Oceans Class Notes PDF

The circulation of the surface ocean http://www.thisiscolossal.com/2014/09/alexander-gerst-cloud-shadows-iss/  The ocean circulation Some important considerations Winds – ocean surface currents are driven by winds in the lowermost atmosphere Density – sinking of dense water masses...

The circulation of the surface ocean http://www.thisiscolossal.com/2014/09/alexander-gerst-cloud-shadows-iss/  The ocean circulation Some important considerations Winds – ocean surface currents are driven by winds in the lowermost atmosphere Density – sinking of dense water masses drives the deep-ocean, overturning thermohaline circulation Friction between ocean layers contributes to Ekman spirals The Coriolis effect modifies the direction of currents relative to surface winds Bathymetry and basin shape also have a significant impact – currently fixed, but different in the past (plate tectonics) Introductory overview: http://oceanexplorer.noaa.gov/edu/learning/player/lesson08.html  Surface currents – schematic ▪ Largely driven by surface winds – patterns are very similar ▪ Warm currents move heat (energy) poleward ▪ Basin-scale “gyres” As with atmosphere, shorter timescale processes important too http://www.geo.hunter.cuny.edu/tbw/wc.notes/3.temperature/ocean_currents.htm  Ekman transport Wind induced surface currents deflected by Coriolis effect (to right in NH) Currents weaken (friction) and are deflected further (Coriolis effect) with depth Net Ekman transport is perpendicular to surface wind stress (to right in NH) Contributes to convergence and higher sea levels in gyre centres http://www.atmosedu.com/Geol390/Life/OceanCirculation/Slide7.JPG  Surface currents – gyres Prevailing winds Net Ekman transport is perpendicular to surface wind stress Leads to convergence Ekman and higher sea levels in gyre centre Resulting pressure gradient force is balanced Ekman by Coriolis effect – flow at surface is geostrophic Weaker Coriolis effect near equator leads to Winds, basin shape and geostrophic narrow, strong Western balance together lead to the gyre structure Boundary Current  Surface currents – gyres The “Great Pacific Garbage Patch” Read more: http://education.nationalgeographic.com/encyclopedia/great-pacific-garbage-patch/ Animations: https://svs.gsfc.nasa.gov/4174  Upwelling Diverging surface currents (or here along coast in SH) Cause upwelling of water from below Recycle nutrients from deeper water Associated with high biological productivity chlorophyll-a concentration  Past surface ocean currents Reminder on importance of bathymetry and basin shape Late Jurassic 150-200 mya Early Tertiary ~50 mya https://www.britannica.com/list/a-journey- through-time-since-the-precambrian  The circulation of the deep ocean http://www.thisiscolossal.com/2014/09/alexander-gerst-cloud-shadows-iss/  Surface density http://en.wikipedia.org/wiki/Thermohaline_circulation#mediaviewer/File:SeaSurfaceDensity.jpg Dense [cold, salty] Dense waters sink, and contribute to deep ocean over- turning circulation Dense [cold, salty]  The Thermohaline circulation (THC) ▪ Global oceanic circulation driven by deep water formation (density-driven, so linked to thermo – heat; haline – salinity) ▪ Arctic deep waters enter the global ocean in the North Atlantic Atlantic cross section North Atlantic Deep Water (NADW) - deep water mass formed mainly in the Norwegian and Greenland Seas Antarctic Bottom Water (AABW) - cold, salty (dense) bottom water mass that forms around the Antarctic continent and spreads northwards https://www.open.edu/openlearn/science-maths-technology/the-oceans/content-section-4.7  THC in ocean properties Temperature (top) and salinity (bottom) along a section of the Atlantic Ocean CTD data AABW NADW AABW NADW https://www.open.edu/openlearn/science-maths-technology/the-oceans/content-section-3.5  The Thermohaline circulation - globally Currents ~100 km/year Transport time ~1000 years View video 12 here: https://www.open.edu/openlearn /science-maths-technology/the- oceans/content-section-4.7 Surface currents Deep currents http://upload.wikimedia.org/wikipedia/commons/thumb/a/a6/Conveyor_belt.svg/2000px-Conveyor_belt.svg.png  Ocean vs atmosphere (2) Have a think and come up with 2 or 3 important points... How are the circulation patterns in the ocean and atmosphere similar? How and why are they different?  The El Nino phenomenon http://www.thisiscolossal.com/2014/09/alexander-gerst-cloud-shadows-iss/  Tropical Pacific – ‘normal’ conditions ▪ East-West sea surface temperature (SST) gradient drives equatorial Walker circulation ▪ The coupled ocean-atmosphere system is called the El Nino- Southern Oscillation (ENSO) – which has different phases Warm SST Cool SST Heffernan, Nature Climate Change, 2014  El Nino Every few years the east and/or central Pacific warms... Unusual warming Similar cooling pattern called La Nina http://www.newscientist.com/data/images/archive/2968/29682401.jpg  El Nino Unusual ocean warming disrupts ‘normal’ atmospheric circulation Warm SST Heffernan, Nature Climate Change, 2014  Recent large El Ninos – 2015, 2023 https://psl.noaa.gov/enso/enso.current.html Common El Nino impacts Global surface temperature increase Global rainfall anomalies (including dry conditions across Indonesia – drought and fires more common) Weaker land sink for anthropogenic carbon emissions (so a larger fraction of those emissions remains in the atmosphere)  El Nino and global temperature anomalies Heat transferred from ocean to atmosphere during El Nino http://www.columbia.edu/~mhs119/Temperature/globalT_1880-1920base.pdf  El Nino and rainfall anomalies “Tele-connections” within atmospheric circulation important https://www.climate.gov/news-features/blogs/enso/how-enso-leads-cascade-global-impacts  El Nino and Southeast Asian “haze” Lower rainfall combined with land use change leads to fire and atmospheric “haze” pollution in SE Asia during El Nino events Koplitz et al. (2018): https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JD028533  ENSO and carbon uptake by land ▪ Shading shows areas where ENSO strongly affects NPP ▪ An “Earth System” – ocean variability, atmospheric circulation patterns, biogeochemical cycles … NPP larger during El Nino NPP larger during La Nina Bastos et al. (2013): https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgrg.20100  The oceans – summary Seawater density determined by temperature and salinity Ocean salinity due to presence of various ions participating in biogeochemical cycles Surface currents driven by winds Westward intensification of subtropical gyres influenced by Coriolis effect Deep-ocean overturning thermohaline circulation driven by density differences Ocean-atmosphere interactions exert strong control on climate Over year-to-year timescales the El Nino-Southern Oscillation (ENSO) phenomenon is the most important climate fluctuation View the currents today here: http://earth.nullschool.net/ https://www.esri.com/arcgis-blog/products/arcgis- pro/mapping/spilhaus-more-like-thrillhaus/  Before the next class 1. Watch the recording that 1) introduces key concepts related to biogeochemical cycles and 2) outlines the C and N cycles 2. Read the article reading on “Planetary Boundaries” by Steffen et al. (2015) on Moodle. You should find that each of the cycles we will cover – C, N, P and S – is mentioned to a greater or lesser extent. Make some notes on how these four cycles are being affected by human activity so that you have several key points of information to share and/or related questions to ask in the next class