Atmospheric Stationary Waves Research Articles

Moderate Indian Ocean Dipole dominates spring fire weather conditions in southern Australia

Patterns of the Indian Ocean Dipole (IOD) exhibit strong diversity, ranging from being dominated by the western tropical Indian ocean (WTIO) to by the eastern tropical Indian ocean. How the different types of the IOD variability patterns affect Australian fires differently is unknown, nor is it certain how the impacts may change under greenhouse warming. Here, we find that the moderate IOD, dominated by WTIO sea surface temperature (SST) variability, plays a primary role in affecting southern Australian fire weather conditions during austral spring. During a positive moderate IOD, broad-scaled warm SST anomalies in WTIO force an atmospheric stationary Rossby wave with a high-pressure anomaly over southern Australia. This elevated pressure and associated anomalous atmospheric conditions provide suitable fire weather with hot, dry, and windy conditions, raising fire risks in southern Australia. Such impact is distinctively different from that strong IOD-induced. As predicted by climate models, decreased moderate IOD variability in the future will result in weakened Australian fire weather responses.

Blocking and General Circulation in GFDL Comprehensive Climate Models

Abstract This study examines the climatology and dynamics of atmospheric blocking, and the general circulation features that influence blocks in GFDL’s atmosphere-only (AM4) and coupled atmosphere–ocean (CM4) comprehensive models. We compare AM4 and CM4 with reanalysis, focusing on winter in the Northern Hemisphere. Both models generate the correct blocking climatology and planetary-scale signatures of the stationary wave. However, at regional scales some biases exist. In the eastern Pacific and over western North America, both models generate excessive blocking frequency and too strong of a stationary wave. In the Atlantic, the models generate too little blocking and a weakened stationary wave. A block-centered compositing analysis of block-onset dynamics reveals that the models 1) produce realistic patterns of high-frequency (1–6-day) eddy forcing and 2) capture the notable differences in the 500-hPa geopotential height field between Pacific and Atlantic blocking. However, the models fail to reproduce stronger wave activity flux convergence in the Atlantic compared to the Pacific. Overall, biases in the blocking climatology in terms of location, frequency, duration, and area are quite similar between AM4 and CM4 despite the models having large differences in sea surface temperatures and climatological zonal circulation. This could suggest that other factors could be more dominant in generating blocking biases for these GCMs. Significance Statement Atmospheric blocks are persistent high pressure systems that can lead to hazardous weather. Historically, climate models have had trouble capturing blocks, but recent changes in the models might lead to improvements. As such, the work herein investigates the spatial distribution, prevalence, duration, size, and dynamics of wintertime blocking in recent NOAA climate models. Overall, these models capture the long-term-average spatial pattern of blocking, and properly reproduce key dynamical features. However, the models produce too much blocking in the western United States, and too little over the northern Atlantic Ocean and Europe. These blocking biases are consistent with atmospheric stationary waves biases, but not jet stream bias. This downplays the role of jet biases in the models being responsible for blocking biases.

Winter Warming in North America Induced by Urbanization in China

AbstractRapid urbanization in China in the past several decades has been shown to affect surface air temperature (SAT) locally, but its global impact is unclear. Using a global climate model, we have investigated the potential impact of urbanization in China on SAT at the global scale. The expansion of urban areas in China from 1985 to 2017 leads to winter surface air warming by as much as 0.8°C in North America (NA), largely caused by warm temperature advection due to global‐scale atmospheric circulation changes. The East Asian jet stream moves northward in eastern and northeastern China where the expansion of urban areas has increased SAT and sensible heat flux, forcing change of the atmospheric stationary waves and so the structure of the jet streams.

The Impact of SST Biases in the Tropical East Pacific and Agulhas Current Region on Atmospheric Stationary Waves in the Southern Hemisphere

AbstractClimate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) vary significantly in their ability to simulate the phase and amplitude of atmospheric stationary waves in the midlatitude Southern Hemisphere. These models also suffer from a double intertropical convergence zone (ITCZ), with excessive precipitation in the tropical eastern South Pacific, and many also suffer from a biased simulation of the dynamics of the Agulhas Current around the tip of South Africa. The intermodel spread in the strength and phasing of SH midlatitude stationary waves in the CMIP archive is shown to be significantly correlated with the double-ITCZ bias and biases in the Agulhas Return Current. An idealized general circulation model (GCM) is used to demonstrate the causality of these links by prescribing an oceanic heat flux out of the tropical east Pacific and near the Agulhas Current. A warm bias in tropical east Pacific SSTs associated with an erroneous double ITCZ leads to a biased representation of midlatitude stationary waves in the austral hemisphere, capturing the response evident in CMIP models. Similarly, an overly diffuse sea surface temperature gradient associated with a weak Agulhas Return Current leads to an equatorward shift of the Southern Hemisphere jet by more than 3° and weak stationary wave activity in the austral hemisphere. Hence, rectification of the double-ITCZ bias and a better representation of the Agulhas Current should be expected to lead to an improved model representation of the austral hemisphere.

