DOCSLIB.ORG

Uplift of Africa As a Potential Cause for Neogene Intensification of the Benguela Upwelling System

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Uplift of Africa As a Potential Cause for Neogene Intensification of the Benguela Upwelling System LETTERS PUBLISHED ONLINE: 21 SEPTEMBER 2014 | DOI: 10.1038/NGEO2249 Uplift of Africa as a potential cause for Neogene intensification of the Benguela upwelling system Gerlinde Jung*, Matthias Prange and Michael Schulz The Benguela Current, located o the west coast of southern negligible Neogene uplift of the South African Plateau18. Recently, Africa, is tied to a highly productive upwelling system1. Over for East Africa, evidence emerged for a rather simultaneous the past 12 million years, the current has cooled, and upwelling beginning of uplift of the eastern and western branches around has intensified2–4. These changes have been variously linked 25 million years ago19 (Ma), in contrast to a later uplift of the to atmospheric and oceanic changes associated with the western part around 5 Ma as previously suggested20. Palaeoelevation glaciation of Antarctica and global cooling5, the closure of change estimates, for example for the Bié Plateau, during the the Central American Seaway1,6 or the further restriction of past 10 Myr range from ∼150 m (ref. 16) to 1,000 m (ref.7 ). the Indonesian Seaway3. The upwelling intensification also These discrepancies depend strongly on the methods used for occurred during a period of substantial uplift of the African the estimation of uplift, some giving more reliable estimates of continent7,8. Here we use a coupled ocean–atmosphere general the timing of uplift than of uplift rates16, whereas others are circulation model to test the eect of African uplift on Benguela better suited for estimating palaeoelevations but less accurate upwelling. In our simulations, uplift in the East African Rift in the timing7. system and in southern and southwestern Africa induces Despite these uncertainties, there is evidence for uplift processes an intensification of coastal low-level winds, which leads that coincide with the two-step development documented for to increased oceanic upwelling of cool subsurface waters. Benguela upwelling intensification at around 12–10 Ma and after We compare the eect of African uplift with the simulated 5 Ma (ref.2 ). Studies exist that indicate an intensified uplift of the impact of the Central American Seaway closure9, Indonesian South African dome in the past 10 Myr, of the Bié Plateau from 5 to Throughflow restriction10 and Antarctic glaciation11, and find 2 Ma (refs7 ,21), increasing uplift rates during the past 10 Myr for that African uplift has at least an equally strong influence as the Namibia dome7, as well as intensified rifting in the western part each of the three other factors. We therefore conclude that of the East African Rift during the late Miocene and at about 4–2 Ma African uplift was an important factor in driving the cooling and (ref. 22). strengthening of the Benguela Current and coastal upwelling A clear intercomparison of uplift history and timing of upwelling during the late Miocene and Pliocene epochs. intensification is not possible at present, owing to the controversy Today, the atmospheric Benguela jet is found at the eastern flank in timing of uplift and palaeoelevations, but also owing to the