NH Winter Research Articles

Uniform SST Warming Explains Most of the NH Winter Circulation and Blocking Response in a Warmer Climate

Uniform SST Warming Explains Most of the NH Winter Circulation and Blocking Response in a Warmer Climate

On the Nature and Origin of Atmospheric Annual and Semi-Annual Oscillations

This paper proposes a joint analysis of variations of global sea-level pressure (SLP) and of Earth’s rotation (RP), expressed as the coordinates of the rotation pole (m1, m2) and length of day (lod). We retain iterative singular spectrum analysis (iSSA) as the main tool to extract the trend, periods, and quasi periods in the data time series. SLP components are a weak trend, seven quasi-periodic or periodic components (∼130, 90, 50, 22, 15, 4, 1.8 years), an annual cycle, and its first three harmonics. These periods are characteristic of the space-time evolution of the Earth’s rotation axis and are present in many characteristic features of solar and terrestrial physics. The amplitudes of the annual SLP component and its three first harmonics decrease from 93 hPa for the annual to 21 hPa for the third harmonic. In contrast, the components with pseudo-periods longer than a year range between 0.2 and 0.5 hPa. We focus mainly on the annual and, to a lesser extent, the semi-annual components. The annual RP and SLP components have a phase lag of 152 days (half the Euler period). Maps of the first three components of SLP (that together comprise 85% of the data variance) reveal interesting symmetries. The trend is very stable and forms a triskeles structure that can be modeled as Taylor–Couette flow of mode 3. The annual component is characterized by a large negative anomaly extending over Eurasia in the NH summer (and the opposite in the NH winter) and three large positive anomalies over Australia and the southern tips of South America and South Africa in the SH spring (and the opposite in the SH autumn), forming a triskeles. The semi-annual component is characterized by three positive anomalies (an irregular triskeles) in the NH spring and autumn (and the opposite in the NH summer and winter), and in the SH spring and autumn by a strong stable pattern consisting of three large negative anomalies forming a clear triskeles within the 40–60∘ annulus formed by the southern oceans. A large positive anomaly centered over Antarctica, with its maximum displaced toward Australia, and a smaller one centered over Southern Africa, complement the pattern. Analysis of iSSA components of global sea level pressure shows a rather simple spatial distribution with the principal forcing factor being changes in parameters of the Earth’s rotation pole and velocity. The flow can probably best be modeled as a set of coaxial cylinders arranged in groups of three (triskeles) or four and controlled by Earth topography and continent/ocean boundaries. Flow patterns suggested by maps of the three main iSSA components of SLP (trend, annual, and semi-annual) are suggestive of Taylor–Couette flow. The envelopes of the annual components of SLP and RP are offset by four decades, and there are indications that causality is present in that changes in Earth rotation axis lead force pressure variations.

Remote Aerosol Simulated During the Atmospheric Tomography (ATom) Campaign and Implications for Aerosol Lifetime.

We investigate and assess how well a global chemical transport model (GEOS-Chem) simulates submicron aerosol mass concentrations in the remote troposphere. The simulated speciated aerosol (organic aerosol (OA), black carbon, sulfate, nitrate, and ammonium) mass concentrations are evaluated against airborne observations made during all four seasons of the NASA Atmospheric Tomography Mission (ATom) deployments over the remote Pacific and Atlantic Oceans. Such measurements over pristine environments offer fresh insights into the spatial (Northern [NH] and Southern Hemispheres [SH], Atlantic, and Pacific Oceans) and temporal (all seasons) variability in aerosol composition and lifetime, away from continental sources. The model captures the dominance of fine OA and sulfate aerosol mass concentrations in all seasons. There is a high bias across all species in the ATom-2 (NH winter) simulations; implementing recent updates to the wet scavenging parameterization improves our simulations, eliminating the large ATom-2 (NH winter) bias, improving the ATom-1 (NH summer) and ATom-3 (NH fall) simulations, but producing a model underestimate in aerosol mass concentrations for the ATom-4 (NH spring) simulations. Following the wet scavenging updates, simulated global annual mean aerosol lifetimes vary from 1.9 to 4.0days, depending on species. Aerosol lifetimes in each hemisphere vary by season, and are longest for carbonaceous aerosol during the southern hemispheric fire season. The updated wet scavenging parameterization brings simulated concentrations closer to observations and reduces global aerosol lifetime for all species, indicating the sensitivity of global aerosol lifetime and burden to wet removal processes.