Hypersensitivity of glacial summer temperatures in Siberia

Abstract. Climate change in Siberia is currently receiving a lot of attention because large permafrost-covered areas could provide a strong positive feedback to global warming through the release of carbon that has been sequestered there on glacial–interglacial timescales. Geological evidence and climate model experiments show that the Siberian region also played an exceptional role during glacial periods. The region that is currently known for its harsh cold climate did not experience major glaciations during the last ice age, including its severest stages around the Last Glacial Maximum (LGM). On the contrary, it is thought that glacial summer temperatures were comparable to the present day. However, evidence of glaciation has been found for several older glacial periods. We combine LGM experiments from the second and third phases of the Paleoclimate Modelling Intercomparison Project (PMIP2 and PMIP3) with sensitivity experiments using the Community Earth System Model (CESM). Together, these climate model experiments reveal that the intermodel spread in LGM summer temperatures in Siberia is much larger than in any other region of the globe and suggest that temperatures in Siberia are highly susceptible to changes in the imposed glacial boundary conditions, the included feedbacks and processes, and to the model physics of the different components of the climate model. We find that changes in the circumpolar atmospheric stationary wave pattern and associated northward heat transport drive strong local snow and vegetation feedbacks and that this combination explains the susceptibility of LGM summer temperatures in Siberia. This suggests that a small difference between two glacial periods in terms of climate, ice buildup or their respective evolution towards maximum glacial conditions can lead to strongly divergent summer temperatures in Siberia, allowing for the buildup of an ice sheet during some glacial periods, while during others, above-freezing summer temperatures preclude a multi-year snowpack from forming.

Land-atmosphere-ocean coupling associated with the Tibetan Plateau and its climate impacts.

This paper reviews recent advances regarding land–atmosphere–ocean coupling associated with the Tibetan Plateau (TP) and its climatic impacts. Thermal forcing over the TP interacts strongly with that over the Iranian Plateau, forming a coupled heating system that elevates the tropopause, generates a monsoonal meridional circulation over South Asia and creates conditions of large-scale ascent favorable for Asian summer monsoon development. TP heating leads to intensification and westward extension (northward movement) of the South Asian High (Atlantic Intertropical Convergence Zone), and exerts strong impacts on upstream climate variations from North Atlantic to West Asia. It also affects oceanic circulation and buoyancy fields via atmospheric stationary wave trains and air–sea interaction processes, contributing to formation of the Atlantic Meridional Overturning Circulation. The TP thermal state and atmospheric–oceanic conditions are highly interactive and Asian summer monsoon variability is controlled synergistically by internal TP variability and external forcing factors.

Role of the Tian Shan Mountains and Pamir Plateau in Increasing Spatiotemporal Differentiation of Precipitation over Interior Asia

Numerical simulations were conducted to determine the impact of the Tian Shan Mountains and Pamir Plateau on arid conditions over interior Asia. These topographies are crucial for the differentiation of the precipitation seasonality among the subregions in the west, east, and north of the Tian Shan Mountains and Pamir Plateau, namely, arid central Asia, the Tarim basin, and the northern plains. Before the uplift of the Tian Shan Mountains and Pamir Plateau, the precipitation seasonality over the east arid subregion was consistent with that over the west arid subregion, with maximum rainfall in spring and winter and minimum rainfall in summer. After the uplift of the Tian Shan Mountains and Pamir Plateau, the original precipitation seasonality in the west was strengthened. As the precipitation in the east arid subregion increased in summer but decreased in winter and spring, the precipitation seasonality in the east changed to peak in summer, while the precipitation in the north arid subregion showed the opposite change. The precipitation alteration corresponded well with the change of vertical motion. With the modulation of atmospheric stationary waves, the remote East Asian monsoon was also impacted. Though enhanced southerly wind blew over East Asia, the monsoon precipitation over the east coast of China and subtropical western Pacific Ocean was significantly reduced as an anticyclonic circulation appeared. The Tian Shan Mountains and Pamir Plateau also contributed to the intensification of the East Asian winter monsoon.

