Uncertainty and scale interactions in ocean ensembles: From seasonal forecasts to multidecadal climate predictions

Quarterly Journal of the Royal Meteorological Society (2018)

L Zanna, JM Brankart, M Huber, S Leroux, T Penduff, PD Williams

© 2018 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society. The ocean plays an important role in the climate system on time-scales of weeks to centuries. Despite improvements in ocean models, dynamical processes involving multiscale interactions remain poorly represented, leading to errors in forecasts. We present recent advances in understanding, quantifying, and representing physical and numerical sources of uncertainty in novel regional and global ocean ensembles at different horizontal resolutions. At coarse resolution, uncertainty in 21st century projections of the upper overturning cell in the Atlantic is mostly a result of buoyancy fluxes, while the uncertainty in projections of the bottom cell is driven equally by both wind and buoyancy flux uncertainty. In addition, freshwater and heat fluxes are the largest contributors to Atlantic Ocean heat content regional projections and their uncertainties, mostly as a result of uncertain ocean circulation projections. At both coarse and eddy-permitting resolutions, unresolved stochastic temperature and salinity fluctuations can lead to significant changes in large-scale density across the Gulf Stream front, therefore leading to major changes in large-scale transport. These perturbations can have an impact on the ensemble spread on monthly time-scales and subsequently interact nonlinearly with the dynamics of the flow, generating chaotic variability on multiannual time-scales. In the Gulf Stream region, the ratio of chaotic variability to atmospheric-forced variability in meridional heat transport is larger than 50% on time-scales shorter than 2 years, while between 40 and 48°S the ratio exceeds 50% on on time-scales up to 28 years. Based on these simulations, we show that air–sea interaction and ocean subgrid eddies remain an important source of error for simulating and predicting ocean circulation, sea level, and heat uptake on a range of spatial and temporal scales. We discuss how further refinement of these ensembles can help us assess the relative importance of oceanic versus atmospheric uncertainty in weather and climate.

Remote and local influences in forecasting Pacific SST: a linear inverse model and a multimodel ensemble study

Climate Dynamics (2018) 1-19

DF Dias, A Subramanian, L Zanna, AJ Miller

© 2018 Springer-Verlag GmbH Germany, part of Springer Nature A suite of statistical linear inverse models (LIMs) are used to understand the remote and local SST variability that influences SST predictions over the North Pacific region. Observed monthly SST anomalies in the Pacific are used to construct different regional LIMs for seasonal to decadal predictions. The seasonal forecast skills of the LIMs are compared to that from three operational forecast systems in the North American Multi-Model Ensemble (NMME), revealing that the LIM has better skill in the Northeastern Pacific than NMME models. The LIM is also found to have comparable forecast skill for SST in the Tropical Pacific with NMME models. This skill, however, is highly dependent on the initialization month, with forecasts initialized during the summer having better skill than those initialized during the winter. The data are also bandpass filtered into seasonal, interannual and decadal time scales to identify the relationships between time scales using the structure of the propagator matrix. Moreover, we investigate the influence of the tropics and extra-tropics in the predictability of the SST over the region. The Extratropical North Pacific seems to be a source of predictability for the tropics on seasonal to interannual time scales, while the tropics enhance the forecast skill for the decadal component. These results indicate the importance of temporal scale interactions in improving the predictions on decadal timescales. Hence, we show that LIMs are not only useful as benchmarks for estimates of statistical skill, but also to isolate contributions to the forecast skills from different timescales, spatial scales or even model components.

Eddy-mixing entropy and its maximization in forced-dissipative geostrophic turbulence


TW David, L Zanna, DP Marshall

Diagnosing ENSO and Global Warming Tropical Precipitation Shifts Using Surface Relative Humidity and Temperature

Journal of Climate American Meteorological Society 31 (2018) 1413-1433

A Todd, M Collins, FH Lambert, R Chadwick

El Niño–Southern Oscillation complexity

Nature Springer Science and Business Media LLC 559 (2018) 535-545

A Timmermann, S-I An, J-S Kug, F-F Jin, W Cai, A Capotondi, KM Cobb, M Lengaigne, MJ McPhaden, MF Stuecker, K Stein, AT Wittenberg, K-S Yun, T Bayr, H-C Chen, Y Chikamoto, B Dewitte, D Dommenget, P Grothe, E Guilyardi, Y-G Ham, M Hayashi, S Ineson, D Kang, S Kim, W Kim, J-Y Lee, T Li, J-J Luo, S McGregor, Y Planton, S Power, H Rashid, H-L Ren, A Santoso, K Takahashi, A Todd, G Wang, G Wang, R Xie, W-H Yang, S-W Yeh, J Yoon, E Zeller, X Zhang

Predicting the future is hard and other lessons from a population time series data science competition


GRW Humphries, C Che-Castaldo, PJ Bull, G Lipstein, A Ravia, B Carrion, T Bolton, A Ganguly, HJ Lynch

The Signature of Oceanic Processes in Decadal Extratropical SST Anomalies


CH O'Reilly, L Zanna

Recent multivariate changes in the North Atlantic climate system, with a focus on 2005-2016


J Robson, RT Sutton, A Archibald, F Cooper, M Christensen, LJ Gray, NP Holliday, C Macintosh, M McMillan, B Moat, M Russo, R Tilling, K Carslaw, D Desbruyeres, O Embury, DL Feltham, DP Grosvenor, S Josey, B King, A Lewis, GD McCarthy, C Merchant, AL New, CH O'Reilly, SM Osprey, K Read, A Scaife, A Shepherd, B Sinha, D Smeed, D Smith, A Ridout, T Woollings, M Yang

Seasonal to annual ocean forecasting skill and the role of model and observational uncertainty.

