GEO21011 Ocean Temperature and Salinity - PDF

GEO21011 – Understanding the climate system Ocean temperature and salinity: measurement and distribution Dr Andrew Sole [email protected]...

GEO21011 – Understanding the climate system Ocean temperature and salinity: measurement and distribution Dr Andrew Sole [email protected] | Geography, office F18 Attendance code: VE-EL-TE A one-month ocean surface temperature composite for August 2021 from the Moderate-resolution Imaging Spectroradiometer (AQUA). Outline of the lecture Measuring temperature of ocean water (field and Earth Observation - EO) What is salinity? Measuring salinity of ocean water (field and EO) Global ocean surface temperature distribution Global ocean surface salinity distribution Ocean density & potential temperature Temperature and salinity diagrams: water mass identification Transects of ocean temperature and salinity 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 2 Measuring temperature of ocean water in the field (1) Temperature is a measure of the energy due to the motion of molecules in the ocean The unit of temperature, T, is Kelvin, which has the symbol K (1 K is the same increment as 1 °Celsius, but 0 K = -273.15 °C) Originally ocean temperature was measured by placing a mercury thermometer in a bucket of seawater taken from the ocean surface In 1910 Fridtjof Nansen attached a metal bottle and a pair of Wikipedia minimum thermometers to a cable – the Nansen bottle, and A sailor preparing to its 1921 improvement, the Knudsen bottle, was used until the deploy a Nansen 1960s bottle. Multiple Nansen bottles were deployed at set increments along a weighted cable Once at the allotted depth, and when the thermometers had reached a stable temperature, a messenger weight (i.e. a weight on a sleeve dropped down the cable) triggered the bottles’ closure mechanism 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 3 Measuring temperature of ocean water in the field (2) In 1966, Shale Niskin (American inventor and oceanographer) invented the Niskin bottle, a plastic cylinder with spring loaded end caps. Niskin bottles typically have a reversing thermometer (which can retain the temperature measured at a set depth to be viewed later) attached to their outside Today, multiple Niskin bottles can be mounted in a circular pattern on a frame (forming a ‘rosette’ – see photo opposite) and lowered from a research vessel A rosette of Niskin bottles ready to be Bottle closure is triggered electronically from the deployed on an Antarctic research cruise from surface or when a specific water depth is reached New Zealand by The National Institute of Water and Atmospheric Research (NIWA) (different for each bottle on the rosette) https://niwa.co.nz/our- science/voyages/antarctica- Ship Injection Temperature: The temperature of the 2018/oceanographic-drifters-ctd water drawn into the ship to cool the engines has been recorded routinely for decades, with accuracy of 0.5 – 1˚C 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 4 Measuring sea surface temperature (SST) from space Sea surface temperature (SST) is a vital component of the climate system as it exerts a major influence on the exchanges of energy, momentum and gases between the ocean and atmosphere (more later in the module) The ocean and most other objects with T>0 K emit radiation in the infrared and microwave wavelengths The amplitude of these wavelengths varies with the temperature of the ocean and therefore can be used to measure it Satellite-derived SST represents the water temperature over a depth range that is related to the frequency of electromagnetic (EM) radiation emitted by the satellite sensor, with a maximum of a few mm for microwave sensors (so-called ‘ocean skin temperature’) There are two main types of satellite sensor used to measure SST Satellite microwave radiometers (e.g. Advanced Very High Resolution Radiometer, AVHRR): at microwave frequency of the EM spectrum (4-11 GHz), brightness temperature data (a measure of the radiance of microwave radiation reflected from the Earth) contain a specific SST signature Infrared (IR) radiometers (e.g. MODIS AQUA) convert infrared radiation (300 GHz) into an electrical voltage 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 5 What is salinity? Rivers and the wind deposit material (often terrigenic and lithogenic sediment) into the ocean Some of this material is dissolved in ocean water into its constituent ions (note that this takes hours to weeks depending on the mineralogy of