81 research outputs found
The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Climate Dynamics 33 (2009): 777-793, doi:10.1007/s00382-008-0523-2.An OGCM hindcast is used to investigate the linkages between North Atlantic Ocean
salinity and circulation changes during 1963–2003. The focus is on the eastern subpolar
region consisting of the Irminger Sea and the eastern North Atlantic where a careful
assessment shows that the simulated interannual to decadal salinity changes in the upper
1500 m reproduce well those derived from the available record of hydrographic
measurements. In the model, the variability of the Atlantic meridional overturning
circulation (MOC) is primarily driven by changes in deep water formation taking place in
the Irminger Sea and, to a lesser extent, the Labrador Sea. Both are strongly influenced by
the North Atlantic Oscillation (NAO). The modeled interannual to decadal salinity changes
in the subpolar basins are mostly controlled by circulation-driven anomalies of freshwater
flux convergence, although surface salinity restoring to climatology and other boundary
fluxes each account for approximately 25% of the variance. The NAO plays an important
role: a positive NAO phase is associated with increased precipitation, reduced northward
salt transport by the wind-driven intergyre gyre, and increased southward flows of
freshwater across the Greenland-Scotland ridge. Since the NAO largely controlled deep
convection in the subpolar gyre, fresher waters are found near the sinking region during
convective events. This markedly differs from the active influence on the MOC that salinity
exerts at decadal and longer timescales in most coupled models. The intensification of the
MOC that follows a positive NAO phase by about 2 years does not lead to an increase in
the northward salt transport into the subpolar domain at low frequencies because it is
cancelled by the concomitant intensification of the subpolar gyre which shifts the subpolar
front eastward and reduces the northward salt transport by the North Atlantic Current
waters. This differs again from most coupled models, where the gyre intensification
precedes that of the MOC by several years.Support from NSF Grant
82677800 with the Woods Hole Oceanographic Institution, and (to CF) from the Institut
universitaire de France and European FP6 project DYNAMITE (contract 003903-GOCE)
and (to JD) from the NOAA Office of Hydrologic Development through a scientific
appointment administered by UCAR is gratefully acknowledged
Intercomparison of the northern hemisphere winter mid-latitude atmospheric variability of the IPCC models
We compare, for the overlapping time frame 1962-2000, the estimate of the
northern hemisphere (NH) mid-latitude winter atmospheric variability within the
XX century simulations of 17 global climate models (GCMs) included in the
IPCC-4AR with the NCEP and ECMWF reanalyses. We compute the Hayashi spectra of
the 500hPa geopotential height fields and introduce an integral measure of the
variability observed in the NH on different spectral sub-domains. Only two
high-resolution GCMs have a good agreement with reanalyses. Large biases, in
most cases larger than 20%, are found between the wave climatologies of most
GCMs and the reanalyses, with a relative span of around 50%. The travelling
baroclinic waves are usually overestimated, while the planetary waves are
usually underestimated, in agreement with previous studies performed on global
weather forecasting models. When comparing the results of various versions of
similar GCMs, it is clear that in some cases the vertical resolution of the
atmosphere and, somewhat unexpectedly, of the adopted ocean model seem to be
critical in determining the agreement with the reanalyses. The GCMs ensemble is
biased with respect to the reanalyses but is comparable to the best 5 GCMs.
