Meridional Extent Research Articles

The Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength

Abstract. The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper-ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn–winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas Current and East Australian Current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations in the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent.

A Lagrangian theory of equatorial upwelling

An explicit expression is derived for the speed of equatorial upwelling by filtering out the inertial oscillations from the wind-forced, Lagrangian, dynamics north and south of the equator. The derivation focuses on the poleward motion of the steady states of the meridional dynamics of water columns of unit zonal length forced by a uniform, westward directed, wind stress. A significant advance is achieved by substituting the pseudo angular momentum for the zonal velocity in all equations. The derived expression for the speed is independent of the Ekman layer's depth and varies linearly with the wind stress. Its only free parameter is the initial meridional extent of the water column that straddles the equator. The theoretical speed matches available observations of the upwelling speed when the column's meridional extent is 350–400 km on each side of the equator.

A survey of westward‐propagating mixed Rossby–Gravity waves and quantification of their association with extratropical disturbances

AbstractIn this study we have conducted a survey of westward‐propagating mixed Rossby–Gravity (MRG) wave events in the upper troposphere and quantified their association with the intrusions of extratropical disturbances for the period 1979–2019. MRG wave events are identified by imposing the meridional structure of theoretical MRG waves onto the equatorial meridional winds at 200 hPa. In all, 2390 MRG wave events are identified and the majority (61%) of them occurred during the months of May–October, and 65% of the total MRG wave events occurred over the central–east Pacific and Atlantic Ocean domains. Not only the frequency of occurrence but also the amplitude, wavenumber and trapping scale of the MRG wave events are found to exhibit a clear seasonality. MRG wave events associated with intrusions of extratropical disturbances are identified as when the potential vorticity on the 350‐K isentropic surface at 15° latitude exceeded 1 Potential Vorticity Unit (PVU) in the vicinity of the MRG wave events. We find that 37% of the MRG wave events are intrusion MRG wave events and a large majority (88%) of such events occurred over the central–east Pacific and Atlantic Ocean domains. It is also noteworthy that nearly 70% of such intrusions occurred in the winter hemisphere where the westerly wind ducts are well developed. Over the central–east Pacific during northern hemispheric (NH) winter, it is observed that the intrusion MRG wave events have a bigger amplitude and have a larger meridional extent compared to non‐intrusion MRG wave events. They also exhibit a similar spatial scale as the extratropical disturbances implying that resonant interactions may be a primary mechanism for the genesis of MRG wave events. During NH summer, on the other hand, MRG wave events are primarily triggered by convective processes and the extratropical disturbances may be instrumental in amplifying their amplitude.

An Explanation for the Metric Dependence of the Midlatitude Jet‐Waviness Change in Response to Polar Warming

AbstractArctic amplification has been proposed to promote temperature extremes by slowing the midlatitude jet and increasing the amplitude of its meanders. Observational and modeling studies have used a variety of metrics to quantify jet waviness. These studies show conflicting changes in jet waviness depending on the metric used and period examined. Here, we evaluate common metrics for dry idealized model simulations in which we apply polar warming of varying depth and meridional extent. In all simulations, polar warming increases the spatial extent of jet meanders, but reduces the magnitudes of ridges and troughs within the wave. As a result, geometric and anomaly‐amplitude measures of jet waviness can yield opposing responses. This contrast between metrics is particularly evident when warming extends into the midlatitudes. In all simulations, midlatitude temperature anomalies weaken as the poles warm, suggesting that a wavier jet need not imply stronger temperature extremes.

