Zonal Wind Field Research Articles

Retreat of the Bay of Bengal Summer Monsoon

AbstractThe Bay of Bengal summer monsoon (BOBSM) is the most prominent branch of the Asian summer monsoon system, which exhibits complex interannual variability. While previous studies have focused on the onset conditions of the BOBSM, less attention has been paid to the retreat of the BOBSM. In this study, we propose an index to measure BOBSM retreat, based on the mean zonal wind field at 850 hPa during the summer‐to‐winter monsoon transitions. By analyzing the climatic characteristics and interannual variability of the BOBSM retreat using this index, we find that BOBSM retreat exhibits significant interannual variability, which is closely related to the occurrence of Indian Ocean Dipole (IOD) events. Statistically, when a positive IOD event takes place in the boreal autumn season, the retreat of the summer monsoon occurs earlier correspondingly. Conversely, the retreat is delayed when a negative IOD event occurs.

Relationship between the Pacific Meridional Mode and El Niño–Southern Oscillation on different timescales

Previous studies have shown that the Pacific Meridional Mode (PMM) has a crucial impact on the central Pacific El Niño–Southern Oscillation (ENSO). In this study, we redefined the PMM index used previously by focusing on the sea surface temperature in the subtropical northeastern Pacific (NEP), and found that the correlations between the PMM and ENSO are distinct on different timescales. We refer to this redefined index as PMMNEP. Results show that a positive PMM is weakly related to El Niño, while a negative PMM is more convincingly related to La Niña, specifically, the equatorial negative SSTAs possibly affecting negative PMM events. Moreover, the cause–effect relation between the PMMNEP and ENSO is much more significant on a decadal timescale than on an interannual timescale, and their interaction may be related to the equatorial zonal wind field. Our finding indicates that a La Niña-like decadal pattern is established through the combined actions of the PMM and ENSO, rather than an El Niño-like decadal pattern.

Leading modes of wind field variability over the western Tibet Plateau

Atmospheric circulations bring moisture from above the ocean to the high mountains of the western Tibet Plateau (TP), thus wind variability is of great importance to the water cycle centered on the western TP. This study therefore examines the leading modes of the wind fields over the western TP. We use the multivariate empirical orthogonal function (MV-EOF) analysis method to detect the dominant wind patterns above the western TP. This method extracts the leading modes of combined meridional and zonal wind variability at 200-hPa in the region of 22° N–50° N, 50° E–92° E. We find the first leading mode of the combined zonal and meridional wind field in the annual mean and in most seasons (spring, summer and autumn) over the western TP shows high similarity to the Western Tibetan Vortex (WTV), a large-scale atmospheric vortex-like pattern recently recognized over the western TP. In winter, the WTV, however, is closer to the second leading mode. We estimate the sensitivity of our results by changing the domain of the MV-EOF analysis region surrounding the western. We find the influence of the WTV is less sensitive to analysis location around the western TP. In short, the WTV generally represents the first leading mode of the wind field in most seasons over the western TP. This study augments our knowledge on wind variability over the western TP.

The ICON Earth System Model Version 1.0

AbstractThis work documents the ICON‐Earth System Model (ICON‐ESM V1.0), the first coupled model based on the ICON (ICOsahedral Non‐hydrostatic) framework with its unstructured, icosahedral grid concept. The ICON‐A atmosphere uses a nonhydrostatic dynamical core and the ocean model ICON‐O builds on the same ICON infrastructure, but applies the Boussinesq and hydrostatic approximation and includes a sea‐ice model. The ICON‐Land module provides a new framework for the modeling of land processes and the terrestrial carbon cycle. The oceanic carbon cycle and biogeochemistry are represented by the Hamburg Ocean Carbon Cycle module. We describe the tuning and spin‐up of a base‐line version at a resolution typical for models participating in the Coupled Model Intercomparison Project (CMIP). The performance of ICON‐ESM is assessed by means of a set of standard CMIP6 simulations. Achievements are well‐balanced top‐of‐atmosphere radiation, stable key climate quantities in the control simulation, and a good representation of the historical surface temperature evolution. The model has overall biases, which are comparable to those of other CMIP models, but ICON‐ESM performs less well than its predecessor, the Max Planck Institute Earth System Model. Problematic biases are diagnosed in ICON‐ESM in the vertical cloud distribution and the mean zonal wind field. In the ocean, sub‐surface temperature and salinity biases are of concern as is a too strong seasonal cycle of the sea‐ice cover in both hemispheres. ICON‐ESM V1.0 serves as a basis for further developments that will take advantage of ICON‐specific properties such as spatially varying resolution, and configurations at very high resolution.

