Description: Several international initiatives are working to stitch together data describing solar forcing of Earth’s climate. Their objective is to improve understanding of climate response to solar variability...

Description: Key Points: Daily snapshots of TIL strength; synoptic-Scale behavior of the TIL and shear/curl contributions to relative vorticity; TIL within ridges in midlatitude winter is stronger than polar summer TIL High-resolution GPS radio occultation temperature profiles from the COSMIC satellite mission (2007–2013) are used to obtain daily snapshots of the strength of the extratropical tropopause inversion layer (TIL). Its horizontal structure and day-to-day variability are linked to the synoptic situation at near-tropopause level. The strength of the TIL in cyclonic as well as anticyclonic conditions is investigated by separating relative vorticity into curl and shear terms. The analysis shows that the TIL has high zonal variability, and its strength is instantaneously adjusted to the synoptic situation at near-tropopause level. Our key finding is that the TIL within midlatitude ridges in winter is as strong as or stronger than the TIL in polar summer. The strongest TIL in anticyclonic conditions is related to the shear term, while the weaker TIL in cyclonic conditions is enhanced by the curl term.

Description: The recent variability of the tropopause temperature and the tropopause inversion layer (TIL) are investigated with Global Positioning System Radio Occultation data and simulations with the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM). Over the past decade (2001–2011) the data show an increase of 0.8 K in the tropopause temperature and a decrease of 0.4 K in the strength of the tropopause inversion layer in the tropics, meaning that the vertical temperature gradient has declined, and therefore that the stability above the tropopause has weakened. WACCM simulations with finer vertical resolution show a more realistic TIL structure and variability. Model simulations show that the increased tropopause temperature and the weaker tropopause inversion layer are related to weakened upwelling in the tropics. Such changes in the thermal structure of the upper troposphere and lower stratosphere may have important implications for climate, such as a possible rise in water vapor in the lower stratosphere.

Description: Stratospheric sudden warmings (SSWs) are extreme events in the polar stratosphere that are both caused by and have effects on the tropospheric flow. This means that SSWs are associated with changes in the angular momentum of the atmosphere, both before and after their onset. Because these angular momentum changes are transferred to the solid Earth, they can be observed in the rate of the Earth's rotation and the wobble of its rotational pole. By comparing observed Earth rotation variations to reanalysis data, we find that an anomaly in the orientation of the Earth's rotational pole, up to 4 times as large as the annual polar wobble, typically precedes SSWs by 20-40 days. The polar motion signal is due to pressure anomalies that are typically seen before SSW events and represents a new type of observable that may aid in the prediction of SSWs. A decline in the length of day is also seen, on average, near the time of the SSW wind reversal and is found to be due to anomalous easterly winds generated in the tropical troposphere around this time, though the structure and timing of this signal seems to vary widely from event to event.

Description: Major stratospheric sudden warmings are prominent disturbances of the Northern Hemisphere polar winter stratosphere. Understanding the factors controlling major warmings is required, since the associated circulation changes can propagate down into the troposphere and affect the surface climate, suggesting enhanced prediction skill when these processes are accurately represented in models. In this study we investigate how different natural and anthropogenic factors, namely, the quasi-biennial oscillation (QBO), sea surface temperatures (SSTs), anthropogenic greenhouse gases, and ozone-depleting substances, influence the frequency, variability, and life cycle of major warmings. This is done using sensitivity experiments performed with the National Center for Atmospheric Research's Community Earth System Model (CESM). CESM is able to simulate the life cycle of major warmings realistically. The QBO strengthens the climatological stratospheric polar night jet (PNJ) and significantly reduces the frequency of major warmings through reduction of planetary wave propagation into the PNJ region. Variability in SSTs weakens the PNJ and significantly increases the major warming frequency due to enhanced wave forcing. Even extreme climate change conditions (RCP8.5 scenario) do not influence the total frequency but determine the prewarming phase of major warmings. The amplitude and duration of major warmings seem to be mainly determined by internal stratospheric variability. We also suggest that SST variability, two-way ocean/atmosphere coupling, and hence the memory of the ocean are needed to reproduce the observed tropospheric negative Northern Annular Mode pattern after major warmings.

Description: One decade of time-variable gravity field observations from the GRACE satellite mission reveals low-frequency ocean bottom pressure (OBP) variability of up to 2.5 hPa centered at the northern flank of the subtropical gyre in the North Pacific. From a 145 year-long simulation with a coupled chemistry climate model, OBP variability is found to be related to the prevailing atmospheric sea-level pressure and surface wind conditions in the larger North Pacific area. The dominating atmospheric pressure patterns obtained from the climate model run allow in combination with ERA-Interim sea-level pressure and surface winds a reconstruction of the OBP variability in the North Pacific from atmospheric model data only, which correlates favorably (r=0.7) with GRACE ocean bottom pressure observations. The regression results indicate that GRACE-based OBP observations are indeed sensitive to changes in the prevailing sea-level pressure and thus surface wind conditions in the North Pacific, thereby opening opportunities to constrain atmospheric models from satellite gravity observations over the oceans.

