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James A. Screen
,
Clara Deser
, and
Lantao Sun

Abstract

In early January 2014, an Arctic air outbreak brought extreme cold and heavy snowfall to central and eastern North America, causing widespread disruption and monetary losses. The media extensively reported the cold snap, including debate on whether human-induced climate change was partly responsible. Related to this, one particular hypothesis garnered considerable attention: that rapid Arctic sea ice loss may be increasing the risk of cold extremes in the midlatitudes. Here we use large ensembles of model simulations to explore how the risk of North American daily cold extremes is anticipated to change in the future, in response to increases in greenhouse gases and the component of that response solely due to Arctic sea ice loss. Specifically, we examine the changing probability of daily cold extremes as (un)common as the 7 January 2014 event. Projected increases in greenhouse gases decrease the likelihood of North American cold extremes in the future. Days as cold or colder than 7 January 2014 are still projected to occur in the mid-twenty-first century (2030–49), albeit less frequently than in the late twentieth century (1980–99). However, such events will cease to occur by the late twenty-first century (2080–99), assuming greenhouse gas emissions continue unabated. Continued Arctic sea ice loss is a major driver of decreased—not increased—North America cold extremes. Projected Arctic sea ice loss alone reduces the odds of such an event by one-quarter to one-third by the mid-twenty-first century, and to zero (or near zero) by the late twenty-first century.

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Clara Deser
,
Robert A. Tomas
, and
Shiling Peng

Abstract

The objective of this study is to investigate the transient evolution of the wintertime atmospheric circulation response to imposed patterns of SST and sea ice extent anomalies in the North Atlantic sector using a large ensemble of experiments with the NCAR Community Climate Model version 3 (CCM3). The initial adjustment of the atmospheric circulation is characterized by an out-of-phase relationship between geopotential height anomalies in the lower and upper troposphere localized to the vicinity of the forcing. This initial baroclinic response reaches a maximum amplitude in ∼5–10 days, and persists for 2–3 weeks. Diagnostic results with a linear primitive equation model indicate that this initial response is forced by diabatic heating anomalies in the lower troposphere associated with surface heat flux anomalies generated by the imposed thermal forcing. Following the initial baroclinic stage of adjustment, the response becomes progressively more barotropic and increases in both spatial extent and magnitude. The equilibrium stage of adjustment is reached in 2–2.5 months, and is characterized by an equivalent barotropic structure that resembles the hemispheric North Atlantic Oscillation–Northern Annular Mode (NAO–NAM) pattern, the model’s leading internal mode of circulation variability over the Northern Hemisphere. The maximum amplitude of the equilibrium response is approximately 2–3 times larger than that of the initial response. The equilibrium response is primarily maintained by nonlinear transient eddy fluxes of vorticity (and, to a lesser extent, heat), with diabatic heating making a limited contribution in the vicinity of the forcing.

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Antonietta Capotondi
,
Michael A. Alexander
, and
Clara Deser

Abstract

Observations indicate the existence of two bands of maximum thermocline depth variability centered at ∼10°S and 13°N in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10°S and 13°N are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10°S and 13°N, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10°S and 13°N is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10°S and 13°N, appear to affect the sharpness of the variability maxima at low frequencies. Although the Rossby wave model gives an excellent fit to the OGCM, some discrepancies exist: the amplitude of the thermocline variance is generally underestimated by the simple model, and the variability along 13°N is westward intensified in the wave model but reaches a maximum in the central part of the basin in the OGCM. Short Rossby waves excited by small-scale Ekman pumping features, or the presence of higher-order Rossby wave modes may be responsible for the differences in the zonal variance distribution along 13°N.