Low‐Level Jets and the Convergence of Mars Data Assimilation Algorithms

AbstractData assimilation is an increasingly popular technique in Mars atmospheric science, but its effect on the mean states of the underlying atmosphere models has not been thoroughly examined. The robustness of results to the choice of model and assimilation algorithm also warrants further study. We investigate these issues using two Mars general circulation models (MGCMs), with particular emphasis on zonal wind and temperature fields. When temperature retrievals from the Mars Global Surveyor Thermal Emission Spectrometer (TES) are assimilated into the U.K.‐Laboratoire de Météorologie Dynamique MGCM to create the Mars Analysis Correction Data Assimilation (MACDA) reanalysis, low‐level zonal jets in the winter northern hemisphere shift equatorward and weaken relative to a free‐running control simulation from the same MGCM. The Ensemble Mars Atmosphere Reanalysis System (EMARS) reanalysis, which is also based on TES temperature retrievals, also shows jet weakening (but less if any shifting) relative to a control simulation performed with the underlying Geophysical Fluid Dynamics Laboratory MGCM. Examining higher levels of the atmosphere, monthly mean three‐dimensional temperature and zonal wind fields are in generally better agreement between the two reanalyses than between the two control simulations. In conjunction with information about the MGCMs' physical parameterizations, intercomparisons between the various reanalyses and control simulations suggest that overall the EMARS control run is plausibly less biased (relative to the true state of the Martian atmosphere) than the MACDA control run. Implications for future observational studies are discussed.

Statistical Characteristics of the Low‐Latitude E‐Region Irregularities Observed by the HCOPAR in South China

AbstractStatistical characteristics of the E‐region irregularities observed by the HCOPAR in the low‐latitudes of China are presented and analyzed. The data come from the observations between September 2015 and August 2016. Benefit from the high data acquisition rate and the relatively high time and height resolutions, we have found many interesting and unreported details in the statistical characteristics of the E‐region irregularities. The lowest occurrence rate of the E‐region irregularities appears at around sunrise and another low rate appears at around noon. The daytime irregularities are generated in the region where the zonal winds turn around at the E‐region altitudes. There is clear boundary between the irregularities occurring in the morning and afternoon. The nighttime irregularities appear at higher altitudes with steeper background plasma density gradient. The higher tidal amplitude is suggested to increase the irregularity occurrence rate and the greater solar radiation is suggested to decrease it. The steep plasma density gradient in the night may be the main reason for the nighttime irregularities through gradient drift instability. The turning around of the zonal wind field is considered to generate the irregularities in the day through Kelvin‐Helmholtz instability.

Variations in Stratospheric Gravity Waves Derived from Temperature Observations of Multi-GNSS Radio Occultation Missions

The spatial–temporal distribution of the global gravity wave (GW) potential energy (Ep) at the lower stratosphere of 20–35 km is studied using the dry temperature profiles from multi- Global Navigation Satellite System (GNSS) radio occultation (RO) missions, including CHAMP, COSMIC, GRACE, and METOP-A/B/C, during the 14 years from 2007 to 2020, based on which the linear trends of the GW Ep and the responses of GW Ep to solar activity, quasi biennial oscillation (QBO), and El Niño-Southern Oscillation (ENSO) are analyzed using the multivariate linear regression (MLR) method. It is found that the signs and the magnitudes of the trends of GW Ep during each month vary at different altitude ranges and over different latitudes. At 25–35 km of the middle and high latitudes, GW Ep values generally show significant negative trends in almost all months, and the values of the negative trends become smaller in the regions closer to the poles. The distribution of the deseasonalized trends in the monthly zonal-mean GW Ep demonstrates that the GW activities are generally declining from 2007 to 2020 over the globe. The responses of GW Ep to solar activity are found to be mostly positive at 20–35 km over the globe, and the comparison between the distribution pattern of the deseasonalized trends in the GW activities and that of the responses of GWs to solar activity indicates that the sharp decline in solar activity from 2015 to 2017 might contribute to the overall attenuation of gravity wave activity during the 14 years. Significant negative responses of GW Ep to QBO are found at 30–35 km over 30° S–25° N, and the negative responses extend to the mid and high latitudes in the southern hemisphere at 20–30 km. The responses of GW Ep to QBO change to be significantly positive at 20–30 km over 15° S–15° N, which demonstrates that the zonal wind field should be the main factor affecting the GW activities at 20–30 km over the tropics. The responses of GW Ep at 20–35 km to ENSO are found to be positive over 15° S–15° N, while at 30–35 km over 15° N–30° N and at 20–35 km near 50° N, significant negative responses of GW Ep to ENSO exist.