Opposite Changes in Monsoon Precipitation and Low Pressure System Frequency in Response to Orographic Forcing

Abstract Monsoon low pressure systems (LPS) are synoptic-scale disturbances that form along the quasi-stationary trough of the larger-scale South Asian summer monsoon, producing a large fraction of total monsoon precipitation. Here, we use an Earth system model to investigate the influence of Tibetan and Himalayan orography (THO) on mean monsoon strength and LPS activity. The influence of THO height on LPS activity has not been investigated before. The model simulates decreased mean monsoon circulation and precipitation when THO is removed, but the number of LPS and the meridional extent of LPS activity increase; this is an unexpected, important finding given that LPS are principal rain-bearing systems of the monsoon. The decreased mean monsoon circulation and precipitation on removal of THO are attributed to enhanced mixing of dry extratropical air into the humid monsoon domain, as demonstrated by prior studies. The increase in LPS frequency and the meridional extent of LPS activity is attributed to the increase in magnitude and meridional extent of the low-level meridional shear of zonal wind, which previous work showed amplifies LPS through barotropic growth. Specifically, as monsoon precipitation decreases, the sensible heat-driven low-level trough intensifies and shifts equatorward; this strengthens the shear zone in which LPS grow. Conversely, increasing THO height decreases the magnitude and meridional extent of cyclonic shear over India, decreasing LPS frequency and the spatial extent of LPS activity while increasing total monsoon precipitation. These results demonstrate that LPS activity and total monsoon rainfall can undergo large, opposing changes in response to imposed forcings. Significance Statement Monsoon low pressure systems (LPS) are propagating atmospheric vortices that deliver a large fraction of seasonal-mean precipitation to agricultural regions of India. The influence of Tibetan and Himalayan orography (THO) on the South Asian monsoon is an active area of research, but its effect on LPS has remained unexplored. Here, a global climate model is used to simulate LPS characteristics for different heights of THO. Flattening the THO reduces total summer monsoon precipitation but increases the number of LPS and the spatial extent of LPS activity. We attribute these LPS changes to a decrease in the magnitude and meridional extent of the hydrodynamically unstable low-level cyclonic shear zone over South Asia that occurs when orography is flattened.

Determination of the chemical equator from GEOS-Chem model simulation: a focus on the tropical western Pacific region

Abstract. The tropical western Pacific (TWP) plays an important role in global stratosphere–troposphere exchange and is an active region of the interhemispheric transport (IHT). Common indicators for transport between the hemispheres like the tropical rain belt are too broad or lack precision in the TWP. In this paper, we provide a method to determine the atmospheric chemical equator (CE), which is a boundary for air mass transport between the two hemispheres in the tropics. This method used the model output from an artificial passive tracer simulated by the chemical transport model GEOS-Chem in the troposphere. We investigated the movement of the CE in the tropics, which indicates the migration of atmospheric circulation systems and air mass origins. Our results show the CE on different timescales, suggesting that the different features of the IHT in different regions are highly related to the variation in the circulation systems. We compared the CE with the tropical wind fields, indicating that the region of IHT does not coincide with the convergence of the 10 m wind fields in the tropical land sectors and the TWP region. We compared the CE with the atmospheric composition such as satellite data of CH4 and model simulation of sulfur hexafluoride (SF6). The results show that the CE and north–south gradient of CH4 in the Indian Ocean in January are well consistent with each other, which indicates the CE has good potential to estimate the IHT inferred by observations. We discussed the vertical extent and the meridional extent of the IHT. We find that the vertical structure above 2 km has a slight northern tilt in the Northern Hemisphere (NH) winter season and a southern tilt in the NH summer, meaning the seasonality of the migration of the CE at the lower altitude is larger than that at the higher altitude. The meridional extent of the CE indicates a narrow transition zone where IHT happens throughout the year. We find that the meridional extent above South America is larger compared to other regions. The distribution of the land–sea contrast plays an important role in the meridional extent of IHT. We focus on the TWP region and further compared the tropical rain belt with the CE. There is a broad region of high precipitation occurring in the TWP region, and it is difficult to determine the IHT by the rain belt. In the NH winter, the CE is not consistent with the tropical rain belt in the TWP but is confined to the southern branch of the peak of the rain belt. For the other seasons, both indicators of IHT agree.

