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  • 1
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    Physical controls of Southern Ocean deep-convection variability in CMIP5 models and the Kiel Climate Model (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (13). pp. 6951-6958.
    Details
    Publication Date: 2020-02-06
    Description: Global climate models exhibit large biases in the Southern Ocean. For example, in models Antarctic bottom water is formed mostly through open-ocean deep-convection rather than through shelf convection. Still, the timescale, region, and intensity of deep-convection variability vary widely among models. We investigate the physical controls of this variability in the Atlantic sector of the Southern Ocean, where most of the models simulate recurring deep-convection events. We analyzed output from eleven exemplary CMIP5 models and four versions of the Kiel Climate Model (KCM). Of several potential physical control parameters that we tested, the ones shared by all these models are: Stratification in the convection region influences the timescale of the deep-convection variability, i.e. models with a strong (weak) stratification vary on long (short) timescales. And, sea ice volume affects the modeled mean state in the Southern Ocean: large (small) sea ice volume is associated with a non-convective (convective) predominant regime.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    DOI: 10.1002/2017GL074087
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Evolution of Eastern Equatorial Pacific Seasonal and Interannual Variability in response to orbital forcing during the Holocene and Eemian from Model Simulations (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 45 (18). pp. 9843-9851.
    Details
    Publication Date: 2021-02-08
    Description: Characteristics of the seasonal and interannual sea surface temperature (SST) variability in the eastern equatorial Pacific (EEP) over last two interglacials, the Holocene and Eemian, are analyzed using transient climate simulations with the Kiel Climate Model (KCM). There is a tendency towards a strengthening of the seasonal as well as the El Niño/Southern Oscillation‐ (ENSO) related variability from the early to the late interglacials. The weaker EEP SST annual cycle during the early interglacials is mainly result of insolation‐forced cooling during its warm phase and dynamically‐induced warming during its cold phase. Enhanced convection over northern South America weakens northeasterlies in the EEP leading to weaker equatorial upwelling, deeper thermocline and subsequent warming in this region. We show that a negative ENSO modulation of the annual cycle operates only on short timescales and does not affect their evolution on orbital time scales where both ENSO and annual cycle show similar tendencies to increase.
    Type: Article , PeerReviewed
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    DOI: 10.1029/2018GL079337
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Expanding Greenland Ice Sheet Enhances Sensitivity of Plio-Pleistocene Climate to Obliquity Forcing in the Kiel Climate Model (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (19). pp. 9957-9966.
    Details
    Publication Date: 2020-02-06
    Description: Proxy data suggest the onset of Northern Hemisphere glaciation during the Plio-Pleistocene transition from 3.2 to 2.5 Ma resulted in enhanced climate variability at the obliquity (41 kyr) frequency. Here, we investigate the influence of the expanding Greenland ice sheet (GrIS) on the mean climate and obliquity-related variability in a series of climate model simulations. These suggest that an expanding GrIS weakens the Atlantic Meridional Overturning Circulation (AMOC) by ~1 Sv, mainly due to reduced heat loss in the Greenland-Iceland-Norwegian Sea. Moreover, the growing GrIS amplifies the Hadley circulation response to obliquity forcing driving variations in freshwater export from the tropical Atlantic and in turn variations of the AMOC. The stronger AMOC response to obliquity forcing, by about a factor of two, results in a stronger global-mean near-surface temperature response. We conclude that the AMOC response to obliquity forcing is important to understand the enhanced climate variability at the obliquity frequency during the Plio-Pleistocene transition.
    Type: Article , PeerReviewed
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    DOI: 10.1002/2017GL074835
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    What Controls ENSO-Amplitude Diversity in Climate Models? (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 45 (4). pp. 1989-1996.
    Details
    Publication Date: 2021-02-08
    Description: Climate models depict large diversity in the strength of the El Niño/Southern Oscillation (ENSO) (ENSO amplitude). Here we investigate ENSO-amplitude diversity in the Coupled Model Intercomparison Project Phase 5 (CMIP5) by means of the linear recharge oscillator model, which reduces ENSO dynamics to a two-dimensional problem in terms of eastern equatorial Pacific sea surface temperature anomalies (T) and equatorial Pacific upper ocean heat content anomalies (h). We find that a large contribution to ENSO-amplitude diversity originates from stochastic forcing. Further, significant interactions exist between the stochastic forcing and the growth rates of T and h with competing effects on ENSO amplitude. The joint consideration of stochastic forcing and growth rates explains more than 80% of the ENSO-amplitude variance within CMIP5. Our results can readily explain the lack of correlation between the Bjerknes Stability index, a measure of the growth rate of T, and ENSO amplitude in a multimodel ensemble.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: archive
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    DOI: 10.1002/2017GL076849
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Potential of Equatorial Atlantic Variability to Enhance El Nino Prediction (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 . pp. 2278-2283.