Assessing the Impact of ICON/MIGHTI Zonal and Meridional Winds on Upper Atmosphere Weather Specification in a Whole Atmosphere Data Assimilation System: An Observing System Simulation Experiment

AbstractThermospheric data assimilation is limited due to the lack of continuous observation of the neutral state. Recently, the thermospheric wind data from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) on NASA's Ionospheric CONnection Explorer (ICON) became available. ICON/MIGHTI provides near‐continuous observations of the mid‐ and low‐latitude thermospheric meridional and zonal winds ICON/MIGHTI observes the thermosphere zonal and meridional winds at low‐middle latitudes between 90 and 300 km during the daytime, and 90–105 and above 210 km during the nighttime. This study assesses the impact of assimilating ICON/MIGHTI winds in the National Center for Atmospheric Research Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) + Data Assimilation Research Testbed (DART) on the specification and short‐term forecasting of the thermosphere. Observing system simulation experiments of WACCM‐X + DART with and without assimilating synthetic ICON/MIGHTI meridional and zonal wind profiles are performed. The result shows that this new data set can correct the wind specification throughout the mid‐ and low‐latitude thermosphere, especially around the 90–160 km altitude region. A notable impact is also shown in the region above 300 km altitude, which is above the altitude of ICON/MIGHTI wind observations. The impact of ICON/MIGHTI data on the zonal wind field is larger than on the meridional wind field. The errors of the ensemble mean from the truth of meridional and zonal wind fields in the mid‐and low‐latitude region are reduced by 6% and 12%, respectively, with the help of ICON/MIGHTI wind data.

Mapping the zonal winds of Jupiter’s stratospheric equatorial oscillation

Context. Since the 1950s, quasi-periodic oscillations have been studied in the terrestrial equatorial stratosphere. Other planets of the Solar System present (or are expected to present) such oscillations; for example the Jupiter equatorial oscillation and the Saturn semi-annual oscillation. In Jupiter’s stratosphere, the equatorial oscillation of its relative temperature structure about the equator is characterized by a quasi-period of 4.4 yr. Aims. The stratospheric wind field in Jupiter’s equatorial zone has never been directly observed. In this paper, we aim to map the absolute wind speeds in Jupiter’s equatorial stratosphere in order to quantify vertical and horizontal wind and temperature shear. Methods. Assuming geostrophic equilibrium, we apply the thermal wind balance using almost simultaneous stratospheric temperature measurements between 0.1 and 30 mbar performed with Gemini/TEXES and direct zonal wind measurements derived at 1 mbar from ALMA observations, all carried out between March 14 and 22, 2017. We are thus able to self-consistently calculate the zonal wind field in Jupiter’s stratosphere where the JEO occurs. Results. We obtain a stratospheric map of the zonal wind speeds as a function of latitude and pressure about Jupiter’s equator for the first time. The winds are vertically layered with successive eastward and westward jets. We find a 200 m s−1 westward jet at 4 mbar at the equator, with a typical longitudinal variability on the order of ~50 m s−1. By extending our wind calculations to the upper troposphere, we find a wind structure that is qualitatively close to the wind observed using cloud-tracking techniques. Conclusions. Almost simultaneous temperature and wind measurements, both in the stratosphere, are a powerful tool for future investigations of the JEO (and other planetary equatorial oscillations) and its temporal evolution.

