Effectiveness of the Bjerknes stability index in representing ocean dynamics

The El Niño-Southern Oscillation (ENSO) is a naturally occurring coupled phenomenon originating in the tropical Pacific Ocean that relies on ocean–atmosphere feedbacks. The Bjerknes stability index (BJ index), derived from the mixed-layer heat budget, aims to quantify the ENSO feedback process in order to explore the linear stability properties of ENSO. More recently, the BJ index has been used for model intercomparisons, particularly for the CMIP3 and CMIP5 models. This study investigates the effectiveness of the BJ index in representing the key ENSO ocean feedbacks—namely the thermocline, zonal advective, and Ekman feedbacks—by evaluating the amplitudes and phases of the BJ index terms against the corresponding heat budget terms from which they were derived. The output from Australian Community Climate and Earth System Simulator Ocean Model (a global ocean/sea ice flux-forced model) is used to calculate the heat budget in the equatorial Pacific. Through the model evaluation process, the robustness of the BJ index terms are tested. We find that the BJ index overestimates the relative importance of the thermocline feedback to the zonal advective feedback when compared with the corresponding terms from the heat budget equation. The assumption of linearity between variables in the BJ index formulation is the primary reason for these differences. Our results imply that a model intercomparison relying on the BJ index to explain ENSO behavior is not necessarily an accurate quantification of dynamical differences between models that are inherently nonlinear. For these reasons, the BJ index may not fully explain underpinning changes in ENSO under global warming scenarios.     

Stratigraphic evidence for millennial-scale temporal clustering of earthquakes on a continental-interior fault: Holocene Mississippi River floodplain deposits, New Madrid seismic zone, USA

The earthquake cycles that characterize continental-interior areas that are far from active plate boundaries have proven highly cryptic and difficult to resolve. We used a novel paleoseismic proxy to address this issue. Namely, we reconstructed Holocene Mississippi River channels from maps of floodplain strata in order to identify channel perturbations reflective of major displacement events on the high-hazard and mid-plate Reelfoot thrust fault, New Madrid seismic zone, U.S.A. Only three discrete slip events are currently documented for the Reelfoot fault ( AD 900,  AD 1450, and AD 1812). This study extends this record and, thus, illustrates the utility of stratigraphic proxies as paleoseismic tools. We concurrently offer here some of the first quantified response times for tectonically induced channel pattern changes in large alluvial rivers.

We identified at least two cycles of pervasive meandering that were interrupted by channel-straightening responses occurring upstream of the Reelfoot fault scarp. These straightening responses initiated at 2244 BC +/− 269 to 1620 BC +/− 220 and  AD 900, respectively, and each records initiation of a period of Reelfoot fault slip after millennia of relative tectonic quiescence. The second (or New Madrid) straightening response was triggered by the previously known  AD 900 fault slip event, and this initial low sinuosity has been protracted until the modern day by the latter  AD 1450 and AD 1812 events. The first (or Bondurant) straightening response began a period of several hundred to  1400 years of low river sinuosity which evidences a similar period of multiple recurrent displacement events on the Reelfoot fault. These Bondurant events predate the existing paleoseismic record for the Reelfoot fault.

These data offer initial evidence that slip events on the Reelfoot fault were temporally clustered on millennial scales and, thus, offers the first direct evidence for millennial-scale clustering of earthquakes on a continental-interior fault. This carries additional ramifications. Namely, faults that have been quiescent and non-hazardous for millennia could re-enter an enduring period of recurrent hazardous earthquakes with little warning. Likewise, the Reelfoot fault also reveals evidence of temporal clustering of earthquakes on short-term cycles (months), as well as evidence for longer-term reactivation cycles (10⁴–10⁶ years). This introduces the possibility that temporal clustering could be hierarchical on some continental-interior faults.     

