Investigating the Impact of CO2 on Low-Frequency Variability of the AMOC in HadCM3

Abstract This study investigates the impact of CO2 on the amplitude, frequency, and mechanisms of Atlantic meridional overturning circulation (AMOC) variability in millennial simulations of the HadCM3 coupled climate model. Multichannel singular spectrum analysis (MSSA) and empirical orthogonal functions (EOFs) are applied to the AMOC at four quasi-equilibrium CO2 forcings. The amount of variance explained by the first and second eigenmodes appears to be small (i.e., 11.19%); however, the results indicate that both AMOC strength and variability weaken at higher CO2 concentrations. This accompanies an apparent shift from a predominant 100–125-yr cycle at 350 ppm to 160 yr at 1400 ppm. Changes in amplitude are shown to feed back onto the atmosphere. Variability may be linked to salinity-driven density changes in the Greenland–Iceland–Norwegian Seas, fueled by advection of anomalies predominantly from the Arctic and Caribbean regions. A positive density anomaly accompanies a decrease in stratification and an increase in convection and Ekman pumping, generating a strong phase of the AMOC (and vice versa). Arctic anomalies may be generated via an internal ocean mode that may be key in driving variability and are shown to weaken at higher CO2, possibly driving the overall reduction in amplitude. Tropical anomalies may play a secondary role in modulating variability and are thought to be more influential at higher CO2, possibly due to an increased residence time in the subtropical gyre and/or increased surface runoff driven by simulated dieback of the Amazon rain forest. These results indicate that CO2 may not only weaken AMOC strength but also alter the mechanisms that drive variability, both of which have implications for climate change on multicentury time scales.

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Modes of the BiOS-driven Adriatic Sea thermohaline variability

10.21203/rs.3.rs-990898/v1 ◽

2021 ◽

Author(s):

Cléa Lumina Denamiel ◽

Iva Tojčić ◽

Petra Pranić ◽

Ivica Vilibić

Keyword(s):

Time Series ◽

Thermohaline Circulation ◽

Climate Model ◽

Adriatic Sea ◽

Decadal Variability ◽

Current Speed ◽

Empirical Orthogonal Functions ◽

Climate Simulation ◽

Orthogonal Functions ◽

The Impact

Abstract In this study the impact of the Adriatic-Ionian Bimodal Oscillating System (BiOS) on the interannual to decadal variability of the Adriatic Sea thermohaline circulation is quantified during the 1987-2017 period with the numerical results of the Adriatic Sea and Coast (AdriSC) historical kilometer-scale climate simulation. The time series associated with the first five Empirical Orthogonal Functions (EOFs) computed from the salinity, temperature and current speed monthly detrended anomalies at 1-km resolution are correlated to the BiOS signal. First, it is found that the AdriSC climate model is capable to reproduce the BiOS-driven phases derived from in-situ observations along a long-term monitoring transect in the middle Adriatic. Then, for the entire Adriatic basin, high correlations to the 2-year delayed BiOS signal are obtained for the salinity and current speed first two EOF time series at 100 m depth and the sea-bottom Finally, the physical interpretation of the EOF spatial patterns reveals that Adriatic bottom temperatures are more influenced by the dense water circulation than the BiOS. These findings confirmed and generalized the known dynamics derived previously from observations, and the AdriSC climate model can thus be used to better understand the past and future BiOS-driven physical processes in the Adriatic Sea.

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On the Nonlinearity of Winter Northern Hemisphere Atmospheric Variability

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0182.1 ◽

2019 ◽

Vol 76 (1) ◽

pp. 333-356 ◽

Cited By ~ 3

Author(s):

A. Hannachi ◽

W. Iqbal

Keyword(s):

