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Clara Deser
and
Michael S. Timlin

Abstract

Large-scale atmosphere–ocean interaction over the North Atlantic and North Pacific during winter using a 14-yr record of weekly sea surface temperature and atmospheric circulation fields is examined. Singular Value Decomposition is used to quantify objectively the degree of coupling between the sea surface temperature and 500-mb geopotential height fields as a function of time lag, from −4 weeks to +4 weeks. The authors show that the air–sea coupling is strongest when 500-mb height leads sea surface temperature by 2–3 weeks—twice as strong as the simultaneous covariability and nearly four times as large as when sea surface temperature leads 500-mb height by a few weeks. The authors believe the 2–3-week timescale may be a reflection of high-frequency stochastic forcing by the atmosphere on the ocean mixed layer, in line with the theoretical model of Frankignoul and Hasselmann. Sensible and latent energy fluxes at the sea surface are shown to be an important component of the atmospheric forcing. The close spatial and temporal correspondence between the fluxes and SST tendencies on weekly timescales is a testament to the quality of the datasets.

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

Abstract

A newly available, extensive compilation of upper-ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400-m depth in the North Pacific during 1970–1991. A prominent decade-long perturbation in climate occurred during this time period: surface waters cooled by ∼1°C in the central and western North Pacific and warmed by about the same amount along the west coast of North America from late 1976 to 1988. Comparison with data from COADS suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly.

The vertical structure of seasonal thermal anomalies in the central North Pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400-m depth. Individual cold events descend rapidly (∼100 m yr−1), superimposed upon a slower cooling (∼15 m yr−1). The interdecadal climate change, while evident at the surface, is most prominent below ∼150 m where interannual variations are small. Unlike the central North Pacific, the temperature changes along the west coast of North America appear to be confined to approximately the upper 200–250 m. The structure of the interdecadal thermal variations in the eastern and central North Pacific appears to be consistent with the dynamics of the ventilated thermocline. In the western North Pacific, strong cooling is observed along the axis of the Kuroshio Current Extension below ∼200 m depth during the 1980s.

Changes in mixed layer depth accompany the SST variations, but their spatial distribution is not identical to the pattern of SST change. In particular, the decade-long cool period in the central North Pacific was accompanied by a ∼20 m deepening of the mixed layer in winter, but no significant changes in mixed layer depth were found along the west coast of North America. It is suggested that other factors such as stratification beneath the mixed layer and synoptic wind forcing may play a role in determining the distribution of mixed layer depth anomalies.

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

Abstract

The reemergence mechanism, whereby temperature anomalies extending over the deep winter mixed layer are stored beneath the surface in summer and are reentrained into the mixed layer when it deepens again in the following autumn and winter, is studied in the North Atlantic using approximately 40 years of surface and subsurface data. Reemergence is found to be robust in the Sargasso Sea and the northeast Atlantic, regions where (i) the mixed layer is much deeper in winter than in summer, (ii) currents are relatively weak, and (iii) temperature anomalies are coherent over broad areas. The two leading empirical orthogonal functions of North Atlantic SST anomalies also exhibit strong reemergence signatures. A novel application of empirical orthogonal function analysis to temperature anomalies in the time–depth plane, which also incorporates information from all grid points, is shown to be an efficient and useful approach for detecting reemergence without dependence on specific spatial patterns or prior selection of regions.

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

Abstract

An extension of the simple stochastic climate model of Frankignoul and Hasselman that includes the effects of seasonal variations in upper-ocean mixed layer depth upon the persistence of winter sea surface temperature (SST) anomalies is proposed. Seasonal variations in mixed layer depth allow for the “reemergence mechanism,” whereby thermal anomalies stored in the deep winter mixed layer persist at depth through summer and become partially reentrained into the mixed layer during the following winter. In this way, SST anomalies can recur from winter to winter without persisting through the intervening summer. Reformulating the simple stochastic climate model in terms of an effective ocean thermal capacity given by the depth of the winter mixed layer, thereby implicitly taking into account reemergence, is shown to provide a favorable fit to the observed winter-to-winter SST autocorrelations in the North Atlantic and Pacific, and represents a considerable improvement over the original model. The extended model also compares favorably with results from an entraining bulk ocean mixed layer model coupled to an atmospheric general circulation model. The authors propose that the extended model be adopted as the new “null hypothesis” for interannual SST variability in middle and high latitudes.

