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NASA NTRS1995

Augmented Shock Wave Severance of Materials

This paper describes a new approach for severing or weakening a variety of materials. The technique employs embedding explosive cords into parallel grooves that are cut into a surface of a material. The cords are initiated simultaneously to produce shock waves that progress toward the centerline between the cords and the lower surface of the material. Intersecting incident and reflected waves augment at the centerline to fail or weaken the material in tension. No harmful debris is produced on the opposite side of the material from the explosive cords. The primary focus of the effort described in this paper was to fracture the F-16 aircraft trilaminate canopy. Also, complete severance was achieved in 2024-T4 aluminum plate stock. Possible applications are through canopy egress and crew module severance from military aircraft and separation of rocket vehicle stages and payloads. This approach offers important advantages over explosive methods currently in use.

Bement, Laurence J.↗

NASA NTRS1999

Centrosome and microtubule instability in aging Drosophila cells

Several cytoskeletal changes are associated with aging which includes alterations in muscle structure leading to muscular atrophy, and weakening of the microtubule network which affects cellular secretion and maintenance of cell shape. Weakening of the microtubule network during meiosis in aging oocytes can result in aneuploidy or trisomic zygotes with increasing maternal age. Imbalances of cytoskeletal organization can lead to disease such as Alzheimer's, muscular disorders, and cancer. Because many cytoskeletal diseases are related to age we investigated the effects of aging on microtubule organization in cell cultures of the Drosophila cell model system (Schneider S-1 and Kc23 cell lines). This cell model is increasingly being used as an alternative system to mammalian cell cultures. Drosophila cells are amenable to genetic manipulations and can be used to identify and manipulate genes which are involved in the aging processes. Immunofluorescence, scanning, and transmission electron microscopy were employed for the analysis of microtubule organizing centers (centrosomes) and microtubules at various times after subculturing cells in fresh medium. Our results reveal that centrosomes and the microtubule network becomes significantly affected in aging cells after 5 days of subculture. At 5-14 days of subculture, 1% abnormal out of 3% mitoses were noted which were clearly distinguishable from freshly subcultured control cells in which 3% of cells undergo normal mitosis with bipolar configurations. Microtubules are also affected in the midbody during cell division. The midbody in aging cells becomes up to 10 times longer when compared with midbodies in freshly subcultured cells. During interphase, microtubules are often disrupted and disorganized, which may indicate improper function related to transport of cell organelles along microtubules. These results are likely to help explain some cytoskeletal disorders and diseases related to aging.

Non-NASA Center↗

NASA NTRS1987

Hypogravity-induced atrophy of rat soleus and extensor digitorum longus muscles

Prolonged exposure of humans to hypogravity causes weakening of their skeletal muscles. This problem was studied in rats exposed to hypogravity for 7 days aboard Spacelab 3. Hindlimb muscles were harvested 12-16 hours postflight for histochemical, biochemical, and ultrastructural analyses. The majority of the soleus and extensor digitorum longus fibers exhibited simple cell shrinkage. However, approximately 1% of the fibers in flight soleus muscles appeared necrotic. Flight muscle fibers showed increased glycogen, lower subsarcolemmal staining for mitochondrial enzymes, and fewer subsarcolemmal mitochondria. During atrophy, myofibrils were eroded by multiple focal losses of myofilaments; lysosomal autophagy was not evident. Tripeptidylaminopeptidase and calcium-activated protease activities of flight soleus fibers were significantly increased, implying a role in myofibril breakdown. Simple fiber atrophy appears to account for muscle weakening during spaceflight, but fiber necrosis is also a contributing factor.

manned↗

NASA NTRS2005

[Summary of Research on Relationship Between Core Convective Structure and Intensity Change in Tropical Cyclones]

