Evolution of the equatorial and dipole modes of the sea-surface temperature in the Tropical Atlantic at decadal scale

Summary ?One hundred and thirty six years (1856–1991) of monthly sea-surface temperature (SST) data in the Tropical Atlantic Ocean are used to investigate the propagating signals of the SST at a decadal (DD) time scale. The first and the third evolving modes show a relationship between the equatorial and the inter-hemispheric patterns, one evolving into the other mode and vice-versa. These modes describe two different evolutions of the SST at DD time-scale. The first EEOF features a 12-year period oscillatory regime with a strong 2-year duration inter-hemispheric pattern evolving into a weak 1-year duration equatorial pattern and vice-versa. This mode exhibits also a northward displacement of the anomalies in the band 15° S–15° N. The third EEOF also shows an oscillatory regime, but with a period of 10 years and with a relatively strong 2-year duration equatorial pattern evolving into a weak 1-year duration inter-hemispheric pattern and vice-versa. For this mode, the SST anomalies show a southward displacement in the band 15° S–15° N. These results have not yet been documented in previous works and explain some of the previous findings on the DD variability in the Tropical Atlantic. Received December 31, 2001; revised April 9, 2002; accepted September 4, 2002 Published online: March 20, 2003     

Description and evaluation of the bergen climate model: ARPEGE coupled with MICOM

A new coupled atmosphere–ocean–sea ice model has been developed, named the Bergen Climate Model (BCM). It consists of the atmospheric model ARPEGE/IFS, together with a global version of the ocean model MICOM including a dynamic–thermodynamic sea ice model. The coupling between the two models uses the OASIS software package. The new model concept is described, and results from a 300-year control integration is evaluated against observational data. In BCM, both the atmosphere and the ocean components use grids which can be irregular and have non-matching coastlines. Much effort has been put into the development of optimal interpolation schemes between the models, in particular the non-trivial problem of flux conservation in the coastal areas. A flux adjustment technique has been applied to the heat and fresh-water fluxes. There is, however, a weak drift in global mean sea-surface temperature (SST) and sea-surface salinity (SSS) of respectively 0.1 °C and 0.02 psu per century. The model gives a realistic simulation of the radiation balance at the top-of-the-atmosphere, and the net surface fluxes of longwave, shortwave, and turbulent heat fluxes are within observed values. Both global and total zonal means of cloud cover and precipitation are fairly close to observations, and errors are mainly related to the strength and positioning of the Hadley cell. The mean sea-level pressure (SLP) is well simulated, and both the mean state and the interannual standard deviation show realistic features. The SST field is several degrees too cold in the equatorial upwelling area in the Pacific, and about 1 °C too warm along the eastern margins of the oceans, and in the polar regions. The deviation from Levitus salinity is typically 0.1 psu – 0.4 psu, with a tendency for positive anomalies in the Northern Hemisphere, and negative in the Southern Hemisphere. The sea-ice distribution is realistic, but with too thin ice in the Arctic Ocean and too small ice coverage in the Southern Ocean. These model deficiencies have a strong influence on the surface air temperatures in these regions. Horizontal oceanic mass transports are in the lower range of those observed. The strength of the meridional overturning in the Atlantic is 18 Sv. An analysis of the large-scale variability in the model climate reveals realistic El Niño – Southern Oscillation (ENSO) and North Atlantic–Arctic Oscillation (NAO/AO) characteristics in the SLP and surface temperatures, including spatial patterns, frequencies, and strength. While the NAO/AO spectrum is white in SLP and red in temperature, the ENSO spectrum shows an energy maximum near 3 years.     

Climatological features of blocking anticyclones: a study of Northern Hemisphere CCM1 model blocking events in present-day and double CO2 concentration atmospheres

 Using output made with the National Center for Atmospheric Research (NCAR) Community Climate Model Version 1 (CCM1), the characteristics of blocking events over the Northern Hemisphere in a ten-year present day control simulation with a CO₂ concentration of 330 ppm were compared to those in a previously analyzed observational three-year climatology. The characteristics of blocking events in a double present-day CO₂ concentration simulation were then compared to those in the control simulation in order to evaluate how these characteristics might change in an increased CO₂ atmosphere. The results demonstrated that in the Northern Hemisphere the CCM1 correctly simulated many characteristics of blocking events such as average annual number of occurrences, annual variations is size and intensity, and preferred formation regions. A more detailed analysis (i.e., by region and season) revealed some differences between the CCM1 and observed blocking events for characteristics such as mean frequency of occurrence, intensity, size and duration. In addition, the model failed to capture adequately the occurrence of blocking events over the western Asian continent. A comparison of the double CO₂ concentration run to the control showed that, in general, blocking events were more persistent and weaker, but of similar size in the increased CO₂ atmosphere. Also, some statistically significant regional and seasonally dependent changes were found in the frequency of occurrence, duration, and intensity. Finally, a correlation between block size and intensity, significant at the 99% confidence level, was found in each climatology. This result is similar to a correlation found in the analysis of observations. Received: 8 May 1995 / Accepted: 20 September 1996     

On the heat budget of the Arabian Sea

Summary The rate of oceanic heat storage of the upper 200m of the Arabian Sea is explained in terms of net air-sea heat flux (Q _F), heat change due to horizontal divergence and vertical motion (Q _V) and heat change due to lateral advection (Q _A). The analysis revealed that the heat storage of the Arabian Sea is mainly controlled byQ _V while the effect ofQ _A is much larger than expected. Parameterisation of summer cooling revealed that the depletion of energy from the mixed layer is mainly due to upwelling and horizontal advection though large amount of heat is accumulated due to net air-sea heat flux. The annual heat balance of the upper 200m of the Arabian Sea suggested large heat gain by air-sea exchange processes. About two third of this heat gain is compensated by horizontal advection and one third by vertical advection.With 4 Figures     

