Joël Hirschi

National Oceanography Centre

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Adam T. Blaker

National Oceanography Centre

Bablu Sinha

National Oceanography Centre

Simon A. Josey

National Oceanography Centre

Aurélie Duchez

Établissement Français du Sang

Gerard McCarthy

National University of Ireland, Maynooth

Andrew C. Coward

National Oceanography Centre

Robert Marsh

University of Southampton

Jochem Marotzke

Max Planck Institute for Meteorology

Ivan D. Haigh

University of Southampton

Stuart A. Cunningham

Scottish Association For Marine Science

Cooperative Institutions

National Oceanography Centre

120

University of Southampton

Met Office

Centre National de la Recherche Scientifique

University of Reading

Institut de Recherche pour le Développement

National Centre for Atmospheric Science

Woods Hole Oceanographic Institution

Ifremer

University of Oxford

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Spin‐up of UK Earth System Model 1 (UKESM1) for CMIP6

Journal of Advances in Modeling Earth Systems (2020)

Andrew Yool Julien Palmiéri Colin Jones Alistair Sellar L. de Mora

Abstract For simulations intended to study the influence of anthropogenic forcing on climate, temporal stability of the Earth's natural heat, freshwater, and biogeochemical budgets is critical. Achieving such coupled model equilibration is scientifically and computationally challenging. We describe the protocol used to spin‐up the UK Earth system model (UKESM1) with respect to preindustrial forcing for use in the sixth Coupled Model Intercomparison Project (CMIP6). Due to the high computational cost of UKESM1's atmospheric model, especially when running with interactive full chemistry and aerosols, spin‐up primarily used parallel configurations using only ocean/land components. For the ocean, the resulting spin‐up permitted the carbon and heat contents of the ocean's full volume to approach equilibrium over 5,000 years. On land, a spin‐up of 1,000 years brought UKESM1's dynamic vegetation and soil carbon reservoirs toward near‐equilibrium. The end‐states of these parallel ocean‐ and land‐only phases then initialized a multicentennial period of spin‐up with the full Earth system model, prior to this simulation continuing as the UKESM1 CMIP6 preindustrial control (piControl). The realism of the fully coupled spin‐up was assessed for a range of ocean and land properties, as was the degree of equilibration for key variables. Lessons drawn include the importance of consistent interface physics across ocean‐ and land‐only models and the coupled (parent) model, the extreme simulation duration required to approach equilibration targets, and the occurrence of significant regional land carbon drifts despite global‐scale equilibration. Overall, the UKESM1 spin‐up underscores the expense involved and argues in favor of future development of more efficient spin‐up techniques.

Earth system science

Forcing (mathematics)

Biogeochemical Cycle

10.1029/2019ms001933

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Citations (75)

Increasing ocean model resolution reveals impact of tuning eddy permitting models

AGUFM (2019)

Maike Sonnewald A. J. George Nurser Yvonne L. Firing Joël Hirschi Andrew C. Coward

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Seasonal to interannual variability in density around the Canary Islands and their influence on the Atlantic meridional overturning circulation at 26°N

Journal of Geophysical Research Oceans (2014)

Aurélie Duchez Eleanor Frajka‐Williams Natalia Castro Joël Hirschi Andrew C. Coward

The meridional interior flow obtained from the RAPID array is determined by horizontal density fluctuations at the eastern and western boundary of 26°N. The physical causes of these density variations are responsible for fluctuations in the Atlantic Meridional Overturning Circulation (AMOC) and through it, the meridional heat transport of the Atlantic. In this modeling study, a high-resolution ocean model is used to investigate the source and origin of the AMOC variability associated with the density fluctuations at the eastern boundary. The AMOC in the model is in good agreement with the RAPID observations and appears to adequately represent the smaller scale features of variability around the Canary Islands. In this paper, we identify a robust relationship between the density structure south of the Canary Islands, the local wind stress curl (WSC) around these islands and the AMOC using an empirical orthogonal functions analysis, wavelet transform, and wavelet coherence. We find that the deep density fluctuations at the eastern boundary of 26°N arise from the pumping effect of the spatial pattern of WSC south of the islands. These deep density fluctuations drive the AMOC both on seasonal and interannual time scales, through their influence on the basinwide tilt of the thermocline. At seasonal time scales, the density fluctuations south of the islands are driven by the WSC and directly influence the AMOC. At interannual time scales, a significant coherence is found between the density fluctuation and the southward Upper Mid-Ocean (UMO) transport although the origin of these density fluctuations is not explained by the direct pumping caused by the WSC.