Genesis of Upper-Tropospheric Anticyclones over the Asian–Western Pacific Sector from Tropical–Extratropical Interaction Perspective

Abstract Upper-tropospheric anticyclones (UTACs) emerge throughout the seasons with changing location and intensity. Here, the formation mechanisms of these UTACs, especially in the Asian–Australian–western Pacific sector, were investigated based on the diagnosis of the vorticity equation as well as the contribution of the planetary waves. During June–July–August (JJA), a vigorous UTAC corresponding to the South Asian high (SAH) forms over South Asia, to the south of the Tibetan Plateau, where intense heating associated with the Asian summer monsoon rainfall and the resultant baroclinic Rossby response are the important physical processes. Meanwhile, the produced anticyclonic vorticity is farther transported by the interhemispheric divergent wind toward the Southern Hemisphere (SH), creating the SH UTAC centered over the Maritime Continent. During December–January–February (DJF), two zonally elongated UTACs reside on each side of the equator (∼10° poleward), mainly over the Maritime Continent–western Pacific sector. Upon a closer look at the NH winter, we observed that the northern parts of UTAC cannot be explained by this vorticity balance alone. Diagnosis of the wave activity flux indicated that planetary waves emanating from the cold Eurasian continent converges around the northern parts of the UTAC with its peak in the NH winter, which weakens the subtropical jet, thus generating UTAC. Configuration of the SH summer (DJF) UTAC bears resemblance with that of SAH. These results suggest that the creation of anticyclonic vorticity and its interhemispheric transportation as well as the propagation of planetary wave are the selectively important agents for the genesis of seasonally varying UTACs. Significance Statement Recent studies have provided evidence that the South Asian high (formerly Tibetan high) is not a purely thermally driven system only maintained over the elevated Tibetan Plateau. This study aims to understand the physical processes responsible for the genesis of the upper-tropospheric anticyclone, especially in the Asian–Australian–western Pacific sector, throughout the season. During summer in the Northern Hemisphere, deep heating caused by South Asian monsoon rainfall plays a crucial role in the genesis of the South Asian high. The wintertime anticyclone emerging over the subtropical western North Pacific is caused via remote influences anchored with tropical convection and the cold Eurasian continent in which atmospheric teleconnections are important. These findings provide new avenues for research on tropical–extratropical interactions with respect to the formation and variability of important climate phenomena.

Transient Extratropical Response to Solar Ultraviolet Radiation in the Northern Hemisphere Winter

AbstractAn intermediate complexity general circulation model is used to investigate the transient response of the NH winter stratosphere to modulated ultraviolet (UV) radiation by imposing a stepwise, deliberately exaggerated UV perturbation and analyzing the lagged response. Enhanced UV radiation is accompanied by an immediate warming of the tropical upper stratosphere. The warming then spreads into the winter subtropics due to an accelerated Brewer–Dobson circulation in the tropical upper stratosphere. The poleward meridional velocity in the subtropics leads to an increase in zonal wind in midlatitudes between 20° and 50°N due to Coriolis torque. The increase in midlatitude zonal wind is accompanied by a dipole in Eliassen–Palm flux convergence, with decreased convergence near the winter pole and increased convergence in midlatitudes (where winds are strengthening due to the Coriolis torque); this dipole subsequently extends the anomalous westerlies to subpolar latitudes within the first 10 days. The initial radiatively driven acceleration of the Brewer–Dobson circulation due to enhanced shortwave absorption is replaced in the subpolar winter stratosphere by a wave-driven deceleration of the Brewer–Dobson circulation, and after a month the wave-driven deceleration of the Brewer–Dobson circulation encompasses most of the winter stratosphere. Approximately a month after UV is first modified, a significant poleward jet shift is evident in the troposphere. The results of this study may have implications for the observed stratospheric and tropospheric responses to solar variability associated with the 27-day solar rotation period, and also to solar variability on longer time scales.

