The role of stratosphere-troposphere coupling on North Atlantic air-sea interactions

Dr Hua Lu (BAS), Dr Bablu Sinha, Prof Sybren Drijfhout, Prof Adam Scaife (Met Office), Prof John King (BAS)
Rationale: 

Lu et al. (2021) has developed a new index based on Ertel’s potential vorticity in the upper stratosphere. They show that the early winter conditions in the upper stratosphere play a key role in processing disturbances from the troposphere and then transmitting the signals back to the surface. The significance of this linkage is yet to be assessed and is the main topic of this project.

UK weather and climate are affected by the North Atlantic Oscillation (NAO), which accounts for variability of the mid-latitude jet stream that blows eastward across the North Atlantic Ocean. Air-sea interactions play an important role in the NAO. Climate models however do not capture the interactions well and exhibit weaker-than-observed signal. A misrepresentation of the upper stratosphere and its feedback processes could be one of the causes.

This project assesses the mechanism revealed by Lu et al. (2021) using model simulations from the Met Office’s global seasonal forecast system, supplemented by targeted simulations with specific ocean perturbations.  The aim is to improve the skill of long-range forecasts by providing better process understanding of the feedbacks between the upper stratosphere and ocean anomalies. Project outcome will benefit mitigation decisions and adaptation preparations for weather-related damage.

Methodology: 

This project addresses two main questions:

Q1: How sensitive is the NAO to the early winter upper stratospheric flow regimes and vice versa?

Q2: Can the weaker-than-observed variability in ocean-atmospheric coupling in models be explained by misrepresentations of stratosphere-troposphere coupling?

which will be addressed as follows.  Q1 will be addressed by analyzing the hindcast and forecast ensembles generated by Met Office’s seasonal forecast system GloSea6 to investigate feedbacks among tropospheric precursors, regime development in the upper stratosphere, and the NAO response. Model skill in predicting the NAO and relevant atmospheric circulation anomalies will be assessed by separating the hindcasts / forecasts according to upper stratospheric conditions (defined by Lu et al. 2021) and comparing with observations using ERA5 global atmospheric reanalysis. Q2 will be addressed by selecting a representative subset of case studies to evaluate how air-sea interaction over the North Atlantic Ocean affects both the wave driving in early winter and the circulation response in middle to late winter using the ocean-atmosphere coupled ensemble simulations produced by the National Centre of Oceanography (NOC). The student will use existing Matlab- and Python-based diagnostics developed by the supervisory team, develop these tools further, and write up the results for publication.

 

Location: 
British Antarctic Survey (BAS), Cambridge
Training: 

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the British Antarctic Survey, with placement opportunities at the Met Office and prospect of using a world-leading seasonal prediction system.

Specific training includes large-scale atmosphere/ocean circulation, air-sea interactions, climate variability, analysis of big data using MATLAB and Python, supplemented by summer schools, workshops, and seminars held by BAS and Cambridge University. The student will gain important research skills, i.e. literature review, scientific writing/publication, and oral presentation. The cross-disciplinary supervisory team provides an exceptional learning and network opportunity.

Attending international conferences will be facilitated and encouraged. 

Eligibility & Funding Details: 

Please see https://inspire-dtp.ac.uk/how-apply for details.

Background Reading: 

Lu, H., Gray, L. J., Martineau, P., King, J. C., & Bracegirdle, T. J. (2021). Regime behavior in the upper stratosphere as a precursor of stratosphere-troposphere coupling in the northern winter, Journal of Climate, https://doi.org/10.1175/JCLI-D-20-0831.1

Josey, S.A., J.J.-M. Hirschi, B. Sinha, A. Duchez, J.P. Grist, M. Robert, 2018: The recent Atlantic cold anomaly: Causes, consequences, and related phenomena, Annual Review of Marine Science, 10:1, 475-501.

Scaife A.A., A. Arribas, E. Blockley, A. Brookshaw, R. T. Clark, N. Dunstone, R. Eade, D. Fereday, C. K. Folland, M. Gordon, L. Hermanson, J. R. Knight, D. J. Lea, C. MacLachlan, A. Maidens, M. Martin, A. K. Peterson, D. Smith, M. Vellinga, E. Wallace, J. Waters and A. Williams, 2014. Skilful Long Range Prediction of European and North American Winters. Geophys. Res. Lett., 41, 2514-2519, DOI:10.1002/2014GL059637.

 

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