Statistical Characteristics of Inertial Gravity Waves Over a Tropical Station in the Western Pacific Based on High‐Resolution GPS Radiosonde Soundings

AbstractThe characteristics of gravity wave activity in tropical areas have been extensively explored, but at present, less attention has been paid to the activity of inertial gravity waves (IGWs) in the Western Pacific based on in‐situ detection data. Based on the radiosonde data over Guam (13.3°N, 144.4°E) for 6 years (2013–2018), the hodograph method and spectrum analysis are used to analyze the statistical characteristics of IGWs in the troposphere (2–14 km) and stratosphere (18–28 km). The gravity wave (GW) energy in the troposphere has clear annual cycle, with the strongest activity in winter and the weakest in monsoon. In the stratosphere, the periodic variation of wave energy is interrupted due to the 2016 quasi‐biennial oscillation (QBO) disruption reflected in the zonal wind field. In the troposphere, the quantity of up‐propagating GWs is close to that of the down‐propagating GWs, while in the stratosphere, they are almost up‐propagating GWs. The horizontal propagation direction of GWs is mainly northeast and southwest in both the troposphere and the stratosphere, since the weak background wind field does not produce significant filtering effect. The spectral amplitude based on the normalized temperature perturbation has clear uniform annual variations in the troposphere and stratosphere, which are the largest in winter and the smallest in monsoon. Due to the “saturation” process in the up‐propagating process of GWs, the spectral slope is more negative in the stratosphere.

On the Influence of Sea Surface Temperature Distributions on the Development of Extratropical Cyclones

AbstractThe sea surface temperature (SST) distribution can modulate the development of extratropical cyclones through sensible and latent heat fluxes. However, the direct and indirect effects of these surface fluxes, and thus the SST, are still not well understood. This study tackles this problem using idealized channel simulations of moist baroclinic development under the influence of surface fluxes. The model is initialized with a zonal wind field resembling the midlatitude jet and a different SST distribution for each experiment, where the absolute SST, the SST gradient, and the meridional position of the SST front are varied. The surface latent heat flux associated with the absolute SST plays a key role in enhancing the moist baroclinic development, while the sensible heat fluxes associated with the SST gradient play a minor role that can be detrimental for the development of the cyclone. The additional moisture provided by the latent heat fluxes originates from about 1000 km ahead of the cyclone a day prior to the time of the most rapid deepening. When the SST in this region is higher than 16°C, the additional latent heat is conducive for explosive cyclone development. For SSTs above 20°C, the cyclones feature characteristics of hybrid cyclones with latent heat release close to their core, maintaining their intensity for a longer period due to continuous and extensive moisture supply from the surface. A high absolute SST with a weak SST gradient, however, can lead to a delay of the deepening stage, because of unorganized convection at early stages reducing environmental baroclinicity.

Dynamics and Electrodynamics of an Ultra‐Fast Kelvin Wave (UFKW) Packet in the Ionosphere‐Thermosphere (IT)

AbstractNumerical experiments are performed using a suite of general circulation models that enable the interaction between a Kelvin wave packet and the ionosphere‐thermosphere (IT) to be elucidated. Focus is on an eastward‐propagating ultra‐fast Kelvin wave (UFKW) packet with periods between 2 and 4 days and zonal wavenumber s=−1 during day of year (DOY) 266–281, 2009. Dissipative processes modify the classic UFKW dynamics (equatorially trapped, small meridional wind component) in three ways: (1) molecular diffusion acts to spread the UFKW zonal (u) and meridional (v) wind fields meridionally, pole to pole, as u and v, respectively, decay and grow with increasing height; (2) due to molecular diffusion, the UFKW spectrum at longer periods and with shorter vertical wavelengths preferentially dissipates with height; and (3) interaction with the diurnally varying IT introduces a westward‐propagating s=+2 component to the wind field that significantly modifies its longitude‐UT structure to include a diurnal modulation. The F‐region ionosphere also responds with s=+2, which originates from the influence of diurnally varying E‐region conductivity on E×B drifts. Additional spectral peaks in v and ionospheric parameters arise due to longitude variations in the magnetic field. Maximum excursions in NmF2 (as compared with those from a simulation without UFKW forcing) achieve values as large as ±50% but more commonly occur in the range of ±20–30%. The combination of positive and negative responses, and their relative magnitudes, depends on the phasing of the UFKW as it moves zonally relative to the Sun‐synchronous diurnal variation of the ionosphere, in addition to its changing amplitude between DOY 266 and 282. Modifications of order 10 ms−1 and −7% to zonal‐mean zonal winds and NmF2, respectively, also result from dissipation of the UFKW packet.

