Zonal Wind Direction Research Articles

Process-Informed Subsampling Improves Subseasonal Rainfall Forecasts in Central America.

Subseasonal rainfall forecast skill is critical to support preparedness for hydrometeorological extremes. We assess how a process-informed evaluation, which subsamples forecasting model members based on their ability to represent potential predictors of rainfall, can improve monthly rainfall forecasts within Central America in the following month, using Costa Rica and Guatemala as test cases. We generate a constrained ensemble mean by subsampling 130 members from five dynamic forecasting models in the C3S multimodel ensemble based on their representation of both (a) zonal wind direction and (b) Pacific and Atlantic sea surface temperatures (SSTs), at the time of initialization. Our results show in multiple months and locations increased mean squared error skill by 0.4 and improved detection rates of rainfall extremes. This method is transferrable to other regions driven by slowly-changing processes. Process-informed subsampling is successful because it identifies members that fail to represent the entire rainfall distribution when wind/SST error increases.

Responses of the African and American Equatorial Ionization Anomaly (EIA) to 2014 Arctic SSW Events

AbstractAside from the influence of forcing from above on the ionosphere during space weather, forcing from below also have significant influence on the ionosphere. This study investigates responses of the equatorial ionization anomaly (EIA) in the African and American longitudinal sectors to the combined effects of 2014 Sudden Stratospheric Warming (SSW) events and geomagnetic storms that coexisted with them. The study locations cover ±40° geomagnetic latitudes in both sectors. A multiinstrument approach with models was adopted. During the SSW events, a hemispherical asymmetry in TEC distribution was observed, with higher plasma ionization in the Northern Hemisphere (NH). Generally, in both sectors, EIA crests locations shifted to higher latitudes during peak phases of SSW, except in the SH of the African sector, where crests locations shifted to lower latitudes. Reversal of stratospheric zonal mean wind direction supported reversed fountain effect. TEC responded positively to SSW peak phases and daytime or nighttime orientation of Prompt Penetration Electric Field (PPEF) and PPEF strength played major role on TEC responses to storms. PPEF values were generally weak, but comparatively higher in the American sector. TEC were predominant in the American sector than the African sector due to the comparative higher electrodynamics over the American sector. EIA crests were generally located at higher latitudes on the days of SSW peaks than on the days of geomagnetic storms, except in the NH of the American sector. In both sectors, geomagnetic storms modified ionospheric irregularities by weakening or enhancing them, while the major SSW event weakened irregularities.

Interannual Variability of the Lena River Plume Propagation in 1993–2020 during the Ice-Free Period on the Base of Satellite Salinity, Temperature, and Altimetry Measurements

The Lena River plume significantly affects the thermohaline, optical and chemical properties of the eastern Arctic seas. We use sea surface salinity (SSS), temperature (SST), and altimetry measurements to study features of the Lena plume propagation during 1993–2020. A comparison of Soil Moisture Active Passive (SMAP) SSS measurements with in situ data obtained using the flow-through system in oceanographic surveys in 2018–2019 demonstrates good coincidence with correlation ~ 0.96 and RMSD ~ 1 psu. The SMAP data were used to reconstruct the plume evolution in 2015–2020 and to identify three main types of Lena plume propagation, which are mainly related to the variability of dominant zonal wind direction: «northern»—the plume moves to the north from the delta up to 78° N; «eastern»—the plume moves eastward along the Siberian coast up to 180° E; «mixed» between two main types. Brackish plume waters were characterized by increased temperature and sea level, which provides the opportunity for studying the Lena plume dynamics using satellite altimetry and infrared measurements. These data were analyzed to study the interannual variability of plume propagation during the ice-free period of 1993–2020. The obtained results show that the «northern» type is observed twice more often than the «eastern» one, but the «eastern» type has intensified since 2010.