The downshear reformation of Tropical Storm Gabrielle (2001) was investigated using radar reflectivity and lightning data that were nearly continuous in time, as well as frequent aircraft reconnaissance flights. Initially the storm was a marginal tropical storm in an environment with strong 850-200 hPa vertical wind shear of 12-13 meters per second and an approaching upper tropospheric trough. Both the observed outflow and an adiabatic balance model calculation showed that the radial-vertical circulation increased with time as the trough approached. Convection was highly asymmetric, with almost all radar return located in one quadrant left of downshear in the storm. Reconnaissance data show that an intense mesovortex formed downshear of the original center. This vortex was located just south of, rather than within, a strong downshear left lightning outbreak, consistent with tilting of the horizontal vorticity associated with the vertical wind shear. The downshear mesovortex contained a 972 hPa minimum central pressure, 20 hPa lower than minimum pressure in the original vortex just three hours earlier. The mesovortex became the new center of the storm, but weakened somewhat prior to landfall. It is argued that dry air carried around the storm from the region of upshear subsidence, as well as the direct effects of the shear, prevented the reformed vortex from continuing to intensify. Despite the subsequent weakening of the reformed center, it reached land with greater intensity than the original center. It is argued that this intensification process was set into motion by the vertical wind shear in the presence of an environment with upward motion forced by the upper tropospheric trough. In addition, the new center formed much closer to the coast and made landfall much earlier than predicted. Such vertical shear-induced intensity and track fluctuations are important to understand, especially in storms approaching the coast. The structures of the highly sheared tropical storm Chantal During CAMEX-4 is also discussed.

Source record↗

NASA NTRS2005

Near Infrared Spectra of Mixtures Relevant to Icy Satellites

We will present near IR spectra of ice mixtures and review the differences between spectra of mixtures and those of pure solids: the creation of new features, weakening and shifting of bands, dependence on concentration, and changes with temperature. The forbidden CO2 (2nu3) overtone at 2.134 microns (4685/cm) is absent from the spectrum of pure CO2, but present in all of the following mixtures: H2O/CO2 = 5 and = 25, H2O:CH3OH:CO2 = 100:2.5:1, and CH3OH:CO2 = 5. Also, in mixtures of H2O and any other material, we see a feature at 1.89 microns (5290/cm) that is possibly related to the "dangling OH" feature at 2.73 microns (3360/cm). The features of a material in H2O are generally weaker and shifted to longer wavelength in comparison to the pure substance. For example, the largest near-IR absorption of pure solid CH4 is located at 2.324 microns (4303/cm) but is broader and at slightly longer wavelength in samples mixed with H2O. The degree of shifting and weakening depends on the ratio of the mixture. The mixture mentioned above was at a ratio of H2O/ CH4 = 3. When the ratio rises to H2O/ CH4 = 87, the CH4 feature at 2.324 microns is shifted to shorter wavelength and is much broader and weaker. In CH4/ H2O mixtures the peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H2O are irreversible. In short, mixtures created in the lab produce spectra that are very different from modeled combinations of end member species. Recent Cassini VIMS observations show the CO2 fundamental at 4.255 microns (2350/cm) on Iapetus [l] and at 4.26 microns (2347/cm) on Phoebe [2], while Galileo NIMS observed it at 4.25 microns (2353/cm) on Ganymede [3]. Since pure CO2 is located at 4.266 (2344/cm), the CO2 must be mixed with something else to produce the shift. A mixture of CH3OH:CO2 = 5 at 90 K shifts the fundamental to 4.262 microns (2346/cm). The shifts in the feature between satellites could indicate variations in concentration and temperature, or the introduction of a new compound, encouraging further lab investigation.

Mastrapa, Rachel M. E.↗

NASA NTRS2007

Evidence for Interannual to Decadal Variations in Hadley and Walker Circulations and Links to Water and Energy Fluxes