of the subtropical South Atlantic anticyclone just off the coast of much lower temporal resolution of uplift history dating compared southern Africa. This low-level wind maximum is closely connected with oceanic palaeotemperature and productivity proxy time series. to the area of maximum oceanic upwelling12 and is influenced However, considering the possible co-occurrence of uplift and by the land–sea thermal contrast and the presence of a coastal coastal upwelling intensification, we reason that uplift of Africa mountain chain. As the thermal land–sea contrast increases with was a likely factor in driving long-term changes in the Benguela strengthened upwelling, the intensity of upwelling and that of the upwelling system. Benguela jet are connected through a positive feedback12. Model- We test this hypothesis by means of a coupled general based sensitivity studies for the California jet, which shares common circulation model. Although it has been suggested that uplift altered features with the Benguela jet12, revealed a substantial alteration of atmosphere and ocean dynamics as well as coastal upwelling in other jet strength and shape due to coastal topography13. For the Benguela regions23, no modelling study has investigated the impact of uplift on region it has also been discussed that the steep slope along the the Benguela Current and upwelling system so far. coast leads not only to strengthening, but also a reduced seasonal Our simulations were performed with the Community Climate variability of the wind. This stabilizes the flow and hence favours System Model version 3 (CCSM3; ref. 24). The model simulations do oceanic upwelling1. not represent a specific time in Earth history. Instead we carried out Southern and eastern African plateaux are considered to form a series of sensitivity experiments to give an insight into processes part of the so-called African superswell14, which was emplaced in that are important for coastal upwelling intensification due to uplift. the early Miocene15 and successively lifted during the Neogene16,17 Four experiments were performed, a control run with present-day with phases of major uplift since the late Miocene7,8. The exact topography (HIGH), a simulation identical to that, except for a timing of uplift as well as regional uplift rates and estimates of lowered eastern and southern African topography to half of the palaeoelevations, however, remain controversial. For the Namibian present-day level (LOW), and two additional simulations lowering dome, a continuous uplift with linearly increasing uplift rates East (EALOW) and southern African (SALOW) topographies since the late Miocene was estimated whereas the Bié Plateau and separately (Supplementary Methods). the South African dome show maxima in uplift rates in the late According to our model simulations, the uplift of southern and Miocene/early Pliocene period7. In contrast, another study suggests eastern Africa leads to a strengthening of the horizontal wind at MARUM - Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, 28334 Bremen, Germany. *e-mail: [email protected] NATURE GEOSCIENCE | VOL 7 | OCTOBER 2014 | www.nature.com/naturegeoscience 741 © 2014 Macmillan Publishers Limited. All rights reserved LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2249 abLOW HIGH 500 500 600 600 700 700 800 800 Pressure (hPa) Pressure (hPa) Pressure 