Error in depth determination from resistivity soundings due to non-identification of suppressed layers

The magnitude of errors in the determination of depth to bedrock from Wenner and Schlumberger resistivity sounding curves, caused by the non-identification of a suppressed layer, has been investigated. The principal objective is to evaluate how the layer thicknesses and resistivities affect the accuracy of depth estimates. In the computations, the intermediate layer in a 3-layer model, in which the resistivity increases with depth, is removed and the 2-layer sounding curve that is electrically equivalent to the 3-layer curve is generated. The results indicate that there is a possibility for large depth underestimations when the resistivity contrast between layers 1 and 2 is very large. This is manifested in a steeply rising terminal branch on the sounding curve. There is a slight decrease in the depth underestimation as the resistivity contrast between layers 2 and 3 increases. Conversely, if the intermediate layer is fairly thick and the resistivity contrasts are not too large, the best-fit 2-layer curve shows large deviations from the 3-layer curve. In such cases, the intermediate layer can be identified, resulting in reliable depth estimates. A field example from Nigeria is presented in which the sounding data has been interpreted so as to account for a prebasement layer of intermediate resistivity, indicative of a fractured granite.     

Cell-zonal textures of tinguaites from the Kola Peninsula

Textures of tinguaite dykes cutting the alkaline Khibiny massif in the Kola Peninsula, Russia, are described. They are characterized by a combination of a fractal microfracture network dividing rock into pencil-like cells and of a concentric rhythmical zonation in almost all of them. The latter is formed by interchange of volatile-enriched and volatile-depleted mineral zones. The location of the textures only where dykes contact host khibinites appears to point out the contraction nature of the microfracture network. The cooling time calculated agrees well with this hypothesis. The zonation appears to have arisen through autometamorphic processes with two main competing factors, namely (a) overall cooling of the system and (b) periodic depletion of it in some elements, mainly Na and K. Another mechanism which may be applied to explain the zonation is the known Marangoni Instability effect at the early stage of evolution of the volatile-saturated phonolite melt. Thus, tinguaite textures are caused by nonspecific influences external to the system and may be regarded as an example of self-organization in nature.     

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia

Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions ( T  ≈650  °C and P ≈12.0  kbar). Sm–Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as ⁴⁰Ar–³⁹Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c .  550  Ma. Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c .  550  Ma was c .  16  °C  km⁻¹ and at c .  535  Ma was c .  18  °C  km⁻¹, based on P – T  estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to ≥1.5  mm year⁻¹ are comparable with other orogenic belts, rather than cratonic areas elsewhere.     

Two stages of zircon and monazite growth in anatectic leucogneiss: SHRIMP constraints on the duration and intensity of Pan-African metamorphism in Prydz Bay, East Antarctica

SHRIMP U–Pb data from two samples of foliated anatectic leucogneiss bracket the high-grade metamorphic history of metasediments from southern Prydz Bay, East Antarctica. Magmatic zircons in these two samples have mean crystallization ages of 535 ± 13 and 536 ± 35 Myr, and provide a good estimate for the timing of peak metamorphism and partial melting. Low-Th monazite grains in the same samples have mean ages of 528 ± 4 and 527 ± 11 Myr, which are within error of the zircon ages but may reflect a lower blocking temperature. High-Th monazite rims with younger mean ages of 518 ± 3 and 512 ± 13 Myr occur around low-Th cores in both samples, and were precipitated from hydrous fluids released during the crystallization of residual melts. These magmatic fluids were channelled along the retrograde foliation present in both samples. All mineral assemblages and fabrics preserved in the metasediments reflect intense Pan-African metamorphism and deformation, associated with high degrees of partial melting and significant vertical displacements of the crust. Continental reconstructions assuming limited Pan-African tectonism in East Antarctica should be reassessed in the light of these new data.     

Elasticity of tetragonal end-member majorite and solid solutions in the system Mg4Si4O12-Mg3Al2Si3O12

 The adiabatic elastic moduli of polycrystalline En₅₀Py₅₀ and En₈₀Py₂₀ majorite-garnet solid solutions (where En=4MgSiO₃, Py=Mg₃Al₂Si₃O₁₂) and the end-member En₁₀₀ tetragonal majorite were determined at ambient conditions using Brillouin spectroscopy. The adiabatic bulk modulus, K, and shear modulus, μ, of En₅₀Py₅₀ were found to be K=173.1 (20) and μ=92.3 (10) GPa, and are indistinguishable from those of the end-member pyrope, Py₁₀₀. The moduli of the more majorite-rich samples are significantly lower and are virtually identical to each other (K=162.6(11) and μ=85.7(7) for En₈₀Py₂₀; K=166(5) and μ=88(2) for En₁₀₀). In combination with previously reported moduli for this system, we conclude that both K and μ are constant over the compositional range from Py₁₀₀ to a majorite content of about 70–80%, whereupon the moduli decrease substantially. For compositions ranging from En₈₀Py₂₀ to the end-member majorite, the moduli are also approximately independent of composition, but at a lower value. An alternative model with a continuous decrease in moduli with increasing majorite content cannot be excluded, within the uncertainties of existing measurements. The contrast in moduli between aluminous pyrope garnet and Al-free majorite are small compared with the modulus changes accompanying the pyroxene – majorite phase transformation. Received August 16, 1995 /Revised, accepted January 12, 1996