Impacts of the Cryosphere and Atmosphere on Observed Microseisms Generated in the Southern Ocean
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manuscript submitted to JGR: Earth Surface Impacts of the cryosphere and atmosphere on observed microseisms generated in the Southern Ocean Ross J. Turner1;2, Martin Gal1;3, Mark A. Hemer4, and Anya M. Reading1;2 1School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, TAS 7001, Australia 2Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, TAS 7004, Australia 3Institute of Mine Seismology, 50 Huntingfield Avenue, Huntingfield, TAS 7055, Australia 4CSIRO Oceans and Atmosphere Flagship, Hobart, TAS 7004, Australia Key Points: • Seasonal sea ice coverage is the dominant control on the microseism intensity in East Antarctica • Microseism events with extremal intensity are most prevalent in East Antarctica during March-April • East Antarctic extremal microseism events increase during the austral autumn for a negative SAM climate index arXiv:2108.08590v1 [physics.geo-ph] 19 Aug 2021 Corresponding author: R. J. Turner, [email protected] {1{ manuscript submitted to JGR: Earth Surface Abstract The Southern Ocean (in the region 60-180◦E) south of the Indian Ocean, Aus- tralia and the West Pacific is noted for the frequent occurrence and severity of its storms. These storms give rise to high-amplitude secondary microseisms from sources including the deep ocean regions, and primary microseisms where the swells impinge on submarine topographic features. A better understanding of the varying microseism wavefield enables improvements to seismic imaging, and development of proxy observ- ables to complement sparse in-situ wave observations and hindcast models of the global ocean wave climate. We analyse 12-26 years of seismic data from 11 seismic stations either on the East Antarctic coast or sited in the Indian Ocean, Australia and New Zealand. The power spectral density of the seismic wavefield is calculated to explore how the time-changing microseism intensity varies with: i) sea ice coverage surround- ing Antarctica; and ii) the Southern Annular Mode (SAM) climate index. Variations in sea ice extent are found to be the dominant control on the microseism intensity at Antarctic stations, which exhibit a seasonal pattern phase-shifted by 4-5 months com- pared to stations in other continents. Peaks in extremal intensity at East Antarctic stations occur in March-April, with the highest peaks for secondary microseisms oc- curring during negative SAM events. This relationship between microseism intensity and the SAM index is opposite to that observed on the Antarctic Peninsula. This work informs the complexity of microseism amplitudes in the Southern Hemisphere and as- sists ongoing interdisciplinary investigations of interannual variability and long-term trends. 1 Introduction Microseisms are ubiquitously observed on seismometers around the world and arise due to ocean swell and wave activity coupling with the solid earth. Previously considered as `noise' in the study of earthquakes and other seismic events, the useful- ness of seismic ambient wavefield (Nakata, 2019) is now recognised since it provides a relatively high frequency energy source for Earth imaging (Shapiro et al., 2005; Pilia et al., 2015; Shen et al., 2018). Seismic noise caused by Earth surface or oceanic phe- nomena also have the potential to provide an independent data stream to progress the understanding of the given system and its changes. Examples of the use of ambient seismic noise include volcano monitoring (Sens-Sch¨onfelder& Wegler, 2011), the study of stream flow (Tsai et al., 2012) and glacier activity (Winberry et al., 2013), and very significant progress in understanding and using the relationship between microseisms and ocean waves (Ardhuin et al., 2012; Obrebski et al., 2012). Microseisms generated by ocean sources are described in terms of the observed frequency (Ardhuin et al., 2019). The longest period signals ( 50-500 s), known as `hum', are generated through the interaction of atmosphere, ocean∼ and sea floor (Rhie & Romanowicz, 2004; Traer et al., 2012). Primary microseisms ( 10-30 s) are generated through interaction with the sea bed, notably at a continental∼ slope (Hasselmann, 1963). Secondary microseisms ( 1-10 s) are generated through the interaction of two different wave groups in the∼ deep ocean (Longuet-Higgins, 1950). The alternate name `double frequency' may be understood through the interaction of opposing wave groups of similar frequency (Ardhuin et al., 2015). Recent work has further developed the understanding of microseism generation mechanisms and also the way in which energy from the water column is coupled to the solid-Earth (Gualtieri et al., 2015; Ardhuin, 2018). Microseism studies have been carried out using observatory