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  • 1
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    Estimating Climatically Relevant Singular Vectors for Decadal Predictions of the Atlantic Ocean (2011)
    American Meteorological Society
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    Publication Date: 2011-01-01
    Description: A key aspect in designing an efficient decadal prediction system is ensuring that the uncertainty in the ocean initial conditions is sampled optimally. Here one strategy for addressing this issue is considered by investigating the growth of optimal perturbations in the third climate configuration of the Met Office Unified Model (HadCM3) global climate model (GCM). More specifically, climatically relevant singular vectors (CSVs)—the small perturbations of which grow most rapidly for a specific set of initial conditions—are estimated for decadal time scales in the Atlantic Ocean. It is found that reliable CSVs can be estimated by running a large ensemble of integrations of the GCM. Amplification of the optimal perturbations occurs for more than 10 yr, and possibly up to 40 yr. The identified regions for growing perturbations are found to be in the far North Atlantic, and these perturbations cause amplification through an anomalous meridional overturning circulation response. Additionally, this type of analysis potentially informs the design of future ocean observing systems by identifying the sensitive regions where small uncertainties in the ocean state can grow maximally. Although these CSVs are expensive to compute, ways in which the process could be made more efficient in the future are identified.
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    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    CORRIGENDUM (2013)
    American Meteorological Society
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    Publication Date: 2013-10-29
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    Topics: Geography , Geosciences , Physics
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  • 3
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    The Interpretation and Use of Biases in Decadal Climate Predictions (2014)
    American Meteorological Society
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    Publication Date: 2014-04-10
    Description: Decadal climate predictions exhibit large biases, which are often subtracted and forgotten. However, understanding the causes of bias is essential to guide efforts to improve prediction systems, and may offer additional benefits. Here the origins of biases in decadal predictions are investigated, including whether analysis of these biases might provide useful information. The focus is especially on the lead-time-dependent bias tendency. A “toy” model of a prediction system is initially developed and used to show that there are several distinct contributions to bias tendency. Contributions from sampling of internal variability and a start-time-dependent forcing bias can be estimated and removed to obtain a much improved estimate of the true bias tendency, which can provide information about errors in the underlying model and/or errors in the specification of forcings. It is argued that the true bias tendency, not the total bias tendency, should be used to adjust decadal forecasts. The methods developed are applied to decadal hindcasts of global mean temperature made using the Hadley Centre Coupled Model, version 3 (HadCM3), climate model, and it is found that this model exhibits a small positive bias tendency in the ensemble mean. When considering different model versions, it is shown that the true bias tendency is very highly correlated with both the transient climate response (TCR) and non–greenhouse gas forcing trends, and can therefore be used to obtain observationally constrained estimates of these relevant physical quantities.
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    Topics: Geography , Geosciences , Physics
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  • 4
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    An Anatomy of the Cooling of the North Atlantic Ocean in the 1960s and 1970s (2014)
    American Meteorological Society
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    Publication Date: 2014-10-24
    Description: In the 1960s and early 1970s, sea surface temperatures in the North Atlantic Ocean cooled rapidly. There is still considerable uncertainty about the causes of this event, although various mechanisms have been proposed. In this observational study, it is demonstrated that the cooling proceeded in several distinct stages. Cool anomalies initially appeared in the mid-1960s in the Nordic Seas and Gulf Stream extension, before spreading to cover most of the subpolar gyre. Subsequently, cool anomalies spread into the tropical North Atlantic before retreating, in the late 1970s, back to the subpolar gyre. There is strong evidence that changes in atmospheric circulation, linked to a southward shift of the Atlantic ITCZ, played an important role in the event, particularly in the period 1972–76. Theories for the cooling event must account for its distinctive space–time evolution. The authors’ analysis suggests that the most likely drivers were 1) the “Great Salinity Anomaly” of the late 1960s; 2) an earlier warming of the subpolar North Atlantic, which may have led to a slowdown in the Atlantic meridional overturning circulation; and 3) an increase in anthropogenic sulfur dioxide emissions. Determining the relative importance of these factors is a key area for future work.
