Ch. 2 The System

The 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 components:

Tectonic, , scales large influence scales Smaller geological: ≥106 yrs ocean, land, , -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 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

Why? • The oceans covers 72% of the earth’s surface, mass ~ 250 of the atmosphere; regulate the earth’s temperature • Main source of vapor for the atmosphere • Main source of climate variability on interannual, decadal, multi-decadal, centennial and millennial 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 , its density increase with T from 0°-4°C. – Question: Why both sea and 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 . 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 : 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 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. maximum in August–September, also contributing to the From 1955 or from 1978 to the mid-1990s, this region decrease of the SSS seasonal minimum. Conversely, exhibits a significant increase in salinity (not shown), southward of 25°S, in the region of high salinity, the consistent with Delcroix et al. (2007) findings who also increase in salinity is stronger in July–August, during the detected an increasing trend along a shipping line, located SSS seasonal maximum. Interestingly, the SSS seasonal slightly southward, whose data stopped in the mid-1990s. cycles have been amplified in recent decades. During recent decades, it therefore seems that geographical contrasts in SSS have been amplified. 3.2.2 In the Warm Pool Linear SST and SSS trends have also been computed for spring, summer, autumn and winter (not shown). The As noticed above, the warming observed in the Warm Pool warming appears to be similar in each season. Regarding region is a very robust feature, observed whatever the SSS, the freshening appears to be stronger under the SPCZ chosen time period and SST product. In the Warm Pool in boreal spring, and stronger under the ITCZ (where data region, our estimate of warming ranges from 0.2°C to 1°C

123 Ocean circulation: winds Surface currents: • What similarity do you see between the ocean surface winds and currents? • Ocean surface currents mainly follow winds with about 45ο to the right (left) in N.H (S.H.). Ekman effect (due to earth’s rotation and friction). currents

http://oceanmotion.org/html/background/patterns-of-circulation.htm Global ocean circulation: – Driven by both buoyancy and winds – Transport heat, nutrients and chemical tracers between surface and deep ocean layers. Role in carbon cycle:

• Deep ocean stores far more carbon than the terrestrial including soil. • ~1/4 of the anthropogenic emission of CO2 is absorbed by marine ecosystem; As ocean temperature increases, less would be absorbed. • On time scale 100-1000 yrs, ocean carbon cycle becomes more important than terrestrial carbon cycle. Distribution of marine biosphere and its connection to circulation:

• What cause an increase of carbon and decrease of oxygen with depth in the ocean surface layer? • What cause ocean desert in the subtropical gyro regions? • Why is PP higher along coastal regions, northern high- latitudinal oceans and equatorial eastern Pacific and Atlantic oceans? Summary: • How does ocean influence climate? – Exchange , water and carbon with the atmosphere – Interannual variability of the ocean dominates interannual to millennial climate variabilities. • What drive ocean surface currents and deep circulation? – Winds and density/buoyancy change • What control sea water density change? How do these controlling factors vary geographically and why? – Primarily T and S – The influence of S increase as T decreases, from tropics to high latitudes • What control changes of temperature and salinity of the ocean? – Surface heat and water fluxes, up- and down-welling, ocean currents • What control primary productivity of the ocean? – Mixing with water from deep oceans forced by storms, coastal and equatorial upwelling.

2.1.2 Cryosphere

• What is the roles of cryosphere in climate? • What is the distribution of different components of cryosphere? • How has cryosphere changed in recent decades? How would it affect climate in the 21st century?

• What is cryosphere? – Cryo (frozen), a component of the earth’s comprised of water in its state. It consists of • & ice sheets, • , • (continuous and discontinuous) • (perennial and seasonal). • Largest fresh water reservoir on earth

Cryospheric component ! Area (% of earth surface) !Mass (103 kg/m2)! 2.7 53 ice sheet 0.35 5 Alpine glaciers 0.01 0.2 Sea-ice (in season of maximal extent) 7 0.01 Seasonal snow cover 9 <0.01 Permafrost 5 1 Cryospheric component ! Area (% of earth surface) !Mass (103 kg/m2)! 2.7 53 0.35 5 Alpine glaciers 0.01 0.2 Sea-ice (in season of maximal extent) 7 0.01 Seasonal snow cover 9 <0.01 Permafrost 5 1

What can we infer from ice mass listed above? – Surface surface area: 5.1X1014 m2, total land area: 1.45X1014 m2! – 103 kg/m2: equivalent to depth of liquid water in meter per unit area.! • If Antarctic ice sheet melted, it would create 53 m deep water layer over entire earth.! • How much would sea-level rise?! 76M = 53mX5.1/(5.1-1.45)

Role in climate system:

• Largest fresh water storage: – Influence sea-level rise – Water resources – Influence ocean circulation • Regular earth’s change • Regular regional-global climate How do we estimate water in snow and ice?

• Snow (ice) equivalent depth: snow is porous and its porosity depends on temperature and age of the snow. A measure of liquid water contained in snow is water equivalent depth, hm: hm=ρs/ρw·hs ρs,ρw: density of the snow and water, respectively. hs: depth of the snow/ice layer hm: The depth of water that will resulted from complete melt of snow/ice.

Snow relative density, ρs/ρw ranges from 0.15-0.4

Snow/ice albedo (whiteness): • Albedo: ratio of the reflected vs. incident radiative flux. It is a function of wavelength.

