Near-equatorial deep circulation in the Indian and Pacific oceans

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1990Theory and observations of deep circulation in the near-equatorial Atlantic, Indian and Pacific Oceans are reviewed. Flow of deep and bottom water in the near-equatorial Indian and Pacific oceans, the two oceans with only a southern source of bottom water, is described through analysis of recent CTD data. Zero-velocity surfaces are chosen through use of water-mass properties and transports are estimated. Effects of basin geometry, bottom bathymetry and vertical diffusivity as well as a model meridional inertial current on a sloping bottom near the equator are all discussed in conjunction with the flow patterns inferred from observations. In the western equatorial Indian Ocean, repeat CTD surveys in the Somali Basin at the height of subsequent northeast and southwest monsoons show only small differences in the strength of the circulation of the bottom water (potential temperature θ ≤1.2°C). A deep western boundary current (DWBC) carrying about 4x106 m3 s-1 of this water is observed moving north along the continental rise of Africa at 3°S. The cross-equatorial sections suggest that the current turns eastward at the equator. The northern sections show a large mass of the coldest water in the interior east of the Chain Ridge, augmenting the evidence that the DWBC observed south of the equator turns east at the equator rather than remaining on the boundary, and feeds the interior circulation in the northern part of the basin from the equator. The circulation of deep water (1.2°C< θ ≤ 1.7°C) in the Somali and Arabian Basins is also analyzed. A DWBC flowing southward along the Carlsberg ridge in the Arabian Basin is described. In the central equatorial Pacific Ocean a recent zonal CTD section at 10°N, allows estimation that 5.0x106 m3 s-1 of Lower Circumpolar Water (LCPW, θ ≤ 1.2°C) moves northward as a DWBC along the Caroline Seamounts in the East Mariana Basin. In the Central Pacific Basin, 8.1x106 m3 s-1 of LCPW is estimated to move northward along the Marshal Seamounts as a DWBC at this latitude. An estimated 4.7x106 m3 s-1 of the LCPW moves back southward across 10°N in the Northeast Pacific Basin along the western flank of the East Pacific Rise and an equatorial jet is observed to flow westward from 138°W to 148°W shifting south of the Line Islands at 2.5°S, 159°W. The net northward flow of LCPW across 10°N in the Pacific Ocean is estimated at 8.4x106 m3 s-I. The net southward flow of the silica-rich North Pacific Deep Water (NPDW, 1.2 < θ ≤ 2.0°C) in the central Pacific Ocean estimated at 2.7x106 m3 s-1 is also discussed. In the Indian Ocean, the eastward equatorial flow in the the bottom water of the Somali Basin differs from the prediction of a flat-bottom uniform-upwelling Stommel-Arons calculation with realistic basin geometry and source location. The behavior of a uniform potential vorticity meridional jet on a sloping bottom is examined in an attempt to explain the observed behavior at the equator. The inertial jet does not cross the equator in a physically plausible fashion owing to the constraint of conservation of potential vorticity. Mass and heat budgets for the bottom water of the Somali Basin are of interest with respect to the equatorial feature. Upwelling through the θ = 1.2°C surface is estimated at 12±4x10-5 cm s-1 and a rough heat budget for the deep Somali Basin results in an estimate of vertical diffusivity of 9±5 cm2 s-1 at 3800 m. Numerical model results indicate that large vertical diffusivities result in eastward jets in the bottom water at the equator. In the Pacific Ocean the DWBC observed flowing northward south of the equator crosses the equator with transport nearly intact, albeit split into two at 10°N by the tortuous bathymetry. However the southward flow along the East Pacific Rise in the Northeast Pacific Basin and the westward equatorial jet this flow feeds are puzzling. The basin depth decreases equatorward and eastward, which may allow some southeastward flow in the Stommel-Arons framework. However, the equatorial jet is still unexplained. The estimated vertical velocity and diffusivity at 3600 db of 2±2x10-5 cm s-1 and 4±3 cm2 s-1 for the area between 12°8 and 10°N are much smaller than estimates in the Somali Basin. Thus the two oceans, similar in their single southern source of bottom water, have DWBC's which behave remarkably differently near the equator. In the Somali Basin of the Indian Ocean the DWBC appears to turn eastward at the equator, with large vertical upwelling velocity and large vertical diffusivity estimates for the bottom water of the basin. In the Pacific Ocean the DWBC appears to cross the equator, but there is a puzzling westward flowing equatorial jet in the bottom water of the Northeast Pacific Basin.The author began this research in the M.I.T.-W.H.O.I Joint Program while supported by the U. S. Offce of Naval Research through a Secretary of the Navy Graduate Fellowship in Oceanography. Support for collection and analysis of the data taken during R.R.S. Charles Darwin cruises 86-19 and 87-25 was provided by the U. S. National Science Foundation under grants OCE8800135 and OCE8513825 to D. B. Olson at the University of Miami and by the U. S. Offce of Naval Research under contract N00014-87-K-0001, NR083-004 and grant N00014-89-J-1076 to B. A. Warren at W.H.O.I. Collection of data taken during R.Y. Moana Wave cruise 89- 3 was supp6rted by the U. S. National Science Foundation under grant OCE881691O to H. L. Bryden and J. M. Toole at W.H.O.I. Collection of data taken during the U.S.-P.R.C. Toga cruises was supported by N.O.A.A. under grant NA85AA-DACU7