records from single three-component stations (Aster et al., 2010) and also using array recordings which afford the ability to locate multiple, simultaneous sources of secondary microseisms (Gal et al., 2016, 2017). Extending the array analysis to three {2{ manuscript submitted to JGR: Earth Surface manuscript submitted to JGR: Earth Surface decreased summer rain increased summer rain increased winter rain decreased winter rain and cooler temperatures and warmer temperatures warmer autumn temperatures cooler autumn temperatures decreased storm activity increased storm activity Negative SAM polarity Positive SAM polarity Figure 1. Seasonal impacts of the Southern Annular Mode (SAM) on East Antarctica and Figure 1. Seasonal impacts of the Southern Annular Mode (SAM) on East Antarctica and Australia. TheAustralia. solid The white solid lines white linesare anare anapproximate approximate location location of the of circumpolar the circumpolar westerly winds. westerly winds. Regions shadedRegions in shaded red have in red warmer have warmer temperatures, temperatures, lower lower precipitation precipitation and reduced and reduced storm activity; storm activity; regions in blueregions have in blue cooler have coolertemperatures, temperatures, increased increased precipitation precipitation and heightened and heightened storm activity. storm activity. The peak seasonThe peak for season these for anomalies these anomalies due due to to variations variations in in the the SAM SAM is indicated is stated in text. in the overlaid text. 81 The SAM has a relatively short characteristic timescale of around 10 days, but the fre- components82 quency where of the suitable positive recording polarity also exists has increased has helped slowly on to decadal reveal timescales the complexity due to of the ambient83 wavefield;stratospheric e.g. ozone the depletion previously (Thompson undetected et al., 2011).Sn Thephase positive (Gal mode et al.,is associated 2016). Where84 with seismic the intensification ambient and signals contraction are recorded of the westerly over wind longer, belt towards preferably Antarctica multidecadal re- time frames,85 sulting the in ambient increased polar wavefield storm activity also provides (Hurrell & a van means Loon, 1994;of investigating Meehl et al., 1998; Earth sys- tem components that impact the generation of microseisms (Stutzmann et al., 2009; 86 Thompson & Wallace, 2000; Marshall et al., 2017). Meanwhile, the wind belt expands Aster et al., 2010; Gal et al., 2015). Seismic records are now being utilised to improve the homogeneity87 towards the of oceanequator inwave the negative model polarity hindcasts (Marshall, (Stopa 2007; et Marshall al., 2019), et al., particularly2017) lead- for the Southern88 ing to Hemisphere increased storm where activity other over the observations Australian continent which (Hemer might et be al., used 2010; forMar- hindcast calibration are sparse (Babanin et al., 2019). Cyclical features of global climate are 89 shall & Thompson, 2016; Wandres et al., 2018). The SAM is recognised to influence global of particular interest, for example the Southern Annular Mode (SAM) climate in- 90 ocean surface waves (Hemer et al., 2010; Marshall et al., 2018) and surface temperatures dex, or Antarctic Oscillation (AO), which is associated with variability in wave power (Bromirski91 (Marshall, et al., 2013; 2007) which Bromirski in turn a↵ &ect Cayan, sea ice and 2015). ice shelves The (Greene SAM et concernsal., 2017; Mas- the north- south92 movementsom et al., of 2018). westerly winds that influence the strength and position of cold fronts and mid-latitude systems. SAM indices are defined using either the first empiri- cal orthogonal93 functionThe microseism, (EOF) or seismic of a Southern noise, environment Hemisphere of Antarctica extratropical was first investigated climate variable (e.g. geopotential94 at a single height station (Dumont and mean d’Urville) sea level as part pressure), of a global orstudy the undertaken difference by Stutzmann in zonal mean pressure between 40 and 65◦S (Ho et al., 2012). The positive mode is associated 95 et al. (2009). Subsequently, the microseism wavefield has been investigated in more de- with the intensification and contraction of the westerly wind belt towards Antarctica resulting96 intail increased as part of continent-wide polar storm appraisals activity by (Hurrell both Grob &et al. van (2011) Loon, and Anthony 1994; Meehl et al. et al., 1998;97 Thompson(2014). They & Wallace, reported systematic 2000; Marshall noise level variationset al., 2017). for stations Meanwhile, with di↵erent the instal- wind belt expands towards the equator in the negative polarity (Marshall, 2007; Marshall et al., 2017) leading to increased storm activity over–4– the Australian continent (Hemer et al., 2010;