Tag: keenlyside

Pacific oceanic front amplifies the impact of Atlantic oceanic front on North Atlantic blocking

Cheung, HN., Omrani, NE., Ogawa, F., Keenlyside, N., Nakamura, H., Zhou, W. 2023: Pacific oceanic front amplifies the impact of Atlantic oceanic front on North Atlantic blocking. npj Clim Atmos Sci 6, 61. https://doi.org/10.1038/s41612-023-00370-x

Summary: Atmospheric blocking is a crucial driver of extreme weather events, but its climatological frequency is largely underestimated in state-of-the-art climate models, especially around the North Atlantic. While air-sea interaction along the North Atlantic oceanic frontal region is known to influence Atlantic blocking activity, remote effects from the Pacific have been less studied. Here we use semi-idealised experiments with an atmospheric general circulation model to demonstrate that the mid-latitude Pacific oceanic front is crucial for climatological Atlantic blocking activity. The front intensifies the Pacific eddy-driven jet that extends eastward towards the North Atlantic. The eastward-extended Pacific jet reinforces the North Atlantic circulation response to the Atlantic oceanic front, including the storm track activity and the eddy-driven jet. The strengthening of the eddy-driven jet reduces the Greenland blocking frequency. Moreover, the Pacific oceanic front greatly strengthens the stationary planetary-scale ridge in Europe. Together with a stronger northeastward extension of the Atlantic storm track, enhanced interaction between extratropical cyclones and the European ridge favours the occurrence of Euro-Atlantic blocking. Therefore, the North Atlantic circulation response amplified remotely by the Pacific oceanic front substantially increases Euro-Atlantic blocking frequency while decreasing Greenland blocking frequency.

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Trends and internal variability in Brazilian hydropower catchments (Master’s thesis)

Byermoen, Emilie. 2023: Trends and internal variability in Brazilian hydropower catchments. Master’s thesis, University of Bergen, Norway. https://bora.uib.no/bora-xmlui/handle/11250/3071878

Summary: Hydropower is a major energy source in Brazil, and long-term hydropower production planning is crucial both for maintaining energy and water security in the country. The amount of water that is available to electricity production in the reservoirs have changed in the recent years, and there is an urgent need to understand the cause(s) of these changes, and whether observed stream flow trends will persist, reverse or amplify in the future. In this thesis, I therefore separate externally forced precipitation and evaporation trends and variability from internal variations originating in the ocean for three hydrographic catchments in Brazil: Óbidos catchment in Amazon, Propria catchment in São Francisco and Porto Murtinho catchment in Paraguay. I compare an ocean anomaly assimilation experiment of Norwegian Climate Prediction Model (NorCPM) to an externally forced historical experiment and observed stream flow, precipitation and evaporation in the catchments. The results indicate that the multi-decadal increasing stream flow trend in Amazon is (partly) externally forced, and might therefore persist, but that the SON stream flow is tightly connected to JJA precipitation variation which is shown to be driven by ocean variation, and may therefore reverse in the future. The long-term decrease of precipitation in São francisco is likely to be caused by internal variability, and is therefore likely to (partly) restore in the future, but results indicate that decadal stream flow variations in the basin is substantially impacted by other factors than precipitation as well. São Francisco catchment is found to be strongly connected to DJF precipitation variations that the model is unable to replicate. In Paraguay, I find that the austral summer stream flow is tightly connected to inter-annual precipitation variability that originates in the ocean in austral winter and spring. The steep significant decrease in stream flow over the last decades in Paraguay catchment is likely to have additional causes than precipitation, according to the results. All the results have implications for hydropower and water management planning in the three catchments in Brazil.

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Phytoplankton abundance in the Barents Sea is predictable up to five years in advance

Fransner, F., Olsen, A., Årthun, M., Counillon, F., Tjiputra, J., Samuelsen, A., Keenlyside, N. 2023: Phytoplankton abundance in the Barents Sea is predictable up to five years in advance. Commun Earth Environ. https://doi.org/10.1038/s43247-023-00791-9

Summary: The Barents Sea is a highly biologically productive Arctic shelf sea with several commercially important fish stocks. Interannual-to-decadal predictions of its ecosystem would therefore be valuable for marine resource management. Here, we demonstrate that the abundance of phytoplankton, the base of the marine food web, can be predicted up to five years in advance in the Barents Sea with the Norwegian Climate Prediction Model. We identify two different mechanisms giving rise to this predictability; 1) in the southern ice-free Atlantic Domain, skillful prediction is a result of the advection of waters with anomalous nitrate concentrations from the Subpolar North Atlantic; 2) in the northern Polar Domain, phytoplankton predictability is a result of the skillful prediction of the summer ice concentration, which influences the light availability. The skillful prediction of the phytoplankton abundance is an important step forward in the development of numerical ecosystem predictions of the Barents Sea.