Correction to “Recent western South Atlantic bottom water warming”

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 33 (2006): L21604, doi:10.1029/2006GL028294

Recent interannual upper ocean variability in the deep southeastern Bering Sea

Recent seasonal and interannual variability of the upper ocean in the southeast Aleutian Basin of the Bering Sea is related to air sea fluxes, ocean advection, and mixing. Between the winter of 2001/2002 and that of 2002/2003 a warming and freshening of the upper ocean was observed in data from a regional array of profiling Conductivity-Temperature-Depth (CTD) floats. The mild winter of 2002/2003 resulted in an unusually warm, fresh, light, and shallow winter mixed layer and a weakly ventilated temperature minimum layer. These unusual winter conditions contributed to a substantial reduction in the subsurface temperature inversion characteristic of the southeast Aleutian Basin. Heat budget analysis, one-dimensional upper ocean model runs, and altimeter sea-surface height anomalies suggest that a combination of atypical ocean advection and anomalous atmospheric forcing contributed to the unusual upper ocean conditions in 2002/2003. The observed warming and disappearance of the temperature minimum in 2002/2003 appears to have preconditioned the water column toward a similar structure in 2003/2004, despite a return to more normal atmospheric forcing

Relative contributions of temperature and salinity to seasonal mixed layer density changes and horizontal density gradients

Temperature and salinity both contribute to ocean density, including its seasonal cycle and spatial patterns in the mixed layer. Temperature and salinity profiles from the Argo Program allow construction and analysis of a global, monthly, mixed layer climatology. Temperature changes dominate the seasonal cycle of mixed layer density in most regions, but salinity changes are dominant in the tropical warm pools, Arctic, and Antarctic. Under the Intertropical Convergence Zone, temperature and salinity work in concert to increase seasonal stratification, but the seasonal density changes there are weak because the temperature and salinity changes are small. In the eastern subtropics, seasonal salinity changes partly compensate those in temperature and reduce seasonal mixed layer density changes. Besides a hemispheric seasonal reversal, the times of maximum and minimum mixed layer density exhibit regional variations. For instance, the equatorial region is more closely aligned with Southern Hemisphere timing, and much of the North Indian Ocean has a minimum density in May and June. Outside of the tropics, the maximum mixed layer density occurs later in the winter toward the poles, and the minimum earlier in the summer. Finally, at the times of maximum mixed layer density, some of the ocean has horizontal temperature and salinity gradients that work against each other to reduce the horizontal density gradient. However, on the equatorial sides of the subtropical salinity maxima, temperature and salinity gradients reinforce each other, increasing the density gradients there. Density gradients are generally stronger where either salinity or temperature gradients are dominant influences

An equatorial ocean bottleneck in global climate models

Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 25 (2012): 343–349, doi:10.1175/JCLI-D-11-00059.1.The Equatorial Undercurrent (EUC) is a major component of the tropical Pacific Ocean circulation. EUC velocity in most global climate models is sluggish relative to observations. Insufficient ocean resolution slows the EUC in the eastern Pacific where nonlinear terms should dominate the zonal momentum balance. A slow EUC in the east creates a bottleneck for the EUC to the west. However, this bottleneck does not impair other major components of the tropical circulation, including upwelling and poleward transport. In most models, upwelling velocity and poleward transport divergence fall within directly estimated uncertainties. Both of these transports play a critical role in a theory for how the tropical Pacific may change under increased radiative forcing, that is, the ocean dynamical thermostat mechanism. These findings suggest that, in the mean, global climate models may not underrepresent the role of equatorial ocean circulation, nor perhaps bias the balance between competing mechanisms for how the tropical Pacific might change in the future. Implications for model improvement under higher resolution are also discussed.KBK gratefully acknowledges the J. Lamar Worzel Assistant Scientist Fund. GCJ is supported by NOAA’s Office of Oceanic and Atmospheric Research. RM gratefully acknowledges the generous support and hospitality of the Divecha Centre for Climate Change and CAOS at IISc, Bangalore, and partial support by NASA PO grants.2012-07-0