Arctic warming induced by the Laurentide Ice Sheet topography

Abstract. It is well known that ice sheet–climate feedbacks are essential for realistically simulating the spatiotemporal evolution of continental ice sheets over glacial–interglacial cycles. However, many of these feedbacks are dependent on the ice sheet thickness, which is poorly constrained by proxy data records. For example, height estimates of the Laurentide Ice Sheet (LIS) topography at the Last Glacial Maximum (LGM; ∼ 21 000 years ago) vary by more than 1 km among different ice sheet reconstructions. In order to better constrain the LIS elevation it is therefore important to understand how the mean climate is influenced by elevation discrepancies of this magnitude. Here we use an atmospheric circulation model coupled to a slab-ocean model to analyze the LGM surface temperature response to a broad range of LIS elevations (from 0 to over 4 km). We find that raising the LIS topography induces a widespread surface warming in the Arctic region, amounting to approximately 1.5 ∘C per km of elevation increase, or about 6.5 ∘C for the highest LIS. The warming is attributed to an increased poleward energy flux by atmospheric stationary waves, amplified by surface albedo and water vapor feedbacks, which account for about two-thirds of the total temperature response. These results suggest a strong feedback between continental-scale ice sheets and the Arctic temperatures that may help constrain LIS elevation estimates for the LGM and explain differences in ice distribution between the LGM and earlier glacial periods.

Spring Land Surface and Subsurface Temperature Anomalies and Subsequent Downstream Late Spring‐Summer Droughts/Floods in North America and East Asia

AbstractSea surface temperature (SST) variability has been shown to have predictive value for land precipitation, although SSTs are unable to fully predict intraseasonal to interannual hydrologic extremes. The possible remote effects of large‐scale land surface temperature (LST) and subsurface temperature (SUBT) anomalies in geographical areas upstream and closer to the areas of drought/flood have largely been ignored. Here evidence from climate observations and model simulations addresses these effects. Evaluation of observational data using Maximum Covariance Analysis identifies significant correlations between springtime 2‐m air temperature (T2 m) cold (warm) anomalies in both the western U.S. and the Tibetan Plateau and downstream drought (flood) events in late spring/summer. To support these observational findings, climate models are used in sensitivity studies, in which initial LST/SUBT anomaly is imposed to produce observed T2 m anomaly, to demonstrate a causal relationship for two important cases: between spring warm T2 m/LST/SUBT anomalies in western U.S. and the extraordinary 2015 flood in Southern Great Plains and adjacent regions and between spring cold T2 m/LST/SUBT anomalies in the Tibetan Plateau and the severe 2003 drought south of the Yangtze River region. The LST/SUBT downstream effects in North America are associated with a large‐scale atmospheric stationary wave extending eastward from the LST/SUBT anomaly region. The effects of SST in these cases are also tested and compared with the LST/SUBT effects. These results suggest that consideration of LST/SUBT anomalies has the potential to add value to intraseasonal prediction of dry and wet conditions, especially extreme drought/flood events. The results suggest the importance of developing land data and models capable of preserving observed soil memory.

A new classification of large-scale climate regimes around the Tibetan Plateau based on seasonal circulation patterns

This study aims to develop a large-scale climate classification for investigating the characteristics of the climate regimes around the Tibetan Plateau based on seasonal precipitation, moisture transport and moisture divergence using in situ observations and ERA40 reanalysis data. The results indicate that the climate can be attributed to four regimes around the Plateau. They situate in East Asia, South Asia, Central Asia and the semi-arid zone in northern Central Asia throughout the dryland of northwestern China, in addition to the Köppen climate classification. There are different collocations of seasonal temperature and precipitation: 1) in phase for the East and South Asia monsoon regimes, 2) anti-phase for the Central Asia regime, 3) out-of-phase for the westerly regime. The seasonal precipitation concentrations are coupled with moisture divergence, i.e., moisture convergence coincides with the Asian monsoon zone and divergence appears over the Mediterranean-like arid climate region and westerly controlled area in the warm season, while it reverses course in the cold season. In addition, moisture divergence is associated with meridional moisture transport. The northward/southward moisture transport corresponds to moisture convergence/divergence, indicating that the wet and dry seasons are, to a great extent, dominated by meridional moisture transport in these regions. The climate mean southward transport results in the dry-cold season of the Asian monsoon zone and the dry-warm season, leading to desertification or land degradation in Central Asia and the westerly regime zone. The mean-wind moisture transport (MMT) is the major contributor to total moisture transport, while persistent northward transient eddy moisture transport (TEMT) plays a key role in dry season precipitation, especially in the Asian monsoon zone. The persistent TEMT divergence is an additional mechanism of the out-of-phase collocation in the westerly regime zone. In addition, the climate-mean MMT and TEMT are associated with the atmospheric stationary wave and storm track, which results from the uplift of orography and land-sea thermal contrast. Therefore, the paleoclimate changes in mid-latitude arid-semi-arid regions are linked to the different phases of uplift of mountains and plate motion tied to the evolution of the Mediterranean.