Boundary current

Wind Stress

10.1002/2013jc009416

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Citations (35)

Relationship between changes in the AMOC, North Atlantic heat content and SST

2018 Ocean Sciences Meeting (2018)

Ben Moat Bablu Sinha Simon A. Josey Jon Robson Pablo Ortega

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Predicting the 2015 North Atlantic Cold Blob

Sybren Drijfhout Jenny Mecking Joël Hirschi Alex Megann

<p>Leading up to and during the summer of 2015 sea surface temperatures (SSTs) in the eastern North Atlantic Subpolar Gyre reached anomalously low values while in the subtropical gyre just to the SSTs were anomalously warm.&#160;Recent observation and modelling studies have found evidence showing that these SST anomalies can be linked to the heat wave experienced over Europe that summer.&#160; The latest observation based data still shows anomalously cold temperatures, as well as the anomalously fresh conditions that went along the 2015 cold blob in the upper layers of the eastern North Atlantic Subpolar gyre.&#160; A second heat wave over Europe occurred in the summer of 2018 where the SSTs reached another minimum value.&#160; Therefore, being able to predict the development, enhancement and persistence of such an anomaly is essential for good seasonal and longer predictions.&#160; At present several modelling systems have had difficulties in simulating/maintaining the 2015 cold blob.&#160;In this work we apply a novel initialization technique using anomalous initialization from a forced ocean simulation to simulate the 2015 cold blob.&#160; Initial results show that the model is able to maintain the cold blob as well as have a strengthening of the cold blob, however, it has difficulties capturing the timing of this strengthening.</p>

Initialization

Anomaly (physics)

10.5194/egusphere-egu2020-6802

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Western boundary circulation and coastal sea-level variability in Northern Hemisphere oceans

Ocean science (2021)

Samuel Tiéfolo Diabaté Didier Swingedouw Joël Hirschi Aurélie Duchez Philip J. Leadbitter

Abstract. The northwest basins of the Atlantic and Pacific oceans are regions of intense western boundary currents (WBCs): the Gulf Stream and the Kuroshio. The variability of these poleward currents and their extensions in the open ocean is of major importance to the climate system. It is largely dominated by in-phase meridional shifts downstream of the points at which they separate from the coast. Tide gauges on the adjacent coastlines have measured the inshore sea level for many decades and provide a unique window on the past of the oceanic circulation. The relationship between coastal sea level and the variability of the western boundary currents has been previously studied in each basin separately, but comparison between the two basins is missing. Here we show for each basin that the inshore sea level upstream of the separation points is in sustained agreement with the meridional shifts of the western boundary current extension over the period studied, i.e. the past 7 (5) decades in the Atlantic (Pacific). Decomposition of the coastal sea level into principal components allows us to discriminate this variability in the upstream sea level from other sources of variability such as the influence of large meanders in the Pacific. Our result extends previous findings limited to the altimetry era and suggests that prediction of inshore sea-level changes could be improved by the inclusion of meridional shifts of the western boundary current extensions as predictors. Long-duration tide gauges, such as Key West, Fernandina Beach or Hosojima, could be used as proxies for the past meridional shifts of the western boundary current extensions.