Mass and energy transports associated with the Hadley and Walker circulations are important components of the earth s climate system and are strongly linked to hydrologic processes. Interannual to decadal variation in these flows likely signify a combination of natural climate noise as well as a response to anthropgenic forcing. There remains considerable uncertainty in quantifying variations in these flows. Evidence in the surface pressure record supports a weakening of the Walker circulation over the Pacific in recent decades. Conversely the NCEP / NCAR and ERA 40 reanalyses indicate that the Hadley circulation has increased in strength over the last two decades, though these analyses depict significantly different mass circulation changes. Interestingly, the NCEP - II / DOE reanalysis contains essentially no Hadley circulation changes. Most climate model integrations anticipate a weakening of both tropical circulations associated with stronger static stability. Clearly there is much uncertainty not only with the mass transports, but also how they are linked to water and energy balance of the planet through variations in turbulent heat and radiative fluxes and horizontal exports / imports of energy. Here we examine heat and water budget variations from a number of reanalysis products and focus on the linear and nonlinear response of ENSO warm and cold events as opportunities to study budget variations over the past 15-20 years. Our analysis addresses such questions as To what extent do Hadley and Walker Cell variations compensate each other on mass and energy transport? Do static stability adjustments appear to constrain fractional precipitation response vs. fractional water vapor response? We appeal to constraints offered by GPCP precipitation, SSWI ocean evaporation estimates, and ISCCP-FD radiative fluxes, and other satellite data sets to interpret and confirm reanalysis-based diagnostics. Using our findings we also attempt to place in context the recent findings that tropical ocean evaporation increased by order 5% or more during the 1990s, reconciling this with GPCP precipitation variations.

Robertson, Franklin↗

NASA NTRS2007

Initiation of Solar Eruptions

We consider processes occurring just prior to and at the start of the onset of flare- and CME-producing solar eruptions. Our recent work uses observations of filament motions around the time of eruption onset as a proxy for the evolution of the fields involved in the eruption. Frequently the filaments show a slow rise prior to fast eruption, indicative of a slow expansion of the field that is about co explode. Work by us and others suggests that reconnection involving emerging or canceling flux results in a lengthening of fields restraining the filament-carrying field, and the consequent upward expansion of the field in and around the filament produces the filament's slow rise: that is, the reconnection weakens the magnetic "tethers" ("tether-weakening" reconnection), and results in the slow rise of the filament. It is still inconclusive, however, what mechanism is responsible for the switch from the slow rise to the fast eruption.

Sterling, Alphonse C.↗

NASA NTRS2009

Life of a Six-Hour Hurricane

Hurricane Claudette developed from a weak vortex in 6 h as deep convection shifted from downshear into the vortex center, despite ambient vertical wind shear exceeding 10 m/s. Six hours later it weakened to a tropical storm, and 12 h after the hurricane stage a circulation center could not be found at 850 hPa by aircraft reconnaissance. At hurricane strength the vortex contained classic structure seen in intensifying hurricanes, with the exception of 7-12 C dewpoint depressions in the lower troposphere upshear of the center. These extended from the 100-km radius to immediately adjacent to the eyewall, where equivalent potential temperature gradients reached 6 K/km. The dry air was not present prior to intensification, suggesting that it was associated with vertical shear-induced subsidence upshear of the developing storm. It is argued that weakening of the vortex was driven by cooling associated with the mixing of dry air into the core, and subsequent evaporation and cold downdrafts. Evidence suggests that this mixing might have been enhanced by eyewall instabilities after the period of rapid deepening. The existence of a fragile, small, but genuinely hurricane-strength vortex at the surface for 6 h presents difficult problems for forecasters. Such a "temporary hurricane" in strongly sheared flow might require a different warning protocol than longer-lasting hurricane vortices in weaker shear.

Shelton, Kay L.↗

NASA NTRS2009

The Dynamics and Structure of Titan's Middle Atmosphere

Titan, after Venus, is the second example in the solar system of an atmosphere with a global cyclostrophic circulation. The origin and maintenance of these superrotating atmospheres is not well understood, but Titan has a strong seasonal modulation in the middle atmosphere, and the seasonal changes in the winds may offer clues. The pole in winter and early spring is characterized by temperatures 20-30 K cooler at 100-170 km than those at low latitudes, and strong circumpolar winds as high as 190 m/s at 200-250 km. At these levels the polar region is characterized by enhanced concentrations of several organic gases, and also detectable condensates. All this suggests that the polar vortex provides a mixing barrier between winter polar and lower-latitude air masses, analogous to the winter polar vortices on Earth. Because the concentrations of organic gases increase with altitude in the middle atmosphere, the observed enhancements suggest subsidence over the winter pole. Consistent with this are the observed temperatures approx.200 K at the winter-polar stratopause (280 km), making it the warmest part of the atmosphere. The warm stratopause likely results from adiabatic heating associated with the subsidence. Recent observations in late northern winter and early spring indicate that the warm anomaly at the winter-polar stratopause is weakening, and the strong zonal winds are weakening. Curiously, the stratospheric zonal winds and temperatures in both hemispheres are symmetric about a pole that is offset from the surface pole by 4 degrees. The cause of this is not well understood, but it may reflect the response of a cyclostrophic circulation to the offset between the equator, where the distance to the rotation axis is greatest, and the seasonally varying subsolar latitude.