Projected future changes in Marine Cold-Air Outbreaks associated with Polar Lows in the Northern North-Atlantic Ocean

Marine Cold-Air Outbreaks (MCAO) can be used as a large-scale measure of the potential for development of Polar Lows in the Northern North Atlantic Ocean during NH winter. We applied an MCAO index to 30 members of the Community Earth System Model Large Ensemble to investigate model-projected future changes in the Nordic and Barents Seas in response to anthropogenic climate change. In agreement with previous studies we found an overall decrease in the MCAO index due to increased tropospheric static stability. We also found a changed seasonal profile, with a stronger decrease in December–January than in February–March, effectively leading to the peak occurring in February rather than January in the Nordic Seas. In the Barents Sea, the reductions were only statistically significant in the autumn and spring, with the winter reduction due to increased static stability was partly balanced by a retracting sea-ice edge. The contribution from circulation changes in mean sea-level pressure was assessed by a cluster analysis in lower-dimensional phase-space, spanned by the projections onto the four leading Empirical Orthogonal Functions. While there was a small but statistically significant increase of the Atlantic Ridge-like pattern, the overall changes in MCAO were dominated by changes in temperature at the sea surface and aloft.

Role of the North Atlantic Oscillation in decadal temperature trends

Global temperatures have undergone periods of enhanced warming and pauses over the last century, with greater variations at local scales due to internal variability of the climate system. Here we investigate the role of the North Atlantic Oscillation (NAO) in decadal temperature trends in the Northern Hemisphere for periods with large decadal NAO trends. Using a regression based technique we find a best estimate that trends in the NAO more than halved (reduced by 57%, 5%–95%: 47%–63%) the winter warming over the Northern Hemisphere extratropics (NH; 30N–90N) from 1920–1971 and account for 45% (±14%) of the warming there from 1963–1995, with larger impacts on regional scales. Over the period leading into the so-called warming hiatus, 1989–2013, the NAO reduced NH winter warming to around one quarter (24%; 19%–31%) of what it would have been, and caused large negative regional trends, for example, in Northern Eurasia. Warming is more spatially uniform across the Northern Hemisphere after removing the NAO influence in winter, and agreement with multi-model mean simulated trends improves. The impact of the summer NAO is much weaker, but still discernible over Europe, North America and Greenland, with the downward trend in the summer NAO from 1988–2012 reducing warming by about a third in Northern Europe and a half in North America. A composite analysis using CMIP5 control runs suggests that the ocean response to prolonged NAO trends may increase the influence of decadal NAO trends compared to estimates based on interannual regressions, particularly in the Arctic. Results imply that the long-term NAO trends over the 20th century alternately masked or enhanced anthropogenic warming, and will continue to temporarily offset or enhance its effects in the future.

Similarities and dissimilarities between the last two deglaciations and interglaciations in the North Atlantic region

