Wintertime Northern Research Articles

The impact of stratospheric aerosol intervention on the North Atlantic and Quasi-Biennial Oscillations in the Geoengineering Model Intercomparison Project (GeoMIP) G6sulfur experiment

Abstract. As part of the Geoengineering Model Intercomparison Project a numerical experiment known as G6sulfur has been designed in which temperatures under a high-forcing future scenario (SSP5-8.5) are reduced to those under a medium-forcing scenario (SSP2-4.5) using the proposed geoengineering technique of stratospheric aerosol intervention (SAI). G6sulfur involves introducing sulfuric acid aerosol into the tropical stratosphere where it reflects incoming sunlight back to space, thus cooling the planet. Here, we compare the results from six Earth-system models that have performed the G6sulfur experiment and examine how SAI affects two important modes of natural variability, the northern wintertime North Atlantic Oscillation (NAO) and the Quasi-Biennial Oscillation (QBO). Although all models show that SAI is successful in reducing global mean temperature as designed, they are also consistent in showing that it forces an increasingly positive phase of the NAO as the injection rate increases over the course of the 21st century, exacerbating precipitation reductions over parts of southern Europe compared with SSP5-8.5. In contrast to the robust result for the NAO, there is less consistency for the impact on the QBO, but the results nevertheless indicate a risk that equatorial SAI could cause the QBO to stall and become locked in a phase with permanent westerly winds in the lower stratosphere.

Equatorward Medium to Large‐Scale Traveling Ionospheric Disturbances of High Latitude Origin During Quiet Conditions

AbstractThis article presents observations of medium to large‐scale traveling ionospheric disturbances (TIDs) originating from high latitudes, and propagating across the equator into the opposite hemisphere in the African‐European sector during geomagnetically quiet conditions between 2010 and 2018. During the study period, four geomagnetically quiet days were selected each month. The Global Navigation Satellite Systems (GNSS) total electron content (TEC) data were used to obtain the two dimensional (2‐D) TEC residuals. We have identified seven transhemispheric TIDs from the days that were analyzed with five originating in the southern and the rest in northern wintertime hemisphere. The TIDs meridional propagation velocities, periods and wavelengths are in the range of 270–322 m/s, 48–80 min and 802–1,296 km, respectively. The horizontal propagation velocities and horizontal wavelengths ranges are cH = 120–274 m/s and λH = 379–1,104 km, respectively. Analysis of the conditions of the space weather environment rules out space weather as a source of the TIDs. While observations of the 4.3 μm brightness temperature (BT) from the Atmospheric Infrared Sounder (AIRS) instrument on board the NASA Aqua satellite show the existence of atmospheric gravity waves (AGWs) in the troposphere‐stratosphere region, it is suggested that the tertiary GWs from orographic forcing may be the likely source of the TIDs observed in this study.

The nighttime winter anomaly phenomenon observed by the in situ electron density measurements from the CSES satellite

The nighttime winter anomaly (NWA) phenomenon is studied using the in situ electron density measurements from the CSES (China Seismo-Electronmagnetic Satellite) satellite orbiting above the F2 layer peak height (hmF2) region. Observations in the present study indicates that: (1) The NWA phenomenon can appear at mid-latitudes in large longitude ranges above the hmF2 region during low solar activity period, with the most obvious part being located at the American longitude sector during northern winter time and at the Asian longitude sector during the southern winter time as mention in previous studies. (2) The NWA phenomenon can also appear in the equatorial region. The equatorial NWA is located at the American to Eurasian longitude sectors during northern winter time and at the eastern Pacific longitude sector during southern winter time, and its occurrence and distribution vary with solar activity, which is similar to that of the mid-latitude NWA. (3) The formation mechanism for the mid-latitude NWA can be explained by the ionosphere-plasmasphere coupling process, and the formation mechanism for the equatorial NWA can be explained by the wind-driven plasma transporting process. These results are helpful for the in-depth understanding of the topside ionosphere, and the in situ electron density measurements from the CSES satellite provide us good opportunities to study the ionospheric climatological phenomena due to its special and unified observation local times.

Comparing the effects of solar-related and terrestrial drivers on the northern polar vortex

Northern polar vortex experiences significant variability during Arctic winter. Solar activity contributes to this variability via solar irradiance and energetic particle precipitation. Recent studies have found that energetic electron precipitation (EEP) affects the polar vortex by forming ozone depleting NOxcompounds. However, it is still unknown how the EEP effect compares to variabilities caused by, e.g., solar irradiance or terrestrial drivers. In this study we examine the effects of EEP, solar irradiance, El-Niño-Southern Oscillation (ENSO), volcanic aerosols and quasi-biennial oscillation (QBO) on the northern wintertime atmosphere. We use geomagneticAp-index to quantify EEP activity, sunspot numbers to quantify solar irradiance, Niño 3.4 index for ENSO and aerosol optical depth for the amount of volcanic aerosols. We use a new composite dataset including ERA-40 and ERA-Interim reanalysis of zonal wind and temperature and multilinear regression analysis to estimate atmospheric responses to the above mentioned explaining variables in winter months of 1957–2017. We confirm the earlier results showing that EEP and QBO strengthen the polar vortex. We find here that the EEP effect on polar vortex is stronger and more significant than the effects of the other drivers in almost all winter months in most conditions. During 1957–2017 the considered drivers together explain about 25–35% of polar vortex variability while the EEP effect alone explains about 10–20% of it. Thus, a major part of variability is not due to the linear effect by the studied explaining variables. The positive EEP effect is particularly strong if QBO-wind at 30 hPa has been easterly during the preceding summer, while for a westerly QBO the EEP effect is weaker and less significant.