Flasar, F. Michael↗

NASA NTRS2011

Exploring the Use of Radar for Physically-Based Nowcasting of Lightning Cessation

NASA's Marshall Space Flight Center and the University of Alabama in Huntsville (UAHuntsville) are collaborating with the 45th Weather Squadron (45WS) at Cape Canaveral Air Force Station (CCAFS) to enable improved nowcasting of lightning cessation. This project centers on use of dual-polarimetric radar capabilities, and in particular, the new C-band dual polarimetric weather radar acquired by the 45WS. Special emphasis is placed on the development of a physically-based operational algorithm to predict lightning cessation. While previous studies have developed statistically based lightning cessation algorithms driven primarily by trending in the actual total lightning flash rate, we believe that dual polarimetric radar variables offer the possibility to improve existing algorithms through the inclusion of physically meaningful trends reflecting interactions between in-cloud electric fields and ice-microphysics. Specifically, decades of polarimetric radar research using propagation differential phase has demonstrated the presence of distinct phase and ice crystal alignment signatures in the presence of strong electric fields associated with lightning. One question yet to be addressed is: To what extent can propagation phase-based ice-crystal alignment signatures be used to nowcast the cessation of lightning activity in a given storm? Accordingly, data from the UAHuntsville Advanced Radar for Meteorological and Operational Research (ARMOR) along with the NASA-MSFC North Alabama Lightning Mapping Array are used in this study to investigate the radar signatures present before and after lightning cessation. Thus far our case study results suggest that the negative differential phase shift signature weakens and disappears after the analyzed storms ceased lightning production (i.e., after the last lightning flash occurred). This is a key observation because it suggests that while strong electric fields may still have been present, the lightning cessation signature was encompassed in the period of the polarimetric negative phase shift signature. To the extent this behavior is repeatable in other cases, even if only in a substantial fraction of those cases, the analysis suggests that differential propagation phase may prove to be a useful parameter for future lightning cessation algorithms. Indeed, a preliminary analysis of 15+ cases has shown additional indications of the weakening and disappearance of this ice alignment signature with lightning cessation. A summary of these case-study results is presented.

Schultz, Elise V.↗

NASA NTRS2011

Single-Event Gate Rupture in Power MOSFETs: A New Radiation Hardness Assurance Approach

Almost every space mission uses vertical power metal-semiconductor-oxide field-effect transistors (MOSFETs) in its power-supply circuitry. These devices can fail catastrophically due to single-event gate rupture (SEGR) when exposed to energetic heavy ions. To reduce SEGR failure risk, the off-state operating voltages of the devices are derated based upon radiation tests at heavy-ion accelerator facilities. Testing is very expensive. Even so, data from these tests provide only a limited guide to on-orbit performance. In this work, a device simulation-based method is developed to measure the response to strikes from heavy ions unavailable at accelerator facilities but posing potential risk on orbit. This work is the first to show that the present derating factor, which was established from non-radiation reliability concerns, is appropriate to reduce on-orbit SEGR failure risk when applied to data acquired from ions with appropriate penetration range. A second important outcome of this study is the demonstration of the capability and usefulness of this simulation technique for augmenting SEGR data from accelerator beam facilities. The mechanisms of SEGR are two-fold: the gate oxide is weakened by the passage of the ion through it, and the charge ionized along the ion track in the silicon transiently increases the oxide electric field. Most hardness assurance methodologies consider the latter mechanism only. This work demonstrates through experiment and simulation that the gate oxide response should not be neglected. In addition, the premise that the temporary weakening of the oxide due to the ion interaction with it, as opposed to due to the transient oxide field generated from within the silicon, is validated. Based upon these findings, a new approach to radiation hardness assurance for SEGR in power MOSFETs is defined to reduce SEGR risk in space flight projects. Finally, the potential impact of accumulated dose over the course of a space mission on SEGR susceptibility is explored. SEGR evaluation of gamma-irradiated power MOSFETs suggests a non-significant SEGR susceptibility enhancement due to accumulated dose from gamma rays. During SEGR testing, an unexpected enhanced dose effect from heavy-ion irradiation was detected. We demonstrate that this effect could be due to direct ionization by two or more ions at the same channel location. The probability on-orbit for such an occurrence is near-zero given the low heavy-ion fluence over a typical mission lifetime, and did not affect SEGR susceptibility. The results of this work can be used to bound the risk of SEGR in power MOSFETs considered for insertion into spacecraft and instruments.

