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David W. J. Thompson
and
David J. Lorenz

Abstract

The extratropical annular modes are coupled with a distinct pattern of climate anomalies that spans the circulation of the tropical troposphere. The signature of the annular modes in the tropical troposphere exhibits a high degree of equatorial symmetry. It is associated with upper-tropospheric zonal wind anomalies centered about the equator, midtropospheric temperature anomalies located ∼20°N and 20°S, and opposing mean meridional circulation anomalies that span the subtropics of both hemispheres. The linkages between the annular modes and the tropical circulation are only evident during the cold season months, and are most robust in association with the Northern Hemisphere annular mode (NAM).

The coupling between the annular modes and the circulation of the tropical troposphere is consistent with forcing by waves originating at extratropical latitudes. Both annular modes are characterized by anomalies in the eddy momentum flux convergence at tropical latitudes that act to reinforce the changes in the tropical wind and temperature fields. The most pronounced tropical anomalies lag indices of the annular modes by ∼2 weeks and are found over the eastern tropical Pacific, where climatological westerlies permit extratropical waves to propagate into the deep Tropics. The linkages between the NAM and the tropical tropospheric circulation are most pronounced during the cold phase of the El Niño–Southern Oscillation cycle.

The recent trend in the NAM is linearly congruent with a ∼0.1-K cooling of the tropical troposphere over the past two decades during the Northern Hemisphere winter season.

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David W. J. Thompson
and
Susan Solomon

Abstract

The long-term, global-mean cooling of the lower stratosphere stems from two downward steps in temperature, both of which are coincident with the cessation of transient warming after the volcanic eruptions of El Chichón and Mount Pinatubo. Previous attribution studies reveal that the long-term cooling is linked to ozone trends, and modeling studies driven by a range of known forcings suggest that the steps reflect the superposition of the long-term cooling with transient variability in upwelling longwave radiation from the troposphere. However, the long-term cooling of the lower stratosphere is evident at all latitudes despite the fact that chemical ozone losses are thought to be greatest at middle and polar latitudes. Further, the ozone concentrations used in such studies are based on either 1) smooth mathematical functions fit to sparsely sampled observations that are unavailable during postvolcanic periods or 2) calculations by a coupled chemistry–climate model.

Here the authors provide observational analyses that yield new insight into three key aspects of recent stratospheric climate change. First, evidence is provided that shows the unusual steplike behavior of global-mean stratospheric temperatures is dependent not only upon the trend but also on the temporal variability in global-mean ozone immediately following volcanic eruptions. Second, the authors argue that the warming/cooling pattern in global-mean temperatures following major volcanic eruptions is consistent with the competing radiative and chemical effects of volcanic eruptions on stratospheric temperature and ozone. Third, it is revealed that the contrasting latitudinal structures of recent stratospheric temperature and ozone trends are consistent with large-scale increases in the stratospheric overturning Brewer–Dobson circulation.

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Ying Li
and
David W. J. Thompson

Abstract

The signatures of large-scale annular variability on the vertical structure of clouds and cloud radiative effects are examined in vertically resolved CloudSat and other satellite and reanalysis data products. The northern and southern “barotropic” annular modes (the NAM and SAM) have a complex vertical structure. Both are associated with a meridional dipole in clouds between subpolar and middle latitudes, but the sign of the anomalies changes between upper, middle, and lower tropospheric levels. In contrast, the northern and southern baroclinic annular modes have a much simpler vertical structure. Both are linked to same-signed anomalies in clouds extending throughout the troposphere at middle to high latitudes. The changes in cloud incidence associated with both the barotropic and baroclinic annular modes are consistent with dynamical forcing by the attendant changes in static stability and/or vertical motion. The results also provide the first observational estimates of the vertically resolved atmospheric cloud radiative effects associated with hemispheric-scale extratropical variability. In general, the anomalies in atmospheric cloud radiative effects associated with the annular modes peak in the middle to upper troposphere, and are consistent with the anomalous trapping of longwave radiation by variations in upper tropospheric clouds. The southern baroclinic annular mode gives rise to periodic behavior in longwave cloud radiative effects at the top of the atmosphere averaged over Southern Hemisphere midlatitudes.