900 900 1,000 1,000 010203040 010203040 Longitude (° E) Longitude (° E) 0246810 12 0246810 12 Wind speed (m s−1) Wind speed (m s−1) cdLOW HIGH 500 500 600 600 700 700 800 800 Pressure (hPa) Pressure (hPa) Pressure 900 900 1,000 1,000 0 10 20 30 40 0 10 20 30 40 Longitude (° E) Longitude (° E) −6 −2 2 61014182226 −6 −2 2 61014182226 Temperature (°C) Temperature (°C) Figure 1 | Vertical cross-sections at 25◦ S of annual mean horizontal wind speed (representing the low-level Benguela jet) and air temperature. a, Horizontal wind speed for low African topography (LOW). b, Horizontal wind speed for present-day topography (HIGH). c, Temperature for low African topography (LOW). d, Temperature for present-day topography (HIGH). the eastern limb of the subtropical anticyclone with the strongest intensification and pattern change is the elevated heating source intensification occurring in the vicinity of the southwest African over land in the present-day altitude simulation (HIGH). We find coast in the region of the Benguela jet. The maximum intensi- that zonal temperature gradients near the coast increase with uplift fication is seasonally varying in magnitude (around 2.5 m s−1 to (Fig.1 c,d). Consequently, a deeper layer of baroclinity develops, more than 4 m s−1) and in location, leading to a stabilization of which strengthens the along-coast winds, similar to the mechanism the flow (Supplementary Discussion). On average it is up to 3 m s−1 discussed for the California coastal jet involving thermal wind and found at approximately 25◦ S (Fig.1 a,b). Uplift also leads to considerations13,25. Another cause for increased coastal baroclinity a slight change in wind direction with a weakening of the east- with mountain uplift is the barrier effect that leads to a mechanical erly component and hence the wind becoming more parallel to blocking of onshore intrusion of higher momentum air, as was the coast (Fig.2 ). One factor contributing to this regional wind also demonstrated for the California coastal jet25. An important 742 NATURE GEOSCIENCE | VOL 7 | OCTOBER 2014 | www.nature.com/naturegeoscience © 2014 Macmillan Publishers Limited. All rights reserved NATURE GEOSCIENCE DOI: 10.1038/NGEO2249 LETTERS abLOW HIGH−LOW 10 10 10 2 1081 20 1081 20 1084 1084 Latitude (° S) Latitude (° S) 1085 30 30 1085 1087 1087 40 40 01020 30 0102030 Longitude (° E) Longitude (° E) −1 −0.5 0 0.5 1 −0.3−0.2 −0.1 0 0.1 0.2 0.3 Ekman pumping velocity (m d−1) Δ(Ekman pumping velocity) (m d−1) cdHIGH−SALOW HIGH−EALOW 10 10 2 2 1081 20 20 1081 1084 1084 Latitude (° S) Latitude (° S) 1085 30 1085 30 1087 1087 40 40 0102030 0102030 Longitude (° E) Longitude (° E) −0.3−0.2 −0.1 0 0.1 0.2 0.3 −0.3−0.2 −0.1 0 0.1 0.2 0.3 Δ(Ekman pumping velocity) (m d−1) Δ(Ekman pumping velocity) (m d−1) 1 Figure 2 | Annual mean wind vectors at 1,000 hPa (m s− ) and Ekman pumping velocity. a, For lowered African topography (LOW). b, Eect of eastern and southern African uplift (HIGH–LOW). c, Eect of southern African uplift (HIGH–SALOW). d, Eect of eastern African uplift (HIGH–EALOW); positive values indicate a strengthening of upwelling. Note the dierent scaling of the panels. The black circles and the associated numerical identifiers indicate the positions of Ocean Drilling Program sites that show Benguela upwelling intensification during the late Neogene.
Load more

Recommended publications