Subtropical Impact on the Tropical Double-ITCZ Bias in the GFDL CM2.1 Model

Abstract The double–intertropical convergence zone (ITCZ) bias has been an outstanding problem among climate models for two decades. However, it remains unclear how much of this tropical bias is attributed to the extratropics and tropics itself, respectively. Applying the regional data assimilation (RDA) method, we follow up a previous study with a more advanced model of GFDL CM2.1 to quantify the influence of extratropical atmosphere on the double-ITCZ bias. Our study reveals that this tropical bias is influenced to a large extent by the extratropics between 20° and 30°, with little impact from the extratropics poleward of 30°. This vital role of subtropics in the double-ITCZ bias is likely determined by the meridional extent of Hadley circulation from zonal-mean perspective. Besides, the vital role of subtropics is also supported by wind–evaporation–SST feedback in the subtropical southeastern Pacific from a regional perspective.

Impact of the Latitude of Stratospheric Aerosol Injection on the Southern Annular Mode

AbstractThe impacts of Stratospheric Aerosol Injection (SAI) strategies on the Southern Annular Mode (SAM) are analyzed with the Community Earth System Model. Using a set of simulations with fixed single‐point SO2 injections we demonstrate the first‐order dependence of the SAM response on the latitude of injection, with the northern hemispheric and equatorial injections driving a response corresponding to a positive phase of SAM and the southern hemispheric injections driving a negative phase of SAM. We further demonstrate that the results can to first order explain the differences in the SAM responses diagnosed from the two recent large ensembles of geoengineering simulations utilizing more complex injection strategies – Geoengineering Large Ensemble and Assessing Responses and Impacts of Solar climate intervention on the Earth system with Stratospheric Aerosol Injection (GLENS and ARISE‐SAI) – as driven by the differences in the simulated sulfate aerosol distributions. Our results point to the meridional extent of aerosol‐induced lower stratospheric heating as an important driver of the sensitivity of the SAM response to the injection location.

Stirring and Mixing in the Wake of Angaur and Peleliu Islands, Palau

AbstractTemperature or salinity variability on isopycnals is referred to as spice, which acts as a tracer in the absence of mixing. The westward North Equatorial Current and eastward North Equatorial Countercurrent are incident upon Palau, where the meridional extent of the topography exceeds 100 km and the thermohaline structure on the up‐ and downstream sides of the islands can be distinct. Westward flow of high salinity water past two islands at the south point of Palau injects a spice anomaly into the wake in the form of a 15 km wide jet with changes of salinity of 0.1 psu across the headland and the jet. With three repeated surveys, several terms in the spice balance are estimated for the jet. Horizontal diffusivity has a typical value of about 11 m2 s−1 for 15‐km length scales, while the vertical diffusivity is about 10−5 m2 s−1 from the balance, with similar maximum estimates for the latter from a shear‐based parameterization. Typical vertical diffusivities at this latitude are about 10−6 m2 s−1, which is the area mean from the shear‐based parameterization. Amidst this typical background diffusion, the spice variance increases linearly in the wake over three successive surveys. The surveys occurred over similar but not identical areas, following the shifting flow. The spice anomaly is detected 100–200 km downstream and is consistent with the mesoscale flow obtained from altimetric absolute geostrophic currents. Spice anomalies are a useful way of tracing flows encountering abrupt topography.

Daytime F Region Echoes at Equatorial Ionization Anomaly Crest During Geomagnetic Quiet Period: Observations From Multi‐Instruments

AbstractBy using Qujing very high frequency radar (25.6°N, 103.7°E, magnetic latitude 16.1°N, magnetic longitude 177.0°E), daytime F region echoes were reported at equatorial ionization anomaly crest on 26 June 2020. Radar interferometry experiment was performed during the observation period. The observed results show that the spatial distributions of daytime F region echo pattern were about 150 km, 200 km, and 180 km in the zonal, meridional, and height extents, respectively. In addition, we observed a remarkable eastward drift of F region echoes with a mean velocity of about 50 m/s. To investigate the possible generation mechanism of daytime F region echoes, simultaneous occurrences of ionospheric disturbance and atmospheric gravity wave activities were presented by combining with the observations of ionosonde, global position system stations and FY‐4A satellite. The existence of gravity wave from the deep convective activities in the lower atmosphere was confirmed by using FY‐4A satellite measurements. The gravity wave signatures were also observed in the time series of virtual height deviations of sporadic E (dhES) and total electron content near the region where the F region echoes. We surmise that ionospheric disturbance in the F region could be excited during atmospheric gravity wave activities. Then the large‐scale ionospheric disturbance could further evolve into small‐scale irregularity producing radar echoes through the non‐linear cascade process.