Sea surface temperatures (SSTs) recorded by alkenones and oxygen isotopes in the Alboran basin are used here to describe, at an unprecedented fine temporal resolution, the present interglaciation (PIG, initiated at 11.7 ka BP), the last interglaciation (LIG, onset approximately at 129 ka) and respective deglaciations. Similarities and dissimilarities in the progression of these periods are reviewed in comparison with ice cores and stalagmites. Cold spells coeval with the Heinrich events (H) described in the North Atlantic include multi-decadal scale oscillations not previously obvious (up to 4 °C in less than eight centuries within the stadials associated with H1 and H11, ca 133 ka and 17 ka respectively). These abrupt oscillations precede the accumulation of organic rich layers deposited when perihelion moves from alignment with NH spring equinox to the summer solstice, a reference for deglaciations. Events observed during the last deglaciation at 17 ka, 14.8 ka and 11.7 ka are reminiscent of events occurred during the penultimate deglaciation at ca 136 ka, 132 ka and 129 ka, respectively. The SST trend during the PIG is no more than 2 °C (from 20 °C to 18 °C; up to −0.2 °C/ka). The trend is steeper during the LIG, i.e. up to a 5 °C change from the early interglaciation to immediately before the glacial inception (from 23 °C to 18 °C; up to −0.4 °C/ka). Events are superimposed upon a long term trend towards colder SSTs, beginning with SST maxima followed by temperate periods until perihelion aligned with the NH autumn equinox (before ca 5.3 ka for the PIG and 121 ka for the LIG). A cold spell of around eight centuries at 2.8 ka during the PIG was possibly mimicked during the LIG at ca 118 ka by a SST fall of around 1 °C in a millennium. These events led interglacial SST to stabilise at around 18 °C. The glacial inception, barely evident at the beginning ca 115 ka (North Atlantic event C25, after perihelion passage in the NH winter solstice), culminated with a SST drop of at least 2 °C in two millennia (event C24, ca 111 ka). The Little Ice Age (0.7 ka) also occurred after the latest perihelion passage in the NH winter solstice and could be an example of how a glacial pre-inception event following an interglaciation might be.

The Contribution of Radiative Feedbacks to Orbitally Driven Climate Change

Abstract Radiative feedbacks influence Earth's climate response to orbital forcing, amplifying some aspects of the response while damping others. To better understand this relationship, the GFDL Climate Model, version 2.1 (CM2.1), is used to perform idealized simulations in which only orbital parameters are altered while ice sheets, atmospheric composition, and other climate forcings are prescribed at preindustrial levels. These idealized simulations isolate the climate response and radiative feedbacks to changes in obliquity and longitude of the perihelion alone. Analysis shows that, despite being forced only by a redistribution of insolation with no global annual-mean component, feedbacks induce significant global-mean climate change, resulting in mean temperature changes of −0.5 K in a lowered obliquity experiment and +0.6 K in a NH winter solstice perihelion minus NH summer solstice perihelion experiment. In the obliquity experiment, some global-mean temperature response may be attributable to vertical variations in the transport of moist static energy anomalies, which can affect radiative feedbacks in remote regions by altering atmospheric stability. In the precession experiment, cloud feedbacks alter the Arctic radiation balance with possible implications for glaciation. At times when the orbital configuration favors glaciation, reductions in cloud water content and low-cloud fraction partially counteract changes in summer insolation, posing an additional challenge to understanding glacial inception. Additionally, several systems, such as the Hadley circulation and monsoons, influence climate feedbacks in ways that would not be anticipated from analysis of feedbacks in the more familiar case of anthropogenic forcing, emphasizing the complexity of feedback responses.

Diagnosing Northern Hemisphere Jet Portrayal in 17 CMIP3 Global Climate Models: Twentieth-Century Intermodel Variability

Abstract The present study focuses on diagnosing the intermodel variability of nonzonally averaged NH winter jet stream portrayal in 17 global climate models (GCMs) from phase three of the Coupled Model Intercomparison Project (CMIP3). Relative to the reanalysis, the ensemble-mean 300-hPa Atlantic jet is too zonally extended and located too far equatorward in GCMs. The Pacific jet varies significantly between modeling groups, with large biases in the vicinity of the jet exit region that cancel in the ensemble mean. After seeking relationships between twentieth-century model wind biases and 1) the internal modes of jet variability or 2) tropical sea surface temperatures (SSTs), it is found that biases in upper-level winds are strongly related to an ENSO-like pattern in winter-mean tropical Pacific Ocean SST biases. The spatial structure of the leading modes of variability of the upper-level jet in the twentieth century is found to be accurately modeled in all 17 GCMs. Also, it is shown that Pacific model biases in the longitude of EOFs 1 and 2 are strongly linked to the modeled longitude of the Pacific jet exit, indicating that the improved characterization of the mean state of the Pacific jet may positively impact the modeled variability. This work suggests that improvements in model portrayal of the tropical Pacific mean state may significantly advance the portrayal of the mean state of the Pacific and Atlantic jets, which will consequently improve the modeled jet stream variability in the Pacific. To complement these findings, a companion paper examines the twenty-first-century GCM projections of the nonzonally averaged NH jet streams.