Layer-thickness dependence of the conductive properties of Mo/Si multilayers

We report new measurements of the conductance and superconducting transition temperature of a set of Mo/Si multilayers, as a function of the metal layer thickness (from 7-85 Angstrom) for a constant semiconductor layer thickness of 22 Angstrom. Unlike previously reported measurements, we do not observe oscillations in either the resistivity, resistivity ratio, or the superconducting transition temperature with the metal layer thickness. Rather, we observe monotonic variations in the transport properties as the metal layer thickness increases. The sheet conductance and its change between 10 and 300 K both vary approximately linearly with the metal layer thickness, above a threshold thickness. The conductance starts to grow with metal layer thickness at approximately 10 Angstrom, whereas the temperature coefficient of resistance changes sign at approximately 25 Angstrom, exhibiting a Mooij correlation with a crossover resistivity of 125 mu Omega cm. The observed temperature dependence of the conductance rules out localization as the origin of the negative temperature coefficient of resistance. The conductance data are analyzed using a simple phenomenological model involving transport in interfacial and metallic layers, whose relative contribution to the conductance depends on the metal layer thickness and the temperature. The model is applied to separate two competing contributions that determine the overall temperature dependence of the conductance. We attribute the differences between our measurements and previous measurements to differences in bulk metallic conductivities and interface morphologies, due to differences in thermal evaporation versus sputtering fabrication processes. Our results show that the level and nature of disorder is an important ingredient in any theory that explains the cause of the observed oscillations

Effects of carbon dioxide on the searching behaviour of the root-feeding clover weevil <i>Sitona lepidus</i> (Coleoptera: Curculionidae)

The respiratory emission of CO2 from roots is frequently proposed as an attractant that allows soil-dwelling insects to locate host plant roots, but this role has recently become less certain. CO2 is emitted from many sources other than roots, so does not necessarily indicate the presence of host plants, and because of the high density of roots in the upper soil layers, spatial gradients may not always be perceptible by soil-dwelling insects. The role of CO2 in host location was investigated using the clover root weevil Sitona lepidus Gyllenhall and its host plant white clover (Trifolium repens L.) as a model system. Rhizochamber experiments showed that CO2 concentrations were approximately 1000 ppm around the roots of white clover, but significantly decreased with increasing distance from roots. In behavioural experiments, no evidence was found for any attraction by S. lepidus larvae to point emissions of CO2, regardless of emission rates. Fewer than 15% of larvae were attracted to point emissions of CO2, compared with a control response of 17%. However, fractal analysis of movement paths in constant CO2 concentrations demonstrated that searching by S. lepidus larvae significantly intensified when they experienced CO2 concentrations similar to those found around the roots of white clover (i.e. 1000 ppm). It is suggested that respiratory emissions of CO2 may act as a ‘search trigger’ for S. lepidus, whereby it induces larvae to search a smaller area more intensively, in order to detect location cues that are more specific to their host plant.<br/

Variable Hard X-ray Emission from the Candidate Accreting Black Hole in Dwarf Galaxy Henize 2-10

We present an analysis of the X-ray spectrum and long-term variability of the nearby dwarf starburst galaxy Henize 2-10. Recent observations suggest that this galaxy hosts an actively accreting black hole with mass ~10^6 M_sun. The presence of an AGN in a low-mass starburst galaxy marks a new environment for active galactic nuclei (AGNs), with implications for the processes by which "seed" black holes may form in the early Universe. In this paper, we analyze four epochs of X-ray observations of Henize 2-10, to characterize the long-term behavior of its hard nuclear emission. We analyze observations with Chandra from 2001 and XMM-Newton from 2004 and 2011, as well as an earlier, less sensitive observation with ASCA from 1997. Based on detailed analysis of the source and background, we find that the hard (2-10 keV) flux of the putative AGN has decreased by approximately an order of magnitude between the 2001 Chandra observation and exposures with XMM-Newton in 2004 and 2011. The observed variability confirms that the emission is due to a single source. It is unlikely that the variable flux is due to a supernova or ultraluminous X-ray source, based on the observed long-term behavior of the X-ray and radio emission, while the observed X-ray variability is consistent with the behavior of well-studied AGNs.Comment: 7 pages, 4 figures, 2 tables; accepted for publication in Ap