Influence of High Latitude Sudden Stratospheric Warming on Tropical Weather: Observations From a 205 MHz Stratosphere Troposphere Radar and Surface Meteorological Parameters

AbstractThe study illustrates the evidence of dynamical coupling between the high‐latitude sudden stratospheric warming (SSW) events that occurred in three consecutive winter seasons and the concomitant changes in the wind pattern in the lower stratosphere and troposphere observed with a state‐of‐the‐art 205 MHz stratosphere‐troposphere wind profiling Radar, located at Cochin (10.03°N, 77.33°E), a tropical station in southwest peninsular India. Associated with the occurrence of SSW, the tropical region experiences a change in zonal wind speed and direction at upper tropospheric altitudes after the central day of warming. The zonal wind in the troposphere also reversed its prevailing direction. Downward propagation of zonal wind during the SSW period is noticed, which is also supported by the reanalysis data set. In the lower troposphere, vertical wind experienced sudden fluctuations with varying amplitudes soon after the peak day of the SSW event. The surface meteorological parameters in an area (8°–12°N, 74°–78°E), centered at Cochin, from ERA‐Interim are also examined. An abrupt fall in outgoing longwave radiation, followed by convection and unusual rainfall in the tropical region 5–6 days before the peak of SSW events, is a unique and exciting feature noted in the study. Detailed analysis of the past 20 years of SSW events confirmed the observational evidence of unusual rainfall over the low latitude region related with the onset of SSW. Comprehensive observational and numeric modeling studies are needed to understand the mechanism for the SSW events in high latitudes and the associated convective activity and precipitation in the low latitude region.

The phytoplankton variability in the Pearl River estuary based on VIIRS imagery

A support vector machine (SVM) regression algorithm was developed for estimating the chlorophyll-a concentration (Cchla) in the turbid estuarine waters of the Pearl River estuary (PRE) based on visible infrared imaging radiometer suite (VIIRS) ocean color data. The proposed SVM algorithm outperformed the OC3 algorithm in the turbid waters. The temporal variation and spatial distribution of SVM-based monthly Cchla products were determined during 2012 and 2018. By analyzing the monthly average Cchla products and ancillary data, the underlying mechanisms of the Cchla coastal summer plume jet and winter bloom were revealed. The Cchla coastal plume jet was often observed in June, when the zonal wind direction changed from westward to eastward and the river runoff reached its maximum value of the year. The winter bloom in the ocean area was attributed to strong mixing in the upper layer, which resulted from cooling-induced buoyancy changes.

4DVD visualization and delivery of the 20th century reanalysis data: methods and examples

This paper describes the use of a web application system, called 4-Dimensional Visual Delivery (4DVD), to visualize and deliver climate data. The focus of this paper is on the monthly data from the NOAA-CIRES-DOE Twentieth Century Reanalysis (20CR) project Version 2c. The 20CR is a global model at a spatial resolution about 2° latitude-longitude. The temporal coverage is from January 1851 to December 2014. The monthly data include numerous climate variables, such as temperature, pressure, precipitation rate, zonal wind, meridional wind, albedo, and longwave radiation flux. We show how to use the 4DVD system to conveniently visualize, deliver, and analyze the 20CR data of these parameters on a globe or on a 2D latitude-longitude map. We analyze the temporal and spatial patterns of zonal wind and precipitation over Nino 4 region (5° N-5° S, 160° E-150° W). The monthly NASA Global Precipitation Climatology Project (GPCP) data are used to compare the results from 20CR. GPCP data used in this 4DVD application are defined on 2.5° latitude-longitude grid boxes from January 1979 to January 2019. The 4DVD figures successfully show the reverse of zonal wind direction from the east-to-west wind to the west-to-east wind on the grid box (2° S, 176° W) in the Nino 4 region during an El Nino event. The spectral analysis shows the annual and semi-annual cycles in the zonal wind data on this grid box. The GPCP data are used to support the analysis results of 20CR zonal data. The precipitation data over the grid box (1.25° S, 176.25° W) in the Nino 4 region show clear El Nino signals (persistent extreme precipitation more than 6 mm/day) and clear La Nina signals (persistent “dry” with near-zero precipitation). The spectral analysis of the precipitation data time series over this grid box shows no annual cycle.

