Ch. 2 The Earth System The Ocean and cryosphere • Text, Ch. 2.1.1, Ch. 2.1.2 • Supplementary reading (required for graduate students, optional for undergraduate students): – Cravatte et al. 2009 – Saenger et al. 2009 – Shepherd and Wingham 2007 Scales and climate components: Tectonic, Atmosphere, large scales Smaller scales influence geological: ≥106 yrs ocean, land, biosphere, Glacier-interglacer: 104-5 yrs cryosphere millennium: 103 yrs centennial: 100 yrs Atmosphere, ocean, land, climate climate decadal: 10-50 yrs biosphere, cryosphere Interannual : 2-7 yrs seasonal: 1 yr intraseasonal: 20-90 days scales smaller influence Large-scale Synoptic: days-a week Diurnal scale: day weather weather Mesoscale: a few hours Atmosphere, surface Microscales: a few minutes conditions 2.1.1: The Oceans Why? • The oceans covers 72% of the earth’s surface, mass ~ 250 times of the atmosphere; regulate the earth’s temperature • Main source of water vapor for the atmosphere • Main source of climate variability on interannual, decadal, multi-decadal, centennial and millennial time scales; e.g., ENSO, AMO, • Delayed climate response to change of the forcing; • Key part of carbon cycle Current state of the ocean: • Sea water density ~ 1.02-1.03×103 kg/m3 depends on its temperature, salinity and pressure (T, S, P). • To focus on the small change that is matter for ocean circulation, that is the depart from 1×103 kg/m3, we use, σ, which represents the departure from 103 kg/m3, in unit of g/kg, to describe sea water density. • For example, σ=35 means that sea water density is 35 g/kg higher than 1.002X103 kg/m3 Density distribution in Atlantic Lynn and Reid (1968) source:http://oceanworld.tamu.edu/resources/ ocng_textbook/chapter06/chapter06_05.htm Potential density: • Potential density σθ is the density a parcel of water would have if it were raised adiabatically to the surface without change in salinity. σθ = σ( s, θ, 0) σθ is especially useful because it is a conserved thermodynamic property Change of potential density, T and S as a function of depth from a sounding in the subtropical Atlantic Ocean. Numbers along the curve indicate depth in unit of 100 m. How do T and S influence density? • Ocean temperature, T, ranges from -2°C-32°C globally. – σê with Té for sea water monotonically. But for fresh water, its density increase with T from 0°-4°C. – Question: Why both sea and lake ice float at top of the water? • Salinity, S, of the sea water ranges from 34-36 g/kg of water (or °/°°). – Question: Where do you expect S be highest and lowest, respectively, on earth? • Salinity has stronger influence on σ near the freezing point. Its influence decreases rapidly from 0-5C, reduces to 1/3 to ¼ when T>5C. • Question: Is salinity change more or less important in tropics vs. sub-polar region? ΔT equivalent to 1g/kg of ΔS in terms of its influence on σ at sea level. Pycnocline, thermocline, halocline: • Pycnocline is referred to a layer with strong density gradient. • Thermocline: a layer with strong temperature gradient. • Halocline: layers with fresh water above and salt water below (stable). • In tropics: pycnocline follows thermocline because vertical T gradient prohibit mixing of water with different densities. • In high latitudes: pycnocline is often parallel to halocline Global distributions of T and S: Why? Global SST distribution: Global salinity distribution: Fig. 2.11 Annual mean sea surface temperature. Top panel: the total field. Bottom panel: the departure of the local sea surface temperature at each location from the zonally averaged field. [Based on data from the Comprehensive Ocean-Atmosphere Data Set. Courtesy of Todd P. Mitchell.] Vertical and seasonal changes of T and S: • T and S decrease with depth of the ocean. Why? • Greatest T and S changes occur near ocean surface. Why? Vertical profiles of T and S and potential density (contours) from a sounding in subtropical Atlantic Ocean. • What are the sources of influences on ocean surface T and s? • Net heat flux at the surface: – Solar, infrared, sensible and latent fluxes • Mixing of cooler water from deeper ocean • Heat advection by ocean current • Net fresh water at the surface • Mixing with deeper water from below • Advect water with a different S precip evap IR, SH, LH solar adv adv Mixing of deeper water Mixing of cooler water Example: Determine the influence of weather on S and T in the ocean surface layer: • A heavy tropical storm dumps 20 cm of rainfall in a region of ocean where S=35.00 g/kg and the water is well mixed in the top 50 m and this layer is referred to as the ocean surface layer. How would this storm influence the salinity of the ocean surface layer? The density of pure water is 1.004X103 kg/m3. • After the storm, persistent wind causes evaporation at the rate of 10 cm/month. How long it would take to increase salinity back to 35 g/kg assuming no rainfall during this period? How change of rainfall influences ocean surface S. Cravatte et al.: Observed freshening and warming of the western Pacific Warm Pooland circulation? 