Some features of water vapor mixing ratio in tropical upper troposphere and lower stratosphere: Role of convection

Spatial and temporal variation of water vapor mixing ratio (WVMR) is examined for its association with the convective activity in upper troposphere and lower stratosphere over tropical region particularly Asian monsoon region (AMR) and Indonesian–Australian West Pacific region (IAWPR) using WVMR obtained from MLS satellite with simultaneous daily mean OLR from NOAA and daily mean wind from NCEP reanalysis. An examination of WVMR at various pressure levels during high water vapor regime (moist Phase) indicates that water vapor (WV) transport, in troposphere, is rather fast up to a level of ~147hPa. Seasonal variation of WVMR over tropical lower stratosphere (TLS) is noted to be closely associated with seasonal northward movement of intertropical convergence zone (ITCZ). Convection activity over AMR appears to be a prominent contributor to the moist phase of WVMR seasonal cycle in TLS. However, other tropical regions may also be contributing to the seasonal variability of WVMR. Low WV (dry) phase of the WVMR seasonal cycle in TLS observed during NH winter and early spring months may be caused by the appearance of extreme cold temperatures (≤191K) close to tropopause heights over IAWPR. Mechanisms that could cause such low temperatures over IAWPR are discussed. Intraseasonal oscillations with period of 30–40days are observed in WVMR at various pressure levels. At 100hPa level such oscillations are noted to be closely associated with similar oscillation in OLR and temperature. These observations suggest that variations in OLR (proxy of convection activity) produce such oscillation in WVMR. Present analysis thus report signature of convection in upward transport of WV, seasonal and intraseasonal oscillation in WVMR in upper troposphere and lower stratosphere (UTLS).

WACCM simulations of the mean circulation and trace species transport in the winter mesosphere

[1] Downwelling of air in the high latitude winter middle atmosphere causes perturbations in chemical composition. Species with sources in the mesosphere and lower thermosphere, such as nitric oxide and carbon monoxide, can be locally enhanced in the polar region. We investigate the origin of the downwelling air and the timescale for transport using the Whole Atmosphere Community Climate Model (WACCM). Analysis of the middle atmosphere transformed Eulerian mean (TEM) circulation, also called the residual circulation, is presented. This circulation gives the net air motion in the meridional and vertical directions due to the combined effects of the zonally averaged winds and the wave transport. The summer to winter circulation in the upper mesosphere during the solstice seasons includes strong upward motion at the summer pole and downward motion at the winter pole. During most winters, the air that is transported down by the circulation originates from low to middle latitudes in the upper mesosphere, 85–95 km; the timescale for this transport is 2–3 months. Trace species transport by molecular and eddy diffusion can bring high concentrations of thermospheric molecules into the winter mesosphere, where they are then brought to lower altitudes by the circulation. The magnitudes of the net tendencies due to advection by the TEM circulation and diffusion are similar. A case study of an active NH winter shows that the origin of the downwelling air in the TEM circulation is similar to that during average winters although the downwelling velocity is stronger by about a factor of two.