Analysis and Simulation of the Stratospheric Quasi-zero Wind Layer over Korla, Xinjiang Province, China

The stratospheric quasi-zero wind layer (QZWL) is a transition region with low zonal wind speeds in the lower stratosphere at an altitude of ~20 km. The zonal wind direction above the QZWL layer is opposite to that below the QZWL layer and the north–south wind component is small. The atmospheric wind field near the stratospheric QZWL is an important factor affecting the flight altitude and dynamic control of stratospheric airships. It is therefore necessary to study the stratospheric QZWL to provide better environmental information for these aircraft. High-resolution radiosonde data were used to analyze the characteristics of the stratospheric QZWL over Korla, Xinjiang Province, China. A weak wind layer in which the wind direction suddenly reversed from westerly to easterly was observed at ~20 km in the lower stratosphere, characteristic of the stratospheric QZWL. The Weather Research and Forecasting model was used to simulate the profiles of the horizontal wind speed and direction over Korla. The forcing effect of each diagnostic term in the equation on the zonal wind speed was analyzed. The results showed that the advection term was the dominant factor forcing the zonal wind speed. The wave term had a secondary forcing role, although the forcing effect of the wave term on the zonal wind speed was significant in some regions.

Investigating the semiannual oscillation on Mars using data assimilation

A Martian semiannual oscillation (SAO), similar to that in the Earth’s tropical stratosphere, is evident in the Mars Analysis Correction Data Assimilation reanalysis dataset (MACDA) version 1.0, not only in the tropics, but also extending to higher latitudes. Unlike on Earth, the Martian SAO is found not always to reverse its zonal wind direction, but only manifests itself as a deceleration of the dominant wind at certain pressure levels and latitudes. Singular System Analysis (SSA) is further applied on the zonal-mean zonal wind in different latitude bands to reveal the characteristics of SAO phenomena at different latitudes. The second pair of principal components (PCs) is usually dominated by a SAO signal, though the SAO signal can be strong enough to manifest itself also in the first pair of PCs. An analysis of terms in the Transformed Eulerian Mean equation (TEM) is applied in the tropics to further elucidate the forcing processes driving the tendency of the zonal-mean zonal wind. The zonal-mean meridional advection is found to correlate strongly with the observed oscillations of zonal-mean zonal wind, and supplies the majority of the westward (retrograde) forcing in the SAO cycle. The forcing due to various non-zonal waves supplies forcing to the zonal-mean zonal wind that is nearly the opposite of the forcing due to meridional advection above ∼3 Pa altitude, but it also partly supports the SAO between 40 Pa and 3 Pa. Some distinctive features occurring during the period of the Mars year (MY) 25 global-scale dust storm (GDS) are also notable in our diagnostic results with substantially stronger values of eastward and westward momentum in the second half of MY 25 and stronger forcing due to vertical advection, transient waves and thermal tides.Mars; Mars, atmosphere; Mars, climate; Atmospheres, dynamics; Atmospheres, structure; Meteorology.

Observed response of stratospheric and mesospheric composition to sudden stratospheric warmings

In this study we investigate and quantify the statistical changes that occur in the stratosphere and mesosphere during 37 sudden stratospheric warming (SSW) events from 1989 to 2016. We consider changes in the in-situ ozonesonde observations of the stratosphere from four sites in the northern hemisphere (Ny-Ålesund, Sodankylä, Lerwick, and Boulder). These data are supported by Aura/MLS satellite observations of the ozone volumetric mixing ratio above each site, and also ground-based total-column O3 and NO2, and mesospheric wind measurements, measured at the Sodankylä site. Due to the long-time periods under consideration (weeks/months) we evaluate the observations explicitly in relation to the annual mean of each data set. Following the onset of SSWs we observe an increase in temperature above the mean (for sites usually within the polar vortex) that persists for >∼40 days. During this time the stratospheric and mesospheric ozone (volume mixing ratio and partial pressure) increases by ∼20% as observed by both ozonesonde and satellite instrumentation. Ground-based observations from Sodankylä demonstrate the total column NO2 does not change significantly during SSWs, remaining close to the annual mean. The zonal wind direction in the mesosphere at Sodankylä shows a clear reversal close to SSW onset. Our results have broad implications for understanding the statistical variability of atmospheric changes occurring due to SSWs and provides quantification of such changes for comparison with modelling studies.

