Geophysical Research Letters

Gunell, H.; Kallio, E.; Jarvinen, R.; Janhunen, P.; Holmström, M.; Dennerl, K.

doi: 10.1029/2006GL028602pmid: N/A

A hybrid simulation of the solar wind‐Venus interaction and a test particle simulation of heavy ion trajectories near Venus are used to compute the contribution from solar wind charge exchange processes to the X‐ray emission from Venus. It is found that the X‐ray halo caused by the solar wind charge exchange process at Venus is confined to lower altitudes than at Mars, and that the structure on the ion gyro radius scale that is seen in X‐ray images of Mars is absent at Venus.

Lambert, F. Hugo; Faull, Nicholas E.

doi: 10.1029/2006GL028124pmid: N/A

The responses of the HadSM3 and NCAR CCM3 general circulation models to a change in solar insolation are compared to their responses to a doubling of CO2 concentration. In both models, it is found that the important difference is a rapid adjustment of the troposphere in the solar case that reduces the value of effective radiative forcing by about 25%. Clear‐sky warming appears to make the major contribution. Subsequent warming of the coupled troposphere, land‐surface, ocean mixed‐layer system occurs with a very similar sensitivity to that expected under a CO2 forcing of the reduced value. Because of adjustment, the overall precipitation response to solar forcing is similar to, or less than the response to CO2 forcing, despite being smaller per unit temperature change.

Niranjan, K.; Madhavan, B. L.; Sreekanth, V.

doi: 10.1029/2006GL028199pmid: N/A

Aerosol back scatter profiles obtained using a micro pulse lidar at Visakhapatnam, a station located on the east coast of peninsular India show certain high altitude aerosol layers during the months of March/April 2005, 2006. Co‐located aerosol optical depth measurements show an increase in AOD by 0.05 to 0.25 during the event when the layers were observed. The prevailing meteorology does not indicate any possible local entrainment of aerosol leading to the formation of elevated layers due to boundary layer dynamics. The 7 day back trajectory analysis shows that the possible origin of the layers could be from Arabia in 60% of the cases while it is from Indian sub‐continent during the rest of the events. When the air mass flow is from the Indian sub‐continent, there is a proportional increase in MODIS derived aerosol column fine mode fraction though it is not observed at the surface level. During the events when air mass flow is from Arabia, such an increase in column fine mode fraction was not observed but the angstrom size index which is a measure of the aerosol size distribution was low indicating that the elevated layers of Arabian origin could contain significant fraction of dust aerosol.

Kodera, K.; Coughlin, K.; Arakawa, O.

doi: 10.1029/2006GL027827pmid: N/A

The Indian Ocean Dipole (IOD) mode and El Niño/Southern Oscillation (ENSO) exhibit a substantial correlation during boreal autumn. Although they are separate phenomena, coupling occurs under certain conditions. This study reveals that ENSO‐related variability extends into the Indian Ocean, led by a developed South Indian Ocean (SIO) anticyclone, during periods of low solar (LS) activity. During periods of high solar (HS) activity, anomalous sea surface temperatures are confined to the Pacific with little amplification of the anticyclone in the South Indian Ocean. The direct cause of the difference in the SIO anticyclone arises from a shift in the location of the descending branch of the anomalous Walker circulation. This may be attributable to a change in the background Indian Ocean monsoon circulation which can be modulated by changes in solar irradiance.

Zhou, X.‐Y.; Lummerzheim, D.; Gladstone, R.; Gunapala, S.

doi: 10.1029/2006GL028611pmid: N/A

This letter reports a feasibility study on dayside auroral observation using a near‐infrared (NIR) InGaAs camera from high‐altitude, long‐duration balloons. This effort is motivated by science interest in the dayside aurora. Model predictions indicate daytime observations are possible because the sky brightness significantly decreases towards longer wavelengths at certain altitude in the upper atmosphere, and the NIR ambient sky brightness substantially decreases with increasing altitude. To address the question whether aurora can indeed be detected in the upper atmosphere, auroral test observations were conducted at the Poker Flat Research Range from March 22 to April 26, 2005. The InGaAs camera used in this test covers a waveband from 900 to 1700 nm with a 320 × 256 focal plane array, 30 × 30 μm pixels, and a 9° field of view. For one evening event, we estimate the NIR sky brightness using the MODTRAN atmospheric radiance model and compare it with the expected sky brightness at 30–50 km altitudes. The comparison indicates that the N2+ Meinel emission at ∼1100 nm should be seen at 35–40 km altitudes and above, and therefore, confirms that such dayside auroral observations are feasible. Auroral studies using this method include, e.g., studies of interplanetary shock and pressure pulse induced auroras as well as conjugate auroras.

