Abstract
Cold conditions in the upper layer of the subpolar North Atlantic Ocean, at a time of pervasive warming elsewhere, have provoked significant debate. Uncertainty arises both from potential ...causes (surface heat loss and ocean circulation changes) and characteristic timescales (interannual to multidecadal). Resolution of these uncertainties is important as cold conditions have been linked to recent European weather extremes and a decline in the Atlantic overturning circulation. Using observations, supported by high resolution climate model analysis, we show that a surprisingly active ocean regularly generates both cold and warm interannual anomalies in addition to those generated by surface heat exchange. Furthermore, we identify distinct sea surface temperature patterns that characterise whether the ocean or atmosphere has the strongest influence in a particular year. Applying these new insights to observations, we find an increasing role for the ocean in setting North Atlantic mixed layer heat content variability since 1960.
The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, ...we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events.
Abstract
Global sea-level rise caused by a warming climate increases flood risk from storm surge events for those who live in coastal and low-lying areas. Estimates of global thermosteric sea-level ...rises are well constrained by model projections, but local variability in dynamic sea-level arising from seasonal and interannual changes is less well characterised. In this paper we use satellite altimetry observations coupled with CMIP6 model projections to understand drivers of change in dynamic sea-level over the UK shelf seas. We find a northward shift in the atmospheric jet stream and a weakening of the Atlantic meridional overturning circulation to be the key drivers of local dynamic sea-level variability. Using a storyline approach to constrain climate system responses to changes in atmospheric greenhouse gas concentrations, we find that dynamic sea-level is predicted to rise between 15–39 cm by 2080–2099 along the east coast of England (ECE). Under a worst-case scenario, assuming maximum variability as seen in the CMIP6 projections, ECE dynamic sea-level rise could reach 58 cm by 2100. We illustrate the impact of this dynamic sea-level rise in addition to non-dynamic components on the risks posed by storm surge events in ECE using an idealised example. If a storm surge event of the magnitude of the one experienced in ECE on the 5th of December 2013 was to occur in 2100, an additional 1414 km
2
of land would potentially be affected in our worst-case idealised example, 22.4% of which can be attributed to dynamic sea-level rise.
Sensitivity to nonlinear equations may be a characteristic feature of biological models, particularly those that are complex. A complex marine ecosystem model (PlankTOM5.2) that incorporates multiple ...plankton functional types (PFTs) was embedded in a global ocean general circulation model (OGCM) and its performance assessed for four different formulations of multiple-prey zooplankton functional response: Michaelis-Menten (MM: Holling Type II), Sigmoidal (S: Holling Type III), Blackman (B) and Ivlev (Iv). Predictions of the four simulations were compared for the North Atlantic and North Pacific oceans. Remarkable differences were seen in both spatial extent and magnitude of predicted distributions of PFTs, as well as bulk properties, highlighting how the choice of functional response has a major impact on the resulting ecosystem structure. The range of average concentration of diatoms in surface waters was particularly marked, varying between 0.04mgm super(-3) (B and MM) and 0.13mgm super(-3) (S) in spring and between 0.01mgm super(-3) (B) and 0.07mgm super(-3) (S) in autumn. Differences in ecosystem structure affected predicted export flux, which varied by more than 25% among the simulations. Overall, our work highlights that accuracy is required in ecosystem formulation if reliable predictions are to be made when using complex marine ecosystem models embedded in OGCMs and therefore the need for further studies, with appropriate validation, that address structural sensitivity.
The control of atmospheric storm tracks by ocean dynamics, orography, and their interaction is investigated using idealized experiments with a simplified coupled atmosphere–ocean climate model. The ...study focuses on the quasi–steady state for the storm tracks in the Northern Hemisphere winter mean. The experiments start with a background state without mountains and ocean dynamics, and in separate stages include orography and a dynamic ocean to obtain a more realistic control simulation. The separate effects of ocean dynamics, orography, and their nonlinear interaction are identified for the storm tracks and the surface thermodynamic forcing over the ocean.