Lauenstein, Jean-Marie↗

NASA NTRS2011

Exploring the Use of Radar for a Physically Based Lightning Cessation Nowcasting Tool

NASA s Marshall Space Flight Center (MSFC) and the University of Alabama in Huntsville (UAHuntsville) are collaborating with the 45th Weather Squadron (45WS) at Cape Canaveral Air Force Station (CCAFS) to enable improved nowcasting of lightning cessation. This project centers on use of dual-polarimetric radar capabilities, and in particular, the new C-band dual-polarimetric weather radar acquired by the 45WS. Special emphasis is placed on the development of a physically based operational algorithm to predict lightning cessation. While previous studies have developed statistically based lightning cessation algorithms, we believe that dual-polarimetric radar variables offer the possibility to improve existing algorithms through the inclusion of physically meaningful trends reflecting interactions between in-cloud electric fields and hydrometeors. Specifically, decades of polarimetric radar research using propagation differential phase has demonstrated the presence of distinct phase and ice crystal alignment signatures in the presence of strong electric fields associated with lightning. One question yet to be addressed is: To what extent can these ice-crystal alignment signatures be used to nowcast the cessation of lightning activity in a given storm? Accordingly, data from the UAHuntsville Advanced Radar for Meteorological and Operational Research (ARMOR) along with the NASA-MSFC North Alabama Lightning Mapping Array are used in this study to investigate the radar signatures present before and after lightning cessation. Thus far, our case study results suggest that the negative differential phase shift signature weakens and disappears after the analyzed storms ceased lightning production (i.e., after the last lightning flash occurred). This is a key observation because it suggests that while strong electric fields may still have been present, the lightning cessation signature encompassed the period of the polarimetric negative phase shift signature. To the extent this behavior is repeatable in other cases, even if only in a substantial fraction of those cases, the case analyses suggests that differential propagation phase may prove to be a useful parameter for future lightning cessation algorithms. Indeed, analysis of 15+ cases has shown additional indications of the weakening and disappearance of this ice alignment signature with lightning cessation. A summary of results will be presented.

Schultz, Elise V.↗

NASA NTRS2010

Atmospheric Circulation Anomalies During Two Persistent North American Droughts: 1932-1939 and 1948-1957

