Description: Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 90 (2009):459-480, doi:10.1175/2008BAMS2608.1.

Description: The Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25°N by the Asian landmass. Seasonal heating and cooling of the land sets the stage for dramatic monsoon wind reversals, strong ocean–atmosphere interactions, and intense seasonal rains over the Indian subcontinent, Southeast Asia, East Africa, and Australia. Recurrence of these monsoon rains is critical to agricultural production that supports a third of the world's population. The Indian Ocean also remotely influences the evolution of El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), North American weather, and hurricane activity. Despite its importance in the regional and global climate system though, the Indian Ocean is the most poorly observed and least well understood of the three tropical oceans. This article describes the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA), a new observational network designed to address outstanding scientific questions related to Indian Ocean variability and the monsoons. RAMA is a multinationally supported element of the Indian Ocean Observing System (IndOOS), a combination of complementary satellite and in situ measurement platforms for climate research and forecasting. The article discusses the scientific rationale, design criteria, and implementation of the array. Initial RAMA data are presented to illustrate how they contribute to improved documentation and understanding of phenomena in the region. Applications of the data for societal benefit are also described.

Description: Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 90 (2009): ES5-ES8, doi:10.1175/2008BAMS2608.2.

Description: [1] We study seismic velocity and attenuation structures in the top 400 km of the Earth's inner core based on modeling of differential traveltimes, amplitude ratios, and waveforms of the PKiKP-PKIKP phases observed at the epicentral distance range of 120°–141° and the PKPbc-PKIKP phases observed at the distance range of 146°–160° along equatorial paths. Our data are selected from the seismograms recorded in the Global Seismographic Network from 1990 to 2001 and many regional seismic networks. The observed PKiKP-PKIKP and PKPbc-PKIKP phases exhibit distinctive “east-west” hemispheric patterns: (1) At the distance ranges of 131°–141° and 146°–151°, PKIKP phases arrive about 0.3 s earlier than the theoretical arrivals based on the Preliminary Reference Earth Model (PREM) for the PKIKP phases sampling the “eastern hemisphere” (40°E–180°E) of the inner core and about 0.4 s later for those sampling the “western hemisphere” (180°W–40°E). At the distance range of 151°–160°, PKIKP phases arrive about 0.7 s earlier than the predicted arrivals based on PREM for those sampling the eastern hemisphere and about 0.1 s later for those sampling the western hemisphere. (2) Amplitude ratios of the PKIKP/PKiKP phases at the distance range of 131°–141° and of the PKIKP/PKPbc phases at the distance range of 146°–151° are, in general, smaller for the PKIKP phases sampling the eastern hemisphere than for those sampling the western hemisphere. At distances greater than 151°, the PKIKP/PKPbc amplitude ratios become indistinguishable for the two hemispheres. These observations can be best explained by two different types of seismic velocity and attenuation models along equatorial paths, one for each hemisphere, in the top 400 km of the inner core. For the eastern hemisphere, the velocity structure has a velocity increase of 0.748 km/s across the inner core boundary (ICB), a small velocity gradient of 0.0042 (km/s)/100 km in the top 235 km, followed by a steeper velocity gradient of 0.1 (km/s)/100 km extending from 235 km to 375 km, and a velocity gradient of 0.01 (km/s)/100 km in the deeper portion of the inner core; the attenuation structure has an average Q value of 300 in the top 300 km and an average Q value of 600 in the deeper portion of the inner core. For the western hemisphere, the velocity structure has a velocity increase of 0.645 km/s across the ICB and a velocity gradient of 0.049 (km/s)/100 km in the top 375 km; the attenuation structure has an average Q value of 600 in the top 375 km of the inner core. Our results suggest that the inner core hemispheric variations in velocity extend deeper than 375 km below the ICB and the top 235 km of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core in having a small velocity gradient, high velocity, and high attenuation.

