Tag: omrani

Nonstationarity in the NAO–Gulf Stream SST Front Interaction

Famooss Paolini, L., Omrani, N.-E., Bellucci, A., Athanasiadis, P.J., Ruggieri, P., Patrizio, C.R., Keenlyside, N. 2024: Non-stationarity in the NAO–Gulf Stream SST front interaction. J Clim. https://doi.org/10.1175/JCLI-D-23-0476.1

Summary: The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of the Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of nonstationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO. This study assesses the possible nonstationarity in the NAO–GSF interaction and the mechanisms underlying this interaction during 1950–2020, using reanalysis data. Results show that the NAO and GSF indices covary on the decadal time scale but only during 1972–2018. A secondary peak in the NAO–GSF covariability emerges on multiannual time scales but only during 2005–15. The nonstationarity in the decadal NAO–GSF covariability is also manifested in variations in their lead–lag relationship. Indeed, the NAO tends to lead the GSF shifts by 3 years during 1972–90 and by 2 years during 1990–2018. The response of the GSF to the NAO at the decadal time scale can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only during 1972–90. Here it is suggested that the nonstationarity of Rossby wave propagation caused the time lag between the NAO and the GSF shifts on the decadal time scale to differ between the two time periods.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Identifying quasi-periodic variability using multivariate empirical mode decomposition: a case of the tropical Pacific

Boljka, L., Omrani, N.-E., Keenlyside, N. S. 2023: Identifying quasi-periodic variability using multivariate empirical mode decomposition: a case of the tropical Pacific. Weather Clim Dynam. https://wcd.copernicus.org/articles/4/1087/2023/wcd-4-1087-2023-discussion.html

Summary: A variety of statistical tools have been used in climate science to gain a better understanding of the climate system’s variability on various temporal and spatial scales. However, these tools are mostly linear, stationary, or both. In this study, we use a recently developed nonlinear and nonstationary multivariate time series analysis tool – multivariate empirical mode decomposition (MEMD). MEMD is a powerful tool for objectively identifying (intrinsic) timescales of variability within a given spatio-temporal system without any timescale pre-selection. Additionally, a red noise significance test is developed to robustly extract quasi-periodic modes of variability. We apply these tools to reanalysis and observational data of the tropical Pacific. This reveals a quasi-periodic variability in the tropical Pacific on timescales ∼ 1.5–4.5 years, which is consistent with El Niño–Southern Oscillation (ENSO) – one of the most prominent quasi-periodic modes of variability in the Earth’s climate system. The approach successfully confirms the wellknown out-of-phase relationship of the tropical Pacific mean thermocline depth with sea surface temperature in the eastern tropical Pacific (recharge–discharge process). Furthermore, we find a co-variability between zonal wind stress in the western tropical Pacific and the tropical Pacific mean thermocline depth, which only occurs on the quasi-periodic timescale. MEMD coupled with a red noise test can therefore successfully extract (nonstationary) quasi-periodic variability from the spatio-temporal data and could be used in the future for identifying potential (new) relationships between different variables in the climate system.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

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.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Thermohaline patterns of intrinsic Atlantic Multidecadal Variability

Zanchettin, D., Fang, S.-W., Khodri, M., Omrani, N.-E., Rubinetti, S., Rubino, A., Timmreck, C., Jungclaus, J.-H. 2023: Thermohaline patterns of intrinsic Atlantic Multidecadal Variability. Clim Dyn. https://link.springer.com/article/10.1007/s00382-023-06679-w

Summary: A vivid scientific debate exists on the nature of the Atlantic Multidecadal Variability (AMV) as an intrinsic rather than predominantly forced climatic phenomenon, and on the role of ocean circulation. Here, we use a multi-millennial unperturbed control simulation and a Holocene simulation with slow-varying greenhouse gas and orbital forcing performed with the low-resolution version of the Max Planck Institute Earth System Model to illustrate thermohaline conditions associated with twelve events of strong AMV that are comparable, in the surface anomalies, to observations in their amplitudes (~ 0.3 °C) and periods (~ 80 years). The events are associated with recurrent yet spatially diverse same-sign anomalous sea-surface temperature and salinity fields that are substantially symmetric in the warm-to-cold and following cold-to-warm transitions and only partly superpose with the long-term spatial AMV pattern. Subpolar cold-fresh anomalies develop in the deep layers during the peak cold phase of strong AMV events, often in association with subtropical warm-salty anomalies yielding a meridional dipole pattern. The Atlantic meridional overturning circulation (AMOC) robustly weakens during the warm-to-cold transition of a strong AMV event and recovers thereafter, with surface salinity anomalies being potential precursors of such overturning changes. A Holocene simulation with the same model including volcanic forcing can disrupt the intrinsic AMV–AMOC connection as post-eruption periods often feature an AMOC strengthening forced by the volcanically induced surface cooling. Overall, our results support the AMV as a potential intrinsic feature of climate, whose episodic strong anomalous events can display different shades of spatial patterns and timings for the warm-to-cold and subsequent cold-to-warm transitions. Attribution of historical AMV fluctuations thus requires full consideration of the associated surface and subsurface thermohaline conditions and assessing the AMOC–AMV relation.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