Structure and Inter-Annual Variability of the Freshened Surface Layer in the Laptev and East-Siberian Seas During Ice-Free Periods

This work is focused on the structure and inter-annual variability of the freshened surface layer (FSL) in the Laptev and East-Siberian seas during ice-free periods. This layer is formed mainly by deltaic rivers among which the Lena River contributes about two thirds of the inflowing freshwater volume. Based on in situ measurements, we show that the area of this FSL during certain years is much greater than the area of FSL in the neighboring Kara Sea, while the total annual freshwater discharge to the Laptev and East-Siberian seas is 1.5 times less than to the Kara Sea (mainly from the estuaries of the Ob and Yenisei rivers). This feature is caused by differences in morphology of the estuaries and deltas. Shallow and narrow channels of the Lena Delta are limitedly affected by sea water. As a result, undiluted Lena discharge inflows to sea from multiple channels and forms relatively shallow plume, as compared to the Ob-Yenisei plume, which mixes with subjacent saline sea water in deep and wide estuaries. Due to small vertical extents of FSL in the Laptev and East-Siberian seas, wind conditions strongly affect its spreading and determine its significant inter-annual variability, as compared to relatively stable FSL in the Kara Sea. During years with prevailing western and northern winds, FSL is localized in the southern parts of the Laptev and East-Siberian seas due to southward Ekman transport, meridional extent (<250 km) and area (∼250,000 km2) of FSL are relatively small. During years with strong eastern winds FSL spreads northward over large area (up to 500,000 km2), its meridional extent increases up to 500–700 km. At the same time, area and position of FSL do not show any dependence on significant variability of the annual river discharge volume and ice coverage during warm season.

The interaction of deep convection with the general circulation in Titan’s atmosphere. Part 2: Impacts on the climate

The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts–Miller (SBM) moist convection parameterization scheme. We vary the reference relative humidity (RHSBM) and relaxation timescale of convection (τ) parameters of the SBM scheme. Titan’s atmosphere is mostly insensitive to changes in τ, but convective instability and precipitation are highly impacted by changes in RHSBM. Convection behavior changes from infrequent (<1 per Titan year), intense events at summer solstice that quickly encompass the entire globe at low RHSBM to near-continuous precipitation at the poles during summer at high RHSBM (85%). The intermediate regime (RHSBM=70%–80%) consists of frequent events (∼10 per Titan year) of moderate intensity that are limited in meridional extent to their respective hemisphere. Using results from the Titan Regional Atmospheric Modeling System (TRAMS) and observations, we tune the parameters of the SBM parameterization with optimum values of RH=80% and τ=28800 s. We present a simulated decadal climatology that qualitatively matches observations of Titan’s humidity and cloud activity and generally resembles previous results with TAM. Comparing this simulation to one without moist convection demonstrates that convection strengthens the meridional circulation, warms the mid-levels and cools the surface at the poles, and magnifies zonal-mean global moisture anomalies.

Climate variability of the East Asian winter monsoon and associated extratropical-tropical interaction: a review.

The East Asian winter monsoon (EAWM) is one of the most important climate systems during boreal winter. It has a large meridional extent and provides a vital bridge between the mid-to-high latitudes and the tropics. This paper reviews the latest research on the climate variability of the EAWM, with particular emphasis on the associated processes of extratropical-tropical interaction. Some new results have been achieved in understanding the variation of the EAWM on interdecadal, interannual, and intraseasonal time scales, such as the interdecadal strengthening in the early 21st century, the unstable impact of El Niño-South Oscillation (ENSO) on the EAWM, and the intraseasonal strong cold events in East Asia. In addition, understanding of the extratropical-tropical interactions related to the EAWM has improved in several aspects, including the impacts of the EAWM on ENSO-related climate effects, and the variation of regional Hadley circulation over the western Pacific and its climate impacts in the Asia-Australia region. Moreover, recent work has systematically evaluated the capacity of climate models to simulate the EAWM, including its climatology, interannual to interdecadal variations, and its relationship with ENSO. Finally, some scientific issues regarding our understanding of the EAWM are proposed for future investigation.