An evaluation of the classical and extended Rossby wave theories in explaining spectral estimates of the first few baroclinic modes in the South Pacific Ocean

Previous literature has suggested that multiple peaks in sea level anomalies (SLA) detected by two-dimensional Fourier Transform (2D-FT) analysis are spectral components of multiple propagating signals, which may correspond to different baroclinic Rossby wave modes. We test this hypothesis in the South Pacific Ocean by applying a 2D-FT analysis to the long Rossby wave signal determined from filtered TOPEX/Poseidon and European Remote Sensing-1/2 satellite altimeter derived SLA. The first four baroclinic mode dispersion curves for the classical linear wave theory and the Killworth and Blundell extended theory are used to determine the spectral signature and energy contributions of each mode. South of 17°S, the first two extended theory modes explain up to 60% more of the variance in the observed power spectral energy than their classical linear theory counterparts. We find that Rossby wave modes 2–3 contribute to the total Rossby wave energy in the SLA data. The second mode contributes significantly over most of the basin. The third mode is also evident in some localized regions of the South Pacific but may be ignored at the large scale. Examination of a selection of case study sites suggests that bathymetric effects may dominate at longer wavelengths or permit higher order mode solutions, but mean flow tends to be the more influential factor in the extended theory. We discuss the regional variations in frequency and wave number characteristics of the extended theory modes across the South Pacific basin.     

Book reviews

The weather and climate of Australia and New Zealand

A.P. Sturman & N.J. Tapper, 1996

Oxford University Press, Melbourne

18 × 24 cm, xi + 476pp., ISBN 0–19–553393–3, A$55.00 (soft)

Australians and the environment

W. McLennan, 1996

Australian Bureau of Statistics (Catalogue No. 4601.0), Melbourne

21 × 30 cm, viii + 415pp., ISBN 0–642–20690–2, A$44.00 (soft)

The information age: economy, society and culture. Vol.1—The rise of the network society

M. Castells, 1996

Blackwell, Cambridge MA/Oxford

15 × 23 cm, xvii + 556 pp., ISBN 1–155786–617–1, US$25.95/£15.99 (soft)

Retailing, consumption and capital: towards the new retail geography

N. Wrigley & M. Lowe (eds), 1996

Longman, Harlow

16 × 23 cm, vi + 352 pp., ISBN 0–582–22824–7, £17.99 (soft)     

ENSO to multi-decadal time scale changes in East Australian Current transports and Fort Denison sea level: Oceanic Rossby waves as the connecting mechanism

The connection between East Australian Current (EAC) transport variability and Australia’s east coast sea level has received little treatment in the literature. This is due in part to the complex interacting physical processes operating in the coastal zone combined with the sparsity of observations available to improve our understanding of these possible connections. This study demonstrates a statistically significant (at the >90% level) relationship between interannual to decadal time scale variations in observed estimates of the EAC transport changes and east coast sea level measured at the high-quality, long record Fort Denison tide-gauge in Sydney Harbour, Australia (33°51′18″S, 151°13′32″E). We further demonstrate, using a linear reduced-gravity ocean model, that ENSO to decadal time-scale variations and the ocean-adjusted multi-decadal trend (approx. 1 cm/decade) in observed sea level at Fort Denison are strongly connected to modulations of EAC transports by incoming westward propagating oceanic Rossby waves. We show that EAC transport and Fort Denison sea level vary in a manner expected from both Tasman Sea generated Rossby waves, which account for the interannual and multi-annual variability, and remotely forced (from east of New Zealand) Rossby wave connections through the mid-latitudes, accounting for the ocean-adjusted multi-decadal trend observed at the New South Wales coast - with the regional-Tasman Sea forcing explaining the greatest overall proportion of EAC transport and sea-level variances.     