North Atlantic ◽

Polar Vortex ◽

Principal Component ◽

Empirical Orthogonal Functions ◽

The Arctic ◽

Climate Change Signal ◽

Two Dimensional ◽

Orthogonal Functions ◽

Local Flow ◽

The North

Abstract Nonlinearity in the Northern Hemisphere’s wintertime atmospheric flow is investigated from both an intermediate-complexity model of the extratropics and reanalyses. A long simulation is obtained using a three-level quasigeostrophic model on the sphere. Kernel empirical orthogonal functions (EOFs), which help delineate complex structures, are used along with the local flow tendencies. Two fixed points are obtained, which are associated with strong bimodality in two-dimensional kernel principal component (PC) space, consistent with conceptual low-order dynamics. The regimes reflect zonal and blocked flows. The analysis is then extended to ERA-40 and JRA-55 using daily sea level pressure (SLP) and geopotential heights in the stratosphere (20 hPa) and troposphere (500 hPa). In the stratosphere, trimodality is obtained, representing disturbed, displaced, and undisturbed states of the winter polar vortex. In the troposphere, the probability density functions (PDFs), for both fields, within the two-dimensional (2D) kernel EOF space are strongly bimodal. The modes correspond broadly to opposite phases of the Arctic Oscillation with a signature of the negative North Atlantic Oscillation (NAO). Over the North Atlantic–European sector, a trimodal PDF is also obtained with two strong and one weak modes. The strong modes are associated, respectively, with the north (or +NAO) and south (or −NAO) positions of the eddy-driven jet stream. The third weak mode is interpreted as a transition path between the two positions. A climate change signal is also observed in the troposphere of the winter hemisphere, resulting in an increase (a decrease) in the frequency of the polar high (low), consistent with an increase of zonal flow frequency.

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ENSO impact on simulated South American hydro-climatology

Advances in Geosciences ◽

10.5194/adgeo-6-227-2006 ◽

2006 ◽

Vol 6 ◽

pp. 227-236 ◽

Cited By ~ 4

Author(s):

J. Stuck ◽

A. Güntner ◽

B. Merz

Keyword(s):

Climate Model ◽

Initial Conditions ◽

Circulation Model ◽

Empirical Orthogonal Functions ◽

South American ◽

The South ◽

Time Space ◽

The Pacific ◽

Hydro Climatology ◽

The Impact

Abstract. The variability of the simulated hydro-climatology of the WaterGAP Global Hydrology Model (WGHM) is analysed. Main object of this study is the ENSO-driven variability of the water storage of South America. The horizontal model resolution amounts to 0.5 degree and it is forced with monthly climate variables for 1961-1995 of the Tyndall Centre Climate Research Unit dataset (CRU TS 2.0) as a representation of the observed climate state. Secondly, the model is also forced by the model output of a global circulation model, the ECHAM4-T42 GCM. This model itself is driven by observed monthly means of the global Sea Surface Temperatures (SST) and the sea ice coverage for the period of 1903 to 1994 (GISST). Thus, the climate model and the hydrological model represent a realistic simulated realisation of the hydro-climatologic state of the last century. Since four simulations of the ECHAM4 model with the same forcing, but with different initial conditions are carried out, an analysis of variance (ANOVA) gives an impression of the impact of the varying SST on the hydro-climatology, because the variance can be separated into a SST-explained and a model internal variability (noise). Also regional multivariate analyses, like Empirical Orthogonal Functions (EOF) and Canonical Correlation Analysis (CCA) provide information of the complex time-space variability. In particular the Amazon region and the South of Brazil are significantly influenced by the ENSO-variability, but also the Pacific coastal areas of Ecuador and Peru are affected. Additionally, different ENSO-indices, based on SST anomalies (e.g. NINO3.4, NINO1+2), and its influence on the South American hydro-climatology are analysed. Especially, the Pacific coast regions of Ecuador, Peru and Chile show a very different behaviour dependant on those indices.