  • Navier-Stokes Equation
    ,90HWHRURORJLFDO'\QDPLFV ,9 ,QWURGXFWLRQ ,9)RUFHV DQG HTXDWLRQ RI PRWLRQV ,9$WPRVSKHULFFLUFXODWLRQ IV/1 ,90HWHRURORJLFDO'\QDPLFV ,9 ,QWURGXFWLRQ ,9)RUFHV DQG HTXDWLRQ RI PRWLRQV ,9$WPRVSKHULFFLUFXODWLRQ IV/2 Dynamics: Introduction ,9,QWURGXFWLRQ y GHILQLWLRQ RI G\QDPLFDOPHWHRURORJ\ ÎUHVHDUFK RQ WKH QDWXUHDQGFDXVHRI DWPRVSKHULFPRWLRQV y WZRILHOGV ÎNLQHPDWLFV Ö VWXG\ RQQDWXUHDQG SKHQRPHQD RIDLU PRWLRQ ÎG\QDPLFV Ö VWXG\ RI FDXVHV RIDLU PRWLRQV :HZLOOPDLQO\FRQFHQWUDWH RQ WKH VHFRQG SDUW G\QDPLFV IV/3 Pressure gradient force ,9)RUFHV DQG HTXDWLRQ RI PRWLRQ K KKdv y 1HZWRQµVODZ FFm==⋅∑ i i dt y )ROORZLQJDWPRVSKHULFIRUFHVDUHLPSRUWDQW ÎSUHVVXUHJUDGLHQWIRUFH 3*) ÎJUDYLW\ IRUFH ÎIULFWLRQ Î&RULROLV IRUFH IV/4 Pressure gradient force ,93UHVVXUHJUDGLHQWIRUFH y 3UHVVXUH IRUFHDUHD y )RUFHIURPOHIW =⋅ ⋅ Fpdydzleft ∂p F=− p + dx dy ⋅ dz right ∂x ∂∂pp y VXP RI IRUFHV FFF= + =−⋅⋅⋅=−⋅ dxdydzdV pleftrightx ∂∂xx ∂∂ y )RUFHSHUXQLWPDVV −⋅pdV =−⋅1 p ∂∂ρ xdmm x ρ = m m V K 11K y *HQHUDO f=− ∇ p =− ⋅ grad p p ρ ρ mm 1RWHXQLWLV 1NJ IV/5 Pressure gradient force ,93UHVVXUHJUDGLHQWIRUFH FRQWLQXHG K 11K f=− ∇ p =− ⋅ grad p p ρ ρ mm K K ∇p y SUHVVXUHJUDGLHQWIRUFHDFWVÄGRZQKLOO³RI WKHSUHVVXUHJUDGLHQW y ZLQG IRUPHGIURPSUHVVXUHJUDGLHQWIRUFHLVFDOOHG(XOHULDQ ZLQG y WKLV W\SH RI ZLQGVDUHIRXQG ÎDW WKHHTXDWRU QR &RULROLVIRUFH ÎVPDOOVFDOH WKHUPDO FLUFXODWLRQ NP IV/6 Thermal circulation ,93UHVVXUHJUDGLHQWIRUFH FRQWLQXHG y7KHUPDOFLUFXODWLRQLVFDXVHGE\DKRUL]RQWDOWHPSHUDWXUHJUDGLHQW Î([DPSOHV RYHQ ZDUP DQG ZLQGRZ FROG RSHQILHOG ZDUP DQG IRUUHVW FROG FROGODNH DQGZDUPVKRUH XUEDQUHJLRQ
    [Show full text]
  • North Sea Barotropic Tides Abstract Strategy of Mixed Meshes
    Modelling of coastal long-wave dynamics on unstructured mixed meshes under strong nonlinearity and pronounced baroclinity Alexey Androsov, Sergey Danilov, Vera Fofonova, Natalja Rakowsky, Karen Helen Wiltshire and Sven Harig Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Postfach 12-01-61, 27515 Bremerhaven, Germany, ([email protected] +49(0471) 4831-2106)‏ Abstract Results: North Sea barotropic tides Numerical modelling of coastal zone dynamics provides basis for solving a wide range of hydrogeological, engineering and ecological problems. A novel three-dimensional unstructured-mesh model is applied to simulate the dynamics of the density field and turbulence characteristics. The model is based on a finite-volume discretization and works on mixed unstructured meshes composed of triangles and quads. Although triangular meshes are Fig. 1 Fragments of the quads (left) and mixed (right) mesh used in numerical simulations. most flexible geometrically, quads are more efficient numerically and do not support spurious Fig. 2 Tidal map of the M2 wave: the amplitude in cm and phase (solid line) in inertial modes of triangular cell-vertex discretization. Mixed meshes composed of triangles and Here we present results of simulations of M2 tide in the degrees. The triangles indicate the station locations. A dashed rectangle shows North Sea. A large amount of observations is available the domain where the mesh is refined; the transition zone is over its periphery. quads combine benefits of both. In particular, triangular transitional zones can be used to join for this region helping to validate the model. Computations have been performed on three meshes quadrilateral meshes of differing resolution.