Southwest Pacific subtropical mode water: A climatology

The large-scale distribution and changes in Southwest Pacific subtropical mode water (STMW) are investigated and discussed. The paper presents for the first time geographic maps showing the spatial distribution of STMW thicknesses, with a vertical temperature gradient <2.0 °C/100 m occupying the 14–20 °C range below the mixed layer depth, across the entire Southwest Pacific region. STMW changes in areal thickness extent, vertical cross-sectional area along selected transects, and total volume, are examined on seasonal and interannual time scales between 1973 and 1988.We find that STMW extends across the entire width of the Tasman Sea in a very broad swath between the Tropical Convergence in the north (just to the south of New Caledonia), the southeast Australian coast in the west to as far south as 39°S (likely due to the southward extension of the EAC), and eastwards along the Southern STMW boundary in a meandering pathway that broadly follows the Tasman Front. The total STMW volume across the region (i.e., west of 180°) varies seasonally by a factor of more than three between the estimated maximum of 6.6 (±0.5) × 10¹⁴ m³ in October and minimum of 1.9 (±0.4) × 10¹⁴ m³ in May. Interannual variations O (±0.5 × 10¹⁴ m³) are also observed in the spatial extent of the thick mode water and its total volume. El Niño composite maps show an anomalous thickening of the STMW during the El Niño year with October positive thickness anomalies in excess of +20 m (total volume anomaly of +0.6 × 10¹⁴ m³) manifested throughout the subtropical gyre interior as far north as New Caledonia. Total volume anomalies tend to be positive from January of the El Niño year through to the July following (18 months). The maximum correlation coefficient r = −0.3 between 3-monthly STMW volume anomalies and the Southern Oscillation index is statistically significant at the 95% confidence level. We conclude that during the anomalous cooling of the upper Southwest Pacific Ocean in the El Niño year, winter-time convection and STMW formation is enhanced across the region resulting in an El Niño – Southern Oscillation climate signal that is identifiable below the mixed layer by the increased STMW volume which persists through to the following winter. Finally, some evidence for the possible decadal modulation of the STMW variability is also discussed.     

The Origins and Consequences of democratic citizens' Policy Agendas: A Study of Popular Concern about Global Warming

This article proposes and tests a model of the causes and consequences of Americans’ judgments of the national seriousness of global warming. The model proposes that seriousness judgments about global warming are a function of beliefs about the existence of global warming, attitudes toward it, the certainty with which these beliefs and attitudes are held, and beliefs about human responsibility for causing global warming and people’s ability to remedy it. The model also proposes that beliefs about whether global warming is a problem are a function of relevant personal experiences (with the weather) and messages from informants (in this case, scientists), that attitudes toward global warming are a function of particular perceived consequences of global warming, and that certainty about these attitudes and beliefs is a function of knowledge and prior thought. Data from two representative sample surveys offer support for all of these propositions, document effects of national seriousness judgments on support for ameliorative efforts generally and specific ameliorative policies, and thereby point to psychological mechanisms that may be responsible for institutional and elite impact on the public’s assessments of national problem importance and on public policy preferences.     

Mantle thermal structure and active upwelling during continental breakup in the North Atlantic

Seismic reflection and refraction data acquired on four transects spanning the Southeast Greenland rifted margin and Greenland–Iceland Ridge (GIR) provide new constraints on mantle thermal structure and melting processes during continental breakup in the North Atlantic. Maximum igneous crustal thickness varies along the margin from >30 km in the near-hotspot zone (<500 km from the hotspot track) to 18 km in the distal zone (500–1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800±300 to 600±50 km³/km/Ma in the near-hotspot zone and from 700±200 to 300±50 km³/km/Ma in the distal zone. Comparison of our data with the British/Faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist. Joint consideration of crustal thickness and mean crustal seismic velocity suggests that along-margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature. The thermal anomaly (ΔT) at breakup was modest (100–125°C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125±50°C) and upwelling ratio (4 times passive) of the Iceland hotspot have remained roughly constant since 56 Ma. Our results are consistent with a plume–impact model, in which (1) a plume of radius 300 km and ΔT of 125°C impacted the margin around 61 Ma and delivered warm material to distal portions of the margin; (2) at breakup (56 Ma), the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin; and (3) by 45 Ma, both the remaining plume head and the distal warm layer were exhausted, with excess magmatism thereafter largely confined to a narrow (<200 km radius) zone immediately above the Iceland plume stem. Alternatively, the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre-existing thermal anomaly unrelated to the plume.