sediment and ocean water properties, and the net short-term effect of river input to the oceans is freshening) Salinity is the net concentration of these dissolved ions (i.e. the total amount of dissolved material in grams in one kilogram of sea water) Thus, salinity is a dimensionless quantity - i.e. mass/mass it has no units Typical salinity values are 32 – 38 (g/kg or parts per thousand) The range of salinity for most of the ocean’s water below 4˚C is from 34.60 to 34.80 (i.e. 0.2) parts per thousand, or 200 parts per million So we need to measure it very precisely to be able to discern spatial and temporal variability (Stewart, 2008) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 6 Principal ions in ocean water The predominant ions are sodium (Na+) and chloride (Cl-) Cations (g/kg) Anions (g/kg) There are many others that contribute to Na+ 10.8 Cl- 19.4 ocean salinity, e.g. Calcium Ca2+, magnesium Mg2+ 1.3 SO42- 2.7 Mg2+, potassium K+, carbonate CO32-, Ca2+ 0.4 HCO3- 0.1 bicarbonate HCO3-, sulphate SO42- K+ 0.4 These ions make up 99.7% of salinity The ratios of the major ions, and some minor ones, in the ocean are fixed because the mixing time of the global ocean (~1000-2000 years) is much less than the residence time of the ions In the case of conservative ions like Na+ this can be order 100 million years – the age of an ocean basin – so that the ions are well mixed throughout the oceans Trace ions that play a major role in ocean chemistry or biogeochemistry (e.g. derived from iron, Fe or Phosphorus, P) are not conservative and may vary in concentration significantly in time and space 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 7 Measuring salinity of ocean water in the field Salinity (chloride ion concentration) was initially measured using chemical titration of ocean water with silver salts (salinity accuracy of ±0.02). These days salinity is derived from the conductivity of a sample of ocean water Conductivity is determined based on the current that flows when there is a known voltage applied between (usually) platinum electrodes in a known volume of ocean water (salinity accuracy of ±0.005). Conductivity depends on the amount of dissolved ions per volume (salinity) and the mobility of the ions (related to temperature and pressure) Conductivity is measured in mS cm-1 (milli-Siemens per centimetre) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 8 Salinity scales The chlorinity scale (1966): S = 1.8066[Cl], where S = salinity (g/L) and Cl = chlorinity (g/L). The units were ‰ (parts per thousand) The Practical Salinity Scale (1978): defines a water sample’s salinity as the ratio of its conductivity to that of a solution of 32.4356 g of potassium chloride (KCl) at 15°C in a 1 kg solution of ocean water (psu). A 1 kg sample of seawater at 15°C with a conductivity equal to this KCl solution has a salinity of exactly 35 practical salinity units (psu). Being a ratio, the psu is non-dimensional. In practice 35 psu = 35 ‰ in the 1966 chlorinity scale. For international standardisation, salinity is determined from empirical relationships between temperature and the conductivity ratio of a sample to International Association for the Physical Sciences of the Ocean (IAPSO) Standard Seawater (sampled from the Atlantic Ocean). 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 9 Measuring salinity of surface ocean water from space The observed relationship between conductivity and salinity enables remote sensing of salinity from space As the conductivity of ocean surface waters change (with salinity) there are minute detectable changes in the brightness temperature of the surface in the microwave part of the EM spectrum This is also the part of the EM spectrum where SST measurements can be made so it is very difficult to differentiate variations in microwave brightness due to differences in SST and conductivity (Martin-Neira et al. 2014) – but we can use IR wavelength data of SST to isolate salinity effects ESA Soil Moisture and Ocean Salinity (SMOS) mission (2009 - present): interferometric radiometry which uses brightness temperature and phase differences (don’t worry about the details) NASA Aquarius mission (2011-2015): 3 passive microwave radiometers and an active scatterometer to measure the ocean waves that affected the precision of the salinity measurement NASA Soil Moisture Active Passive (SMAP) mission (2015 – present): newer version of Aquarius 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 10 Conductivity Temperature Depth (CTD) sensors