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David W. J. Thompson
and
Susan Solomon

Abstract

The global structure of recent stratospheric climate trends is examined in radiosonde data. In contrast to conclusions published in previous assessments of stratospheric temperature trends, it is demonstrated that in the annual mean the tropical stratosphere has cooled substantially over the past few decades. The cooling of the tropical stratosphere is apparent in both nighttime and adjusted radiosonde data, and seems to be robust to changes in radiosonde instrumentation. The meridional structure of the annual-mean stratospheric trends is not consistent with our current understanding of radiative transfer and constituent trends but is consistent with increased upwelling in the tropical stratosphere.

The annual-mean cooling of the tropical stratosphere is juxtaposed against seasonally varying trends in the extratropical stratosphere dominated by the well-known springtime cooling at polar latitudes. The polar stratospheric trends are accompanied by similarly signed trends at tropospheric levels in the Southern Hemisphere but not in the Northern Hemisphere.

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Laura M. Ciasto
and
David W. J. Thompson

Abstract

The authors provide a detailed examination of observed ocean–atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM), and the El Niño–Southern Oscillation (ENSO). Results are examined separately for the warm (November–April) and cold (May–October) seasons and for monthly and weekly time scales. It is shown that the signatures of the SAM and ENSO in the SH SST field vary as a function of season, both in terms of their amplitudes and structures. The role of surface turbulent and Ekman heat fluxes in driving seasonal variations in the SAM- and ENSO-related SST anomalies is investigated. Analyses of weekly data reveal that variability in the SAM tends to precede anomalies in the SST field by ∼1 week, and that the e-folding time scale of the SAM-related SST field anomalies is at least 4 months. The persistence of the SAM-related SST anomalies is consistent with the passive thermal response of the Southern Ocean to variations in the SAM, and seasonal variations in the persistence of the SAM-related SST anomalies are consistent with the seasonal cycle in the depth of the ocean mixed layer.

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John M. Wallace
and
David W. J. Thompson

Abstract

The leading empirical orthogonal function (EOF) of the sea level pressure (SLP) field, referred to as the Arctic Oscillation (AO) or Northern Hemisphere annular mode (NAM), consists of a dipole between the polar cap region and the surrounding zonal ring centered along 45°N. Embedded within the outer ring are centers of action over the Euro-Atlantic and Pacific sectors in which SLP fluctuates in phase. That the observed SLP fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the Pacific center in the annular mode could be an artifact of EOF analysis.

It is argued that sea level pressure fluctuations at the Pacific and Euro-Atlantic centers of action of the AO/NAM would be more strongly correlated were it not for the fact that SLP variability over the North Pacific is dominated by a pattern in which fluctuations over the North Atlantic and North Pacific are inversely related. Evidence of the coexistence of such a pattern, which resembles an augmented version of the Pacific–North American pattern, is presented.

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David W. J. Thompson
and
John M. Wallace

Abstract

The leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep, zonally symmetric or “annular” structures, with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 45° latitude. The structure and dynamics of the Southern Hemisphere (SH) annular mode have been extensively documented, whereas the existence of a Northern Hemisphere (NH) mode, herein referred to as the Arctic Oscillation (AO), has only recently been recognized. Like the SH mode, the AO can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields. In this paper the structure and seasonality of the NH and SH modes are compared based on data from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis and supplementary datasets.

The structures of the NH and SH annular modes are shown to be remarkably similar, not only in the zonally averaged geopotential height and zonal wind fields, but in the mean meridional circulations as well. Both exist year-round in the troposphere, but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave–mean flow interaction: midwinter in the NH and late spring in the SH. During these “active seasons,” the annular modes modulate the strength of the Lagrangian mean circulation in the lower stratosphere, total column ozone and tropopause height over mid- and high latitudes, and the strength of the trade winds of their respective hemispheres. The NH mode also contains an embedded planetary wave signature with expressions in surface air temperature, precipitation, total column ozone, and tropopause height. It is argued that the horizontal temperature advection by the perturbed zonal-mean zonal wind field in the lower troposphere is instrumental in forcing this pattern.

A companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades.

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Samantha M. Wills
and
David W. J. Thompson

Abstract

Observational analyses reveal that wintertime variations in sea surface temperatures (SST) in the Kuroshio–Oyashio Extension (KOE) region of the North Pacific are associated with two distinct and robust patterns of atmospheric variability: 1) a pattern that peaks in amplitude approximately 2–3 weeks prior to large KOE SST anomalies and is consistent with atmospheric forcing of the SST field and 2) a very different pattern that lags SST anomalies in the KOE region by approximately a month. The latter pattern is dominated by low sea level pressure anomalies and turbulent heat fluxes directed into the atmosphere over warm SST anomalies and is interpreted as the transient atmospheric response to SST anomalies over the KOE region. The results contribute to a growing body of evidence that suggests variations in SSTs in the midlatitude oceans are capable of significantly influencing the large-scale atmospheric circulation, especially near western boundary currents.

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Casey R. Patrizio
and
David W. J. Thompson

Abstract

Understanding the role of the ocean in climate variability requires first understanding the role of ocean dynamics in the ocean mixed layer and thus sea surface temperature variability. However, key aspects of the spatially and temporally varying contributions of ocean dynamics to such variability remain unclear. Here, the authors quantify the contributions of ocean dynamical processes to mixed layer temperature variability on monthly to multiannual time scales across the globe. To do so, they use two complementary but distinct methods: 1) a method in which ocean heat transport is estimated directly from a state-of-the-art ocean state estimate spanning 1992–2015 and 2) a method in which it is estimated indirectly from observations between 1980–2017 and the energy budget of the mixed layer. The results extend previous studies by providing quantitative estimates of the role of ocean dynamics in mixed layer temperature variability throughout the globe, across a range of time scales, in a range of available measurements, and using two different methods. Consistent with previous studies, both methods indicate that the ocean-dynamical contribution to mixed layer temperature variance is largest over western boundary currents, their eastward extensions, and regions of equatorial upwelling. In contrast to previous studies, the results suggest that ocean dynamics reduce the variance of Northern Hemisphere mixed layer temperatures on time scales longer than a few years. Hence, in the global mean, the fractional contribution of ocean dynamics to mixed layer temperature variability decreases at increasingly low frequencies. Differences in the magnitude of the ocean dynamical contribution based on the two methods highlight the critical need for improved and continuous observations of the ocean mixed layer.

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Casey R. Patrizio
and
David W. J. Thompson

Abstract

In a recent paper, we argued that ocean dynamics increase the variability of midlatitude sea surface temperatures (SSTs) on monthly to interannual time scales, but act to damp lower-frequency SST variability over broad midlatitude regions. Here, we use two configurations of a simple stochastic climate model to provide new insights into this important aspect of climate variability. The simplest configuration includes the forcing and damping of SST variability by observed surface heat fluxes only, and the more complex configuration includes forcing and damping by ocean processes, which are estimated indirectly from monthly observations. It is found that the simple model driven only by the observed surface heat fluxes generally produces midlatitude SST power spectra that are too red compared to observations. Including ocean processes in the model reduces this discrepancy by whitening the midlatitude SST spectra. In particular, ocean processes generally increase the SST variance on <2-yr time scales and decrease it on >2-yr time scales. This happens because oceanic forcing increases the midlatitude SST variance across many time scales, but oceanic damping outweighs oceanic forcing on >2-yr time scales, particularly away from the western boundary currents. The whitening of midlatitude SST variability by ocean processes also operates in NCAR’s Community Earth System Model (CESM). That is, midlatitude SST spectra are generally redder when the same atmospheric model is coupled to a slab rather than dynamically active ocean model. Overall, the results suggest that forcing and damping by ocean processes play essential roles in driving midlatitude SST variability.

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