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Framework for an Ocean-Connected Supermodel of the Earth System

Counillon, F., Keenlyside, N., Wang, S., Devilliers, M., Gupta, A., Koseki, S., Shen, M.-L. 2023: Framework for an Ocean-Connected Supermodel of the Earth System. JAMES. https://doi.org/10.1029/2022MS003310

Summary: Observed and future winter Arctic sea ice loss is strongest in the Barents Sea. However, the anthropogenic signal of the sea ice decline is superimposed by pronounced internal variability that represents a large source of uncertainty in future climate projections. A notable manifestation of internal variability is rapid ice change events (RICEs) that greatly exceed the anthropogenic trend. These RICEs are associated with large displacements of the sea ice edge which could potentially have both local and remote impacts on the climate system. In this study we present the first investigation of the frequency and drivers of RICEs in the future Barents Sea, using multi-member ensemble simulations from CMIP5 and CMIP6. A majority of RICEs are triggered by trends in ocean heat transport or surface heat fluxes. Ice loss events are associated with increasing trends in ocean heat transport and decreasing trends in surface heat loss. RICEs are a common feature of the future Barents Sea until the region becomes close to ice-free. As their evolution over time is closely tied to the average sea ice conditions, rapid ice changes in the Barents Sea may serve as a precursor for future changes in adjacent seas.

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ENSO teleconnections in terms of non-NAO and NAO atmospheric variability

King, M.P., Keenlyside, N., Li, C. 2023: ENSO teleconnections in terms of non-NAO and NAO atmospheric variability. Clim Dyn. https://doi.org/10.1007/s00382-023-06697-8

Summary: The validity of the long-held understanding or assumption that El Niño-Southern Oscillation (ENSO) has a remote influence on the North Atlantic Oscillation (NAO) in the January–February–March (JFM) months has been questioned recently. We examine this claim further using atmospheric data filtered to separate the variability orthogonal and parallel to NAO. This decomposition of the atmospheric fields is based on the Principal Component/Empirical Orthogonal Function method whereby the leading mode of the sea-level pressure in the North Atlantic sector is recognised as the NAO, while the remaining variability is orthogonal (unrelated) to NAO. Composite analyses indicate that ENSO has statistically significant links with both the non-NAO and NAO variability at various atmospheric levels. Additional bootstrap tests carried out to quantify the uncertainty and statistical significance confirm these relationships. Consistent with previous studies, we find that an ENSO teleconnection in the NAO-related variability is characterised by lower-stratospheric eddy heat flux anomalies (related to the vertical propagation of planetary waves) which appear in November–December and strengthen through JFM. Under El Niño (La Niña), there is constructive (destructive) interference of anomalous eddy heat flux with the climatological pattern, enhancing (reducing) fluxes over the northern Pacific and Barents Sea areas. We further show that the teleconnection of extreme El Niño is essentially a non-NAO phenomenon. Some non-linearity of the teleconnections is suggested, with El Niño including more NAO-related variability than La Niña, but the statistical significance is degraded due to weaker signals and smaller sample sizes after the partitioning. Our findings have implications for the general understanding of the nature of ENSO teleconnections over the North Atlantic, as well as for refining methods to characterise and evaluate them in models.

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Upcoming workshop: External versus internal variability on decadal and longer time scales

On Wednesday 14th September, the CLIVAR Climate Dynamics Panel (CDP) will launch the first of an intended series of annual CDP workshops. This year’s workshop will target our understanding of internal and externally forced variability in the climate system, their interaction on decadal timescales and longer, and the effects of variability on extreme events. To foster discussion that will stimulate focused research on this important topic, the workshop aims to tackle the following overarching questions:

  • How to isolate the relative contributions of external and internal variability to observed decadal and longer variability?
  • How do the various external forcings modulate internal variability?
  • How to progress in narrowing observational and modeling uncertainties in external and internal variability?
  • What are the effects of external and internal variability on extreme events?

The workshop will be online, and consist of six, weekly 2-hour sessions, from September 14th to October 19th, 2022. The sessions will be on Wednesdays with the timings varying to accommodate participation from different time zones.

Workshop program and further event information: https://www.clivar.org/events/clivar-climate-dynamics-panel-cdp-annual-workshop-external-versus-internal-variability

 

Weakening of the Atlantic Niño variability under global warming

Crespo, L.R., Prigent, A., Keenlyside, N., Koseki, S., Svendsen, L., Richter, I., Sánchez-Gómez, E. 2022: Weakening of the Atlantic Niño variability under global warming. Nat. Clim. Chang. https://doi.org/10.1038/s41558-022-01453-y

Summary: The Atlantic Niño is one of the most important patterns of interannual tropical climate variability, but how climate change will influence this pattern is not well known due to large climate model biases. Here we show that state-of-the-art climate models robustly predict a weakening of Atlantic Niños in response to global warming, mainly due to a decoupling of subsurface and surface temperature variations as the upper equatorial Atlantic Ocean warms. This weakening is predicted by most (>80%) models in the Coupled Model Intercomparison Project Phases 5 and 6 under the highest emission scenarios. Our results indicate a reduction in variability by the end of the century by 14%, and as much as 24–48% when accounting for model errors using a simple emergent constraint analysis. Such a weakening of Atlantic Niño variability will potentially impact climate conditions and the skill of seasonal predictions in many regions.

