In the pacemaker experiments, sea surface temperature (SST) is constrained to observations in the eastern equatorial Pacific through a q-flux that measures the contribution of ocean dynamics to SST variability, while the AGCM is coupled to the slab model. An ensemble of pacemaker experiments is analyzed using a multivariate EOF analysis to identify the two major modes of variability of the EASM. The results show that the pacemaker experiments simulate a substantial amount (around 45 %) of the variability of the first mode (the Pacific-Japan pattern) in ERA40 from 1979 to 1999. Different from previous work, the pacemaker experiments also simulate a large part (25 %) of the variability of the second mode, related to rainfall variability over northern China. Furthermore, we find that the lower (850 hPa) and the upper (200 hPa) tropospheric circulation of the first mode display the same degree of reproducibility whereas only the lower part of the second mode is reproducible. The basis for the success of the pacemaker experiments is the ability of the experiments to reproduce the observed relationship between El Niño Southern Oscillation (ENSO) and the EASM.
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2019-09-24
    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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  • 3
    Publication Date: 2019-01-23
    Description: The Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG) are important elements in mechanisms for multidecadal variability in models in the North Atlantic Ocean. In this study, a 2000-year long global ocean model integration forced with the atmospheric patterns associated with a white noise North Atlantic Oscillation (NAO) index is shown to have three distinct timescales of North Atlantic Ocean variability. First, an interannual timescale with variability shorter than 15 years, that can be related to Ekman dynamics. Second, a multidecadal timescale, on the 15- to 65-year range, that is mainly concentrated in the SPG region and is controlled by constructive interference between density anomalies around the gyre and the changing NAO forcing. Finally, the centennial timescales, with variability longer than 65 years, that can be attributed to the ocean being in a series of quasi-equilibrium states. The relationship between the ocean’s response and the NAO index differs for each timescale; the 15-year and shorter timescales are directly related to the NAO of the same year, 15- to 65-year timescales are dependent on the NAO index in the last 25–30 years in a sinusoidal sense while the 65-year and longer timescales relate to a sum of the last 50–80 years of the NAO index.
    Type: Article , PeerReviewed
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  • 4
    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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  • 5
    Publication Date: 2019-09-23
    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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  • 6
    Publication Date: 2019-09-23
    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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