Vinogradova, Nadya T.; Ponte, Rui M.; Stammer, Detlef

doi: 10.1029/2006GL028588pmid: N/A

The relation between large‐scale sea level and bottom pressure variability is studied using long (50‐yr) simulations of a general circulation model under realistic forcing. Admittance and coherence analyses are used to characterize the bottom pressure and sea level relationship as a function of period, horizontal spatial scale, and location. At the model grid scale (1°), bottom pressure is found to be essentially equivalent to sea level at periods <30 days, except in the tropics. This equivalence still holds for longer periods (∼100 days), but only at high latitudes (>60°) and in shallow depths (<200 m). Elsewhere, bottom pressure and sea level fields can differ significantly. Results indicate an increase of the importance of baroclinic signals with decreasing latitude and spatial scale, with significant baroclinic signals at intra‐seasonal and longer periods present in many subtropical and mid‐latitude regions. Variability is clearly baroclinic at inter‐annual periods, regardless of location and spatial scale. Results have broad implications for the interpretation and processing of both satellite altimetry and gravity data and for their assimilation into numerical models.

Siswanto, Eko; Ishizaka, Joji; Yokouchi, Katsumi; Tanaka, Katsuhisa; Tan, Chun Knee

doi: 10.1029/2006GL028368pmid: N/A

Using multiplatform satellite data and 13 typhoon passages from 1998 through 2004, an empirical relationship to assess typhoon‐forced primary production (PP) enhancement in the outer shelf of the East China Sea is introduced. The constructed relationship requires variables of typhoon maximum sustained wind, typhoon transit speed and bottom depth of the location below the typhoon track. As this relationship does not require ocean color data, it enabled us to reconstruct the interannual and interdecadal variations of typhoon‐forced PP enhancement from 2004 back to 1980. Typhoon‐forced PP enhancements and typhoon contributions to summer‐fall new production during El Niño years tended to be higher than those during La Niña years. Typhoon‐enhanced PP also has shown a tendency to be higher during the 1991–2004 period than during the 1980–1990 one. The method we detail in this study is applicable for other tropical cyclone trajectory basins.

Visser, K.; Lebedev, S.; Trampert, J.; Kennett, B. L. N.

doi: 10.1029/2006GL028671pmid: N/A

Love wave phase velocities for fundamental and higher modes are difficult to measure because the different modes cannot easily be separated. Following Yoshizawa and Kennett (2002), we generate suites of path specific one‐dimensional shear wave velocity profiles using the Neighbourhood Algorithm of Sambridge (1999a). From this family of O(104) models both fundamental and higher mode phase velocities with mutually consistent uncertainties are calculated. We have fully automated the method and analysed over forty thousand Love wave seismograms from the GDSN and GEOSCOPE global networks from 1994–2004. Our phase velocity measurements agree remarkably well with previous studies, but we have been able to enlarge the available dataset dramatically. We present global Love wave phase velocity maps (up to the fifth overtone) with unprecedented resolution due to the improved path coverage. Comparing these maps to existing tomographic models, we discern evidence of significant anisotropy in the lower mantle around a depth of 1000 km in the Pacific.

Strong, Mel; Sharp, Zachary D.; Gutzler, David S.

doi: 10.1029/2006GL028307pmid: N/A

Water vapor transport paths into the American Southwest were deduced from a high temporal resolution record of hydrogen isotope compositions of atmospheric water vapor (δDwv) collected over a six‐week period in late spring, 2005, at Albuquerque, New Mexico. Daily fluctuations of δDwv routinely exceeded 20‰ in magnitude, while δDwv variations up to 80‰ occurred on the time scale of weather (a few hours to ∼ a week). Vertical profiles of δDwv in the lower troposphere exhibited considerable structure that cannot be ascertained from standard meteorological measurements. Trajectory analyses provide consistent evidence that the large temporal variations of surface δDwv and vertical variations of δDwv are primarily due to advection of water from different source regions. The lack of mixing inferred from our analyses indicates that δDwv can be used as a sensitive tracer of the moisture transport history of air parcels.

Spanswick, E.; Donovan, E.; Friedel, R.; Korth, A.

doi: 10.1029/2006GL028329pmid: N/A

Using data from an array of ground‐based single beam riometers and the CRRES Medium Energy B (MEB) particle detector we examine the signature of dispersionless electron injections. We find that it is possible to distinguish a dispersionless electron injection by it's integrated (>30 keV) flux signature at in situ satellites with a simple rise time criteria. If the total electron flux >30 keV exhibits a dramatic rise within 3 minutes then the signature is guaranteed to correspond to a dispersionless injection. Since the Kennel‐Petschek limit for strong pitch angle scattering is almost certainly met during substorm injections and riometers respond to the >30 keV energy range, this translates to an equivalent criteria for riometer absorption. We present results obtained using our selection criteria for the ground‐based identification of dispersionless electron injections including MLT dependence and initial spatial extent of the electron injection region.