Impact of increasing resolution and a warmer climate on extreme weather from Northern Hemisphere extratropical cyclones

The effect of a warmer climate on the properties of extratropical cyclones is investigated using simulations of the ECHAM5 global climate model at resolutions of T213 (60 km) and T319 (40 km). Two periods representative of the end of the 20th and 21st centuries are investigated using the IPCC A1B scenario. The focus of the paper is on precipitation for the NH summer and winter seasons, however results from vorticity and winds are also presented. Similar number of events are identified at both resolutions. There are, however, a greater number of extreme precipitation events in the higher resolution run. The difference between maximum intensity distributions is shown to be statistically significant using a Kolmogorov—Smirnov test. A generalized Pareto distribution is used to analyse changes in extreme precipitation and wind events. In both resolutions, there is an increase in the number of extreme precipitation events in a warmer climate for all seasons, together with a reduction in return period. This is not associated with any increased vertical velocity, or with any increase in wind intensity in the winter and spring. However, there is an increase in wind extremes in the summer and autumn associated with tropical cyclones migrating into the extratropics.

Stratospheric Bimodality: Can the Equatorial QBO Explain the Regime Behavior of the NH Winter Vortex?

Abstract The Northern Hemisphere extended winter mean stratospheric vortex alternates between a strong and a weak state, which is manifested in a statistically significant bimodal distribution. In the end of the 1970s a regime change took place, increasing the frequency of the strong phase relative to the weak phase. This paper investigates the connection between the regime behavior of the vortex and the equatorial quasi-biennial oscillation (QBO) in three different datasets. Although there are some differences between the datasets, they agree regarding the general picture. It is found that stratospheric equatorial wind between 70 and 8 hPa shows a bimodal structure in the Northern Hemisphere winter. Such bimodality is nontrivial as it requires only weak variability in the amplitude. Unimodality is found above 8 hPa, where the semiannual oscillation dominates. A strong coincidence is found between strong (weak) vortex winters and winter in the westerly (easterly) QBO regime. Furthermore, the change of the vortex in the late 1970s can be related to a change in the QBO from a period with strong bimodality to a period with weak bimodality. Careful consideration of the statistical significance shows that this change in the QBO can be a random process simply related to the annual sampling of the QBO. Finally, previous findings of phase locking between the QBO and the annual cycle are considered; it is shown that the phase locking is related to the seasonal variations in the bimodality of the QBO.

Seasonal Influence of the Quasi-Biennial Oscillation on Stratospheric Jets and Rossby Wave Breaking

Abstract The influence of the stratospheric quasi-biennial oscillation (QBO) on the polar night jets (PNJs), subtropical easterly jets (SEJs), and associated Rossby wave breaking (RWB) is investigated using global meteorological analyses spanning 10 recent QBO cycles. The seasonal dependence of the descent of the QBO is shown by using five layered shear indices. It is found that the influence of the QBO is distinctive for each combination of QBO phase, season, and hemisphere (NH or SH). The following QBO westerly (W) minus easterly (E) differences in the PNJs were found to be significant at the 97% level: When a QBO W (E) maximum is in the lower stratosphere (∼500 K or ∼50 hPa), the NH winter PNJ is stronger (weaker), in agreement with previous results (mode A). Mode A does not appear to operate in other seasons in the NH besides DJF or in the SH in any season. When a QBO W (E) maximum is in the middle stratosphere (∼700–800 K or ∼10–20 hPa), the PNJ in the SH spring is stronger (weaker), also in agreement with previous results (mode B). It is found that mode B also operates in the NH spring. A third distinctive mode is found during autumn in both hemispheres: a QBO W (E) maximum in the middle stratosphere coincides with a weaker (stronger) PNJ (mode C). The signs of wind anomalies are the same at low and high latitudes for modes A and B, but are opposite for mode C. This sensitive dependence on QBO phase and season is consistent with the nonlinear nature of the interaction between planetary waves and the shape of the seasonal wind structures. During the solstices the meridional circulation associated with QBO connects primarily with the winter hemisphere, whereas during the equinoxes it is more symmetric about the equator. QBO W enhance the equatorial potential vorticity (PV) gradient maximum, but the time-mean maximum may be related to chronic instabilities in the subtropics. The equatorial PV gradient maximum and flanking RWB tend to be more pronounced in the Eastern Hemisphere in stratospheric analyses. When QBO W are in the middle stratosphere, the flanking PV gradient minima (SEJs) are enhanced and RWB is more frequent and symmetric about the equator. When QBO W are in the upper stratosphere, a strong seasonal asymmetry is seen, with enhanced RWB in the summer SEJ, primarily during boreal winter. This is consistent with an upward increase of summer to winter flow and modulation by a strong “first” and weak “second” semiannual oscillation.