Southward Cold Airmass Flux Associated with the East Asian Winter Monsoon: Diversity and Impacts

AbstractThe winter monsoon has strong impacts on East Asia via latitude-crossing southward cold airmass fluxes called cold air outbreaks (CAOs). CAOs have a high diversity in terms of meridional extent and induced weather. Using the daily cold airmass flux normalized at 50° and 30°N during 1958–2016, we categorize the CAOs into three groups: high–middle (H–M), high–low (H–L), and middle–low (M–L) latitude events. The H–L type is found to have the longest duration, and the M–L type is prone to strong CAOs regarding normalized intensity. The H–L and H–M events feature a large-scale dipole pattern of cold airmass flux over high-latitude Eurasia; the former events feature relatively strong anticyclonic circulation over Siberia, while the latter events feature cyclonic circulation over northeastern Asia. In contrast, the M–L events are characterized by a cyclonic anomaly over northeastern Asia but no obvious high-latitude precursor. The H–L events have the greatest cold anomaly in airmasses near the surface, and the M–L events mainly feature a strong northerly wind. As a result, the H–L events induce widespread long-lasting low temperatures over East Asia, while the M–L events induce a sharp temperature drop at mainly low latitudes. Both H–L and M–L events couple with the MJO to enhance rainfall over the South China Sea, while H–M events increase rainfall over southern China. Moreover, the occurrences of H–L and M–L events have experienced a long-term decrease since the 1980s that has induced a stronger warming trend in the cold extremes than in the winter mean temperature at mid–low latitudes over East Asia.

Aircraft-based observation of meteoric material in lower-stratospheric aerosol particles between 15 and 68° N

Abstract. We analyse aerosol particle composition measurements from five research missions between 2014 and 2018 to assess the meridional extent of particles containing meteoric material in the upper troposphere and lower stratosphere (UTLS). Measurements from the Jungfraujoch mountaintop site and a low-altitude aircraft mission show that meteoric material is also present within middle- and lower-tropospheric aerosol but within only a very small proportion of particles. For both the UTLS campaigns and the lower- and mid-troposphere observations, the measurements were conducted with single-particle laser ablation mass spectrometers with bipolar-ion detection, which enabled us to measure the chemical composition of particles in a diameter range of approximately 150 nm to 3 µm. The five UTLS aircraft missions cover a latitude range from 15 to 68∘ N, altitudes up to 21 km, and a potential temperature range from 280 to 480 K. In total, 338 363 single particles were analysed, of which 147 338 were measured in the stratosphere. Of these total particles, 50 688 were characterized by high abundances of magnesium and iron, together with sulfuric ions, the vast majority (48 610) in the stratosphere, and are interpreted as meteoric material immersed or dissolved within sulfuric acid. It must be noted that the relative abundance of such meteoric particles may be overestimated by about 10 % to 30 % due to the presence of pure sulfuric acid particles in the stratosphere which are not detected by the instruments used here. Below the tropopause, the observed fraction of the meteoric particle type decreased sharply with 0.2 %–1 % abundance at Jungfraujoch, and smaller abundances (0.025 %–0.05 %) were observed during the lower-altitude Canadian Arctic aircraft measurements. The size distribution of the meteoric sulfuric particles measured in the UTLS campaigns is consistent with earlier aircraft-based mass-spectrometric measurements, with only 5 %–10 % fractions in the smallest particles detected (200–300 nm diameter) but with substantial (&gt; 40 %) abundance fractions for particles from 300–350 up to 900 nm in diameter, suggesting sedimentation is the primary loss mechanism. In the tropical lower stratosphere, only a small fraction (&lt; 10 %) of the analysed particles contained meteoric material. In contrast, in the extratropics the observed fraction of meteoric particles reached 20 %–40 % directly above the tropopause. At potential temperature levels of more than 40 K above the thermal tropopause, particles containing meteoric material were observed in much higher relative abundances than near the tropopause, and, at these altitudes, they occurred at a similar abundance fraction across all latitudes and seasons measured. Above 440 K, the observed fraction of meteoric particles is above 60 % at latitudes between 20 and 42∘ N. Meteoric smoke particles are transported from the mesosphere into the stratosphere within the winter polar vortex and are subsequently distributed towards low latitudes by isentropic mixing, typically below a potential temperature of 440 K. By contrast, the findings from the UTLS measurements show that meteoric material is found in stratospheric aerosol particles at all latitudes and seasons, which suggests that either isentropic mixing is effective also above 440 K or that meteoric fragments may be the source of a substantial proportion of the observed meteoric material.