Signature of La Niña in interannual variations of the East India Coastal Current during spring

We have investigated interannual variations of the spring (February–April average) East India Coastal Current (EICC) magnitude between 2000 and 2018 using OSCAR (Ocean Surface Current Analysis Real-time) current and a linear, continuously stratified (LCS) model. Interannual variability of the EICC shows significant decrease in magnitude during spring of 2000, 2008 and 2011, the years when high negative ONI (Oceanic Nino Index for sea surface temperature) value has been observed due to dominance of strong La Nina events. Our LCS model also successfully simulates these interannual variability of the spring average EICC between 2000 and 2018. We carried out numerical experiments using LCS model related to local and remote forcing response on EICC. Dynamics of the EICC during spring are dominated by four different forcing processes; local wind along east coast of India, remote forcing response from the eastern and northern boundary of the BoB including islands, interior BoB and the Equatorial Indian Ocean (EIO). During El Nino and normal spring years, strong poleward interannual EICC are due to very weak negligible (order of 0–5 cm s $$^{-1}$$ ) EICC from EIO remote response and in-phase poleward EICC formation using other three forcings. However, during La Nina spring years, weak (order of 0–10 cm s $$^{-1}$$ ) poleward interannual EICC are formed due to destructive interference between equatorward current (order of 10–25 cm s $$^{-1}$$ ) from EIO forcing and in-phase poleward current from other three forcings. We have also found propagation of interannual upwelling (downwelling) favorable Kelvin wave from EIO via eastern and western boundary of the BoB during spring in the El Nino (La Nina) years. The interannual variations in the propagation of EIO Kelvin wave are associated with the changes in the EIO zonal wind direction by climate mode like ENSO (El Nino–Southern Oscillation).

QBO Influence on MJO Amplitude over the Maritime Continent: Physical Mechanisms and Seasonality

AbstractThe quasi-biennial oscillation (QBO) is stratified by stratospheric zonal wind direction and height into four phase pairs [easterly midstratospheric winds (QBOEM), easterly lower-stratospheric winds, westerly midstratospheric winds (QBOWM), and westerly lower-stratospheric winds] using an empirical orthogonal function analysis of daily stratospheric (100–10 hPa) zonal wind data during 1980–2017. Madden–Julian oscillation (MJO) events in which the MJO convective envelope moved eastward across the Maritime Continent (MC) during 1980–2017 are identified using the Real-time Multivariate MJO (RMM) index and the outgoing longwave radiation (OLR) MJO index (OMI). Comparison of RMM amplitudes by the QBO phase pair over the MC (RMM phases 4 and 5) reveals that boreal winter MJO events have the strongest amplitudes during QBOEM and the weakest amplitudes during QBOWM, which is consistent with QBO-driven differences in upper-tropospheric lower-stratospheric (UTLS) static stability. Additionally, boreal winter RMM events over the MC strengthen during QBOEM and weaken during QBOWM. In the OMI, those amplitude changes generally shift eastward to the eastern MC and western Pacific Ocean, which may result from differences in RMM and OMI index methodologies. During boreal summer, as the northeastward-propagating boreal summer intraseasonal oscillation (BSISO) becomes the dominant mode of intraseasonal variability, these relationships are reversed. Zonal differences in UTLS stability anomalies are consistent with amplitude changes of eastward-propagating MJO events across the MC during boreal winter, and meridional stability differences are consistent with amplitude changes of northeastward-propagating BSISO events during boreal summer. Results remain consistent when stratifying by neutral ENSO phase.