Simulation of the climate impact of Mt. Pinatubo eruption using ECHAM5 – Part 1: Sensitivity to the modes of atmospheric circulation and boundary conditions

Abstract. The eruption of Mt. Pinatubo in the Philippines in June 1991 was one of the strongest volcanic eruptions in the 20th century and this well observed eruption can serve as an important case study to understand the subsequent weather and climate changes. In this paper, the most comprehensive simulations to date of the climate impact of Mt. Pinatubo eruption are carried out with prescribed volcanic aerosols including observed SSTs, QBO and volcanically induced ozone anomalies. This is also the first attempt to include all the known factors for the simulation of such an experiment. Here, the climate response is evaluated under different boundary conditions including one at a time, thereby, investigating the radiative and dynamical responses to individual and combined forcings by observed SSTs, QBO and volcanic effects. Two ensembles of ten members each, for unperturbed and volcanically perturbed conditions were carried out using the middle atmosphere configuration of ECHAM5 general circulation model. Our results show that the simulated climate response that may arise solely from aerosol forcing in lower stratospheric temperature is insensitive to the boundary conditions in the tropics and does not show some observed features such as the temperature signature of the QBO phases. Also, statistically significant positive anomalies in the high latitudes in NH winter of 1991/92 seen in our model simulations with prescribed observed SST and QBO phases as boundary conditions are consistent with the observations. To simulate realistically the lower stratospheric temperature response, one must include all the known factors. The pure QBO and ocean signatures in lower stratospheric temperature are simulated consistently with earlier studies. The indirect effect of the volcanic aerosols manifested as the winter warming pattern is not simulated in the ensemble mean of the experiments. Our analysis also shows that the response to El Niño conditions is very strong in the model and that it partially masks the effects due to volcanic forcing.

Response of the northern stratospheric polar vortex to the seasonal alignment of QBO phase transitions

This study considers the strength of the Northern Hemisphere Holton‐Tan effect (HTE) in terms of the phase alignment of the quasi‐biennial oscillation (QBO) with respect to the annual cycle. Using the ERA‐40 Reanalysis, it is found that the early winter (Nov–Dec) and late winter (Feb–Mar) relation between QBO phase and the strength of the stratospheric polar vortex is optimized for subsets of the 44‐year record that are chosen on the basis of the seasonality of QBO phase transitions at the 30 hPa level. The timing of phase transitions serves as a proxy for changes in the vertical structure of the QBO over the whole depth of the tropical stratosphere. The statistical significance of the Nov–Dec (Feb–Mar) HTE is greatest when 30 hPa QBO phase transitions occur 9–14 (4–9) months prior to the January of the NH winter in question. This suggests that there exists for both early and late winter a vertical structure of tropical stratospheric winds that is most effective at influencing the interannual variability of the polar vortex, and that an early (late) winter HTE is associated with an early (late) progression of QBO phase towards that structure. It is also shown that the seasonality of QBO phase transitions at 30 hPa varies on a decadal timescale, with transitions during the first half of the calendar year being relatively more common during the first half of the tropical radiosonde wind record. Combining these two results suggests that decadal changes in HTE strength could result from the changing seasonality of QBO phase transitions.