Description: The inner core boundary (ICB), where melting and solidification of the core occur, plays a crucial role in the dynamics of the Earth's interior. To probe temporal changes near the ICB beneath the eastern hemisphere, I analyze differential times of PKiKP (dt(PKiKP)), double differential times of PKiKP‐PKPdf, and PKiKP coda waves from repeating earthquakes in the southwest Pacific subduction zones. dt(PKiKP) values are mostly within ±30 ms of one another, without systematic temporal dependence. Some observations of PKiKP coda waves have absolute time shifts of 〉50 ms relative to their main phases. The combination of temporal changes in PKiKP coda arrivals and negligible changes in PKiKP arrivals favors a smooth ICB with fine‐scale structures in the upper inner core. dt(PKiKP) values are interpreted in the context of melting‐ or growth‐induced ICB topography, based on dynamic models. Uncertainties in dt(PKiKP) prevent verification of ICB melting or growth on decadal time scales.

Description: This study examines time-dependent inner core structures using waveforms and double differential times of the PKP(bc–df) and PKP(ab–df) phases measured from repeating earthquakes in the southwest Pacific subduction zones. Repeating earthquakes can eliminate potential artefacts of interevent distance and improve the measurement precision of temporal changes in PKPdf phases due to differential rotation of the Earth's inner core. PKPdf waves from the southwest Pacific primarily sample the eastern hemisphere of the inner core along equatorial paths. Time separation of repeating earthquakes ranges from 4 to 14.4 yr. Most observed double differential times of PKP(bc–df) and PKP(ab–df) are within ±70 ms, with no systematic changes as a function of time separation or calendar time. Null temporal changes of the PKPdf wave could indicate a smooth regional-scale lateral velocity gradient in the eastern hemisphere of the inner core. Uncertainties in the data prohibit statistically meaningful estimates of the lateral velocity gradient, temporal trend, inner core differential rotation rate, or decadal oscillations. Synthetic seismograms are used to test the effects of several possible artefacts and to quantify the magnitudes of velocity perturbations relative to previous estimates. These artefacts are quantitatively assessed to determine their expected effects on the measurements.

Description: We report complex seismic anisotropy in the top 80 km of the Earth's inner core beneath Africa. The anisotropy in the top 80 km of the inner core is constrained using differential travel times, amplitude ratios, and waveforms of the PKiKP-PKIKP phases sampling Africa along various directions. The differential PKiKP-PKIKP time residuals (relative to the Preliminary Reference Earth Model [PREM]) along the polar paths are larger than those along the equatorial paths by 0–1.4 s, indicating the presence of seismic anisotropy in the top 80 km of the inner core. Furthermore, the observations along the polar paths show complex regional variations beneath Africa: the differential PKiKP-PKIKP travel time residuals vary from 1.2 s beneath eastern Africa, to −0.1 s beneath central Africa, and to −0.2 to 0.8 s beneath western Africa. A correlation between small PKIKP/PKiKP amplitude ratios and large differential PKiKP-PKIKP travel time residuals is observed. The waveform data are spatially binned into six groups to constrain the regional dependence of velocity and attenuation anisotropy in the top 80 km of the inner core. Overall, the seismic data can be explained by an isotropic upper inner core (UIC) overlying an anisotropic lower inner core (LIC) in the top 80 km of the inner core across Africa. The thickness of the isotropic UIC varies from 0 to 50 km, and the P velocity transition from the isotropic UIC to the anisotropic LIC is sharp, with velocity increases laterally varying from 1.6% to 2.2%. The attenuation structure along the polar paths has a Q value of 600 for the isotropic UIC and Q values varying from 150 to 400 for the anisotropic LIC. The complex seismic anisotropy in the top of the inner core is found in a region where a rapid change of the inner core boundary (ICB) between 1993 and 2003 was discovered (Wen, 2006) and may be explained by complex alignments of iron crystals, resulting from a localized anomalous solidification of the inner core.