We use an early twentieth century (1908-1958) atmospheric reanalysis, based on assimilation of surface and sea level pressure observations, to contrast atmospheric circulation during two periods of persistent drought in North America: 1932-1939 (the Dust Bowl) and 1948-1957. Primary forcing for both droughts is believed to come from anomalous sea surface temperatures (SSTs): a warm Atlantic and a cool eastern tropical Pacific. For boreal winter (October-March) in the 1950s, a stationary wave pattern originating from the tropical Pacific is present, with positive centers over the north Pacific and north Atlantic ocean basins and a negative center positioned over northwest North America and the tropical/subtropical Pacific. This wave train is largely absent for the 1930s drought; boreal winter height anomalies are organized much more zonally, with positive heights extending across northern North America. For boreal summer (April-September) during the 1930s, a strong upper level ridge is centered over the Great Plains; this feature is absent during the 1950s and appears to be linked to a weakening of the Great Plains low-level jet (GPLLJ). Subsidence anomalies are co-located over the centers of each drought: in the central Great Plains for the 1930s and in a band extending from the southwest to the southeastern United States for the 1950s. The location and intensity of this subsidence during the 1948-1957 drought is a typical response to a cold eastern tropical Pacific, but for 1932-1939 deviates in terms of the expected intensity, location, and spatial extent. Overall, circulation anomalies during the 1950s drought appear consistent with the expected response to the observed SST forcing. This is not the case for the 1930s, implying some other causal factor may be needed to explain the Dust Bowl drought anomalies. In addition to SST forcing, the 1930s were also characterized by massive alterations to the land surface, including regional-scale devegetation from crop failures and intensive wind erosion and dust storms. Incorporation of these land surface factors into a general circulation model greatly improves the simulation of precipitation and subsidence anomalies during this drought, relative to simulations with SST forcing alone. Even with additional forcing from the land surface, however, the model still has difficulty reproducing some of the other circulation anomalies, including weakening of the GPLLJ and strengthening of the upper level ridge during AMJJAS. This may be due to either weaknesses in the model or uncertainties in the boundary condition estimates. Still, analysis of the circulation anomalies supports the conclusion of an earlier paper (Cook et al. in Proc Natl Acad Sci 106:4997, 2009), demonstrating that land degradation factors are consistent with the anomalous nature of the Dust Bowl drought.

Cook, Benjamin↗

NASA NTRS2012

On the Influence of North Pacific Sea Surface Temperature on the Arctic Winter Climate

Differences between two ensembles of Goddard Earth Observing System Chemistry-Climate Model simulations isolate the impact of North Pacific sea surface temperatures (SSTs) on the Arctic winter climate. One ensemble of extended winter season forecasts is forced by unusually high SSTs in the North Pacific, while in the second ensemble SSTs in the North Pacific are unusually low. High Low differences are consistent with a weakened Western Pacific atmospheric teleconnection pattern, and in particular, a weakening of the Aleutian low. This relative change in tropospheric circulation inhibits planetary wave propagation into the stratosphere, in turn reducing polar stratospheric temperature in mid- and late winter. The number of winters with sudden stratospheric warmings is approximately tripled in the Low ensemble as compared with the High ensemble. Enhanced North Pacific SSTs, and thus a more stable and persistent Arctic vortex, lead to a relative decrease in lower stratospheric ozone in late winter, affecting the April clear-sky UV index at Northern Hemisphere mid-latitudes.

Hurwitz, Margaret M.↗

NASA NTRS2012

Long-Term Changes in Stratospheric Age Spectra in the 21st Century in the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM)

In this study we investigate the long-term variations in the stratospheric age spectra using simulations of the 21st century with the Goddard Earth Observing System Chemistry- Climate Model (GEOSCCM). Our purposes are to characterize the long-term changes in the age spectra and identify processes that cause the decrease of the mean age in a warming climate. Changes in the age spectra in the 21st century simulations are characterized by decreases in the modal age, the mean age, the spectral width, and the tail decay timescale. Our analyses show that the decrease in the mean age is caused by two processes: the acceleration of the residual circulation that increases the young air masses in the stratosphere, and the weakening of the recirculation that leads to the decrease of tail of the age spectra and the decrease of the old air masses. The weakening of the stratospheric recirculation is also strongly correlated with the increase of the residual circulation. One important result of this study is that the decrease of the tail of the age spectra makes an important contribution to the decrease of the main age. Long-term changes in the stratospheric isentropic mixing are investigated. Mixing increases in the subtropical lower stratosphere, but its impact on the age spectra is outweighed by the increase of the residual circulation. The impacts of the long-term changes in the age spectra on long-lived chemical traces are also investigated. 37 2

Li, Feng↗

NASA NTRS2012

Solar Flare Impacts on Ionospheric Electrodynamics

The sudden increase of X-ray and extreme ultra-violet irradiance during flares increases the density of the ionosphere through enhanced photoionization. In this paper, we use model simulations to investigate possible additional contributions from electrodynamics, finding that the vertical E X B drift in the magnetic equatorial region plays a significant role in the ionosphere response to solar flares. During the initial stage of flares, upward E X B drifts weaken in the magnetic equatorial region, causing a weakened equatorial fountain effect, which in turn causes lowering of the peak height of the F2 region and depletion of the peak electron density of the F2 region. In this initial stage, total electron content (TEC) enhancement is predominantly determined by solar zenith angle control of photoionization. As flares decay, upward E X B drifts are enhanced in the magnetic equatorial region, causing increases of the peak height and density of the F2 region. This process lasts for several hours, causing a prolonged F2-region disturbance and TEC enhancement in the magnetic equator region in the aftermath of flares. During this stage, the global morphology of the TEC enhancement becomes predominantly determined by these perturbations to the electrodynamics of the ionosphere.