Boundary current

Tide gauge

Oceanic basin

10.5194/os-17-1449-2021

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Citations (18)

Variability of the Antilles and Deep Western Boundary Currents at 26N in RAPID observations and a high resolution NEMO simulation

EGUGA (2013)

Aurélie Duchez Harry L. Bryden Joël Hirschi Eleanor Frajka‐Williams

Boundary current

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Major Variations in Sub-Tropical North Atlantic Heat Transport at Short Timescales: Causes and Consequences

EGUGA (2016)

Ben Moat Simon A. Josey Bablu Sinha Adam T. Blaker David A. Smeed

Variability in the North Atlantic ocean heat transport at 26.5N on short (5-day) timescales is identified and contrasted with different behaviour at monthly intervals using a combination of RAPID/MOCHA/WBTS measurements and the NEMO-LIM2 1/12 ocean circulation/sea ice model. Wind forcing plays the leading role in establishing the heat transport variability through the Ekman transport response of the ocean and the associated driving atmospheric conditions vary significantly with timescale. We find that at 5-day timescales the largest changes in the heat transport across 26.5N coincide with north-westerly airflows originating over the American land mass that drive strong southward anomalies in the Ekman flow. During these events the northward heat transport reduces by 0.5-0.7 PW (i.e. about 50% of the mean). In contrast, the Ekman transport response at longer monthly timescales is smaller in magnitude (0.2-0.3 PW) and consistent with expected variations in the leading mode of North Atlantic atmospheric variability, the North Atlantic Oscillation. The north-westerly airflow mechanism can have a prolonged influence beyond the central 5-day timescale and on occasion can reduce the accumulated winter ocean heat transport into the north Atlantic by ~40%.

Ekman transport

Tropical Atlantic

Forcing (mathematics)

Ocean dynamics

Atlantic Equatorial mode

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The impact of resolving the Rossby radius at mid-latitudes in the ocean: results from a high-resolution version of the Met Office GC2 coupled model

Geoscientific model development (2016)

Helene T. Hewitt Malcolm Roberts Pat Hyder Tim Graham Jamie Rae

Abstract. There is mounting evidence that resolving mesoscale eddies and western boundary currents as well as topographically controlled flows can play an important role in air–sea interaction associated with vertical and lateral transports of heat and salt. Here we describe the development of the Met Office Global Coupled Model version 2 (GC2) with increased resolution relative to the standard model: the ocean resolution is increased from 1/4 to 1/12° (28 to 9 km at the Equator), the atmosphere resolution increased from 60 km (N216) to 25 km (N512) and the coupling period reduced from 3 hourly to hourly. The technical developments that were required to build a version of the model at higher resolution are described as well as results from a 20-year simulation. The results demonstrate the key role played by the enhanced resolution of the ocean model: reduced sea surface temperature (SST) biases, improved ocean heat transports, deeper and stronger overturning circulation and a stronger Antarctic Circumpolar Current. Our results suggest that the improvements seen here require high resolution in both atmosphere and ocean components as well as high-frequency coupling. These results add to the body of evidence suggesting that ocean resolution is an important consideration when developing coupled models for weather and climate applications.

Eddy

Boundary current

Atmospheric models

10.5194/gmd-9-3655-2016

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Citations (69)

Continuous observations of the variability of the Atlantic meridional overturning circulation at 26.5°N: Major results and present challenges

EGUGA (2010)

Torsten Kanzow Stuart A. Cunningham William E. Johns Harry L. Bryden Joël Hirschi

(1) Leibniz-Institut fur Meereswissenschaften, Chrstian-Albrechts-Universitat Kiel, Ozeanzirkulation und Klimadynamik, Kiel, Germany (tkanzow@ifm-geomar.de), (2) National Oceanography Centre, University of Southampton, Southampton, U.K., (3) Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, U.S.A, (4) NOAA Atlantic Oceanographic and Meteorological Laboratory, Miami, U.S.A., (5) Max-Planck-Institut fur Meteorologie, Hamburg, Germany

Miami

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Citations (1)