    [Show full text]
  • Downloaded 10/01/21 01:41 AM UTC 442 MONTHLY WEATHER REV1 EW Vol
    July 1967 Richard D. Lindzen 441 PLANETARY WAVES ON BETA-PLANES RICHARD D. LINDZEN National Center for Atmospheric Research, Boulder, Colo. ABSTRACT The problem of linearized oscillations of the gaseous envelope of a rotating sphere (with periods in excess of a day) is considered using the @-planeapproximation. Two particular &planes are used-one centered at the equator, the other at a middle latitude. Both forced and free oscillations are considered. With both 8-planes it is possible to approximate known solutions on a sphere. The use of either 8-plane alone, however, results in an inadequate description. In particular it is shown that the equatorial @-planeprovides good approximations to the positive equiv- alent depths of the solar diurnal oscillation, while the midlatitude 8-plane provides good approximations to the negative equivalent depths. The two 8-planes are also used to describe Rossby-Haurwitz waves on rapidly rotating planets, and the vertical propagatability of planetary waves with periods of a day or longer. 1. INTRODUCTION [ll], Dikii [4], Golitsyn and Dikii [7], Lindzen [14], Kato [12], etc.). To a certain extent, even the recent, extensive One of the simplest general types of problems of numerical investigation of Laplace’s Tidal Equation by importance to atmospheric dynamics is that of linearized Longuet-Higgins [15] suffers from these limitations. The wave motions in the gaseous envelope of a rotating sphere. difficulty of the equation has prevented the development The waves are generally taken to be small perturbations of simple formulae of great generality. on a barotropic, motion-free basic state. The pressure is In this paper we shall show that by the use of two generally assumed to be hydrostatic and the fluid is 0-planes-one centered at the equator, the other at some assumed to be inviscid and adiabatic; the horizontal middle latitude-simple relations may be obtained which component of the Earth’s rotation is neglected.
    [Show full text]
  • UC Irvine Faculty Publications
    UC Irvine Faculty Publications Title The modeled response of the mean winter circulation to zonally averaged SST trends Permalink https://escholarship.org/uc/item/89c8g2h3 Journal Journal of Climate, 14(21) Author Magnusdottir, G. Publication Date 2001-11-01 DOI 10.1175/1520-0442(2001)014<4166:TMROTM>2.0.CO;2 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California 4166 JOURNAL OF CLIMATE VOLUME 14 The Modeled Response of the Mean Winter Circulation to Zonally Averaged SST Trends GUDRUN MAGNUSDOTTIR Department of Earth System Science, University of California, Irvine, Irvine, California (Manuscript received 27 July 2000, in ®nal form 23 May 2001) ABSTRACT The response of the atmospheric winter circulation in both hemispheres to changes in the meridional gradient of sea surface temperature (SST) is examined in an atmospheric general circulation model. Climatological SSTs are employed for the control run. The other runs differ in that a zonally symmetric component is added to or subtracted from the climatological SST ®eld. The meridional structure of the variation in SST gradient is based on the observed change in zonally averaged SST over the last century. The SST trend has maxima of about 1 K at high latitudes of both hemispheres. Elsewhere, the increase in SST over the last century is fairly uniform at about 0.5 K. In both hemispheres the response to decreased SST gradients is decreased baroclinity in the lower troposphere and increased baroclinity in the upper troposphere, with the reverse response when the SST gradient is increased.
    [Show full text]
  • Answer to Referee
    Mixed layer Mesoscales: a parameterization for OGCMs V.M.Canuto et al. OSD , 7, 873, 2010 REPLY TO REFEREE # 1 V.M. Canuto1,2 , M.S.Dubovikov1,3 1NASA, Goddard Institute for Space Studies, 2880 Broadway, New York, NY, 10025 2Dept. Applied of Phys. and Math., Columbia University, New York, NY, 10027 3Center for Climate Systems Research, Columbia Univ., New York, NY, 10025 The primary goal of this Reply is to prove that the key arguments of Referee # 1 (R1) are rooted in an unfortunate misinterpretation of our past work. This invalidates his key conclusions. R1 Problem 1 • R1. The present mesoscale model in the ML is an adaptation of CD6 to the surface mixed layer (ML). I don’t understand the link with the previous papers. Answers: A) There is no “link” because there cannot be one since the deep ocean is adiabatic (CD6 model) while the ML is diabatic (present model). These are fundamentally different physical regimes (with positive and negative turbulent viscosities), described by different dynamical equations whose solutions are different. B) Concerning “adaptation”, what follows is the unphysical result that would ensue if we had “adapted” the CD6 results, as R1 erroneously says we did. In the adiabatic ocean treated in CD6, flows occur almost entirely along isopycnals: a zero diapycnal flux implies that the vertical flux F Fv is given by FvH=⋅F s , where s is the slope of the isopycnals and H is the horizontal flux. Since Fv is the source of the eddy kinetic energy K, one can estimate the latter under the “adaptation” procedure, specifically, we use relations representing the deep adiabatic ocean and “adapt” the slope of the isopycnals to be consistent with the expected values in the ML, say, 10- 100 times larger than in the deep ocean.