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Antonietta Capotondi
,
Michael A. Alexander
, and
Clara Deser

Abstract

Observations indicate the existence of two bands of maximum thermocline depth variability centered at ∼10°S and 13°N in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10°S and 13°N are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10°S and 13°N, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10°S and 13°N is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10°S and 13°N, appear to affect the sharpness of the variability maxima at low frequencies. Although the Rossby wave model gives an excellent fit to the OGCM, some discrepancies exist: the amplitude of the thermocline variance is generally underestimated by the simple model, and the variability along 13°N is westward intensified in the wave model but reaches a maximum in the central part of the basin in the OGCM. Short Rossby waves excited by small-scale Ekman pumping features, or the presence of higher-order Rossby wave modes may be responsible for the differences in the zonal variance distribution along 13°N.

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Antonietta Capotondi
,
Michael A. Alexander
,
Clara Deser
, and
Michael J. McPhaden

Abstract

The output from an ocean general circulation model driven by observed surface forcing (1958–97) is used to examine the evolution and relative timing of the different branches of the Pacific Subtropical–Tropical Cells (STCs) at both interannual and decadal time scales, with emphasis on the 1976–77 climate shift. The STCs consist of equatorward pycnocline transports in the ocean interior and in the western boundary current, equatorial upwelling, and poleward flow in the surface Ekman layer. The interior pycnocline transports exhibit a decreasing trend after the mid-1970s, in agreement with observational transport estimates, and are largely anticorrelated with both the Ekman transports and the boundary current transports at the same latitudes. The boundary current changes tend to compensate for the interior changes at both interannual and decadal time scales. The meridional transport convergence across 9°S and 9°N as well as the equatorial upwelling are strongly correlated with the changes in sea surface temperature (SST) in the central and eastern equatorial Pacific. However, meridional transport variations do not occur simultaneously at each longitude, so that to understand the phase relationship between transport and SST variations it is important to consider the baroclinic ocean adjustment through westward-propagating Rossby waves. The anticorrelation between boundary current changes and interior transport changes can also be understood in terms of the baroclinic adjustment process. In this simulation, the pycnocline transport variations appear to be primarily confined within the Tropics, with maxima around 10°S and 13°N, and related to the local wind forcing; a somewhat different perspective from previous studies that have emphasized the role of wind variations in the subtropics.

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Christophe Cassou
,
Laurent Terray
,
James W. Hurrell
, and
Clara Deser

Abstract

The observed low-frequency winter atmospheric variability of the North Atlantic–European region and its relationship with global surface oceanic conditions is investigated based on the climate and weather regimes paradigm.

Asymmetries between the two phases of the North Atlantic Oscillation (NAO) are found in the position of the Azores high and, to a weaker extent, the Icelandic low. There is a significant eastward displacement or expansion toward Europe for the NAO+ climate regime compared to the NAO− regime. This barotropic signal is found in different datasets and for two quasi-independent periods of record (1900–60 and 1950–2001); hence, it appears to be intrinsic to the NAO+ phase. Strong spatial similarities between weather and climate regimes suggest that the latter, representing long time scale variability, can be interpreted as the time-averaging signature of much shorter time scale processes. Model results from the ARPEGE atmospheric general circulation model are used to validate observed findings. They confirm in particular the eastward shift of the Atlantic centers of action for the NAO+ phase and strongly suggest a synoptic origin as it can be extracted from daily analyses. These results bring together present-day climate variability and scenario studies where such an NAO shift was suggested, as it is shown that the last three decades are clearly dominated by the occurrence of NAO+ regimes when concentrations of greenhouse gases are rapidly increasing. These findings highlight that the displacement of the North Atlantic centers of action should be treated as a dynamical property of the North Atlantic atmosphere and not as a mean longitudinal shift of climatological entities in response to anthropogenic forcings.