575 Table 2 Expansions of the surface area (inside the three boxes drawn shows the time/longitude diagrams of the 4°S–4°N aver- in Fig. 1a) covered by waters warmer than fixed thresholds and aged SST and SSS. The SSS front clearly shifted eastward Cravattededuced from linear regressions et al. 2009: changeduring the past few of decades rainfall (Fig. 7b). A linear and fit to the evaporation have caused a Water HADISST ERSST position of isohalines from 1955 to 2003 (red line) indi- 1955– 1978– 1955– 1978– cates an eastward displacement of 17° ± 3° of longitude freshening2003 2003 2003and2003 warmingper 50 years, whereas of athe linear fit fromPacific 1978 to 2003 indi-warm pool since 1955. cates a much smaller displacement. A linear fit to the Warmer than 28°C 4.2 6.9 4.3 6.6 position of isohalines from 1955 to 1995 indicates an Warmer than 6.0 8.7 5.5 8.5 Why is change of Seastward correlated displacement of 26° ±with1° of longitude change per of T? 28.5°C 50 years. Therefore, it seems that the equatorial salinity Warmer than 29°C 8.9 10.7 7.6 11.5 front is subject to gradual eastward movement and to Warmer than 6.2 8.5 6.7 14.3 29.5°C decadal displacements of about the same amplitude. The Warmer than 30°C 0.8 1.6 1.0 2.8 westward retreat of the eastern edge of the Fresh Pool at the end of the 1990s, correspondingS. Cravatte et to al.: a Observed possible freshening shift to aand warming of the western Pacific Warm Pool 571 Estimates are given for the HADISST (see Fig. 6a) and the ERSST negative PDO phase (Peterson and Schwing 2003), coun- products. Units are 106 km2 per 50 years teracts the eastward gradual(a) movement and may explain in (c) atmosphere interactions. It is known that it migrates zon- part this slowdown of the eastward expansion. ally at ENSO timescales over thousands of kilometers The same calculations are made for 29°C isotherm. Its (Picaut et al. 2001 for a review), but its displacements at eastward shift is similar for the 1955–2003 period, between decadal or lower-frequency timescales have not been 15 and 20° of longitude per 50 years, depending on the documented yet. As reported in Sect. II-3, accessible SST products and the meridional average (0°,2°S–2°N, proxies for this eastern edge are the position of the 34.6 and 4°S–4°N). From 1978 to 2003, the zonal shift is weaker. It 34.8 isohalines, and the position of the 29°C. Figure 7 is negligible for the HadiSST product, but still significant Fig. 7 Longitude-time (a) (b) diagrams of 4°S-4°N averaged SST (a) and SSS (b). Contour intervals are 0.25°C for SST warmer than 28°C and 0.2 for SSS fresher than 35 pss. Linear fits to the position of 29°C isotherm and 34.8 isohaline are shown in red. The time series on (b) (d) the extreme right panel shows the number of full bins in the area (4°S–4°N/140°E–175°W), as expressed in percentage of the total number of bins inside the surface area. SST and SSS data have been filtered with a 25-month Hanning filter to filter out variations shorter or equal to one year Fig. 3 Linear trends in SST (a) and SSS (b). Units are °C/50 years 1976–1985 (green), 1986–1995 (blue) and 1996–2003 (light blue). and pss/50 years. Positions of the 28.5°C isotherms (c) and of the 34.8 The regions where the linear trends are not significant at the 90% isohalines (d), averaged during 1956–1965 (black), 1966–1975 (red), confidence level are hatched in black available, and in the western Coral Sea including the coverage is sufficient) during boreal autumn, in both cases eastern coast of Australia. It is also worth noting that the during the salinity seasonal minimum. Figure 4 illustrates freshening observed in the southeastern tropical Pacific is this feature along 175°E. Around 10°S, under the SPCZ, mainly due to a rather sudden and strong freshening of the freshening is maximum in March–April, just before and about one pss observed at the end of the 1990s, linked to during seasonal SSS minimum. From 5°S to 7°N, it is the mid- to late-1990s climate123 shift referred to earlier.