    [Show full text]
  • 13.4 an Assessment of Buoyancy and Baroclinity Within Select VORTEX2 Storms
    13.4 An Assessment of Buoyancy and Baroclinity within Select VORTEX2 Storms Christopher C. Weiss* Texas Tech University, Lubbock, Texas David C. Dowell NOAA/ESRL, Boulder, Colorado Patrick S. Skinner, Anthony E. Reinhart Texas Tech University, Lubbock, Texas 1. INTRODUCTION results of mass probe tests, which sporadically occurred through the field phase on inactive days. Numerical and observational studies have Data were then subjected to a standard suggested a number of processes relevant to the time-to-space conversion using a constant storm generation, reorientation and amplification of motion, which in these two cases was estimated vorticity within supercell thunderstorms. Baroclinic using the motion of specific radar reflectivity horizontal vorticity generation related to gradients features and, additionally in the case of Seminole, of buoyancy is one area that has received OK, tornado vortex translation. As all time-to- considerable attention over the past couple of space analyses are centered at the time when the decades (e.g., Rotunno and Klemp 1985; Davies- strongest inferred vertical vorticity (from single Jones and Brooks 1993; Adlerman et al. 1999; Doppler radar coverage in both cases) crossed the Davies-Jones 2000; Markowski et al. 2012a,b). StickNet array, errors in the storm motion (and the Though a few field projects have provided inherent assumption of supercell stationarity) opportunities for in situ sampling of surface accrue as one heads outwards from the center of conditions within these storms, mostly using the domain in both directions. That said, various versions of mobile mesonet platforms comparisons of radar radial velocity with StickNet (Straka et al. 1996), up until the Verification of the winds suggest these errors are not terribly Origin of Rotation in Tornadoes Experiment 2 significant, even at times +/- 30 min from the (VORTEX2) there had yet to be a concentrated center of the analysis.
    [Show full text]
  • Threshold in North Atlantic-Arctic Ocean Circulation Controlled by the Subsidence of the Greenland-Scotland Ridge
    ARTICLE Received 4 Feb 2017 | Accepted 19 Apr 2017 | Published 5 Jun 2017 DOI: 10.1038/ncomms15681 OPEN Threshold in North Atlantic-Arctic Ocean circulation controlled by the subsidence of the Greenland-Scotland Ridge Michael Sta¨rz1, Wilfried Jokat1, Gregor Knorr1,2 & Gerrit Lohmann1 High latitude ocean gateway changes are thought to play a key role in Cenozoic climate evolution. However, the underlying ocean dynamics are poorly understood. Here we use a fully coupled atmosphere-ocean model to investigate the effect of ocean gateway formation that is associated with the subsidence of the Greenland–Scotland Ridge. We find a threshold in sill depth (B50 m) that is linked to the influence of wind mixing. Sill depth changes within the wind mixed layer establish lagoonal and estuarine conditions with limited exchange across the sill resulting in brackish or even fresher Arctic conditions. Close to the threshold the ocean regime is highly sensitive to changes in atmospheric CO2 and the associated modulation in the hydrological cycle. For larger sill depths a bi-directional flow regime across the ridge develops, providing a baseline for the final step towards the establishment of a modern prototype North Atlantic-Arctic water exchange. 1 Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Am Handelshafen 12, Bremerhaven 27570, Germany. 2 School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK. Correspondence and requests for materials should be addressed to M.S. (email: [email protected]). NATURE COMMUNICATIONS | 8:15681 | DOI: 10.1038/ncomms15681 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15681 he tectonic evolution of ocean gateways and CO2 changes representing a template used in our ocean gateway studies are key controls of Cenozoic (from 65 Myr ago until (for details on the model scenarios and boundary conditions, Tpresent) climate change and ocean circulation.