The nonstationarity with time of the atmospheric variability is documented. Late-century decades differ from early ones by the predominance of NAO climate regimes versus others. In such a context, comments on the relevance of the station-based NAO index is provided. Both tropical and extratropical sea surface temperature (SST) anomalies alter the frequency distribution of the North Atlantic regimes. Evidence is presented that the so-called ridge regime is preferably excited during La Niña events, while the NAO regimes are associated with the North Atlantic SST tripole. The ARPEGE model results indicate that the tropical branch of the SST tripole affects the NAO regimes occurrence. Warm tropical SST anomalies are more efficient at exciting NAO− regimes than cold anomalies are at forcing NAO+ regimes. The extratropical portion of the North Atlantic SST tripole also seems to play a significant role in the model, tending to counteract the dominant influence of the tropical Atlantic basin on NAO regimes.

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Michael A. Alexander
,
Clara Deser
, and
Michael S. Timlin

Abstract

Sea surface temperature (SST) data and two different upper-ocean temperature analyses are used to study the winter-to-winter recurrence of SST anomalies in the North Pacific Ocean. The SSTs recur when temperature anomalies that form in the deep ocean mixed layer in late winter/early spring are isolated from the atmosphere in the summer seasonal thermocline and then reemerge at the surface when the mixed layer deepens during the following fall/winter. This “reemergence mechanism” is evaluated over the basin by correlating the time series of the leading pattern of ocean temperature anomalies in the summer seasonal thermocline (∼60–85 m in August–September) with SST anomalies over the course of the year. The results indicate that the dominant large-scale SST anomaly pattern that forms in the North Pacific during late winter, with anomalies of one sign in the central Pacific and the opposite sign along the coast of North America, is sequestered in the seasonal thermocline in summer and returns to the surface in the following fall, with little persistence at the surface in summer.

Regions in the east, central, and west Pacific all show signs of the reemergence process but indicate that it is influenced by the timing and amplitude of the mean seasonal cycle in mixed layer depth. The maximum mixed layer depth increases from east to west across the basin: as a result, the thermal anomalies are shallower and return to the surface sooner in the east compared with the west Pacific. At some locations, the reemerging signal is also influenced by when the SST anomalies are created. In the east Pacific, SST anomalies that are initiated in February–March extend through a deeper mixed layer, persist at greater depths in summer, and are then reentrained later in the year compared with those initiated in April–May.

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Clara Deser
,
John E. Walsh
, and
Michael S. Timlin

Abstract

Forty years (1958–97) of reanalysis products and corresponding sea ice concentration data are used to document Arctic sea ice variability and its association with surface air temperature (SAT) and sea level pressure (SLP) throughout the Northern Hemisphere extratropics. The dominant mode of winter (January–March) sea ice variability exhibits out-of-phase fluctuations between the western and eastern North Atlantic, together with a weaker dipole in the North Pacific. The time series of this mode has a high winter-to-winter autocorrelation (0.69) and is dominated by decadal-scale variations and a longer-term trend of diminishing ice cover east of Greenland and increasing ice cover west of Greenland.

Associated with the dominant pattern of winter sea ice variability are large-scale changes in SAT and SLP that closely resemble the North Atlantic oscillation. The associated SAT and surface sensible and latent heat flux anomalies are largest over the portions of the marginal sea ice zone in which the trends of ice coverage have been greatest, although the well-documented warming of the northern continental regions is also apparent. The temporal and spatial relationships between the SLP and ice anomaly fields are consistent with the notion that atmospheric circulation anomalies force the sea ice variations. However, there appears to be a local response of the atmospheric circulation to the changing sea ice cover east of Greenland. Specifically, cyclone frequencies have increased and mean SLPs have decreased over the retracted ice margin in the Greenland Sea, and these changes differ from those associated directly with the North Atlantic oscillation.

The dominant mode of sea ice variability in summer (July–September) is more spatially uniform than that in winter. Summer ice extent for the Arctic as a whole has exhibited a nearly monotonic decline (−4% decade−1) during the past 40 yr. Summer sea ice variations appear to be initiated by atmospheric circulation anomalies over the high Arctic in late spring. Positive ice–albedo feedback may account for the relatively long delay (2–3 months) between the time of atmospheric forcing and the maximum ice response, and it may have served to amplify the summer ice retreat.