Impact of Gravity Waves on the Middle Atmosphere of Mars: A Non‐Orographic Gravity Wave Parameterization Based on Global Climate Modeling and MCS Observations

AbstractThe impact of gravity waves (GW) on diurnal tides and the global circulation in the middle/upper atmosphere of Mars is investigated using a general circulation model (GCM). We have implemented a stochastic parameterization of non‐orographic GW into the Laboratoire de Météorologie Dynamique (LMD) Mars GCM (LMD‐MGCM) following an innovative approach. The source is assumed to be located above typical convective cells ( 250 Pa), and the effect of GW on the circulation and predicted thermal structure above 1 Pa ( 50 km) is analyzed. We focus on the comparison between model simulations and observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter during Martian Year 29. MCS data provide the only systematic measurements of the Martian mesosphere up to 80 km to date. The primary effect of GW is to damp the thermal tides by reducing the diurnal oscillation of the meridional and zonal winds. The GW drag reaches magnitudes of the order of 1 m/s/sol above 10 Pa in the northern hemisphere winter solstice and produces major changes in the zonal wind field (from tens to hundreds of m/s), while the impact on the temperature field is relatively moderate (10–20 K). It suggests that GW‐induced alteration of the meridional flow is the main responsible for the simulated temperature variation. The results also show that with the GW scheme included, the maximum day‐night temperature difference due to the diurnal tide is around 10 K, and the peak of the tide is shifted toward lower altitudes, in better agreement with MCS observations.

Multi-Instrument Observations of the Atmospheric and Ionospheric Response to the 2013 Sudden Stratospheric Warming Over Eastern Asia Region

We investigate the atmospheric and ionospheric response to the 2013 sudden stratospheric warming (SSW) by using multiple instruments located in Eastern Asia. Three meteor radars and five ionosondes are used to investigate the mesospheric zonal wind fields and ionospheric parameters of F-layer virtual height (h'F), F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -layer peak height (h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), and critical frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) at midand low-latitudes (10.7°N to 40.3°N). The vertical total electron content (TEC) data derived from the groundbased global positioning system receiver network are analyzed to study the ionospheric perturbations in the equatorial ionization anomaly (EIA) region. The changes in equatorial electrojet (EEJ) are observed by using the magnetometer data from stations on and off the magnetic equator. The variations of the h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at Sanya and EEJ strength presented the semidiurnal pattern with increase/decrease and eastward/westward currents in the morning/afternoon hours. In addition, the EIA crest moved poleward/equatorward in the morning/afternoon. The f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> showed the most significant enhancements during daytime at Wuhan and Shaoyang but the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at Sanya and Chumphon reduced mildly. Most importantly, based on the time-period wavelet analysis, the diurnal tidal components in the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> over Beijing, Wuhan, and Sanya seemed similar those in zonal winds and the semidiurnal tides in the low-latitude h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> showed the similar temporal variations as those in EEJ strength during the later phase of SSW. Therefore, apart from the local tides propagating from lower atmosphere having influence on the mid- and low-latitude ionosphere directly during the early phase, the equatorial fountain effect modulated by the enhanced tides also disturbed the EIA region.