Ice-sheet configuration in the CMIP5/PMIP3 Last Glacial Maximum experiments

Abstract. We describe the creation of a data set describing changes related to the presence of ice sheets, including ice-sheet extent and height, ice-shelf extent, and the distribution and elevation of ice-free land at the Last Glacial Maximum (LGM), which were used in LGM experiments conducted as part of the fifth phase of the Coupled Modelling Intercomparison Project (CMIP5) and the third phase of the Palaeoclimate Modelling Intercomparison Project (PMIP3). The CMIP5/PMIP3 data sets were created from reconstructions made by three different groups, which were all obtained using a model-inversion approach but differ in the assumptions used in the modelling and in the type of data used as constraints. The ice-sheet extent in the Northern Hemisphere (NH) does not vary substantially between the three individual data sources. The difference in the topography of the NH ice sheets is also moderate, and smaller than the differences between these reconstructions (and the resultant composite reconstruction) and ice-sheet reconstructions used in previous generations of PMIP. Only two of the individual reconstructions provide information for Antarctica. The discrepancy between these two reconstructions is larger than the difference for the NH ice sheets, although still less than the difference between the composite reconstruction and previous PMIP ice-sheet reconstructions. Although largely confined to the ice-covered regions, differences between the climate response to the individual LGM reconstructions extend over the North Atlantic Ocean and Northern Hemisphere continents, partly through atmospheric stationary waves. Differences between the climate response to the CMIP5/PMIP3 composite and any individual ice-sheet reconstruction are smaller than those between the CMIP5/PMIP3 composite and the ice sheet used in the last phase of PMIP (PMIP2).

Effects of the 11 year solar cycle on middle atmospheric stationary wave patterns in temperature, ozone, and water vapor

[1] The influence of the 11 year cycle in solar irradiation on middle atmospheric stationary wave patterns in temperature, ozone, and water vapor, as indicated by the deviations from zonal mean T*, O3*, and H2O*, is investigated on the basis of time-slice simulations with the general circulation and chemistry model HAMMONIA for solar maximum and minimum conditions. For northern winter, the long-term means of the three parameters are characterized by a pronounced wave-one pattern in the middle atmosphere, but for each of the parameters with a different shift in phase with increasing height. We find a significant increase in amplitude and a horizontal shift in phase of these wave-one patterns when changing from solar minimum to maximum, i.e., regional changes of about ±2–3 K in T*, ±4%–5% in O3*, and ±2%–3% in H2O*. We demonstrate that the solar variability induces these changes by modulating the effect of zonally asymmetric radiative heating due to the stationary wave-one patterns in ozone and other absorbers and to subsequent modulations in planetary wave propagation and wave-driven transport. A comparison with ensemble means for solar maximum and minimum derived from European Centre of Medium-Range Weather Forecasts (ECMWF) Reanalysis data (ERA-40) shows reasonable agreement but also some differences in the significance and location of the solar signals, which is discussed in relation to the different setups of the two data sets. Overall, the results indicate a remarkable effect of the solar cycle on local changes in temperature, wave dynamics, and transport at northern midlatitudes and polar latitudes.

Interactions between stationary waves and ice sheets: linear versus nonlinear atmospheric response

This study examines the mutual interaction between topographically-forced atmospheric stationary waves and continental-scale ice sheets using a thermomechanical ice-sheet model coupled to a linear ...