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  • 5
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    Indian Ocean Climate and Dipole Variability in Hadley Centre Coupled GCMs (2005)
    American Meteorological Society
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    Publication Date: 2005-07-01
    Description: Prediction of Indian Ocean interannual variability may be limited by the systematic biases in coupled GCMs or by a lack of resolution of the processes involved. In particular, little is known about the impact of ocean resolution on simulated climate variability. The simulation of Indian Ocean climate and dipole is investigated in Hadley Centre coupled models with different horizontal and vertical ocean resolutions. The mean state of the Indian Ocean is found to improve only slightly when horizontal resolution is increased from 1.25° to ⅓° and when vertical resolution is increased from 20 to 40 vertical levels due to a small reduction of the Maritime Continent warm bias. However, improvements in the simulation of the dipole are more substantial. All versions of the model realistically simulate dipole onset between April and June, peak in September to October, and then rapidly decay between October and January. The SST anomalies are accompanied by realistic equatorial easterly wind anomalies with thermocline shoaling in the east and deepening in the southwest. In the model with the 1.25° ocean and 20 vertical levels, the dipoles do not terminate completely but persist through the austral summer and then frequently reinvigorate the following year. This unrealistic behavior is eliminated when the ocean vertical resolution is increased from around 20 m in the thermocline to 10 m in the whole of the top 135 m and when Java is represented (even at 1.25° resolution). It is hypothesized that the improvement is due to the resolution of the separation between the thermocline and the surface and also due to the small reduction of the Maritime Continent warm bias.
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  • 6
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    Decadal Predictability of the Atlantic Ocean in a Coupled GCM: Forecast Skill and Optimal Perturbations Using Linear Inverse Modeling (2009)
    American Meteorological Society
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    Publication Date: 2009-07-15
    Description: The decadal predictability of three-dimensional Atlantic Ocean anomalies is examined in a coupled global climate model [the third climate configuration of the Met Office Unified Model (HadCM3)] using a linear inverse modeling (LIM) approach. It is found that the evolution of temperature and salinity in the Atlantic, and the strength of the meridional overturning circulation (MOC), can be effectively described by a linear dynamical system forced by white noise. The forecasts produced using this linear model are more skillful than other reference forecasts for several decades. Furthermore, significant nonnormal amplification is found under several different norms. The regions from which this growth occurs are found to be fairly shallow and located in the far North Atlantic. Initially, anomalies in the Nordic seas impact the MOC and the anomalies then grow to fill the entire Atlantic basin, especially at depth, over one to three decades. It is found that the structure of the optimal initial condition for amplification is sensitive to the norm employed, but the initial growth seems to be dominated by MOC-related basin-scale changes, irrespective of the choice of norm. The consistent identification of the far North Atlantic as the most sensitive region for small perturbations suggests that additional observations in this region would be optimal for constraining decadal climate predictions.
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  • 7
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    Causes of the Rapid Warming of the North Atlantic Ocean in the Mid-1990s (2012)
    American Meteorological Society
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    Publication Date: 2012-06-15
    Description: In the mid-1990s, the subpolar gyre of the North Atlantic underwent a remarkable rapid warming, with sea surface temperatures increasing by around 1°C in just 2 yr. This rapid warming followed a prolonged positive phase of the North Atlantic Oscillation (NAO) but also coincided with an unusually negative NAO index in the winter of 1995/96. By comparing ocean analyses and carefully designed model experiments, it is shown that this rapid warming can be understood as a delayed response to the prolonged positive phase of the NAO and not simply an instantaneous response to the negative NAO index of 1995/96. Furthermore, it is inferred that the warming was partly caused by a surge and subsequent decline in the meridional overturning circulation and northward heat transport of the Atlantic Ocean. These results provide persuasive evidence of significant oceanic memory on multiannual time scales and are therefore encouraging for the prospects of developing skillful predictions.