This study suggests serious caveats with respect to the ability of most of the
presently available GCMs in representing the statistics of the global scale
atmospheric dynamics of the present climate and, a fortiori, in the perspective
of modelling climate change.Comment: 39 pages, 8 figures, 2 table
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Changing Northern Hemisphere storm tracks in an ensemble of IPCC climate change simulations
Winter storm-track activity over the Northern Hemisphere and its changes in a greenhouse gas scenario (the Special Report on Emission Scenarios A1B forcing) are computed from an ensemble of 23 single runs from 16 coupled global climate models (CGCMs). All models reproduce the general structures of the observed climatological storm-track pattern under present-day forcing conditions. Ensemble mean changes resulting from anthropogenic forcing include an increase of baroclinic wave activity over the eastern North Atlantic, amounting to 5%–8% by the end of the twenty-first century. Enhanced activity is also found over the Asian continent and over the North Pacific near the Aleutian Islands. At high latitudes and over parts of the subtropics, activity is reduced. Variations of the individual models around the ensemble average signal are not small, with a median of the pattern correlation near r = 0.5. There is, however, no evidence for a link between deviations in present-day climatology and deviations with respect to climate change
Causes and Consequences of Past and Projected Scandinavian Summer Temperatures, 500–2100 AD
Tree rings dominate millennium-long temperature reconstructions and many records originate from Scandinavia, an area for which the relative roles of external forcing and internal variation on climatic changes are, however, not yet fully understood. Here we compile 1,179 series of maximum latewood density measurements from 25 conifer sites in northern Scandinavia, establish a suite of 36 subset chronologies, and analyse their climate signal. A new reconstruction for the 1483–2006 period correlates at 0.80 with June–August temperatures back to 1860. Summer cooling during the early 17th century and peak warming in the 1930s translate into a decadal amplitude of 2.9°C, which agrees with existing Scandinavian tree-ring proxies. Climate model simulations reveal similar amounts of mid to low frequency variability, suggesting that internal ocean-atmosphere feedbacks likely influenced Scandinavian temperatures more than external forcing. Projected 21st century warming under the SRES A2 scenario would, however, exceed the reconstructed temperature envelope of the past 1,500 years
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Synthesis and Assessment Product
This document is part of the Synthesis and Assessment Products (SAP) described in the U.S. Climate Change Science Program (CCSP) Strategic Plan. This report is meant to reduce uncertainty in projections of how the Earth's climate and related systems may change in the future. It provides scientific information for supporting the decision-making audience and the expert scientific and stakeholder community
Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years
Understanding the internal ocean variability and its influence on climate is imperative for society. A key aspect concerns the enigmatic Atlantic Multidecadal Oscillation (AMO), a feature defined by a 60- to 90-year variability in North Atlantic sea-surface temperatures. The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature. Here, we show that distinct, ∼55- to 70-year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years. We test and reject the hypothesis that this climate oscillation was directly forced by periodic changes in solar activity. We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene. Our analyses further suggest that the coupling from the AMO to regional climate conditions was modulated by orbitally induced shifts in large-scale ocean-atmosphere circulation
Interdecadal variability of the meridional overturning circulation as an ocean internal mode
Climate simulations for 1880-2003 with GISS modelE
We carry out climate simulations for 1880-2003 with GISS modelE driven by ten
measured or estimated climate forcings. An ensemble of climate model runs is
carried out for each forcing acting individually and for all forcing mechanisms
acting together. We compare side-by-side simulated climate change for each
forcing, all forcings, observations, unforced variability among model ensemble
members, and, if available, observed variability. Discrepancies between
observations and simulations with all forcings are due to model deficiencies,
inaccurate or incomplete forcings, and imperfect observations. Although there
are notable discrepancies between model and observations, the fidelity is
sufficient to encourage use of the model for simulations of future climate
change. By using a fixed well-documented model and accurately defining the
1880-2003 forcings, we aim to provide a benchmark against which the effect of
improvements in the model, climate forcings, and observations can be tested.
Principal model deficiencies include unrealistically weak tropical El Nino-like
variability and a poor distribution of sea ice, with too much sea ice in the
Northern Hemisphere and too little in the Southern Hemisphere. The greatest
uncertainties in the forcings are the temporal and spatial variations of
anthropogenic aerosols and their indirect effects on clouds.Comment: 44 pages; 19 figures; Final text accepted by Climate Dynamic
Multispectral analysis of Northern Hemisphere temperature records over the last five millennia
Aiming to describe spatio-temporal climate variability on decadal-to-centennial time scales and longer, we analyzed a data set of 26 proxy records extending back 1,000–5,000 years; all records chosen were calibrated to yield temperatures. The seven irregularly sampled series in the data set were interpolated to a regular grid by optimized methods and then two advanced spectral methods—namely singular-spectrum analysis (SSA) and the continuous wavelet transform—were applied to individual series to separate significant oscillations from the high noise background. This univariate analysis identified several common periods across many of the 26 proxy records: a millennial trend, as well as oscillations of about 100 and 200 years, and a broad peak in the 40–70-year band. To study common NH oscillations, we then applied Multichannel SSA. Temperature variations on time scales longer than 600 years appear in our analysis as a dominant trend component, which shows climate features consistent with the Medieval Warm Period and the Little Ice Age. Statistically significant NH-wide peaks appear at 330, 250 and 110 years, as well as in a broad 50–80-year band. Strong variability centers in several bands are located around the North Atlantic basin and are in phase opposition between Greenland and Western Europe
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