Solar‐Related Variations of the Cloud Top Circulation Above Aphrodite Terra From VMC/Venus Express Wind Fields

AbstractWinds derived by a digital tracking technique from ultraviolet (365 nm) images captured by the Venus Monitoring Camera (VMC) onboard the Venus Express spacecraft from 2006 to 2013 were used to study the atmospheric circulation at cloud top level (70 ± 2 km). This data set allows variations of the wind speed with both latitude and longitude to be studied and establishes their correlation with surface topography as well as local time dependence. Both zonal and meridional wind components show some correlation with topography. The minimum zonal wind speed was found at noon above Ovda Regio (10°S, 93°E), the highest region of Aphrodite Terra, one of the largest highlands in the equatorial region. The area of slow zonal wind extends to at least 30°S and shifts in the direction of superrotation in the afternoon and with increasing latitude (poleward). The observed deceleration of cloud top wind was recently attributed to the interaction of the gravity (mountain) waves generated by Aphrodite Terra with the atmospheric circulation. The present study was performed for different local time over the mountainous longitudes. The deceleration pattern in the zonal wind field is mainly conserved within a few hours around noon. Systematic longitude shift is observed in the afternoon in the direction of the evening terminator. Another area of perturbation of both zonal and meridional wind components is observed in the equatorial region around LT = 13–14 hr and may be explained by the solar tide.

Perturbations of O(1D) VER, Temperature, Winds, Atomic Oxygen, and TEC at High Southern Latitudes

AbstractThe study investigates previously unrecognized wind reversals of the daytime thermosphere in the Southern Hemisphere at latitudes of 50°S–70°S employing observations of O(1D) airglow volume emission rates (VER), temperature, and neutral winds at 170–300‐km height by the Wind Imaging Interferometer. Atomic oxygen densities derived from the Wind Imaging Interferometer O+ (732–733 nm) emission observations were also considered together with contemporaneous observations of total electron content by the TOPEX Poseidon satellite mission. The O(1D) VER for fall equinox (March/April 1994) and summer solstice (January 1995) revealed a peak in the VER over the longitude range of 50°E–150°E, for both seasons and daytime local times. The Doppler temperatures also exhibited a maximum at the same location and times. The co‐located zonal wind field shows a westward trough at 50°E–150°E with wind speeds reaching −450 m/s and an eastward peak at 200°E–300°E. Meridional winds show a “peak and trough” structure over the 50°E–150°E region, with a distinct eastern boundary at ~150°E. During summer solstice the atomic oxygen is depleted at the region of the O(1D) VER enhancement in the presence of westward zonal wind. However, the total electron content observations showed a peak that coincided with the O(1D) VER and temperature enhancements.

Resolving the weakening of orographic rainfall over India using a regional climate model RegCM 4.5

In this study, the weakening of the orographic rainfall for the period 2000–2011 during the Indian summer monsoon (JJA) is investigated using the regional climate model RegCM 4.5. Analysis reveals that when sulfate aerosol feedback is included in the model run, the simulated temperature shows a cooling of the continental upper troposphere by 0.3 °C. At the same time, the sulfate aerosol loading in the atmosphere had induced cooling of 0.2 °C of the lower troposphere over the Indian region. Consequently, the simulated zonal wind fields show a weakening at the upper and lower tropospheric levels, particularly where the semi-permanent systems that drive the monsoon circulations are located. It is suggested that the weakening of zonal winds has caused a suppression of the orographic rainfall along the west coast of India (Western Ghats), as evident from the observations. Concurrently, the simulations indicate a sulfate aerosol induced reduction in orographic monsoon rainfall over the southern Western Ghats, whereas their effect is insignificant in controlling the orographic rainfall in the northern Western Ghats. Moreover, our analysis indicates a decline in rainfall over the Himalayan foothills when the sulfate aerosol feedbacks are included, which corroborates well with the observations.

Feature analysis of stratospheric wind and temperature fields over the Antigua site by rocket data

The wind and temperature fields at 20 to 55 km above the Antigua launch site (17°N, 61°W) were analyzed by using sounding rocket data published by the research organization on Stratosphere-Troposphere Processes and their Role in Climate (SPARC). The results showed distinct variations in the wind and temperature fields at different heights from the 1960s to the 1990s. The overall zonal wind speed showed a significant increasing trend with the year, and the overall change in meridional wind speed showed a falling trend from 1976 to 1990, whereas the temperature field showed a significant cooling trend from 1964 to 1990. The times the trends mutated varied at different levels. By taking the altitudes at 20, 35, and 50 km as representatives, we found that the zonal wind speed trend had mutated in 1988, 1986, and 1986, respectively; that the meridional wind speed trend had mutated in 1990, 1986, and 1990, respectively; and that the temperature trend had mutated separately in 1977, 1973, and 1967, respectively. Characteristics of the periodic wind and temperature field variations at different heights were also analyzed, and obvious differences were found in time scale variations across the different layers. The zonal and meridional wind fields were basically characterized as having a significant periodic variation of 5 years across the three layers, and each level was characterized as having a periodic variation of less than 5 years. Temperature field variation at the three levels was basically characterized as occurring in 10-year and 5-year cycles.