    [Show full text]
  • Causes of Reduced North Atlantic Storm Activity in a CCSM3
    15 SEPTEMBER 2009 D O N O H O E A N D B A T T I S T I 4793 Causes of Reduced North Atlantic Storm Activity in a CAM3 Simulation of the Last Glacial Maximum AARON DONOHOE AND DAVID S. BATTISTI Department of Atmospheric Sciences, University of Washington, Seattle, Washington (Manuscript received 7 August 2008, in final form 6 April 2009) ABSTRACT The aim of this paper is to determine how an atmosphere with enhanced mean-state baroclinity can support weaker baroclinic wave activity than an atmosphere with weak mean-state baroclinity. As a case study, a Last Glacial Maximum (LGM) model simulation previously documented to have reduced baroclinic storm activity, relative to the modern-day climate (simulated by the same model), despite having an enhanced midlatitude temperature gradient, is considered. Several candidate mechanisms are evaluated to explain this apparent paradox. A linear stability analysis is first performed on the jet in the modern-day and the LGM simulation; the latter has relatively strong barotropic velocity shear. It was found that the LGM mean state is more unstable to baroclinic disturbances than the modern-day mean state, although the three-dimensional jet structure does stabilize the LGM jet relative to the Eady growth rate. Next, feature tracking was used to assess the storm track seeding and temporal growth of disturbances. It was found that the reduction in LGM eddy activity, relative to the modern-day eddy activity, is due to the smaller magnitude of the upper-level storms entering the North Atlantic domain in the LGM. Although the LGM storms do grow more rapidly in the North Atlantic than their modern-day counterparts, the storminess in the LGM is reduced because storms seeding the region of enhanced baroclinity are weaker.
    [Show full text]
  • An Assessment of Low-Level Baroclinity and Vorticity Within a Simulated Supercell
    FEBRUARY 2013 BECK AND WEISS 649 An Assessment of Low-Level Baroclinity and Vorticity within a Simulated Supercell JEFFREY BECK Centre National de Recherches Me´te´orologiques, Me´te´o-France, Toulouse, France CHRISTOPHER WEISS Atmospheric Science Group, Texas Tech University, Lubbock, Texas (Manuscript received 7 May 2011, in final form 12 July 2012) ABSTRACT Idealized supercell modeling has provided a wealth of information regarding the evolution and dynamics within supercell thunderstorms. However, discrepancies in conceptual models exist, including uncertainty regarding the existence, placement, and forcing of low-level boundaries in these storms, as well as their importance in low-level vorticity development. This study offers analysis of the origins of low-level bound- aries and vertical vorticity within the low-level mesocyclone of a simulated supercell. Low-level boundary location shares similarities with previous modeling studies; however, the development and evolution of these boundaries differ from previous conceptual models. The rear-flank gust front develops first, whereas the formation of a boundary extending north of the mesocyclone undergoes numerous iterations caused by competing outflow and inflow before a steady-state boundary is produced. A third boundary extending northeast of the mesocyclone is produced through evaporative cooling of inflow air and develops last. Con- ceptual models for the simulation were created to demonstrate the evolution and structure of the low-level boundaries. Only the rear-flank gust front may be classified as a ‘‘gust front,’’ defined as having a strong wind shift, delineation between inflow and outflow air, and a strong pressure gradient across the boundary. Trajectory analyses show that parcels traversing the boundary north of the mesocyclone and the rear-flank gust front play a strong role in the development of vertical vorticity existing within the low-level mesocyclone.