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Clara Deser
,
Michael A. Alexander
, and
Michael S. Timlin

Abstract

The spatial and temporal characteristics of oceanic thermal variations in the mixed layer and main thermocline of the midlatitude North Pacific are distinctive, suggesting different physical origins. Within the main thermocline (400-m depth), the variability is dominated by a westward-intensified pattern of decadal scale, indicative of enhanced eastward geostrophic flow along the southern flank of the Kuroshio Current extension during the 1980s relative to the 1970s. The authors argue that the decadal-scale change in the strength of the Kuroshio extension was a result of the dynamical adjustment of the oceanic circulation to a decadal variation in wind stress curl according to Sverdrup theory. Four-times daily wind stress fields from the National Center for Atmospheric Research–National Centers for Environmental Prediction reanalysis project are used to compute the decadal change in Sverdrup transport associated with the 1976/77 climate transition. It is shown that the decadal changes in Sverdrup transport inferred from the wind stress curl field and in observed geostrophic flow inferred from the upper-ocean thermal field are consistent both in terms of spatial pattern and magnitude. The decadal change in depth-averaged geostrophic transport along the Kuroshio extension (referenced to 1 km) is 11.6 Sv, similar to the Sverdrup transport change (11.5–13.9 Sv). The decadal-scale thermocline variation along the western boundary between 30° and 40°N exhibits a lag of approximately 4–5 yr relative to the decadal variation in the basin-wide wind stress curl pattern. This delay may be indicative of the transient adjustment of the gyre-scale circulation to a change in wind stress curl via long baroclinic Rossby waves.

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Clara Deser
,
Antonietta Capotondi
,
R. Saravanan
, and
Adam S. Phillips

Abstract

Simulations of the El Niño–Southern Oscillation (ENSO) phenomenon and tropical Atlantic climate variability in the newest version of the Community Climate System Model [version 3 (CCSM3)] are examined in comparison with observations and previous versions of the model. The analyses are based upon multicentury control integrations of CCSM3 at two different horizontal resolutions (T42 and T85) under present-day CO2 concentrations. Complementary uncoupled integrations with the atmosphere and ocean component models forced by observed time-varying boundary conditions allow an assessment of the impact of air–sea coupling upon the simulated characteristics of ENSO and tropical Atlantic variability.

The amplitude and zonal extent of equatorial Pacific sea surface temperature variability associated with ENSO is well simulated in CCSM3 at both resolutions and represents an improvement relative to previous versions of the model. However, the period of ENSO remains too short (2–2.5 yr in CCSM3 compared to 2.5–8 yr in observations), and the sea surface temperature, wind stress, precipitation, and thermocline depth responses are too narrowly confined about the equator. The latter shortcoming is partially overcome in the atmosphere-only and ocean-only simulations, indicating that coupling between the two model components is a contributing cause. The relationships among sea surface temperature, thermocline depth, and zonal wind stress anomalies are consistent with the delayed/recharge oscillator paradigms for ENSO. We speculate that the overly narrow meridional scale of CCSM3's ENSO simulation may contribute to its excessively high frequency. The amplitude and spatial pattern of the extratropical atmospheric circulation response to ENSO is generally well simulated in the T85 version of CCSM3, with realistic impacts upon surface air temperature and precipitation; the simulation is not as good at T42.

CCSM3's simulation of interannual climate variability in the tropical Atlantic sector, including variability intrinsic to the basin and that associated with the remote influence of ENSO, exhibits similarities and differences with observations. Specifically, the observed counterpart of El Niño in the equatorial Atlantic is absent from the coupled model at both horizontal resolutions (as it was in earlier versions of the coupled model), but there are realistic (although weaker than observed) SST anomalies in the northern and southern tropical Atlantic that affect the position of the local intertropical convergence zone, and the remote influence of ENSO is similar in strength to observations, although the spatial pattern is somewhat different.

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