Multiple Equilibria in a Land–Atmosphere Coupled System

Many low-order modeling studies indicate that there may be multiple equilibria in the atmosphere induced by thermal and topographic forcings. However, most work uses uncoupled atmospheric model and just focuses on the multiple equilibria with distinct wave amplitude, i.e., the high- and low-index equilibria. Here, a low-order coupled land–atmosphere model is used to study the multiple equilibria with both distinct wave phase and wave amplitude. The model combines a two-layer quasi-geostrophic channel model and an energy balance model. Highly truncated spectral expansions are used and the results show that there may be two stable equilibria with distinct wave phase relative to the topography: one (the other) has a lower layer streamfunction that is nearly in (out of) phase with the topography, i.e., the lower layer ridges (troughs) are over the mountains, called ridge-type (trough-type) equilibria. The wave phase of equilibrium state depends on the direction of lower layer zonal wind and horizontal scale of the topography. The multiple wave phase equilibria associated with ridge- and trough-types originate from the orographic instability of the Hadley circulation, which is a pitch-fork bifurcation. Compared with the uncoupled model, the land–atmosphere coupled system produces more stable atmospheric flow and more ridge-type equilibrium states, particularly, these effects are primarily attributed to the longwave radiation fluxes. The upper layer streamfunctions of both ridge- and trough-type equilibria are also characterized by either a high- or low-index flow pattern. However, the multiple wave phase equilibria associated with ridge- and trough-types are more prominent than multiple wave amplitude equilibria associated with high- and low-index types in this study.

The Longitudinal Variations of Upper Thermospheric Zonal Winds Observed by the CHAMP Satellite at Low and Midlatitudes

AbstractIn this work, we investigate the longitudinal patterns of thermospheric zonal winds (~400 km) and their seasonal and solar activity dependence using CHAllenging Minisatellite Payload cross‐track wind data and Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations. CHAllenging Minisatellite Payload data show that there are large longitudinal variations in thermospheric zonal winds. These longitudinal variations do not have significant solar activity dependence. In the northern hemisphere at low and middle latitudes, the daytime zonal winds are more westward between −90° and 90° longitudes and more eastward at other longitudes. The nighttime zonal wind direction with pattern is opposite to that in the daytime. In the southern hemisphere at low and middle latitudes, the local time variation of the longitudinal wind structure, including positive and negative peak locations, is almost the reverse of that in the northern hemisphere. Thus, the zonal wind patterns also show great hemispheric asymmetry. In addition, the longitudinal patterns of zonal winds in both hemispheres show seasonal variations. The longitudinal patterns during the June solstice are significantly different from those in other seasons. These observational results are well reproduced in TIEGCM simulations. To elucidate the physical processes responsible for the observed and simulated longitudinal patterns of the zonal winds, the TIEGCM is also run for different geomagnetic field configurations. A comparison between the TIEGCM results shows that geomagnetic field structure is the main cause of the large longitudinal variations of thermospheric zonal winds and their local time, hemispheric asymmetry, and seasonal changes at low and middle latitudes.

Characteristics of Stratospheric Winds over Jiuquan (41.1°N, 100.2°E) Using Rocketsonde Data in 1967–2004

AbstractStratospheric winds play a significant role in middle atmosphere dynamics, model research, and carrier rocket experiments. For the first time, 65 sets of rocket sounding experiments conducted at Jiuquan (41.1°N, 100.2°E), China, from 1967 to 2004 are presented to study horizontal wind fields in the stratosphere. At a fixed height, wind speed obeys the lognormal distribution. Seasonal mean winds are westerly in winter and easterly in summer. In spring and autumn, zonal wind directions change from the upper to the lower stratosphere. The monthly zonal mean winds have an annual cycle period with large amplitudes at high altitudes. The correlation coefficients for zonal winds between observations and the Horizontal Wind Model (HWM) with all datasets are 0.7. The MERRA reanalysis is in good agreement with rocketsonde data according to the zonal winds comparison with a coefficient of 0.98. The sudden stratospheric warming is an important contribution to biases in the HWM, because it changes the zonal wind direction in the midlatitudes. Both the model and the reanalysis show dramatic meridional wind differences with the observation data.

Change in Yields of Cereal Crops in European Russia Caused by the Quasi-Biennial Oscillation of Atmospheric Processes