Permanent El Niño and the onset of Northern Hemisphere glaciations: Mechanism and comparison with other hypotheses

[1] The tropical Pacific transitioned from a “permanent El Nino” state, that is, without the current equatorial zonal gradient of sea surface temperatures, to one with an equatorial cold tongue in a process that started ∼4 Ma, during the early Pliocene. The latter stages of this transition, around ∼2.75 Ma, marked the onset of Northern Hemisphere glaciations. We explore the hypothesis that this onset of glaciation is causally linked to the development of the cold tongue guided by atmosphere general circulation model (AGCM) simulations forced by various hypothesized climate influences. A unique feature of our simulations is imposing permanent El Nino conditions not by imposing sea surface temperatures anomalies directly but rather through changing the implied ocean heat transport in the slab ocean component of the AGCM, done in such a way as to maintain global ocean heat balance. Similar to previous studies, we find increased near-surface temperature conditions occur during simulated permanent El Nino conditions at the locations of known ice sheet growth during glacial periods. We establish that this tropical-extratropical connection is mediated through atmospheric stationary waves, a well-known atmospheric pathway for today's El Nino influence on North America. The influence of the permanent El Nino condition is compared, through model simulations, to other mechanisms proposed to date to explain the onset of glaciations at ∼2.75 Ma: changes in atmospheric carbon dioxide (CO2) concentrations, closure of the Panama seaway, and changes in North Pacific seasonality. The exact comparison is done by calculating differences in the equilibrium line altitude at the known locations of ice sheet growth, an approach that permits the translation of climate anomalies into a measurement of glaciation trends in terms of glacier mass balance. We find that permanent El Nino and atmospheric CO2 are the leading influences in this comparison, leading us to conjecture that the two influences combined determined the long-term background conditions governing ice sheet growth. Within this long-term background evolution, however, we additionally find that the exact timing of Northern Hemisphere glaciations is likely to have come from favorable orbital conditions. Our results regarding permanent El Nino on Northern Hemisphere glaciations is, however, conditioned on a strong sensitivity of Northern Hemisphere climate changes to how the oceanic heat compensation to the permanent El Nino is achieved.

The impact of topographically forced stationary waves on local ice-sheet climate

AbstractA linear two-level atmospheric model is employed to study the influence of ice-sheet topography on atmospheric stationary waves. In particular, the stationary-wave-induced temperature anomaly is considered locally over a single ice-sheet topography, which is computed using the plastic approximation. It is found that stationary waves induce a local cooling which increases linearly with the ice volume for ice sheets of horizontal extents smaller than ∼1400 km. Beyond this horizontal scale, the dependence of stationary-wave-induced cooling on the ice volume becomes gradually weaker. For a certain ice-sheet size, and for small changes of the surface zonal wind, it is further shown that the strength of the local stationary-wave-induced cooling is proportional to the basic state meridional temperature gradient multiplied by the vertical stratification in the atmosphere. These results are of importance for the nature of the feedback between ice sheets and stationary waves, and may also serve as a basis for parameterizing this feedback in ice-sheet model simulations (e.g. through the Pleistocene glacial/interglacial cycles).

Effect of the climate shift around mid 1970s on the relationship between wintertime Ural blocking circulation and East Asian climate

AbstractBlocking variability over the Ural Mountain region in the boreal winter and its relationship with the East Asian winter climate is investigated. The climate shift around mid 1970s has been shown to exert a significant influence on the blocking pattern. In contrast with the years before 1976/1977, the Ural blocking signal after 1976/1977 is found to propagate less into the stratosphere and more eastward in the troposphere to East Asia, which therefore exerts more influence on the East Asian winter climate. This enhanced Ural blocking–East Asian climate relationship amplifies the impact of Ural blocking on East Asia and, with the background of decreasing Ural blocking, contributes to the higher frequency of warm winters in this region. Further analyses suggest that the NAM‐related stratospheric polar vortex strength and its modulation on the propagation of atmospheric stationary waves can account for this change, with the key area being located in the North Atlantic region. Copyright © 2009 Royal Meteorological Society

