Moisture transport from the Atlantic to the Pacific basin and its response to North Atlantic cooling and global warming

Atmospheric moisture transport from the Atlantic to the Pacific basin plays an important role in regulating North Atlantic salinity and thus the strength of the thermohaline circulation. Potential changes in the strength of this moisture transport are investigated for two different climate-change scenarios: North Atlantic cooling representative of Heinrich events, and increased greenhouse gas (GHG) forcing. The effect of North Atlantic cooling is studied using a coupled regional model with comparatively high resolution that successfully simulates Central American gap winds and other important aspects of the region. Cooler North Atlantic sea surface temperature (SST) in this model leads to a regional decrease of atmospheric moisture but also to an increase in wind speed across Central America via an anomalous pressure gradient. The latter effect dominates, resulting in a 0.13 Sv (1 Sv = 10⁶ m³ s^?1) increase in overall moisture transport to the Pacific basin. In fresh water forcing simulations with four different general circulation models, the wind speed effect is also present but not strong enough to completely offset the effect of moisture decrease except in one model. The influence of GHG forcing is studied using simulations from the Intergovernmental Panel on Climate Change archive. In these simulations atmospheric moisture increases globally, resulting in an increase of moisture transport by 0.25 Sv from the Atlantic to Pacific. Thus, in both scenarios, moisture transport changes act to stabilize the thermohaline circulation. The notion that the Andes effectively block moisture transport from the Atlantic to the Pacific basin is not supported by the simulations and atmospheric reanalyses examined here. This indicates that such a blocking effect does not exist or else that higher resolution is needed to adequately represent the steep orography of the Andes.     

Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation

Most coupled general circulation models (GCMs) perform poorly in the tropical Atlantic in terms of climatological seasonal cycle and interannual variability. The reasons for this poor performance are investigated in a suite of sensitivity experiments with the Geophysical Fluid Dynamics Laboratory (GFDL) coupled GCM. The experiments show that a significant portion of the equatorial SST biases in the model is due to weaker than observed equatorial easterlies during boreal spring. Due to these weak easterlies, the tilt of the equatorial thermocline is reduced, with shoaling in the west and deepening in the east. The erroneously deep thermocline in the east prevents cold tongue formation in the following season despite vigorous upwelling, thus inhibiting the Bjerknes feedback. It is further shown that the surface wind errors are due, in part, to deficient precipitation over equatorial South America and excessive precipitation over equatorial Africa, which already exist in the uncoupled atmospheric GCM. Additional tests indicate that the precipitation biases are highly sensitive to land surface conditions such as albedo and soil moisture. This suggests that improving the representation of land surface processes in GCMs offers a way of improving their performance in the tropical Atlantic. The weaker than observed equatorial easterlies also contribute remotely, via equatorial and coastal Kelvin waves, to the severe warm SST biases along the southwest African coast. However, the strength of the subtropical anticyclone and along-shore winds also play an important role.     

Age and progression of volcanism, Wrangell volcanic field, Alaska

The Wrangell volcanic field covers more than 10 000 km² in southern Alaska and extends uninterrupted into northwest. Yukon Territory. Lavas in the field exhibit medium-K, calc-alkaline affinities, typical of continental volcanic arcs along convergent plate margins. Eleven major eruptive centers are recognized in the Alaskan part of the field. More than 90 K-Ar age determinations in the field show a northwesterly progression of eruptive activity from 26 Ma, near the Alaska-Yukon border, to about 0.2 Ma at the northwest end of the field. A few age determinations in the southeast extension of the field in Yukon Territory, Canada, range from 11 to 25 Ma. The ages indicate that the progression of volcanism in the Alaska part of the field increased from about 0.8 km/Ma, at 25 Ma, to more than 20 km/MA during the past 2 Ma. The progression of volcanic activity and its increased rate of migration with time is attributed to changes in the rate and angle of Pacific plate convergence and the progressive decoupling of the Yakutat terrane from North America. Subduction of Yakutat terrane-Pacific plate and Wrangell volcanic activity ceased about 200 000 years age when Pacific plate motion was taken up by strike-slip faulting and thrusting.     

Turbulent exchange over a surface with chessboard-type inhomogeneities

The results of an experiment carried out in the summer of 1988 in the forest-steppe region of the USSR are given. The aim of the experiment was to study turbulent exchange between the atmosphere and a non-homogeneous underlying surface consisting of a patchwork quilt of agricultural lands sown with different crops (hereafter called chessboard-type inhomogeneities). Simultaneous measurements of turbulent fluxes of heat, momentum and humidity by the eddy-correlation methods and also the measurements of the mean wind velocity and temperature are described. The values of turbulent fluxes calculated from the profile measurements are in good agreement with those obtained from the eddy-correlation measurements. Thus it may be concluded that the universal functions of Monin-Obukhov similarity theory (determined from measurements over a flat and homogeneous underlying surface) pertain also to a non-homogeneous (chessboard-type) underlying surface.     

X-Ray and optical observations of the X-ray source EX0020528+1454.8

We present preliminary results of the EXOSAT X-ray observations and quasisimultaneous and simultaneous optical photometry of the X-ray source EX0020528+1454.8=1E0205+149 found independently as an serendipitous source both with Einstein and EXOSAT satellites. The optical counterpart is a pair of dMe stars. Our results indicate that the object is variable both in X-rays and optical wavelenghts, and probably belongs to dMe flare stars.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Satellite particle exospheres of planets: Application to Earth

Neutral planetary exospheres are built up by three different kinds of gas particles, namely ballistic, hyperbolic and elliptic particles. Elliptic particles have their origin exclusively in exospheric regions of the planet where they are fed into satellite orbits by different physical processes. It has been suggested that elliptic particles that do not enter the collision-dominated planetary gas regions represent an important fraction of the particles constituting the outer parts of planetary exospheres. Here we develop a theoretical concept for a rigorous calculation of elliptic particle distributions using Boltzmann equation kinetic approaches. Taking into account realistic gain and loss processes a general procedure for the determination of satellite particle densities for the terrestrial case is presented. We give representative height profiles of the satellite particle density in the exosphere for weak and strong solar activity. Our results are compared with those obtained by simplified theoretical approaches; pronounced deviations are obvious. It is shown that satellite particles are more relevant in low temperature exospheres leading to an order-of-magnitude difference above 1500 km between the densities for weak and strong solar activity. There is a general tendency for satellite particles to become increasingly important with increasing height.     

Energetic solar particle events in a stream-structured solar wind

Theoretical considerations lead to a solar cosmic ray diffusion coefficient which varies with heliolongitude in a stream-structured solar wind. By solving numerically the time dependent convection-diffusion equation for the particle transport we investigate the effect of the azimuthal variation of the diffusion coefficient on intensity-time profiles as seen by a stationary observer. Depending on the position of the observer relative to the solar wind stream at the time of flare occurrence, completely different intensity-time profiles will be observed. When the spacecraft is at the time of the flare occurrence right at the leading edge of a solar wind stream, the large mean free path leads to rapid steepening of the initial phase of the intensity profile. The longitudinally decreasing mean free path 1 day in front of the leading edge will lead to intensity-time profiles similar to long-time injection events if the event occurs before the stationary observer enters the flux tubes with the decreasing diffusion coefficient.     

Investigating the Sensitivity of Marine Fog to Physical and Microphysical Processes Using Large-Eddy Simulation

Boundary-Layer Meteorology - Over the past few years large-eddy simulation (LES) has demonstrated success in modelling continental radiation fog, and several recent studies have used LES to...