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A centennial-scale Arctic - North Atlantic recharge oscillator in a coupled climate model

10.5194/egusphere-egu21-5888 ◽

2021 ◽

Author(s):

Robin Waldman ◽

Christophe Cassou ◽

Aurore Voldoire

Keyword(s):

Atlantic Ocean ◽

North Atlantic ◽

Climate Model ◽

Natural Variability ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Western Boundary ◽

Coupled Climate Model ◽

Overturning Circulation ◽

Salinity Variability

<p>In global climate models, low-frequency natural variability related to the Atlantic Ocean overturning circulation is a common behaviour. Such intrinsic climate variability is a potential source of decadal climate predictability. However, over longer term scenario simulations, this natural variability becomes a major source of uncertainty. In this study, we document a large and sustained centennial variability in the 3500-year pre-industrial control run of the CNRM-CM6 coupled climate model which is driven by the North Atlantic ocean, and more specifically its meridional overturning circulation (AMOC). We propose a new AMOC dynamical decomposition highlighting the dominant role of mid-depth density anomalies at the western boundary as the driver of this centennial variability. We relate such density variability to deep convection and overflows in the western subpolar gyre, themselves controlled by and intense salinity variability of the upper layers. Finally, we show that such salinity variability is the result of periodic freshwater recharge and descharge events from the Arctic Ocean, themselves triggered by stochastic atmospheric forcing.</p>

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Investigating the impact of Lake Agassiz drainage routes on the 8.2 ka cold event with a climate model

Climate of the Past ◽

10.5194/cp-5-471-2009 ◽

2009 ◽

Vol 5 (3) ◽

pp. 471-480 ◽

Cited By ~ 8

Author(s):

Y.-X. Li ◽

H. Renssen ◽

A. P. Wiersma ◽

T. E. Törnqvist

Keyword(s):

Atlantic Ocean ◽

North Atlantic ◽

Climate Model ◽

North Atlantic Ocean ◽

Sensitivity Study ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Cold Event ◽

The North ◽

The Impact

Abstract. The 8.2 ka event is the most prominent abrupt climate change in the Holocene and is often believed to result from catastrophic drainage of proglacial lakes Agassiz and Ojibway (LAO) that routed through the Hudson Bay and the Labrador Sea into the North Atlantic Ocean, and perturbed Atlantic meridional overturning circulation (MOC). One key assumption of this triggering mechanism is that the LAO freshwater drainage was dispersed over the Labrador Sea. Recent data, however, show no evidence of lowered δ18O values, indicative of low salinity, from the open Labrador Sea around 8.2 ka. Instead, negative δ18O anomalies are found close to the east coast of North America, extending as far south as Cape Hatteras, North Carolina, suggesting that the freshwater drainage may have been confined to a long stretch of continental shelf before fully mixing with North Atlantic Ocean water. Here we conduct a sensitivity study that examines the effects of a southerly drainage route on the 8.2 ka event with the ECBilt-CLIO-VECODE model. Hosing experiments of four routing scenarios, where freshwater was introduced to the Labrador Sea in the northerly route and to three different locations along the southerly route, were performed to investigate the routing effects on model responses. The modeling results show that a southerly drainage route is possible but generally yields reduced climatic consequences in comparison to those of a northerly route. This finding implies that more freshwater would be required for a southerly route than for a northerly route to produce the same climate anomaly. The implicated large amount of LAO drainage for a southerly routing scenario is in line with a recent geophysical modelling study of gravitational effects on sea-level change associated with the 8.2 ka event, which suggests that the volume of drainage might be larger than previously estimated.

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Simple measures of ozone depletion in the polar stratosphere

Atmospheric Chemistry and Physics ◽

10.5194/acp-8-251-2008 ◽

2008 ◽

Vol 8 (2) ◽

pp. 251-264 ◽

Cited By ~ 39

Author(s):

R. Müller ◽

J.-U. Grooß ◽

C. Lemmen ◽

D. Heinze ◽

M. Dameris ◽

...

Keyword(s):