Dependence of Atmospheric Transport Into the Arctic on the Meridional Extent of the Hadley Cell

AbstractRecent studies have shown a large spread in the transport of atmospheric tracers into the Arctic among a suite of chemistry climate models and have suggested that this is related to the spread in the meridional extent of the Hadley Cell (HC). Here we examine the HC‐transport relationship using an idealized model, where we vary the mean circulation and isolate its impact on transport to the Arctic. It is shown that the poleward transport depends on the relative position between the northern edge of the HC and the tracer source, with maximum transport occurring when the HC edge lies near the middle of the source region. Such dependence highlights the critical role of near‐surface transport by the Eulerian mean circulation rather than eddy mixing in the free troposphere and suggests that variations in the HC edge and the tracer source region are both important for modeling Arctic composition.

West African Summer Monsoon Precipitation Variability as Represented by Reanalysis Datasets

Focusing on West Africa, a region riddled with in situ data scarcity, we evaluate the summer monsoon monthly rainfall characteristics of five global reanalysis datasets: ERA5, ERA-Interim, JRA-55, MERRA2, and NCEP-R2. Their performance in reproducing the West African monsoon (WAM) climatology, interannual variability, and long-term trends for the main monsoon months are compared to gauge-only and satellite products. We further examine their ability to reproduce teleconnections between sea surface temperatures and monsoon rainfall. All reanalyses are able to represent the average rainfall patterns and seasonal cycle; however, regional biases can be marked. ERA5, ERA-Interim, and NCEP-R2 underestimate rainfall over areas of peak rainfall, with ERA5 showing the strongest underestimation, particularly over the Guinea Highlands. The meridional northward extent of the monsoon rainband is well captured by JRA-55 and MERRA2 but is too narrow in ERA-Interim, for which rainfall stays close to the Guinea Coast. Differences in rainband displacement become particularly evident when comparing strong El Niño Southern Oscillation (ENSO) years, where all reanalyses except ERA-Interim reproduce wetter Sahelian conditions for La Niña, while overestimating dry conditions at the coast except for NCEP-R2. Precipitation trends are not coherent across reanalyses and magnitudes are generally overestimated compared to observations, with only JRA-55 and NCEP-R2 displaying the expected positive trend in the Sahel. ERA5 generally outperforms ERA-Interim, highlighting clear improvements over its predecessor. Ultimately, we find the strengths of reanalyses to strongly vary across the region.