Impact of gradients at the martian terminator on the retrieval of ozone from SPICAM/MEx

Rapid variations of pressure, temperature and illumination at the day–night terminator have the potential to cause asymmetries in the abundance distribution of the atmosphere constituents along the line of sight (LOS) of a solar occultation experiment. Ozone, in particular, displays steep density gradients across the terminator of Mars due to photolysis. Nowadays, most of the retrieval algorithms for solar and stellar occultations rely on the assumption of a spherically symmetrical atmosphere. However, photochemically induced variations near sunrise/sunset conditions need to be taken into account in the retrieval technique in order to prevent inaccuracies.We investigated the impact of gradients along the LOS of the solar occultation experiment SPICAM/Mars Express for the retrieval of ozone under sunrise/sunset conditions. In order to test the impact of different gradients, we selected four occultations at sunrise and at sunset each. Sunset occultations are located near the equator, while sunrise observations are situated at high latitudes in the South, because of the geometry of the orbit.We used the diurnal variations in the ozone concentration obtained from a three-dimensional General Circulation Model (GEM-Mars) together with an adapted radiative transfer code (ASIMUT). The General Circulation Model (GCM) suggests that ozone variations strongly depend on latitude, altitude, and season. As shown by the model, near the equator and below 25 km, ozone changes only slightly with local time. Around 45 km, the density changes by several orders of magnitude across the terminator. At high latitudes in the South, during northern winter time, ozone variations at the terminator are negligible.The impact of gradients on ozone retrievals is strongly related to the local atmospheric structure as predicted by the GCM. Sunset ozone retrievals are smaller than retrievals obtained assuming a spherically symmetrical atmosphere, with a maximum change of about 20%. At sunrise, the impact of gradients on the retrievals is negligible. This behavior can be explained by the specific geometry of sunrise observations, all situated at high latitudes in the South.

Decadal Covariability of the Northern Wintertime Land Surface Temperature and Atmospheric Circulation

Abstract The decadal covariability of northern wintertime land surface temperature and 500-hPa geopotential anomalies is examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research and the Twentieth-Century Reanalyses over the twentieth century and a 996-yr preindustrial climate simulation from the Canadian Earth System Model. Based on the reanalysis data, the covariability is dominated by two leading maximum covariance analysis (MCA) modes. MCA1 is characterized by temperature anomalies over most of Canada, the eastern United States, Mexico, and Eurasian mid- to high latitudes, accompanied by anomalies of opposite sign elsewhere over northern landmasses. MCA2 features temperature anomalies over most of North America, Eurasia, and Greenland with opposite anomalies elsewhere. In the upper troposphere the synoptic vorticity fluxes reinforce the anomalous circulation, while in the lower troposphere advection by the anomalous mean flow offsets the eddy forcing and maintains the decadal temperature perturbation. The MCA1-associated variability has a broad spectrum over decadal–interdecadal time scales, while the MCA2-related variability has a significant power peak around 20 yr. The change of temperature and geopotential trends around 1990 tends to be a decadal-scale shift in winter and has significant features of the leading mode of the decadal covariability. The climate model has broadly similar decadal covariability, including the leading MCA patterns as well as the temporal evolution of the patterns. The decadal temperature and geopotential anomalies primarily covary with the North Atlantic Oscillation but also with the variability of the North Pacific index, while the Southern Oscillation index variability tends to be the least important predictor for the northern decadal temperature and geopotential anomalies.

Modeling polar ionospheric effects during the October–November 2003 solar proton events

At Ny Ålesund, Svalbard (78°54′N, 11°53′E, L ∼ 18), a narrowband VLF receiver was used to monitor the behavior of the amplitude of several high‐power transmitters located in the Northern Hemisphere under the influence of the solar proton events (SPE) of October/November 2003. We have used Sodankylä ion chemistry (SIC) atmospheric model profiles calculated at the midpoint location of the propagation paths in the northern wintertime polar region to investigate the radio propagation properties of several high‐latitude paths. Different paths showed different responses to the proton precipitation, but propagation modeling was broadly able to account for most of the positive and negative responses observed. Using the SIC‐based electron density profiles, we have been able to develop models of ionospheric effective height (h′) and sharpness (β) in order to describe the D region behavior as a function of proton flux, extending previous work which reported β and h′ values as functions of the X‐ray flux from solar flares. As a result of these models, our understanding of VLF propagation influenced by SPEs is such that VLF observations might be used to predict changes in the ionospheric D region electron density profiles during other particle precipitation events.