The objective of work is to study the influence of quasi-biennial oscillation of atmospheric processes on precipitation and frequency of the spring-summer drought against the background of the statistics of cyclonic and anticyclonic activity in different phases of the quasi-biennial oscillation and, consequently, on the yield of wheat in the European part of Russia. The area of the study is the grain zone of European Russia including the North Caucasus, Central Chernozem zone, Volga region and Southern Urals. The timeseries of values of speed and direction of zonal wind on an isobaric surface of 30 hPa for the period 1953–2011 was accepted as data characterizing the “classical” quasi-biennial (Data of Free University of Berlin, https://climatedataguide.ucar.edu). Decadic precipitation totals, indices of PDSI and SPI, snow depth in March, the frequency of centers of cyclones during the period of vegetation classified by phases of quasi-biennial. The increase of snow depth in March is revealed, as well as significant increase in precipitation and a reduction in the frequency of severe droughts in May associated with the intensification of cyclonic activity during the Western phase in relation to Eastern on a vast territory: the western region of Central Chernozem zone, Rostov region and Krasnodar krai. As a result of changing agro-climatic conditions depending on the phase of the quasi-biennial in the territory, it is found a significant excess of productivity of spring wheat during the Western phase against the Eastern phase. The yield of winter wheat was less sensitive to the quasi-biennial. The results of the study can be used to adequately address the problem of increasing the sustainability of harvests in Russia (improvement of agrometeorological practice, including prognostic aspects, and agroinsurance).

A Global Assessment of the Spatial Distribution of Precipitation Occurrence

AbstractThe spatial distribution of precipitation occurrence has important implications for numerous applications ranging from defining cloud radiative effects to modeling hydrologic runoff, statistical downscaling, and stochastic weather generation. This paper introduces a new method of describing the spatial characteristics of rainfall and snowfall that takes advantage of the high sensitivity and high resolution of the W-band cloud precipitation radar aboard CloudSat. The resolution dependence of precipitation occurrence is described by a two-parameter exponential function defined by a shape factor that governs the variation in the distances between precipitation events and a scale length that represents the overall probability of precipitation and number density of distinct events.Geographic variations in the shape factor and scale length are consistent with large-scale circulation patterns and correlate with environmental conditions on local scales. For example, a large contrast in scale lengths between land and ocean areas reflects the more extensive, widespread nature of precipitation over land than over ocean. An analysis of warm rain in the southeast Pacific reveals a shift from frequent isolated systems to less frequent but more regularly spaced systems along a transect connecting stratocumulus and trade cumulus cloud regimes. A similar analysis during the Amazon wet season reveals a relationship between the size and frequency of convection and zonal wind direction with precipitation exhibiting a more oceanic character during periods of westerly winds. These select examples demonstrate the utility of this approach for capturing the sensitivity of the spatial characteristics of precipitation to environmental influences on both local and larger scales.

Geothermal heating enhances atmospheric asymmetries on synchronously rotating planets

Earth-like planets within the liquid water habitable zone of M type stars may evolve into synchronous rotators. On these planets, the sub-stellar hemisphere experiences perpetual daylight while the opposing anti-stellar hemisphere experiences perpetual darkness. Because the night-side hemisphere has no direct source of energy, the air over this side of the planet is prone to freeze out and deposit on the surface, which could result in atmospheric collapse. However, general circulation models (GCMs) have shown that atmospheric dynamics can counteract this problem and provide sufficient energy transport to the anti-stellar side. Here we use an idealized GCM to consider the impact of geothermal heating on the habitability of synchronously rotating planets. Geothermal heating may be expected due to tidal interactions with the host star, and the effects of geothermal heating provide additional habitable surface area and may help to induce melting of ice on the anti-stellar hemisphere. We also explore the persistence of atmospheric asymmetries between the northern and southern hemispheres, and we find that the direction of the meridional circulation (for rapidly rotating planets) or the direction of zonal wind (for slowly rotating planets) reverses on either side of the sub-stellar point. We show that the zonal circulation approaches a theoretical state similar to a Walker circulation only for slowly rotating planets, while rapidly rotating planets show a zonal circulation with the opposite direction. We find that a cross-polar circulation is present in all cases and provides an additional mechanism of mass and energy transport from the sub-stellar to anti-stellar point. Characterization of the atmospheres of synchronously rotating planets should include consideration of hemispheric differences in meridional circulation and examination of transport due to cross-polar flow.