    Details
    Publication Date: 2017-05-24
    Description: Extraordinarily strong El Niño events, such as those of 1982/83 and 1997/98, have been poorly predicted by operational seasonal forecasts made before boreal spring, despite significant advances in understanding, improved models, and enhanced observational networks. The Equatorial Atlantic Zonal Mode – a phenomenon similar to El Niño but much weaker and peaking in boreal summer – impacts winds over the Pacific, and hence affects El Niño, and also potentially its predictability. Here we use a climate model to perform a suite of seasonal predictions with and without SST in the Atlantic restored to observations. We show for the first time that knowledge of Equatorial Atlantic sea surface temperature (SST) significantly improves the prediction across boreal spring of major El Niño events and also weaker variability. This is because Atlantic SST acts to modulate El Niño variability, rather than triggering events. Our results suggest that better prediction of major El Niño events might be achieved through model improvement in the Equatorial Atlantic.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    DOI: 10.1002/grl.50362
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Improving Climate Model Simulation of Tropical Atlantic Sea Surface Temperature: The Importance of Enhanced Vertical Atmosphere Model Resolution (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 (7). pp. 2401-2408.
    Details
    Publication Date: 2017-04-10
    Description: A long-standing problem in climate modeling is the inaccurate simulation of tropical Atlantic (TA) sea surface temperature (SST), known as the TA SST bias. It has far-reaching consequences for climate prediction in that area as it goes along, among others, with erroneous precipitation patterns. We show that the TA SST bias can be largely reduced by increasing both the atmospheric horizontal and vertical resolution in a climate model. At high horizontal resolution, enhanced vertical resolution is indispensable to substantially improve the simulation of TA SST by enhancing surface wind stress. This also reduces biases in the upper ocean thermal structure and precipitation patterns. Although, enhanced horizontal resolution alone leads to some improvement in the mean climate, typical bias patterns characterized by a reversed zonal SST gradient at the equator and too warm SST in the Benguela upwelling region are mostly unchanged at a coarser vertical resolution.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    DOI: 10.1002/2015GL063310
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    The Impact of Mean State Errors on Equatorial Atlantic Interannual Variability in a Climate Model (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 120 (2). pp. 1133-1151.
    Details
    Publication Date: 2019-04-04
    Description: Observations show that the Equatorial Atlantic Zonal Mode (ZM) obeys similar physics to the El Niño Southern Oscillation (ENSO): positive Bjerknes and delayed negative feedbacks. This implies the ZM may be predictable on seasonal timescales, but models demonstrate little prediction skill in this region. In this study using different configurations of the Kiel Climate Model (KCM) exhibiting different levels of systematic error, we show that a reasonable simulation of the ZM depends on realistic representation of the mean state, i.e., surface easterlies along the equator, upward sloping thermocline to the east, with an equatorial SST cold tongue in the east. We further attribute the differences in interannual variability among the simulations to the individual components of the positive Bjerknes and delayed negative feedbacks. Differences in the seasonality of the variability are similarly related to the impact of seasonal biases on the Bjerknes feedback. Our results suggest that model physics must be enhanced to enable skillful seasonal predictions in the Tropical Atlantic Sector, although some improvement with regard to the simulation of Equatorial Atlantic interannual variability may be achieved by momentum flux correction. This pertains especially to the seasonal phase locking of interannual SST variability.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2014JC010384
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Oxygen minimum zone variations in the tropical Pacific during the Holocene (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 (20). pp. 8530-8537.
    Details
    Publication Date: 2019-09-24
    Description: We performed simulations with a global model of ocean biogeochemistry forced with orbitally driven anomalies of oceanic conditions for the mid-Holocene, known as Holocene climate optimum, to investigate natural variability in the eastern equatorial Pacific oxygen minimum zone (EEP OMZ). While the global mean temperature during the mid-Holocene was likely slightly higher than the 1961–1990 mean, the sea surface temperature in the EEP was slightly lower. Mid-Holocene oxygen concentrations in the EEP OMZ are generally increased, locally by up to 50%, and the EEP OMZ volume was, depending on definition of the OMZ threshold, at least 6% lower. These higher oxygen levels are the combined result of competing physical and biogeochemical processes. Our results imply that mechanisms for past changes in the EEP OMZ intensity and extension can differ from the global warming driven decline in oxygen levels observed for the recent decades and predicted for the future.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2015GL064680
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Comprehensive Earth System Models of the Last Glacial Cycle (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Eos: Earth & Space Science News, 97 .
    Details
    Publication Date: 2018-05-04
    Description: Much of modern climate science fails to consider millennium-scale processes, many of which may prove to be important for predicting the climate trajectory in the shorter term.
    Type: Article , PeerReviewed
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    DOI: 10.1029/2016EO059587
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Amazonian Arthropods Respond to El Nino (1996)
    Wiley
    In:  Biotropica, 28 (3). p. 403.
    Details
    Publication Date: 2020-06-22
    Type: Article , PeerReviewed
    DOI: 10.2307/2389205
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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