Some Diagnostics of the Northern Wintertime Climate Simulated by the ECHAM3 Model

Abstract The control climate of an atmospheric general circulation model, ECHAM3, is validated with emphasis on the wintertime planetary- and cyclone-scale wave propagation and storm tracks in the Northern Hemisphere. By applying a set of time filters, it is shown that ECHAM3 has too much variance in the period range of 6–10 days. Geographically, the North Pacific and North Atlantic storm tracks are too strong and extend too far into the western parts of the continents. This overestimated variance compensates the deficient low-frequency variability in ECHAM3, so that the total variance is well simulated by the model. These problems seem to have a relatively small effect on the simulation of cyclone-scale eddies. The cyclone climatology of ECHAM3 is studied using a simple method in which a cyclone is defined as a local minimum in the 1000-hPa height field. The climatology is found to be reasonably close to that observed. The propagation of planetary- and cyclone-scale waves in ECHAM3 is studied using vario...

The Northern Wintertime Divergence Extrema at 200 hPa and MSLP Cyclones as Simulated in the AMIP Integration by the ECMWF General Circulation Model

Divergence and convergence centers at 200 hPa and mean sea level pressure (MSLP) cyclones were located every 6 hr for a 10-yr general circulation model (GCM) simulation with the ECMWF (Cycle 36) for the boreal winters from 1980 to 1988. The simulation used the observed monthly mean sea surface temperature (SST) for the decade. Analysis of the frequency, location, and strength of these centers and cyclones gives insight into the dynamical response of the model to the varying SST. The results indicate that (1) the model produces reasonable climatologies of upper-level divergence and MSLP cyclones; (2) the model distribution of anomalies of divergence and convergence centers and MSLP cyclones is consistent with observations for the 1982-83 and 1986-87 El Nifio events; (3) the tropical Indian Ocean is the region of greatest divergence activity and interannual variability in the model; (4) the variability of the divergence centers is greater than that of the convergence centers; (5) strong divergence centers occur chiefly over the ocean in the midlatitudes but are more land-based in the tropics, except in the Indian Ocean; and (6) locations of divergence and convergence centers can be a useful tool for the intercomparison of global atmospheric simulations.

Rossby wave propagation into the tropics in two GFDL general circulation models

During the northern winter the eastern Pacific is characterized by upper level westerly flow extending from the equator into the midlatitudes of both hemispheres. Theoretical and simple modeling studies suggest that such a region should favor the penetration of Rossby waves into the tropics from higher latitudes. Observational results by Kiladis and Weickmann using 200 mb data indicate that Rossby waves do indeed propagate freely into the tropical eastern Pacific during the northern winter from the Asian jet exit region. They also confirmed that cross-equatorial dispersion of energy from the Northern into the Southern Hemisphere occurs frequently. The present study examines these interactions in climatological runs of two GFDL GCMs. The northern wintertime mean states of these models are characterized by a rather realistically simulated upper level westerly regime in the tropical Pacific. Despite the relative weakness of the Asian jet and wave activity with respect to observations, propagation of Rossby waves into the tropics is present in both models, and these waves are strongly positively tilted as seen in the observations. A momentum budget of the zonal wind and E vector diagnostics over the tropical Pacific indicate that these transients are an important component of the momentum balance of the equatorial westerlies in both the observations and in the models.

Predictability and finite‐time instability of the northern winter circulation

AbstractThe finite‐time instability and associated predictability of atmospheric of atmospheric circulations are defined in terms of the largest singular values, and associated singular vectors, of the linear evolution operator determined form given equations of motion. These quantities are calculated in both a barotropic and a three‐level quasi‐geostrophic model, using as basic states realistic large‐scale northern wintertime flows that represent the climatological state, regime composites, and specific realizations of these regimes. for time‐invariant basic states, the singular vectors are compared with the corresponding normal‐mode solutions; it is shown that the perturbations defined (at the initial time) by the singular vectors have much larger growth rates than the normal modes, and possess a more localized spatial structure.The regimes studied have opposite values of the Pacific/North American (PNA) index, and growth rates for the barotropic basis states appear to confirm earlier studies that the barotropic instability of the negative PNA states may be larger than the corresponding positive PNA states. The evolution of the singular‐vector perturbations, with emphasis on the vertical structure, is compared for time‐evolving and time‐invariant baroclinic basis states; the effects of nonlinearity are also discusses. It is shown that, in the baroclinic model, interactions between synoptic‐scale eddies in the time‐evolving basic state and in the perturbation field are fundamental for studying the predictability of transitions in the large‐scale circulation. Consequently results obtained from linear calculations using very smooth basic states cannot properly account for such predictability.These results form the basis of a technique used to initialize ensembles of forecasts made with a primitive‐equation model, and are described in the companion paper (Mureau et al. 1993).