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  • 8
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    Predictable Climate Impacts of the Decadal Changes in the Ocean in the 1990s (2013)
    American Meteorological Society
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    Publication Date: 2013-08-23
    Description: During the 1990s there was a major change in the state of the world's oceans. In particular, the North Atlantic underwent a rapid warming, with sea surface temperatures (SSTs) in the subpolar gyre region increasing by 1°C in just a few years. Associated with the changes in SST patterns were changes in the surface climate, in particular, a tendency for warm and dry conditions over areas of North America in all seasons, and warm springs and wet summers over areas of Europe. Here, the extent to which a climate prediction system initialized using observations of the ocean state is able to capture the observed changes in seasonal mean surface climate is investigated. Rather than examining predictions of the mid-1990s North Atlantic warming event itself, this study compares hindcasts started before and after the warming, relative to hindcasts that do not assimilate information. It is demonstrated that the hindcasts capture many aspects of the observed changes in seasonal mean surface climate, especially in North, South, and Central America and in Europe. Furthermore, the prediction system retains skill beyond the first year. Finally, it is shown that, in addition to memory of Atlantic SSTs, successfully predicting Pacific SSTs was likely important for the hindcasts to predict surface climate over North America.
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  • 9
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    Closing the Time-Varying Mass and Heat Budgets for Large Ocean Areas: The Tasman Box (2005)
    American Meteorological Society
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    Publication Date: 2005-07-01
    Description: The role of oceanic advection in seasonal-to-interannual balances of mass and heat is studied using a 12-yr time series of quarterly eddy-resolving expendable bathythermograph (XBT) surveys around the perimeter of a region the authors call the Tasman Box in the southwestern Pacific. The region contains the South Pacific’s subtropical western boundary current system and associated strong mesoscale variability. Mean geostrophic transport in the warm upper ocean (temperature greater than 12°C) is about 3.8 Sv (1 Sv ≡ 106 m3 s−1) southward into the box across the Brisbane, Australia–Fiji northern edge. Net outflows are 3.3 Sv eastward across the Auckland, New Zealand–Fiji edge, and 2.7 Sv southward across Sydney, Australia–Wellington, New Zealand. Mean Ekman convergence of 2.2 Sv closes the mass budget. Net water mass conversions in the upper ocean consist of inflow of waters averaging about 26°C and 35.4 psu balanced by outflow at about 18°C and 35.7 psu, and reflect the net evaporation and heat loss in the formation of South Pacific Subtropical Mode Water. The mean heat balance shows good agreement between ocean heat flux convergence (42.3 W m−2), heat loss to the atmosphere from the NCEP–NCAR reanalysis (39.2 W m−2), and heat storage calculated from data in the box interior (1.3 W m−2). On interannual time scales, volume transport through the box ranges from about 1 to 9 Sv, with heat flux convergence ranging from about 20 to 60 W m−2. An interannual balance in the heat budget of the warm layer is achieved to within about 10 W m−2 (or 6 W m−2 for the upper 100 m alone). Maxima in the advective heat flux convergence occurred in 1993, 1997, and 1999–2000, and corresponded to maxima in air–sea heat loss. The evolution of surface-layer temperature in the region is the residual of nearly equal and opposing effects of ocean heat flux convergence and air–sea exchange. Hence, ocean circulation is a key element in the interannual heat budget of the air–sea climate system in the western boundary current region.
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  • 10
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    Bjerknes Compensation and the Decadal Variability of the Energy Transports in a Coupled Climate Model (2006)
    American Meteorological Society
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    Publication Date: 2006-04-01
    Description: In the 1960s, Jacob Bjerknes suggested that if the top-of-the-atmosphere (TOA) fluxes and the oceanic heat storage did not vary too much, then the total energy transport by the climate system would not vary too much either. This implies that any large anomalies of oceanic and atmospheric energy transport should be equal and opposite. This simple scenario has become known as Bjerknes compensation. A long control run of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) has been investigated. It was found that northern extratropical decadal anomalies of atmospheric and oceanic energy transports are significantly anticorrelated and have similar magnitudes, which is consistent with the predictions of Bjerknes compensation. The degree of compensation in the northern extratropics was found to increase with increasing time scale. Bjerknes compensation did not occur in the Tropics, primarily as large changes in the surface fluxes were associated with large changes in the TOA fluxes. In the ocean, the decadal variability of the energy transport is associated with fluctuations in the meridional overturning circulation in the Atlantic Ocean. A stronger Atlantic Ocean energy transport leads to strong warming of surface temperatures in the Greenland–Iceland–Norwegian (GIN) Seas, which results in a reduced equator-to-pole surface temperature gradient and reduced atmospheric baroclinicity. It is argued that a stronger Atlantic Ocean energy transport leads to a weakened atmospheric transient energy transport.
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    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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