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.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

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.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Assessing the influence of sea surface temperature and arctic sea ice cover on the uncertainty in the boreal winter future climate projections

Cheung, HN., Keenlyside, N., Koenigk, T., Yang, S., Tian, T., Xu, Z., Gao, Y., Ogawa, F., Omrani, N.-E., Qiao, S., Zhou, W. 2022: Assessing the influence of sea surface temperature and arctic sea ice cover on the uncertainty in the boreal winter future climate projections. Clim. Dyn. https://doi.org/10.1007/s00382-022-06136-0

Summary: We investigate the uncertainty (i.e., inter-model spread) in future projections of the boreal winter climate, based on the forced response of ten models from the CMIP5 following the RCP8.5 scenario. The uncertainty in the forced response of sea level pressure (SLP) is large in the North Pacific, the North Atlantic, and the Arctic. A major part of these uncertainties (31%) is marked by a pattern with a center in the northeastern Pacific and a dipole over the northeastern Atlantic that we label as the Pacific–Atlantic SLP uncertainty pattern (PA∆SLP). The PA∆SLP is associated with distinct global sea surface temperature (SST) and Arctic sea ice cover (SIC) perturbation patterns. To better understand the nature of the PA∆SLP, these SST and SIC perturbation patterns are prescribed in experiments with two atmospheric models (AGCMs): CAM4 and IFS. The AGCM responses suggest that the SST uncertainty contributes to the North Pacific SLP uncertainty in CMIP5 models, through tropical–midlatitude interactions and a forced Rossby wavetrain. The North Atlantic SLP uncertainty in CMIP5 models is better explained by the combined effect of SST and SIC uncertainties, partly related to a Rossby wavetrain from the Pacific and air-sea interaction over the North Atlantic. Major discrepancies between the CMIP5 and AGCM forced responses over northern high-latitudes and continental regions are indicative of uncertainties arising from the AGCMs. We analyze the possible dynamic mechanisms of these responses, and discuss the limitations of this work.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.

Understanding the dynamics of recent Norwegian extreme weather events and their influence on energy production

Pecnjak, Martin (2021-08-05). Understanding the dynamics of recent Norwegian extreme weather events and their influence on energy production (Master’s thesis, University of Bergen, Bergen, Norway). https://bora.uib.no/bora-xmlui/handle/11250/2778409 .

Summary: The growing frequency and severity of extreme weather events in the Northern Hemisphere has prompted a lot of research being done on their origin and physical mechanisms. Both simplified and complex approaches have been introduced in defining and understanding these events, where they look into high-amplitude quasi-stationary Rossby waves and their quasi-resonant amplification. However, different approaches exist to investigating extreme events and these were just a motivation for this thesis. Since the resonance method is suit- able mostly for summer events and the events discussed in this thesis have happened in all seasons, a different approach was needed. The events in question were a winter drought, two summer and autumn floods, a winter snowfall and a spring/summer heatwave in the areas of south and southwestern Norway. In order to detect certain features which would help solve this issue, we look into anomalies of different meteorological variables such as geopoten- tial height, surface temperature, precipitation and snowfall rate and zonal and meridional winds. Deep and thorough statistical and dynamical analyses are applied to define the out- comes and the physical origins which would help us obtain a clear picture on the whole case. The finite-amplitude local wave activity (LWA) diagnostic, as a measure of the meandering of the jet stream, has helped to give a clear picture along with the large-scale circulation. This method can be used as a proxy for the strength of the eddy-driven jet and the storm track. It has proven to be the key factor in defining what has exactly caused the events in ques- tion. The results and findings have shown that the LWA is a conclusive tool in determining whether an extreme event was related to a blocking pattern or not, while the LWA budget equation components have shed light on the so far poorly understood dynamical aspects which led to the events. The zonal LWA flux has proven to be a good predictor of blocking with its onset in the early stages of the events, similar to the traffic jam concept introduced by (Nakamura and Huang, 2018). The jet stream has a capacity for the LWA flux similar to how a highway has a capacity for the number of vehicles on it. If the capacity is exceeded, blocking occurs, and this is readily shown in the results and findings of this work. As for the budget equation components, the zonal LWA flux convergence has proven to be the key in maintaining the increase of the LWA as well as also having an early onset in each blocking event in agreement with the LWA flux. On the other hand, the residual in the LWA budget, which represents the non-conservative small-scale processes (diabatic sources and sinks of LWA), dampens the LWA. The LWA method has also proven to be useful in all seasons. The motivation for the thesis also came from the influence of the events on the meteorological variables related to the Norwegian energy production. The results show us clues into possible ways of improving forecasting of such events and minimizing their harmful impacts. They also show possibilities in improving energy management, infrastructure, allocation of resources and preparedness of the society for damages and hazards caused by the events. This was not fully investigated in this thesis and is the next step in the research of this topic.

Link to publication. You are most welcome to contact us or the corresponding author(s) directly, if you have questions.