Mechanical instruments on Nansen bottles were replaced from the 1960s by an electronic instrument, called a CTD sensor, that measures Conductivity, Temperature, and Depth Depth measurements are derived from measurement of hydrostatic pressure often using a strain gauge (diaphragm) in units of decibars (1 db = 104 Pa ≈ 1 m of ocean water depth) A CTD is often attached beneath an array of Niskin bottles on a rosette and can be deployed from a research vessel With a strong titanium housing, CTDs can operate to depths of 10.5 km But they are still only point measurements (limited spatial extent) https://www.seabird.com/eBooks/CTDs-Explained-Sea-Bird-Scientific 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 11 Automatic drifters While global-scale ocean surface temperature and salinity can be measured from space, most deeper ocean temperature and salinity data come from drifters Ocean drifters are robotic instruments that move up and down through the water column measuring CTD Once deployed, they drift with the ocean currents, periodically carrying out depth profiles and transmitting the data to a global network of receiving Argo floats ready for deployment https://oceanbites.org/ stations The international Argo float programme which https://argo.ucsd.edu/ began in 1999 (Wong et al. There are currently 2020) has collected and transmitted over 2 million ~4000 Argo floats in the ocean CTD profiles from the upper 2000 m of the global ocean 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 12 Argo floats sampling and data transmission schedule Argo floats operate on a 10-day cycle of descent, drift, profile collection and data transmission Wong et al. (2020) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 13 Argo float positions in January 2020 Wong et al. (2020) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 14 Example Argo float data Long-term mean salinity (2001 – 2013) at 150 m depth from Argo float data (Schabetsberger et al. 2016). The data were used to help explain the spawning sites for anguillid eels. 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 15 Global ocean surface temperature distribution Monthly mean sea surface temperature (SST) for August 2021 from NASA’s MODIS AQUA satellite. There is a general global distribution of surface Do you see any systematic T with cooler waters at higher latitudes, and warmer waters towards the equator due to variability across (i.e. with surface heat exchange with the atmosphere. longitude) ocean basins? 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 16 Warm western boundary currents Earth’s rotation Coriolis-deflected poleward mid-latitude winds exert drag on the ocean surface, resulting in relatively warm water at higher latitudes at the western edges of ocean basins. Earth’s rotation Monthly mean sea surface temperature (SST) for August 2021 from NASA’s MODIS AQUA satellite. 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 17 Cool eastern boundary currents - upwelling Coriolis-deflected poleward mid-latitude winds exert drag on the ocean surface, resulting in relatively cool water at lower latitudes at the eastern edges of ocean Earth’s rotation basins. Monthly mean sea surface temperature (SST) for August 2021 from NASA’s MODIS AQUA satellite. 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 18 Eastern and western boundary currents - explanation As viewed from vertically above the north pole, Earth turns anti-clockwise. Because the Earth rotates, large-scale (geophysical) fluid flow is deflected. This ‘Coriolis effect’ results in the deflection of major surface ocean currents to the right in the Northern Hemisphere (in a clockwise spiral) and to the left in the Southern Hemisphere (in a anti-clockwise spiral). These large-scale patterns of water motion are called gyres The rotation of the Earth causes an accumulation of energy on the western side of ocean basins, which must be dissipated in boundary currents; this gives the western boundary currents (e.g. Gulf Stream) typical widths of 100 km and typical speeds of 2 m s-1. Western boundary currents are relatively warm, deep, narrow, and fast-flowing currents that form on the west side of ocean basins (adjacent to the eastern coasts of continents) due to western intensification. They carry warm water from the tropics poleward. Eastern boundary currents are relatively shallow, broad and slow-flowing. They are found on the eastern side of oceanic basins (adjacent to the western coasts of continents). Subtropical eastern boundary currents flow equatorward, transporting cold water from higher latitudes to lower latitudes. 