Total Ozone ◽

Climate Model ◽

The Arctic ◽

Total Column Ozone ◽

Ozone Loss ◽

Break Up ◽

Column Ozone ◽

The Impact ◽

Polar Ozone ◽

Daily Total

Abstract. We investigate the extent to which quantities that are based on total column ozone are applicable as measures of ozone loss in the polar vortices. Such quantities have been used frequently in ozone assessments by the World Meteorological Organization (WMO) and also to assess the performance of chemistry-climate models. The most commonly considered quantities are March and October mean column ozone poleward of geometric latitude 63° and the spring minimum of daily total ozone minima poleward of a given latitude. Particularly in the Arctic, the former measure is affected by vortex variability and vortex break-up in spring. The minimum of daily total ozone minima poleward of a particular latitude is debatable, insofar as it relies on one single measurement or model grid point. We find that, for Arctic conditions, this minimum value often occurs in air outside the polar vortex, both in the observations and in a chemistry-climate model. Neither of the two measures shows a good correlation with chemical ozone loss in the vortex deduced from observations. We recommend that the minimum of daily minima should no longer be used when comparing polar ozone loss in observations and models. As an alternative to the March and October mean column polar ozone we suggest considering the minimum of daily average total ozone poleward of 63° equivalent latitude in spring (except for winters with an early vortex break-up). Such a definition both obviates relying on one single data point and reduces the impact of year-to-year variability in the Arctic vortex break-up on ozone loss measures. Further, this measure shows a reasonable correlation (r=–0.75) with observed chemical ozone loss. Nonetheless, simple measures of polar ozone loss must be used with caution; if possible, it is preferable to use more sophisticated measures that include additional information to disentangle the impact of transport and chemistry on ozone.

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Enhanced Seasonal Prediction of European Winter Warming following Volcanic Eruptions

Journal of Climate ◽

10.1175/2009jcli3145.1 ◽

2009 ◽

Vol 22 (23) ◽

pp. 6168-6180 ◽

Cited By ~ 40

Author(s):

A. G. Marshall ◽

A. A. Scaife ◽

S. Ineson

Keyword(s):

Climate Model ◽

Initial Conditions ◽

Polar Vortex ◽

Volcanic Eruptions ◽

Seasonal Prediction ◽

The Arctic ◽

Extended Model ◽

Similar Response ◽

Volcanic Aerosol ◽

The Impact

Abstract The impact of explosive volcanic eruptions on the atmospheric circulation at high northern latitudes is assessed in two versions of the Met Office Hadley Centre’s atmospheric climate model. The standard version of the model extends to an altitude of around 40 km, while the extended version has enhanced stratospheric resolution and reaches 85-km altitude. Seasonal hindcasts initialized on 1 December produce a strengthening of the winter polar vortex and anomalous warming over northern Europe characteristic of the positive phase of the Arctic Oscillation (AO) when forced with volcanic aerosol following the 1963 Mount Agung, 1982 El Chichón, and 1991 Mount Pinatubo eruptions, as is observed. The AO signal in the extended model is of comparable strength to that in the standard model, showing that there is little impact from both increasing the vertical resolution in the stratosphere and extending the model domain to near the mesopause. The presence of this signal in the models, however, is likely due to the persistence of the observed signal from the initial conditions, because a similar set of experiments initiated with the same conditions, but with no volcanic aerosol forcing, exhibits a similar response as the forced runs. This suggests that the model has limited fidelity in capturing the response to volcanic aerosols on its own, consistent with previous studies on the impact of volcanic forcing in long climate simulations, but does support the premise that seasonal winter forecasts are substantially improved with the inclusion of stratospheric information.

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Statistical description of the East Australian Current low-frequency variability from the WOCE PCM3 array

Marine and Freshwater Research ◽

10.1071/mf05058 ◽

2006 ◽

Vol 57 (3) ◽

pp. 273 ◽

Cited By ~ 8

Author(s):

Mauricio M. Mata ◽

Susan Wijffels ◽

John A. Church ◽

Matthias Tomczak

Keyword(s):

Ocean Circulation ◽

World Ocean ◽

Low Frequency ◽

General Rule ◽

Empirical Orthogonal Functions ◽

East Australian Current ◽

Orthogonal Functions ◽

The Pacific ◽

Strong Surface ◽

The Impact

The in situ dataset used in the current study consists of the Pacific Current Meter 3 (PCM3) array, which was a significant part of the Australian contribution to the World Ocean Circulation Experiment to study the variability of the East Australian Current (EAC), and was operational between September 1991 and March 1994. Area-preserving spectral analysis has been used to investigate the typical time scales observed by the current meters. As a general rule, the spectra from the top layers of the shallow (1, 2 and 3) and the deep (4, 5 and 6) moorings have a distinct peak in the temporal mesoscale band (periods between 70 and 170 days), with a general redistribution of energy towards the higher-frequencies near the ocean floor. This peak has been linked with eddy variability of the EAC system, which influences the fluctuations of the current main jet. The vertical modes of the velocity profile show that the strong surface-intensified baroclinic signal of the EAC dominated the variability at mooring 4 location. Further offshore the predominant configuration resembles more closely the barotropic mode. Ultimately, spatial empirical orthogonal functions (EOF) analysis point out the impact of the presence/absence of the EAC jet in the array.