The model study suggests that atmospheric storm tracks are a robust feature of the climate system, occurring at midlatitudes even if there is no orographic forcing or ocean heat transport. Ocean dynamics generally lead to a poleward shift in both the storm track and the maximum in atmospheric northward heat transport and induce a northeastward tilt over the Atlantic. This poleward shift is linked to the extra heat transport by the ocean and the tightening of sea surface temperature gradients on the western side of ocean basins. Orographic forcing causes along-track variability with a weakening of the storm track over the continents and induces a northeastward tilt over the western Pacific, which is associated with a stationary planetary wave train generated by the Tibetan Plateau. The interaction between ocean dynamics and orographic forcing plays a localized role, enhancing the contrast between the Atlantic and Pacific. Much of the response to the forcing is eddy mediated and transient eddies help to spread the influence of orographic and ocean forcing.
Ensemble experiments with a coupled atmosphere‐ocean model show that natural variations in the strength of ocean thermohaline circulation (THC), a key player in the maintenance of the climate of ...Earth, are potentially predictable one to two decades into the future. In contrast to previous studies, these predictable THC variations lead to predictable variations of the climate of Western Europe that could be of considerable benefit to society. Further model experiments also show that, under certain conditions, variations in the strength of the THC may even be predictable for more than 50 years into the future.
Earth system models enable a broad range of climate interactions that physical climate models are unable to simulate. However, the extent to which adding Earth system components changes or improves ...the simulation of the physical climate is not well understood. Here we present a broad multivariate evaluation of the North Atlantic climate system in historical simulations of the UK Earth System Model (UKESM1) performed for CMIP6. In particular, we focus on the mean state and the decadal time scale evolution of important variables that span the North Atlantic climate system. In general, UKESM1 performs well and realistically simulates many aspects of the North Atlantic climate system. Like the physical version of the model, we find that changes in external forcing, and particularly aerosol forcing, are an important driver of multidecadal change in UKESM1, especially for Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation. However, many of the shortcomings identified are similar to common biases found in physical climate models, including the physical climate model that underpins UKESM1. For example, the summer jet is too weak and too far poleward; decadal variability in the winter jet is underestimated; intraseasonal stratospheric polar vortex variability is poorly represented; and Arctic sea ice is too thick. Forced shortwave changes may be also too strong in UKESM1, which, given the important role of historical aerosol forcing in shaping the evolution of the North Atlantic in UKESM1, motivates further investigation. Therefore, physical model development, alongside Earth system development, remains crucial in order to improve climate simulations.
Plain Language Summary
The North Atlantic climate system plays an important role in regulating Earth's climate, and variability within the Atlantic can have important impacts on society. However, we do not understand all the linkages between different parts of the North Atlantic. Furthermore, climate simulations, which are an essential tool for improving our understanding, have shortcomings that can affect their utility. New developments in Earth System climate simulations could remedy these shortcomings. However, we do not understand the extent to which the addition of complex Earth system developments have changed or improved the simulation of the physical climate. Therefore, in this paper, we present a multidisciplinary evaluation of the North Atlantic climate in simulations made with the UK's Earth System Model, UKESM1. We find that simulations made with UKESM1 capture many aspects of the North Atlantic climate and that human activities have a large impact on the North Atlantic in UKESM1. Nevertheless, we also highlight some shortcomings of the simulations, many of which are like those seen in physical climate simulations. Therefore, further development of both the physical and Earth system components is needed to improve climate simulations in the future.