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 19 Selected global boundary currents Canary California Gulf Current Current Stream Kuroshio Current Galápagos Islands: The upwelling associated East with the Humboldt Current leads to very Australian high marine West Current productivity (and most Humboldt/ Benguela northerly penguins - Peru Australian Galápagos penguin) Current Current Current Brazil Earth’s rotation Current Earth’s rotation Monthly mean sea surface temperature (SST) for August 2021 from NASA’s MODIS AQUA satellite. 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 20  Global ocean surface salinity distribution NASA's Soil Moisture Active Passive (SMAP) mission began collecting sea surface salinity data in April 2015. The map shown is the mean for June 2021 gridded at 25 x 25 km resolution, with an approximate spatial resolution of 70 km. The map shows a range of salinities from 33 to 38 using the Practical Salinity Scale. S has a general global distribution, with Do you see higher values towards any the mid-latitudes and lower values at the interesting equator and high features? latitudes 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 21  Seaice and iceberg ice melt NASA's Soil Moisture Active Passive (SMAP) mission began collecting sea surface salinity data in April 2015. The map shown is the mean for June 2021 gridded at 25 x 25 km resolution, with an approximate spatial resolution of 70 km. The map shows a range of salinities from 33 to 38 using the Practical Salinity Scale. 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 22  River freshwater discharge NASA's Soil Moisture Active Passive (SMAP) mission began collecting sea surface salinity data in April 2015. The map shown is the mean for June 2021 gridded at 25 x 25 km resolution, with an approximate spatial resolution of 70 km. The map shows a range of salinities from 33 to 38 using the Practical Salinity Scale. Danube Yukon & others Ganges Niger Mississippi Amazon 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 23  Rainfall NASA's Soil Moisture Active Passive (SMAP) mission began collecting sea surface salinity data in April 2015. The map shown is the mean for June 2021 gridded at 25 x 25 km resolution, with an approximate spatial resolution of 70 km. The map shows a range of salinities from 33 to 38 using the Practical Salinity Scale. Rainfall, glacier runoff and low evapouration rates East Asian Monsoon 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 24  Evapouration NASA's Soil Moisture Active Passive (SMAP) mission began collecting sea surface salinity data in April 2015. The map shown is the mean for June 2021 gridded at 25 x 25 km resolution, with an approximate spatial resolution of 70 km. The map shows a range of salinities from 33 to 38 using the Practical Salinity Scale. Evapouration is greatest in the subtropics, due to Mediterranean patterns of global air circulation (specifically Red Sea & air becoming warmer Persian Gulf and drier as it descends in the Hadley cell). Dry mid- latitudes 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 25 Ocean density & potential temperature Temperature, T, & salinity, S, are important because together they determine the ocean’s density (σt) Salt in the oceans raises the ocean’s density from that of pure water (~1000 kgm-3) to ~1027 kgm-3 (because the dissolved salts increase the mass by a larger proportion than the volume) Spatial variations in density (latitude, longitude and depth) drive ocean currents Pressure, which increases with depth, also affects density, so to remove this relatively uniform depth-dependency, density is normally expressed in terms of potential density Potential density anomaly, σϴ is the density of a water parcel raised to the surface maintaining its temperature and salinity (adiabatically) minus 1000 (i.e. on average, 27 kgm-3) σϴ of a parcel of water is conserved as the pressure experienced by the parcel changes For static stability (i.e. increase of density with water depth) potential density must decrease upward (an example of instability is fresh subglacial runoff plumes) Potential temperature ϴ is defined as the temperature of a water parcel raised to the surface maintaining its density and salinity (adiabatically) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 26 Global distribution of surface ocean density Broad pattern of higher In polar regions, where water is density