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Multiple Regimes and Low-Frequency Oscillations in the Northern Hemisphere’s Zonal-Mean Flow

Journal of the Atmospheric Sciences ◽

10.1175/jas3672.1 ◽

2006 ◽

Vol 63 (3) ◽

pp. 840-860 ◽

Cited By ~ 23

Author(s):

S. Kravtsov ◽

A. W. Robertson ◽

M. Ghil

Keyword(s):

Singular Spectrum Analysis ◽

Low Frequency ◽

Zonal Flow ◽

Empirical Orthogonal Functions ◽

Mean Flow ◽

Orthogonal Functions ◽

Atlantic Sector ◽

Low Frequency Oscillations ◽

Multiple Regimes ◽

Flow States

Abstract This paper studies multiple regimes and low-frequency oscillations in the Northern Hemisphere zonal-mean zonal flow in winter, using 55 yr of daily observational data. The probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct, statistically significant maxima. The two regimes associated with these maxima describe persistent zonal-flow states that are characterized by meridional displacements of the midlatitude jet, poleward and equatorward of its time-mean position. The geopotential height anomalies of either regime have a pronounced zonally symmetric component, but largest-amplitude anomalies are located over the Atlantic and Pacific Oceans. High-frequency synoptic transients participate in the maintenance of and transitions between these regimes. Significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal-flow time series. These oscillations are described by singular spectrum analysis and the multitaper method. The 147-day oscillation involves zonal-flow anomalies that propagate poleward, while the 72-day oscillation only manifests northward propagation in the Atlantic sector. Both modes mainly describe changes in the midlatitude jet position and intensity. In the horizontal plane though, the two modes exhibit synchronous centers of action located over the Atlantic and Pacific Oceans. The two persistent flow regimes are associated with slow phases of either oscillation.

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Robust effects of springtime Arctic ozone depletion on surface climate

10.21203/rs.3.rs-496081/v1 ◽

2021 ◽

Author(s):

Marina Friedel ◽

Gabriel Chiodo ◽

Andrea Stenke ◽

Daniela Domeisen ◽

Stephan Fueglistaler ◽

...

Keyword(s):

Ozone Depletion ◽

Climate Model ◽

Stratospheric Ozone ◽

Polar Vortex ◽

Negative Temperature ◽

Significant Degree ◽

The Arctic ◽

Surface Climate ◽

Northern Hemispheric ◽

The Impact

Abstract Massive spring ozone loss due to anthropogenic emissions of ozone depleting substances is not limited to the austral hemisphere, but can also occur in the Arctic. Previous studies have suggested a link between springtime Arctic ozone depletion and Northern Hemispheric surface climate, which might add surface predictability. However, so far it has not been possible to isolate the role of stratospheric ozone from dynamical downward impacts. For the first time, we quantify the impact of springtime Arctic ozone depletion on surface climate using observations and targeted chemistry-climate model experiments to isolate the effects of ozone feedbacks. We find that springtime stratospheric ozone depletion is followed by surface anomalies in precipitation and temperature resembling a positive Arctic Oscillation. Most notably, we show that these anomalies, affecting large portions of the Northern Hemisphere, cannot be explained by dynamical variability alone, but are to a significant degree driven by stratospheric ozone. The surface signal is linked to reduced shortwave absorption by stratospheric ozone, forcing persistent negative temperature anomalies in the lower stratosphere and a delayed breakup of the polar vortex - analogous to ozone-surface coupling in the Southern Hemisphere.These results suggest that Arctic stratospheric ozone actively forces springtime Northern Hemispheric surface climate and thus provides a source of predictability on seasonal scales.

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