Distinct Eurasian climate anomalies associated with strong and weak MJO events

AbstractThe MJO‐related tropical Indian Ocean outgoing longwave radiation anomalies display a bimodal distribution, indicating two types of MJO events. The present study distinguishes 14 strong and 16 weak MJO events during the 37 winters from 1979 to 2016 and compares the circulation and climate over the mid‐latitude Eurasia between strong and weak MJO events. Cold anomalies appear over Siberia and East Asia during strong MJO events when the MJO‐related convection is located over the tropical Indian Ocean, whereas the temperature anomalies are weak over Eurasia during weak MJO events. The cold anomalies over Eurasia in strong MJO events are associated with the development of surface anticyclonic anomalies and the accompanying northerly wind anomalies. On one hand, the enhancement of surface anticyclonic anomalies can be attributed to the poleward Rossby wave packets triggered by the MJO convection over the tropical Indian Ocean. On the other hand, the strong convective heating over the tropical Indian Ocean can induce anomalous meridional overturning circulation between the tropics and mid‐latitudes. The anomalous divergent winds associated with the convective heating can reach western Siberia and the descending branch of the vertical cell contributes to the enhancement of surface anticyclonic anomalies and northerly wind anomalies, leading to cold anomalies over Siberia and East Asia. In weak MJO events, the poleward Rossby waves are weak and anomalous meridional overturning is of limited meridional extent. As a result, the MJO‐related wind anomalies are weak and the associated temperature anomalies are small over large areas of Eurasia.

Evaluation of the Quasi‐Biennial Oscillation in global climate models for the SPARC QBO‐initiative

AbstractQuasi‐biennial oscillations (QBOs) in thirteen atmospheric general circulation models forced with both observed and annually repeating sea surface temperatures (SSTs) are evaluated. In most models the QBO period is close to, but shorter than, the observed period of 28 months. Amplitudes are within ±20% of the observed QBO amplitude at 10 hPa, but typically about half of that observed at lower altitudes (50 and 70 hPa). For almost all models, the oscillation's amplitude profile shows an overall upward shift compared to reanalysis and its meridional extent is too narrow. Asymmetry in the duration of eastward and westward phases is reasonably well captured, though not all models replicate the observed slowing of the descending westward shear. Westward phases are generally too weak, and most models have an eastward time mean wind bias throughout the depth of the QBO. The intercycle period variability is realistic and in some models is enhanced in the experiment with observed SSTs compared to the experiment with repeated annual cycle SSTs. Mean periods are also sensitive to this difference between SSTs, but only when parametrized non‐orographic gravity wave (NOGW) sources are coupled to tropospheric parameters and not prescribed with a fixed value. Overall, however, modelled QBOs are very similar whether or not the prescribed SSTs vary interannually. A portrait of the overall ensemble performance is provided by a normalized grading of QBO metrics. To simulate a QBO, all but one model used parametrized NOGWs, which provided the majority of the total wave forcing at altitudes above 70 hPa in most models. Hence the representation of NOGWs either explicitly or through parametrization is still a major uncertainty underlying QBO simulation in these present‐day experiments.

Trends in northern midlatitude atmospheric wave power from 1950 to 2099

In a warming climate, atmospheric wave activity and associated weather patterns may change, although conflicting results have been reported on this topic. Additionally, atmospheric wave changes in a future climate have mainly focused on waves of a specified spatial scale, rather than a particular spatiotemporal scale. Here, changes in the variability of Rossby waves of multiple spatiotemporal scales are analyzed using the wavenumber-frequency power spectrum, a tool commonly applied to analyze atmospheric equatorial waves. Daily 500 hPa geopotential height data over 40°–60°N from historical (1950–2005) and future (2006–2099) simulations from 20 models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) under the RCP8.5 scenario were analyzed. When compared to the historical period, the late 21st century climate projections showed a decline in spectral power for both eastward and westward propagating waves with wavenumbers greater than 8 that spanned over all frequencies in all seasons, but an increase in mean power for eastward propagating waves with wavenumbers 1–7 over all frequencies was shown in winter and spring. This increase in power was accompanied by increased variance, i.e., an increased meridional extent of 500 hPa ridges and troughs, and was the result of increases in the mean number of high amplitude events and duration of activity within this wave band. These results indicate that large-scale (~ 104 km) eastward propagating weather systems may intensify with higher amplitudes for ridges and troughs, while short-scale (102–103 km) weather systems may decrease in their intensity due to reduced variability in the late 21st century under the high emissions scenario. Potential mechanisms for these changes are discussed, including enhanced Arctic warming and midlatitude-tropical interactions.