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Mitigating climate biases in the mid-latitude North Atlantic by increasing model resolution: SST gradients and their relation to blocking and the jet

Athanasiadis, P.J., Ogawa, F., Omrani, N.-E., Keenlyside, N., Schiemann, R., Baker, A.J., Vidale, P.L., Bellucci, A., Ruggieri, P., Haarsma, R., Roberts, M., Roberts, C., Novak, L., Gualdi, S. 2022: Mitigating climate biases in the mid-latitude North Atlantic by increasing model resolution: SST gradients and their relation to blocking and the jet. J Clim. https://doi.org/10.1007/s10236-022-01523-x

Summary: Starting to resolve the oceanic mesoscale in climate models is a step change in model fidelity. This study examines how certain obstinate biases in the midlatitude North Atlantic respond to increasing resolution (from 1° to 0.25° in the ocean) and how such biases in sea surface temperature (SST) affect the atmosphere. Using a multi-model ensemble of historical climate simulations run at different horizontal resolutions, it is shown that a severe cold SST bias in the central North Atlantic, common to many ocean models, is significantly reduced with increasing resolution. The associated bias in the time-mean meridional SST gradient is shown to relate to a positive bias in low-level baroclinicity, while the cold SST bias causes biases also in static stability and diabatic heating in the interior of the atmosphere. The changes in baroclinicity and diabatic heating brought by increasing resolution lead to improvements in European blocking and eddy-driven jet variability. Across the multi-model ensemble a clear relationship is found between the climatological meridional SST gradients in the broader Gulf Stream Extension area and two aspects of the atmospheric circulation: the frequency of high-latitude blocking and the southern-jet regime. This relationship is thought to reflect the two-way interaction (with a positive feedback) between the respective oceanic and atmospheric anomalies. These North Atlantic SST anomalies are shown to be important in forcing significant responses in the midlatitude atmospheric circulation, including jet variability and the stormtrack. Further increases in oceanic and atmospheric resolution are expected to lead to additional improvements in the representation of Euro-Atlantic climate.

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Coupled stratosphere-troposphere-Atlantic multidecadal oscillation and its importance for near-future climate projection

Omrani, NE., Keenlyside, N., Matthes, K., Boljka, L., Zanchettin, D., Jungclaus, JH., Lubis, SW. 2022: Coupled stratosphere-troposphere-Atlantic multidecadal oscillation and its importance for near-future climate projection. npj Clim Atmos Sci. https://doi.org/10.1038/s41612-022-00275-1

Summary: Northern Hemisphere (NH) climate has experienced various coherent wintertime multidecadal climate trends in stratosphere, troposphere, ocean, and cryosphere. However, the overall mechanistic framework linking these trends is not well established. Here we show, using long-term transient forced coupled climate simulation, that large parts of the coherent NH-multidecadal changes can be understood within a damped coupled stratosphere/troposphere/ocean-oscillation framework. Wave-induced downward propagating positive stratosphere/troposphere-coupled Northern Annular Mode (NAM) and associated stratospheric cooling initiate delayed thermohaline strengthening of Atlantic overturning circulation and extratropical Atlantic-gyres. These increase the poleward oceanic heat transport leading to Arctic sea-ice melting, Arctic warming amplification, and large-scale Atlantic warming, which in turn initiates wave-induced downward propagating negative NAM and stratospheric warming and therefore reverse the oscillation phase. This coupled variability improves the performance of statistical models, which project further weakening of North Atlantic Oscillation, North Atlantic cooling and hiatus in wintertime North Atlantic-Arctic sea-ice and global surface temperature just like the 1950s–1970s.

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Skilful decadal-scale prediction of fish habitat and distribution shifts

Payne, M.R., Danabasoglu, G., Keenlyside, N., Matei, D., Miesner, A.K., Yang, S., Yeager, S.G. 2022: Skilful decadal-scale prediction of fish habitat and distribution shifts. Nat. Commun. https://doi.org/10.1038/s41467-022-30280-0

Summary: Many fish and marine organisms are responding to our planet’s changing climate by shifting their distribution. Such shifts can drive international conflicts and are highly problematic for the communities and businesses that depend on these living marine resources. Advances in climate prediction mean that in some regions the drivers of these shifts can be forecast up to a decade ahead, although forecasts of distribution shifts on this critical time-scale, while highly sought after by stakeholders, have yet to materialise. Here, we demonstrate the application of decadal-scale climate predictions to the habitat and distribution of marine fish species. We show statistically significant forecast skill of individual years that outperform baseline forecasts 3–10 years ahead; forecasts of multi-year averages perform even better, yielding correlation coefficients in excess of 0.90 in some cases. We also demonstrate that the habitat shifts underlying conflicts over Atlantic mackerel fishing rights could have been foreseen. Our results show that climate predictions can provide information of direct relevance to stakeholders on the decadal-scale. This tool will be critical in foreseeing, adapting to and coping with the challenges of a changing future climate, particularly in the most ocean-dependent nations and communities.

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