Northern Hemisphere Stationary Waves in Future Climate Projections

Abstract The response of the atmospheric large-scale circulation to an enhanced greenhouse gas (GHG) forcing varies among coupled global climate model (CGCM) simulations. In this study, 16 CGCM simulations of the response of the climate system to a 1% yr−1 increase in the atmospheric CO2 concentration to quadrupling are analyzed with focus on Northern Hemisphere winter. A common signal in 14 out of the 16 simulations is an increased or unchanged stationary wave amplitude. A majority of the simulations may be categorized into one of three groups based on the GHG-induced changes in the atmospheric stationary waves. The response of the zonal mean barotropic wind is similar within each group. Fifty percent of the simulations belong to the first group, which is categorized by a stationary wave with five waves encompassing the entire NH and a strengthening of the zonal mean barotropic wind. The second and third groups, respectively consisting of three and two simulations, are characterized by a broadening and a northward shift of the zonal mean barotropic wind, respectively. A linear model of barotropic vorticity is employed to study the importance of these mean flow changes to the stationary wave response. The linear calculations indicate that the GHG-induced mean wind changes explain 50%, 4%, and 37% of the stationary wave changes in each group, respectively. Thus, for the majority of simulations the zonal mean wind changes do significantly explain the stationary wave response.

Climate impacts of systematic errors in the simulation of the path of the North Atlantic Current

[1] Experiments employing an adjustment of the pressure field in the ocean component of a coupled climate system model are undertaken in both ocean-only and coupled experiments to assess the climatic impacts of reducing the systematic errors in the North Atlantic Current. This conservative and adiabatic adjustment process substantially decreases North Atlantic Ocean SST biases and locally reverses the associated surface heat flux balance in both model configurations. Ice concentrations in the Labrador Sea increase as the oceanic surface heat fluxes are displaced by the adjustment. Downstream, in the Nordic Seas, the subsurface ocean responds favorably to this adjustment, as the vertical profiles of potential temperature and salinity converge towards the observations. Atmospheric stationary wave patterns show a modest improvement, with a slight weakening of the excessively deep Icelandic low. Further unresolved errors in the coupled model framework potentially contribute to the continued presence of biases in the North Atlantic.

Upper level atmospheric stationary waves in the twentieth century climate of the Intergovernmental Panel on Climate Change simulations

The upper level stationary waves are defined as the deviations from longitudinal symmetry of the 250 hPa climatological monthly mean stream function. The climatological averaging period is over the years 1980 to 2000. The coupled model simulations are those of the climate of the 20th Century experiment (20C3M) prescribed by IPCC. The model results are compared to the NCEP/NCAR and ERA40 Reanalyses. The comparison shows the following. (1) The amplitude of the modeled waves in the Northern Hemisphere is generally weaker than observed; that is, modeled flow is too zonal. In the Southern Hemisphere, there are phase as well as amplitude discrepancies but no clear biases. (2) The correlation of the waves in the reanalyses and models in the Northern Hemisphere in winter averages about 0.9, but decreases sharply outside of winter. (3) For many models the correlation of the waves in the reanalyses and models outside of the wintertime Northern Hemisphere is poor, sometimes being less than 0.7. The most prominent systematic error, occurring across all models, is the underestimate of the trough/ridge in the Northern Hemisphere winter over the north Atlantic sector, 60°W–0°W. (4) The models having the coarsest horizontal resolution consistently underperform compared to the others. For models of horizontal resolution finer than approximately 2.5°, the relation to horizontal resolution and fidelity of the simulation is somewhat less clear. (5) The interannual variability of the waves is consistently underestimated by almost all the models throughout the year and over the Northern Hemisphere. The differences between the models and the reanalyses are surprisingly large, given the large scale of the features diagnosed and the fairly long averaging period. Comparison to AMIP2 integrations indicates that differences in the ocean simulation in the coupled models are likely not an overwhelming influence on the agreement with reanalyses.

The Effect of ENSO on Tibetan Plateau Snow Depth: A Stationary Wave Teleconnection Mechanism and Implications for the South Asian Monsoons

Abstract An atmospheric stationary wave teleconnection mechanism is proposed to explain how ENSO may affect the Tibetan Plateau snow depth and thereby the south Asian monsoons. Using statistical analysis, the short available record of satellite estimates of snow depth, and ray tracing, it is shown that wintertime ENSO conditions in the central Pacific may produce stationary barotropic Rossby waves in the troposphere with a northeastward group velocity. These waves reflect off the North American jet, turning equatorward, and enter the North African–Asian jet over the eastern Atlantic Ocean. Once there, the waves move with the jet across North Africa, South Asia, the Himalayas, and China. Anomalous increases in upper-tropospheric potential vorticity and increased wintertime snowfall over the Tibetan Plateau are speculated to be associated with these Rossby waves. The increased snowfall produces a larger Tibetan Plateau snowpack, which persists through the spring and summer, and weakens the intensity of the south Asian summer monsoons.