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Clara Deser
,
Michael A. Alexander
, and
Michael S. Timlin
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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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Michael A. Alexander
,
Uma S. Bhatt
,
John E. Walsh
,
Michael S. Timlin
,
Jack S. Miller
, and
James D. Scott

Abstract

The influence of realistic Arctic sea ice anomalies on the atmosphere during winter is investigated with version 3.6 of the Community Climate Model (CCM3.6). Model experiments are performed for the winters with the most (1982/83) and least (1995/96) Arctic ice coverage during 1979–99, when ice concentration estimates were available from satellites. The experiments consist of 50-member ensembles: using large ensembles proved critical to distinguish the signal from noise.

The local response to ice anomalies over the subpolar seas of both the Atlantic and Pacific is robust and generally shallow with large upward surface heat fluxes (>100 W m−2), near-surface warming, enhanced precipitation, and below-normal sea level pressure where sea ice receded, and the reverse where the ice expanded. The large-scale response to reduced (enhanced) ice extent to the east (west) of Greenland during 1982/83 resembles the negative phase of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) with a ridge over the poles and a trough at midlatitudes. The large-scale response was distinctly different in the Pacific, where ice extent anomalies in the Sea of Okhotsk generate a wave train that extends downstream over North America but the wave train response is greatly diminished when the model is driven by ice concentration rather than ice extent anomalies. Comparing the AGCM response to observations suggests that the feedback of the ice upon the atmospheric circulation is positive (negative) in the Pacific (Atlantic) sector. The magnitude of the wintertime response to ice extent anomalies is modest, on the order of 20 m at 500 mb. However, the 500-mb height anomalies roughly double in strength over much of the Arctic when forced by ice concetration anomalies. Furthermore, the NAO-like response increases linearly with the aerial extent of the Atlantic ice anomalies and thus could be quite large if the ice edge retreats as a result of global warming.

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Andrea M. Smith
,
Greg M. McFarquhar
,
Robert M. Rauber
,
Joseph A. Grim
,
Michael S. Timlin
,
Brian F. Jewett
, and
David P. Jorgensen

Abstract

This study used airborne and ground-based radar and optical array probe data from the spiral descent flight patterns and horizontal flight legs of the NOAA P-3 aircraft in the trailing stratiform regions (TSRs) of mesoscale convective systems (MCSs) observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) to characterize microphysical and thermodynamic variations within the TSRs in the context of the following features: the transition zone, the notch region, the enhanced stratiform rain region, the rear anvil region, the front-to-rear flow, the rear-to-front flow, and the rear inflow jet axis. One spiral from the notch region, nine from the enhanced stratiform rain region, and two from the rear anvil region were analyzed along with numerous horizontal flight legs that traversed these zones. The spiral performed in the notch region on 29 June occurred early in the MCS life cycle and exhibited subsaturated conditions throughout its depth. The nine spirals performed within the enhanced stratiform rain region exhibited saturated conditions with respect to ice above and within the melting layer and subsaturated conditions below the melting layer. Spirals performed in the rear anvil region showed saturation until the base of the anvil, near −1°C, and subsaturation below. These data, together with analyses of total number concentration and the slope to gamma fits to size distributions, revealed that sublimation above the melting layer occurs early in the MCS life cycle but then reduces in importance as the environment behind the convective line is moistened from the top down. Evaporation below the melting layer was insufficient to attain saturation below the melting layer at any time or location within the MCS TSRs. Relative humidity was found to have a strong correlation to the component of wind parallel to the storm motion, especially within air flowing from front to rear.

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