Quarterly journal of the Royal Meteorological Society. Royal Meteorological Society (Great Britain) 144 (2018) 1947-1964

S Juricke, D MacLeod, A Weisheimer, L Zanna, TN Palmer

Accurate forecasts of the ocean state and the estimation of forecast uncertainties are crucial when it comes to providing skilful seasonal predictions. In this study we analyse the predictive skill and reliability of the ocean component in a seasonal forecasting system. Furthermore, we assess the effects of accounting for model and observational uncertainties. Ensemble forcasts are carried out with an updated version of the ECMWF seasonal forecasting model System 4, with a forecast length of ten months, initialized every May between 1981 and 2010. We find that, for essential quantities such as sea surface temperature and upper ocean 300 m heat content, the ocean forecasts are generally underdispersive and skilful beyond the first month mainly in the Tropics and parts of the North Atlantic. The reference reanalysis used for the forecast evaluation considerably affects diagnostics of forecast skill and reliability, throughout the entire ten-month forecasts but mostly during the first three months. Accounting for parametrization uncertainty by implementing stochastic parametrization perturbations has a positive impact on both reliability (from month 3 onwards) as well as forecast skill (from month 8 onwards). Skill improvements extend also to atmospheric variables such as 2 m temperature, mostly in the extratropical Pacific but also over the midlatitudes of the Americas. Hence, while model uncertainty impacts the skill of seasonal forecasts, observational uncertainty impacts our assessment of that skill. Future ocean model development should therefore aim not only to reduce model errors but to simultaneously assess and estimate uncertainties.

The Impact of Tropical Precipitation on Summertime Euro-Atlantic Circulation via a Circumglobal Wave Train

JOURNAL OF CLIMATE 31 (2018) 6481-6504

CH O'Reilly, T Woollings, L Zanna, A Weisheimer

Impact of Gulf Stream SST biases on the global atmospheric circulation

CLIMATE DYNAMICS 51 (2018) 3369-3387

RW Lee, TJ Woollings, BJ Hoskins, KD Williams, CH O'Reilly, G Masato

Skilful Seasonal Predictions of Summer European Rainfall


N Dunstone, D Smith, A Scaife, L Hermanson, D Fereday, C O'Reilly, A Stirling, R Eade, M Gordon, C Maclachlan, T Woollings, K Sheen, S Belcher

Flow dependent ensemble spread in seasonal forecasts of the boreal winter extratropics


D MacLeod, C O'Reilly, T Palmer, A Weisheimer

Interdecadal variability of the ENSO teleconnection to the wintertime North Pacific

CLIMATE DYNAMICS 51 (2018) 3333-3350

CH O'Reilly

Challenges and opportunities for improved understanding of regional climate dynamics

NATURE CLIMATE CHANGE 8 (2018) 101-108

M Collins, S Minobe, M Barreiro, S Bordoni, Y Kaspi, A Kuwano-Yoshida, N Keenlyside, E Manzini, CH O'Reilly, R Sutton, S-P Xie, O Zolina

The impact of horizontal resolution on energy transfers in global ocean models

Fluids 2 (2017)

J Kjellsson, L Zanna

© 2016 by the authors; licensee MDPI, Basel, Switzerland. The ocean is a turbulent fluid with processes acting on a variety of spatio-temporal scales. The estimates of energy fluxes between length scales allows us to understand how the mean flow is maintained as well as how mesoscale eddies are formed and dissipated. Here, we quantify the kinetic energy budget in a suite of realistic global ocean models, with varying horizontal resolution and horizontal viscosity. We show that eddy-permitting ocean models have weaker kinetic energy cascades than eddy-resolving models due to discrepancies in the effect of wind forcing, horizontal viscosity, potential to kinetic energy conversion, and nonlinear interactions on the kinetic energy (KE) budget. However, the change in eddy kinetic energy between the eddy-permitting and the eddy-resolving model is not enough to noticeably change the scale where the inverse cascade arrests or the Rhines scale. In addition, we show that the mechanism by which baroclinic flows organise into barotropic flows is weaker at lower resolution, resulting in a more baroclinic flow. Hence, the horizontal resolution impacts the vertical structure of the simulated flow. Our results suggest that the effect of mesoscale eddies can be parameterised by enhancing the potential to kinetic energy conversion, i.e., the horizontal pressure gradients, or enhancing the inverse cascade of kinetic energy.

A deformation-based parametrization of ocean mesoscale eddy reynolds stresses

OCEAN MODELLING 112 (2017) 99-111

JA Anstey, L Zanna

The statistical nature of turbulent barotropic ocean jets

OCEAN MODELLING 113 (2017) 34-49

TW David, DP Marshall, L Zanna

A note on 'Toward a stochastic parameterization of ocean mesoscale eddies'

OCEAN MODELLING 113 (2017) 30-33

I Grooms, L Zanna

Linking the Climate and Thermal Phase Curve of 55 Cancri e


M Hammond, RT Pierrehumbert