    [Show full text]
  • National Weather Service Glossary Page 1 of 254 03/15/08 05:23:27 PM National Weather Service Glossary
    National Weather Service Glossary Page 1 of 254 03/15/08 05:23:27 PM National Weather Service Glossary Source:http://www.weather.gov/glossary/ Table of Contents National Weather Service Glossary............................................................................................................2 #.............................................................................................................................................................2 A............................................................................................................................................................3 B..........................................................................................................................................................19 C..........................................................................................................................................................31 D..........................................................................................................................................................51 E...........................................................................................................................................................63 F...........................................................................................................................................................72 G..........................................................................................................................................................86
    [Show full text]
  • The Effect of Mountain Uplift on Eastern Boundary Currents and Upwelling Systems
    Clim. Past, 16, 161–181, 2020 https://doi.org/10.5194/cp-16-161-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. The effect of mountain uplift on eastern boundary currents and upwelling systems Gerlinde Jung1,2 and Matthias Prange1,2 1MARUM – Center for Marine Environmental Sciences, University of Bremen, 28334 Bremen, Germany 2Faculty of Geosciences, University of Bremen, 28334 Bremen, Germany Correspondence: Gerlinde Jung ([email protected]) Received: 26 March 2019 – Discussion started: 15 April 2019 Revised: 30 September 2019 – Accepted: 12 November 2019 – Published: 22 January 2020 Abstract. All major mountain ranges are assumed to have rich waters (Garcia-Reyes and Largier, 2012). The upwelling been subject to increased uplifting processes during the late systems also influence climatic patterns of the land region be- Miocene and Pliocene. Previous work has demonstrated that cause local sea surface temperature (SST) is strongly linked African uplift is an important element to explain Benguela to rainfall distribution (e.g., Rouault et al., 2003). upper-ocean cooling in the late Miocene–Pliocene. Accord- Coastal upwelling is mainly occurring as a response to ing to proxy records, a surface ocean cooling also oc- Ekman transport by alongshore winds and due to wind- curred in other eastern boundary upwelling regions during stress-curl-driven ocean-surface divergence offshore (in this the late Neogene. Here we investigate a set of sensitivity study we refer to both processes as Ekman pumping and de- experiments altering topography in major mountain regions fine upward pumping, sometimes called “Ekman suction”, to (Andes, North American Cordillera, and southern and East be positive).
    [Show full text]
  • Low-Level Wind Maxima and Structure of the Stably Stratified Boundary Layer in the Coastal Zone
    Low-Level Wind Maxima and Structure of the Stably Stratified Boundary Layer in the Coastal Zone Mahrt, L., Vickers, D., & Andreas, E. L. (2014). Low-Level Wind Maxima and Structure of the Stably Stratified Boundary Layer in the Coastal Zone. Journal of Applied Meteorology and Climatology, 53(2), 363-376. doi:10.1175/JAMC-D-13-0170.1 10.1175/JAMC-D-13-0170.1 American Meteorological Society Version of Record http://cdss.library.oregonstate.edu/sa-termsofuse FEBRUARY 2014 M A H R T E T A L . 363 Low-Level Wind Maxima and Structure of the Stably Stratified Boundary Layer in the Coastal Zone L. MAHRT NorthWest Research Associates, Redmond, Washington DEAN VICKERS College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon EDGAR LANDREAS NorthWest Research Associates, Inc., Lebanon, New Hampshire (Manuscript received 21 May 2013, in final form 12 October 2013) ABSTRACT A Rutan Aircraft Factory Long-EZ aircraft flew numerous low-level slant soundings on two summer days in 2001 off the northeastern coast of the United States. The soundings are analyzed here to study the non- stationary vertical structure of the wind, temperature, and turbulence. An error analysis indicates that fluxes computed from the aircraft slant soundings are unreliable. The first day is characterized by a weakly stable boundary layer in onshore flow capped by an inversion. A low-level wind maximum formed at about 100 m above the sea surface. The second day is characterized by stronger stability due to advection of warm air from the upwind land surface. On this more stable day, the wind maxima are very sharp and the speed and height of the wind maxima increase with distance from the coast.
    [Show full text]