Qian, Liying↗

NASA NTRS2013

The Aqua-planet Experiment (APE): Response to Changed Meridional SST Profile

This paper explores the sensitivity of Atmospheric General Circulation Model (AGCM) simulations to changes in the meridional distribution of sea surface temperature (SST). The simulations are for an aqua-planet, a water covered Earth with no land, orography or sea- ice and with specified zonally symmetric SST. Simulations from 14 AGCMs developed for Numerical Weather Prediction and climate applications are compared. Four experiments are performed to study the sensitivity to the meridional SST profile. These profiles range from one in which the SST gradient continues to the equator to one which is flat approaching the equator, all with the same maximum SST at the equator. The zonal mean circulation of all models shows strong sensitivity to latitudinal distribution of SST. The Hadley circulation weakens and shifts poleward as the SST profile flattens in the tropics. One question of interest is the formation of a double versus a single ITCZ. There is a large variation between models of the strength of the ITCZ and where in the SST experiment sequence they transition from a single to double ITCZ. The SST profiles are defined such that as the equatorial SST gradient flattens, the maximum gradient increases and moves poleward. This leads to a weakening of the mid-latitude jet accompanied by a poleward shift of the jet core. Also considered are tropical wave activity and tropical precipitation frequency distributions. The details of each vary greatly between models, both with a given SST and in the response to the change in SST. One additional experiment is included to examine the sensitivity to an off-equatorial SST maximum. The upward branch of the Hadley circulation follows the SST maximum off the equator. The models that form a single precipitation maximum when the maximum SST is on the equator shift the precipitation maximum off equator and keep it centered over the SST maximum. Those that form a double with minimum on the equatorial maximum SST shift the double structure off the equator, keeping the minimum over the maximum SST. In both situations only modest changes appear in the shifted profile of zonal average precipitation. When the upward branch of the Hadley circulation moves into the hemisphere with SST maximum, the zonal average zonal, meridional and vertical winds all indicate that the Hadley cell in the other hemisphere dominates.

AGCM↗

NASA NTRS2013

Sensitivity of the Atmospheric Response to Warm Pool El Nino Events to Modeled SSTs and Future Climate Forcings

Warm pool El Nino (WPEN) events are characterized by positive sea surface temperature (SST) anomalies in the central equatorial Pacific. Under present-day climate conditions, WPEN events generate poleward propagating wavetrains and enhance midlatitude planetary wave activity, weakening the stratospheric polar vortices. The late 21st century extratropical atmospheric response to WPEN events is investigated using the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM), version 2. GEOSCCM simulations are forced by projected late 21st century concentrations of greenhouse gases (GHGs) and ozone-depleting substances (ODSs) and by SSTs and sea ice concentrations from an existing ocean-atmosphere simulation. Despite known ocean-atmosphere model biases, the prescribed SST fields represent a best estimate of the structure of late 21st century WPEN events. The future Arctic vortex response is qualitatively similar to that observed in recent decades but is weaker in late winter. This response reflects the weaker SST forcing in the Nino 3.4 region and subsequently weaker Northern Hemisphere tropospheric teleconnections. The Antarctic stratosphere does not respond to WPEN events in a future climate, reflecting a change in tropospheric teleconnections: The meridional wavetrain weakens while a more zonal wavetrain originates near Australia. Sensitivity simulations show that a strong poleward wavetrain response to WPEN requires a strengthening and southeastward extension of the South Pacific Convergence Zone; this feature is not captured by the late 21st century modeled SSTs. Expected future increases in GHGs and decreases in ODSs do not affect the polar stratospheric responses to WPEN.

stratosphere↗