Key Points
UKESM1 performs well across a range of variables and components of the North Atlantic climate system when compared with observations
A range of shortcomings are still present in the simulations, however, with many shortcomings similar to those of the physical climate model
Development of the physical model, alongside the Earth system components, is crucial for improved climate simulations
A state-of-the-art complex marine ecosystem model, PlankTOM5.2, simulating the distribution of five plankton functional types (PFTs; mixed phytoplankton, diatoms, coccolithophores, micro and ...mesozooplankton), was implemented separately in two medium resolution (order 1A) global ocean general circulation models (OGCMs), NEMO and OCCAM. In each case, identical formulations and parameter values were used in the ecosystem model, as well as the same biogeochemical forcing (photosynthetically active radiation and input of nutrients by atmospheric dust at the ocean surface) and initial conditions. The two physical models were forced with essentially the same surface boundary conditions, subject to interpolation between the two model grids, and simulations were undertaken for years 1990-1994. Sensitivity of the ecosystem model and associated biogeochemical fields was assessed with respect to differences in the ocean physics of the two OGCMs. Globally integrated bulk properties, notably annual mean primary production and total phytoplankton biomass, were similar in each case (although export showed regional differences between the models), as well as being generally consistent with available observations. In contrast, predicted distributions of individual PFTs varied markedly between the two simulations. Diatoms and microzooplankton were for example predicted to dominate in the North Atlantic, North Pacific and Southern Oceans in OCCAM because of relatively strong mixing which supplied increased nutrients that favoured diatom production and which also increased the mortality of their main predators, the mesozooplankton, which struggled to recover their numbers over winter as phytoplankton were depleted. In NEMO, lower mixing led to a community structure in which mixed phytoplankton and mesozooplankton were the main PFTs in the same areas. Regions of dominance by coccolithophores and mixed phytoplankton were predicted in the tropics, the former group being more extensive in the subtropics in NEMO, with distributions depending primarily on the size and extent of upwelling and downwelling regions predicted by the OGCMs. Results highlight the need for accuracy both when formulating the equations for, and parameterising, PFTs in models, and moreover in the representation of the physico-chemical environment.
Improved representation of air‐sea fluxes afforded by eddy‐rich oceans in high‐resolution coupled ocean‐atmosphere models may modify the tracks and intensity of storms and their response to climate ...change. We examine changes in winter surface ocean conditions and storminess associated with moving from an eddy‐permitting (1/4°, HM) to an eddy‐rich (1/12°, HH) ocean in control and climate change (SSP585) simulations of the HadGEM3‐GC3.1 model in which atmosphere resolution is kept at 25 km. Differences in North Atlantic climate in the control runs stem from a revised location of the Gulf Stream in the eddy‐rich model. Projections reveal greater warming in the western Atlantic in HH than HM and a pronounced increase in eastern Atlantic storminess with changes six times greater than in the eddy‐permitting model. This increase is associated with the distinctive long‐term evolution of the North Atlantic warming hole and the Gulf Stream separation in the eddy‐rich model.
Key Points
Increased ocean resolution affects North Atlantic winter climate by modifying the location of Gulf Stream
Future projection of eddy‐rich model shows a decrease in the Gulf Stream separation from the coast and a stronger warming hole
Eddy‐rich model projects a more pronounced change in east Atlantic storminess and precipitation than eddy‐permitting model
We use a new method based on point correlation maps and self-organising maps (SOMs) to identify teleconnection patterns in 60 yr of National Centres for Environmental Prediction/National Centre for ...Atmospheric Research (NCEP/NCAR) sea level pressure (SLP) re-analysis data. The most prevalent patterns are the El Nino Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the Southern Annular Mode (SAM). Asymmetries are found between base points in opposite centres of action of the NAO and the Pacific North America pattern (PNA). The SOM-based method is a powerful tool that allows us to efficiently assess how realistically teleconnections are reproduced in any climate model. The degree of agreement between modelled and re-analysis-based teleconnections (or between different models) can be summarised in a single plot. Here, we illustrate this by assessing the skill of the medium complexity climate model FORTE (Fast Ocean Rapid Troposphere Experiment). FORTE reproduces some realistic teleconnections, such as the Arctic Oscillation (AO), the NAO, the PNA, the SAM, the African Monsoon and ENSO, along with several other teleconnections, which resemble to varying degrees the corresponding NCEP patterns. However, FORTE tends to underestimate the strength of the correlation patterns and the patterns tend to be slightly too zonal. The accuracy of frequency of occurrence is variable between patterns. The Indian Ocean is a region where FORTE performs poorly, as it does not reproduce the teleconnection patterns linked to the Indian Monsoon. In contrast, the North and equatorial Pacific and North Atlantic are reasonably well reproduced.
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