A Large Annual Cycle in Ozone above the Tropical Tropopause Linked to the Brewer–Dobson Circulation

Abstract Near-equatorial ozone observations from balloon and satellite measurements reveal a large annual cycle in ozone above the tropical tropopause. The relative amplitude of the annual cycle is large in a narrow vertical layer between ∼16 and 19 km, with approximately a factor of 2 change in ozone between the minimum (during NH winter) and maximum (during NH summer). The annual cycle in ozone occurs over the same altitude region, and is approximately in phase with the well-known annual variation in tropical temperature. This study shows that the large annual variation in ozone occurs primarily because of variations in vertical transport associated with mean upwelling in the lower stratosphere (the Brewer–Dobson circulation); the maximum relative amplitude peak in the lower stratosphere is collocated with the strongest background vertical gradients in ozone. A similar large seasonal cycle is observed in carbon monoxide (CO) above the tropical tropopause, which is approximately out of phase with ozone (associated with an oppositely signed vertical gradient). The observed ozone and CO variations can be used to constrain estimates of the seasonal cycle in tropical upwelling.

Seasonal cycles of O3, CO, and convective outflow at the tropical tropopause

Measurements from the SHADOZ ozonesonde network and Aura MLS are used to show that there are seasonal cycles in ozone (O3) and carbon monoxide (CO) at the tropical tropopause (∼17 km). A simple model, driven by cloud free radiative mass fluxes, is used to investigate the origin of these seasonal cycles. The seasonal cycle in O3 is due to seasonal changes in upwelling and high altitude convective outflow. The seasonal cycle in CO at 100 hPa is influenced by both seasonal changes in dynamics and seasonal changes in CO at lower altitudes. The age of air at 17 km varies from 40 days during NH winter to 70 days during NH summer. The seasonal cycle in clear sky radiative heating at the tropical tropopause is mainly due to seasonal cycles in O3 and temperature. The seasonal cycle in O3 can be expected to amplify the seasonal cycle in cold point temperature.

Formation of sedimentary bedding couplets in the Western Interior Seaway of North America—implications from climate system modeling

We use global climate simulations across one precessional cycle to investigate the effect of orbitally induced climatic changes on sedimentation in the Western Interior Seaway (WIS) of North America at the Cenomanian/Turonian boundary. The simulations include a control run with no orbital eccentricity and hence no precession cycle, and four runs with varying precession with an eccentricity of 0.05 having (1) northern spring equinox at perihelion, (2) northern winter solstice at perihelion, (3) northern fall equinox at perihelion, and (4) northern summer solstice at perihelion. These numeric climate simulations and field observations suggest that the WIS at the Cenomanian/Turonian boundary can be divided into three latitudinal units: (1) A northern unit (Alberta–Montana) between 51°N and 71°N paleolatitude where conditions remained constant under the influence of steady inflow of low salinity, cool waters which were devoid of calcareous plankton flowed in from the Arctic, preventing the development of bedding couplets. (2) A central unit (Wyoming–Colorado) between 41°N and 51°N paleolatitude where runoff from Western North America (WNA) was reduced by more than half when the northern hemisphere winter solstice coincided with perihelion, where bedding couplets are well developed. The central part of the WIS was characterized by warm saline waters with abundant calcareous plankton. However it experienced high summer surface runoff from the Sevier Highlands to the west during all orbital configurations except when the winters were unusually warm, with the northern hemisphere winter solstice occurring at perihelion. Seasonal dilution of the surface waters of the seaway may have resulted in formation of a “fresh water lid” with stratification of the water column throughout most of the precession cycle. When the northern hemisphere winter solstice was at perihelion, reduced runoff would promote vertical mixing. Concomitantly, a steady detrital sediment supply would occur in summer throughout the precession cycle except when the NH winter solstice was at perihelion, when it would be much reduced. Thus the marlstone of the limestone–marlstone couplets would represent most of the time of the precession cycle and the limestone layers would represent the time when the NH winter solstice was near perihelion. (3) A southern region (New Mexico–northern Mexico) from 21° to 42°N paleolatitude where the detrital sediment supply was much reduced and couplets are thicker and less well developed.