A new index for the wintertime southern hemispheric split jet

Abstract. One of the most prominent asymmetric features of the southern hemispheric (SH) circulation is the split jet over Australia and New Zealand in austral winter. Previous studies have developed indices to detect the degree to which the upper-level midlatitude westerlies are split and investigated the relationship between split events and the low-frequency teleconnection patterns, viz. the Antarctic Oscillation (AAO) and the El Niño–Southern Oscillation (ENSO). As the results were inconsistent, the relationship between the wintertime SH split jet and the climate variability indices remains unresolved and is the focus of this study. Until now, all split indices' definitions were based on the specific region where the split jet is recognizable. We consider the split jet as hemispheric rather than a regional feature and propose a new, hemispherical index that is based on the principal components (PCs) of the zonal wind field for the SH winter. A linear combination of PC2 and PC3 of the anomalous monthly (JAS) zonal wind is used to identify split-jet conditions. In a subsequent correlation analysis, our newly defined PC-based split index (PSI) indicates a strong coherence with the AAO. However, this significant relationship is unstable over the analysis period; during the 1980s, the AAO amplitude was higher than the PSI, and vice versa in the 1990s. It is probable that the PSI, as well as the AAO, underlie low-frequency variability on the decadal to centennial timescales, but the analyzed period is too short to draw these conclusions. A regression analysis with the Multivariate ENSO Index points to a nonlinear relationship between PSI and ENSO; i.e., split jets occur during both strong positive and negative phases of ENSO but rarely under “normal” conditions. The Pacific South American (PSA) patterns, defined as the second and third modes of the geopotential height variability at 500 hPa, correlate poorly with the PSI (rPSA−1 ≈ 0.2 and rPSA-2= 0.06), but significantly with the individual components (PCs) of the PSI, revealing an indirect influence on the SH split-jet variability. Our study suggests that the wintertime SH split jet is strongly associated with the AAO, while ENSO is to a lesser extent connected to the PSI. We conclude that a positive AAO phase, as well as both flavors of ENSO and the PSA-1 pattern produce favorable conditions for a SH split event.

Exploring Wave‐Wave Interactions in a General Circulation Model

AbstractNonlinear interactions involving Kelvin waves with (periods, zonal wave numbers) = (3.7d, s =− 1) (UFKW1) and = (2.4d, s =− 1) (UFKW2) and s = 0 and s = 1 quasi 9 day waves (Q9DW) with diurnal tides DW1, DW2, DW3, DE2, and DE3 are explored within a National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) simulation driven at its ∼30 km lower boundary by interpolated 3‐hourly output from Modern‐Era Retrospective Analysis for Research and Applications (MERRA). The existence of nonlinear wave‐wave interactions between the above primary waves is determined by the presence of secondary waves (SWs) with frequencies and zonal wave numbers that are the sums and differences of those of the primary (interacting) waves. Focus is on 10–21 April 2009, when the nontidal dynamics in the mesosphere‐lower thermosphere (MLT) region is dominated by UFKW and when identification of SW is robust. Fifteen SWs are identified in all. An interesting triad is identified involving UFKW1, DE3, and a secondary UFKW4 = (1.5d, s =− 2): The UFKW1‐DE3 interaction produces UFKW4, the UFKW4‐DE3 interaction produces UFKW1, and the UFKW1 interaction with UFKW4 produces DE3. At 120 km the dynamic range of the reconstructed latitude‐longitude zonal wind field due to all of the SW is roughly half that of the primary waves, which produced them. This suggests that nonlinear wave‐wave interactions could significantly modify the way that the lower atmosphere couples with the ionosphere.