Surface Typical Albedo

Fresh asphalt 0.04 Conifer forest (Summer) 0.08, 0.09 to 0.15 Worn asphalt 0.12 Deciduous trees 0.15 to 0.18 Bare soil 0.17 Green grass 0.25 Desert sand 0.40 New concrete 0.55 Fresh snow 0.80–0.90 Ice sheets: • The largest storage of global surface fresh water • The greatest contributor to global • A critical process for abrupt • Highly reflective for solar radiation

The largest ice sheets: Antarctic and Greenland

Reservoirs of water !Mass spread over the Earth surface ! !!!(equivalent to water equivalent depth) Atmosphere 0.03 (103 Kg/m2 or m in depth) Fresh water ( and ) 0.6 Fresh water (underground) 15 Greenland ice sheet 5 Antarctic ice sheet 53 Alpine glaciers 0.2 • If the Greenland ice sheet were completely melted, how high would global sea-level rise? How much heat it would absorb (expressed as how many days of sunlight received by the earth).

Reservoirs of water !Mass spread over the !!!!Earth surface Greenland ice sheet 5 (103 Kg/m2 or m in depth) Antarctic ice sheet 53 Global surface area: 5.1X1014 m2; Global oceanic area: 3.65X1014 m2 of fusion: 3.34X105 J/kg

Earth receives ~ 1.8X1017 W at the top of the atmosphere, assuming the earth’s albedo is 0.3 • If the Greenland ice sheet were completely melted, how much heat it would absorb and how high would global sea- level rise?

Complete melt of Greenland ice sheet absorbs: 2 17 2 5 Q=ViceXL=5Kg/m X5.1X10 m X 3.34X10 J/kg =8.5X1023 J of heat

It takes ~ 77.7 days of total solar radiation received by the earth system (assuming 30% of earth’s albedo):

t=Q/S=8.5X1023 J/(1.8X1017J/s)=4.7X106s/(2.4X3.6X104s/day)=77.7 days

meters of sea level rise for complete melt of Greenland ice: 5 mX100%/72%~7 m

• ~ 8.5X1023 J, or 78 days of total solar radiation received by the earth system. 7 m

Antarctic Ice Sheet: P • Creep rate: near zero at the divides of the ice sheet, and >10 m/yr at the periphery; Why? image of the Antarctic ice sheet and the rate of creep of the • Creep rate is especially high ice (m/yr) on a logarithmic scale. in the W. Antarctic. Greenland Ice Sheet: • Lower latitudes and smaller than the Antarctic ice sheet, • S. Greenland is highly vulnerable to climate change because summer temperature reaches melting point (-5C). • However, the danger of complete melt of Greenland is not as high as it was believed during the 21st century, although it is still highly vulnerable to global warming in long-term.

IV Sea Ice:

• Sea ice in covers maximumly 3% of the earth and in Antarctic covers maximumly 4% of the earth’ surface, and about 1-3 m thick (not much mass, 0.01 m) • Sea ice cover in Antarctic varies seasonally from 2 to 14 X1012 m2, and in Arctic varies from 4 - 11 X1012 m2. • Why does sea ice varies more in Antarctic

than in Arctic?

• Sea ice is a fractal field comprised of Fridtjof Nansen (1861-1930) ice floes. • A new pack of ice is formed by freezing of water in newly formed leads in region where wind drag pack ice away from shore; after reach 1 m thick, it is formed by collisions of ice floes; • Sea ice moves with transpolar drift stream.

floes

leads

Floes streaming southward off the east coast of Greenland • Important to North Atlantic Deep Water (NADW) formation: – Ice is formed by fresh water and concentrated salt water is left behind as brine; – Brine mixed with sounding water. This heavy saline surface water sinks in Arctic to form NADW. • Why is sea-ice only a few meters thick? Influence of ice sheet and sea-ice melting on the ocean : • Melting of Greenland Ice Sheet and Arctic sea ice increase fresh water discharge to the source region of North Atlantic Deep Water, reduce surface water density and weaken the deep ocean convection; • Strong melting of the Greenland Ice Sheet in future could weaken the NADW formation and thermohaline circulation. Alpine glaciers: • Alpine glaciers, smaller ice sheets, can exist at any latitudes although their altitudes increase from < 1 km in high latitudes to 4-6 km in tropics; Why? • Alpine glacier retreat has been observed globally.

P T to form glacier for given P

Air T Snow:

Distribution and variations: • Seasonal snow covers ~9% of the global surface, mainly in high latitudes and high altitudes; • Snow cover varies strongly (50%), seasonally, weekly, interannually, decadally; Impact on climate:

• In NH high latitudes, snow can increase surface albedo from <0.2 to 0.5-0.8.

• Good isolator for surface: Thermal conductivity k is a measure of a material’s ability to transfer heat. A high value of k is a good conductor while a low value of k is a good insulator. k has units of Wm-1K-1

Fresh Snow 0.03 (better than fiberglass insulation!) Old Snow 0.4 Ice 2.1

Permafrost: • Structure and distribution of the permafrost: Permafrost: • The top few meters of soil thaws during summer and freezes in winter. • Below a few meters, the soil temperature remains constant around 0˚C. It would takes hundreds of years for the permafrost to adjust to air temperature; • Carbon locked up in the permafrost > carbon stored in global vegetation. Summary:

• What is cryosphere? – Cryo (frozen), a component of the earth’s climate system comprised of water in its solid state. It consists of • glaciers & ice sheets, • snow, • permafrost (continuous and discontinuous) • sea ice (perennial and seasonal). • What is the distribution of different components of cryosphere? – Largest mass in Antarctic and Greenland, 58 m deep of water globally if they melt completely; – Sea ice and land snow cover 8-16% of the earth’s surface – Greenland and W. Antarctic ice sheet, Arctic sea ice and alpine glaciers have retreated rapidly in recent decades. • What is the roles of cryosphere in climate system? – Largest storage of global surface fresh water – Contribute to the thermal inertial of the earth’s climate – Contribute to albedo of the earth – Controls fresh water flux in the polar region, thus influence oceanic thermohaline circulation; – Store more carbon than that by global vegetation