Description: We present results from three multidecadal sensitivity experiments with time-varying solar cycle and quasi-biennial oscillation (QBO) forcings using National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM3.1). The model experiments are unique compared to earlier studies as they use time-varying forcings for the solar cycle only and the QBO, both individually and combined. The results show that the annual mean solar response in the tropical upper stratosphere is independent of the presence of the QBO. The response in the middle to lower stratosphere differs depending on the presence of the QBO and the solar cycle but is statistically indistinguishable in the three experiments. The seasonal evolution of the solar and the combined solar-QBO signals reveals a reasonable agreement with observations only for the experiment in which both the solar cycle and the QBO forcing are present, suggesting that both forcings are important to generate the observed response. More stratospheric warmings occur during solar maximum and QBO west conditions. This appears to be the result of a QBO modulation of the background zonal mean wind climatology, which modifies the solar signal. Depending on the background wind, the small initial early winter solar signal in the subtropical upper stratosphere/lower mesosphere is enhanced during QBO east and diminished during QBO west conditions. This consequently influences the transfer of the solar-QBO signal during winter and results in the observed differences during late winter

Description: The atmospheric response to the solar cycle has been previously investigated with the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM) using prescribed spectral solar UV and ozone changes as well as prescribed equatorial, QBO-like winds. The solar signal is transferred from the upper to the lower stratosphere through a modulation of the polar night jet and the Brewer-Dobson circulation. These model experiments are further investigated here to show the transfer of the solar signal from the lower stratosphere to the troposphere and down to the surface during Northern Hemisphere winter. Analysis focuses on the transition from significant stratospheric effects in October and November to significant tropospheric effects in December and January. The results highlight the importance of stratospheric circulation changes for the troposphere. Together with the poleward-downward movement of zonal wind anomalies and enhanced equatorward planetary wave propagation, an AO-like pattern develops in the troposphere in December and January during solar maximum. In the middle of November, one third of eddy-forced tropospheric mean meridional circulation and surface pressure tendency changes can be attributed to the stratosphere, whereas most of the polar surface pressure tendency changes from the end of November through the middle of December are related to tropospheric mechanical forcing changes. The weakening of the Brewer-Dobson circulation during solar maximum leads to dynamical heating in the tropical lower stratosphere, inducing circulation changes in the tropical troposphere and down to the surface that are strongest in January. The simulated tropospheric effects are identified as indirect effects from the stratosphere because the sea surface temperatures are identical in the solar maximum and minimum experiment. These results confirm those from other simplified model studies as well as results from observations.

Description: The seasonal and interannual variation in the lower mesospheric subtropical jet (LMSJ) and their dependence on the 11-year solar cycle are studied by comparing observational data with simulations by two general circulation models. In the model simulations, a strengthening of the LMSJs is found in both hemispheres during the winter under the solar maximum condition, similar to the observation. However the model responses are substantially smaller except for one case in the southern hemisphere. It is also found that the stronger LMSJ due to an enhanced solar forcing appears during the period which follows an increasing period of interannual variation. Analysis of the observed seasonal march of the LMSJ in each year shows two different regimes of behavior. For a successful simulation, the model should realistically reproduce the observed interannual variability as well as the climatological mean.

Description: We present a set of six 20 year experiments made with a state-of-the-art chemistry-climate model that incorporates the atmosphere from the surface to the lower thermosphere. The response of the middle atmosphere to the 11 year solar cycle, its impact on the troposphere, and especially the role of an externally prescribed stratospheric quasi-biennial oscillation (QBO) is investigated with NCAR's Whole Atmosphere Community Climate Model (WACCM3). The model experiments use either fixed solar cycle inputs or fixed solar cycle together with prescribed QBO phase. The annual mean solar response in temperature and ozone in the upper stratosphere is in qualitative agreement with other modeling and observational studies and does not depend on the presence of the imposed QBO. However, the solar response in the middle to lower stratosphere differs significantly for the two QBO phases. During solar maxima a weaker Brewer-Dobson circulation with relative downwelling, warming, and enhanced ozone occurs in the tropical lower stratosphere during QBO east conditions, while a stronger circulation, cooling, and decreased ozone exists during QBO west conditions. The net ozone increase during QBO east is the combined result of production and advection, whereas during QBO west the effects cancel each other and result in little net ozone changes. Especially during Southern Hemisphere late winter to early spring, the solar response at polar latitudes switches sign between the two QBO phases and qualitatively confirms observations and other recent model studies. During a poleward downward modulation of the polar night jet and a corresponding modulation of the Brewer-Dobson circulation in time, solar signals are detected all the way down to the extratropical troposphere. Possible limitations of the model experiments with respect to the fixed solar cycle conditions or the prescribed QBO phases, as well as the constant sea surface temperatures, are discussed.