Short period gravity wave momentum fluxes observed in the tropical troposphere, stratosphere and mesosphere

Using long-term data (1998–2008) collected from mesosphere–stratosphere–troposphere (MST) radar and Rayleigh Lidar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, vertical flux of the momentum from troposphere to mesosphere associated with the gravity waves of periods in the range 20min to 2h is investigated for the first time. The emphasis is on seasonal variability of mean zonal and meridional momentum fluxes in mesosphere and troposphere and vertical flux of horizontal momentum in the stratosphere. At tropospheric altitudes of 11–16km large enhancement in flux is noticed during equinoxes. In the lower mesosphere in the altitude region 58–62km the maximum values of flux (∼2.8m2/s2) observed are pragmatic in winter and spring. Interestingly, the vertical flux of horizontal momentum estimated from lidar is in the range of those estimated from radar data in the overlap altitude region, though the estimates are from two different techniques. In the mesosphere, large variations with altitude in zonal momentum flux are noticed with a magnitude ∼0–4m2/s2 in summer. In winter and summer the zonal wind direction is opposite to the momentum flux direction between 73 and 80km and in equinoxes zonal wind follows the momentum flux. The meridional fluxes in the mesosphere are higher in equinoxes (∼10–12m2/s2).

Forecast of development of quasi-biennial oscillation in the equatorial stratospheric wind until April 2014

As is well known, the quasi-biennial oscillation (QBO) of wind in the equatorial stratosphere has a significant influence on the processes not only at tropical but also at middle and high latitudes in both stratosphere and troposphere. Consequently, a forecasting the dates of QBO-phase (W or E) transitions is required for the solution of many problems (for instance, for investigation of possible climate changes, for studying of solar activity effects in atmospheric phenomena, for predicting the trends of various atmospheric parameters, etc.). The type of QBO-phase is commonly determined by the zonal wind direction at fixed altitude. The quasi-biennial alternations of wind direction are related with a successive descent of easterly and westerly wind regimes. The paper describes the peculiarities of the wind evolution during QBO-cycle and reveals that the easterly regime propagates downward always with stop at some altitude in the range 20–40hPa. This time period (no less than three months) of the easterly descent interruption is termed as the stagnation stage. Seasonal regularities of stagnation stage cause the discreteness of QBO-cycle period. If the interval between beginnings of successive stagnation stages is identified as a QBO-cycle length, it appears that no QBO-period other than 24, 30, or 36 months can be realized. The QBO-cycle period is unambiguously related with the stagnation duration that allows making an early forecast of the QBO-phase transition dates. The paper discusses the stagnation stage features occurring during QBO-cycles of different period, and describes the technique of forecasting the QBO-cycle evolution by analyzing the vertical profiles of wind speed. The study is based on experimental observation of zonal wind in the equatorial stratosphere during 1953–2011. It is shown that the previous quasi-biennial cycle has begun in January 2009, lasted for 30 months and completed in June 2011. The current cycle has begun in July 2011, will last for 30 months and will end in December 2013. Subsequent cycle will begin in January 2014. The QBO-phase transition dates for different height levels are determined for all months up to April 2014.

The Life Cycle of the South American Monsoon System

Abstract The South American monsoon system (SAMS) life cycle plays an important role in the distribution and duration of the rainy season mainly over southwestern Amazonia, and the central west and southeast Brazil regions, affecting the economy through impacts on the agriculture and hydrology sectors. In this study a new criterion is applied to identify the monsoon onset and demise that was not used before in the SAMS region. This criterion is based on the atmospheric humidity flux over an area recognized as the monsoon core because of zonal wind reversal and changes in humidity from the transition seasons to summer and winter. Areas in Brazil that have a monsoon regime are identified, and several features associated with the life cycle are discussed. The climatological onset and demise are identified as the end of October and the end of March, respectively, and an interannual variability is found in the times of onset/demise. The main observed features in the two phases are discussed, such as the role of the South Atlantic subtropical high displacement, the northwestern moisture flux east of Andes and from the Atlantic Ocean, the zonal wind intensity and direction over central South America, the vertical motion over the continent, and the expansion/reduction of the Bolivian high circulation with associated high-level divergence. The frontal systems contribute to the pressure decrease, wind direction changes, and soil moisture increase previous to the onset. Low-frequency troughs with intraseasonal variability establish conditions favorable to the monsoon onset, and low-frequency ridges are related to late onset.