Description: Numerous studies have indicated that the atmospheric heat source (AHS) over the Tibetan Plateau (TP) is highly correlated with the western North Pacific anomalous anticyclone (WNPAC) in summer. However, such an interannual relationship has been weakened since the late 1990s. The present work shows that the TP AHS was significantly and positively correlated with the WNPAC in 1979–1999 (P1), while this relationship became insignificant hereafter (2000–2020; P2). We identify that the long-term change in the upper-level atmospheric circulation over the TP is responsible for weakening the relationship. An obvious upper-level anticyclonic trend occurred over the northeastern TP in the past four decades, with an easterly trend on the anticyclone’s southern flank, representing anomalous westerlies during P1 but anomalous easterlies during P2 over the main portion of the TP. With the anomalous upper-level westerlies in P1, the abnormal high pressure induced by the TP heating (i.e. AHS) extended downstream in the upper troposphere. Subsequently, anomalous descending motions formed over the northwestern Pacific due to the eastward-extended high pressure, together with the vertical transport of negative relative vorticity, favorable for the enhancement of the WNPAC. Whereas in P2, the TP heating-induced abnormal high pressure was confined over the southern TP due to the anomalous easterlies, suppressing its downstream influence and finally breaking the connection between the TP AHS and the WNPAC. Modeling results from both LBM sensitivity experiments and CESM large ensemble dataset further confirm the important role of the change in background circulation in weakening the relationship.

Description: An extreme drought occurred over Southeast China (SEC) in August 2019. We demonstrate synergistic effects of mid-latitude and tropical circulation on this extreme event and highlight the impacts of the coupling and locking of two cyclones at different latitudes, which are otherwise ignored. We propose the relaying roles of the Tibetan Plateau (TP) and western North Pacific in connection with the tropical convection and SEC precipitation. The equivalent-barotropic anticyclone over the TP and low-tropospheric cyclone over the western North Pacific both resulted from the positive Indian Ocean dipole and El Niño Modoki. The equivalent-barotropic cyclone over Northeast China originated from the dispersion of Rossby waves upstream along the subtropical waveguide associated with the North Atlantic tripole sea surface temperature anomaly pattern and the Rossby wave response to the TP precipitation deficiency. Further, they jointly contributed to this drought by inducing strong northerly wind anomalies in the entire troposphere over East China. These anomalous northerly winds led to decreased warm moisture from the south and substantial sinking motions, which inhibited the occurrence of the SEC local convection and precipitation. The SEC precipitation is closely related to convection over the Maritime Continent from a climate perspective. This relationship is verified by observations, linear baroclinic model experiments, and general circulation model sensitivity experiments with and without the TP, in which precipitation anomalies over the southern TP and Philippine Sea play important bridge roles. The results will advance the prediction of the SEC extreme drought events.

Description: The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on NASA’s TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite measure vertical profiles of kinetic temperature, pressure, geopotential height and volume mixing ratios of minor species such as CO2, H2O (two important greenhouse gases) since January 2001. With its two-solar-cycle record, we can quantify the long term changes of global temperature, CO2, and H2O and distinguish their solar cycle variations. In this talk, we will review the SABER measured linear trends of temperature, CO2 and H2O. Global temperature in the stratosphere and mesosphere has been cooling due to anthropogenic greenhouse gas, i.e., CO2. CO2 and H2O have also been observed to increase due to anthropogenic CO2 and CH4 emission and warming tropopause. Meanwhile, we will discuss the lessons we learnt while calculating the trends from SABER.

Description: Super typhoons (SuperTYs) generated in the Northwest Pacific (WNP) constantly undergo a sharp weakening after crossing the Philippines-Taiwan into the South China Sea (SCS), thus acting as a natural “buffer” to protect the south-eastern coast of China from severe typhoons due to the blockage of the mountains and the unique atmospheric and oceanic environmental fields. This study examines the determinants of this buffer zone and speculates on the response of this part of the SuperTYs in future climate change. Here, we show that the strong vertical wind shear accompanying the South China Sea summer monsoon is a determining factor in the weakening of SuperTYs into the buffer zone, with a linear correlation up to 0.71. Although most studies suggest that the risk of severe typhoons will increase with global warming, our diagnosis of the latest Sixth Coupled Model Intercomparison Project (CMIP6) multi-model ensemble shows that the decisive factor, vertical wind shear, will not change in trend even in the worst scenario. Therefore, the risk of a severe typhoon making landfall off the southeast coast of China is not getting any worse in future global warming.