at the poles, and cold, density variations are lower density at the controlled by salinity equator In the tropics, where water is warm, density variations are controlled by temperature 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 27 Temperature and salinity (T-S) diagrams Potential temperature (on the y-axis) is plotted against salinity (on the x-axis). Potential temperature and salinity combine to determine the potential density of seawater Contours of constant potential density, σϴ, are often shown on T-S diagrams T-S diagrams are used to identify water masses (which manifest as clusters of points) Data shown opposite are from Cape São Tomé, São Tomé and Príncipe, Gulf of Guinea, off the western equatorial coast of Central Africa (Carlos Pinto de Macedo- Soares et al., 2015) – note the Subantarctic Shelf water transported north by the Plata Plume Water (PPW), Subtropical Shelf Water (STSW), Shelf Water (SW), Tropical Water (TW), Subantarctic Shelf Water (SASW) Benguela Current and South Atlantic Central Water (SACW). Only stations to 130 m depth are displayed (Carlos Pinto de Macedo-Soares et al., 2015) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 28 Atlantic Ocean north-south transect Permanent thermocline AAIW (steep temperature gradient) at NADW GS ~1000 m depth ABW: Antarctic Bottom Water AIW: Antarctic Intermediate Water GS: Greenland Sea (water) MOW: Mediterranean Outflow Water NADW: North Atlantic Deep Water Schlitzer (2000) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 29 Indian Ocean north-south transect ABW: Antarctic CDW Bottom Water ABW AIW: Antarctic Intermediate Water CDW: Circumpolar Deep Water CDW ABW Schlitzer (2000) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 30 Pacific Ocean north-south transect (from Eqt) AIW NPIW AIW: Antarctic Intermediate Water CDW: CDW Circumpolar Deep Water NPIW: North Pacific Intermediate Water AIW NPIW CDW Schlitzer (2000) 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 31 Summary Salinity S, and temperature T, are crucial oceanographic variables, as combined, they determine the density of the ocean Potential T (ϴ) and potential density (σϴ) are the T and σ of a water parcel raised to the surface maintaining its temperature and salinity (adiabatically) Both T and S can be measured in various ways in the field and from satellites There is a general global distribution of surface T with cooler waters at higher latitudes, and warmer waters towards the equator S also has a general global distribution, with higher values towards the mid-latitudes (evapouration) and lower values at high latitudes (rivers, sea ice, ice sheets) and towards the equator (rainfall) The general latitudinal patterns of S and T are also affected by ocean gyres (due to the rotation of the Earth), rainfall, river discharge, evapouration and sea ice & iceberg melt Water masses can be differentiated by plotting T vs S in a T-S diagram Ocean basin transects of T and S reveal broad-scale distributions of water masses and help to explain the global ocean circulation 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 32 Suggested reading (* = prioritize) Carlos Pinto de Macedo-Soareset al. (2015): Temperature-salinity diagram from Cape São Tomé to Chuí for late austral spring and summer seasons.. PLOS ONE. Figure. https://doi.org/10.1371/journal.pone.0091241.g002 Martin-Neira et al. (2014), Microwave interferometric radiometry in remote sensing: An invited historical review, Radio Sci., 49, 415–449, doi:10.1002/2013RS005230. Schabetsberger et al. (2016) The hydrographic features of anguillid spawning areas potential signposts for migrating eels, Marine Ecology Progress Series, Vol. 554: 141–155, doi: 10.3354/meps11824 Schlitzer, R., Electronic Atlas of WOCE Hydrographic and Tracer Data Now Available, Eos Trans. AGU, 81(5), 45, 2000. https://www.ewoce.org/gallery/eWOCE_Tables *Stewart (2008) Introduction to Physical Oceanography, Ch. 6 Temperature, Salinity, and Density. Department of Oceanography, Texas A&M University Vinogradova et al. (2019), Satellite Salinity Observing System: Recent Discoveries and the Way Forward, Frontiers in Marine Science 7, doi: 10.3389/fmars.2019.00243 Wong et al. (2020), Argo Data 1999–2019: Two Million Temperature-Salinity Profiles and Subsurface Velocity Observations From a Global Array of Profiling Floats, Frontiers in Marine Science 7, doi: 10.3389/fmars.2020.00700 13/11/2024 GEO21011 Understanding the climate system - Ocean temperature and salinity: distribution and measurement 33