Typical Distributions of Water Characteristics in the Oceans

1. Typical Distributions of Water Characteristics in the Oceans

2. Ocean Mean State Sea Surface Properties Vertical Structure Thermocline

3. Global Statistics • The mean temperature of the world ocean is 3.5oC and the mean salinity is 34.7. (Even at the equator the average temperature is as low as 4.9°C). • 75% of the total volume of the ocean water has properties within the range from 0o to 6oC in temperature and 34 to 35 in salinity. • 50% of the total volume of the oceans has properties between 1.3oC and 3.8oC and between 34.6 and 34.8. • Both temperature and salinity are quite uniformly distributed below 1000 m depth. Volumetric temperature-salinity diagram of the world ocean. 75% of the ocean's water have a temperature and salinity within the green region, 99% have a temperature and salinity within the region colored in cyan. The warm water outside the 75% region is confined to the upper 1000 m of the ocean. From M. Tomczak, 1996: Introduction to Physical Oceanography (http://gaea.es.flinders.edu.au/~mattom/IntroOc)

4. Mean Sea Surface Temperature (SST) • roughly zonal (east-west) isotherm • meridional gradient follows solar radiation • diverted north-south near the eastern coast (cold tongues) • warm pool: SST > 28oC • sea ice at high latitudes (about -1.8oC) CPC Analysis, January 1982-December 2001

5. NODC WOA98, levitus et al Mean Sea Surface Salinity (SSS) • SSS distribution is mostly zonal (range: 33-37) • Minimum north of the equator (ITCZ) • Maximum in subtropics (trade winds) • Lower around coast (river) and polar region (melting ice) • Mediterranean, 39 and Red Sea, 41 (large evaporation )

6. SSS distribution in open ocean is closely linked to precipitation and sea surface evaporation From Pickard and Emery: Descriptive Physical Oceanography: An Introduction

7. Surface density (σt) σt, 22 at equator, 26-27, 50o-60o latitude, decreases slightly further poleward σt is affected by both the T and S distribution The temperature influence is more dominant

8. Ocean Stratification Potential density (σθ) as a function of latitude and depth in the Pacific Ocean. Note the scale difference between the depths about and below the upper 1000m. The density is quite uniformity below 1000 m depth in the range of 27.50-28.00.

9. subsurface density and pycnocline Density increases with depth (The gradient is not uniform) In the tropics, there is a shallow upper layer of nearly uniform density, then a layer where density increases rapidly (pycnocline), below this the density increases slowly in the deep zone. In deep water σt=27.9 (with little change with latitude)

10. Temperature and Salinity Distributions Temperature (top) and salinity (bottom) as functions of latitude and depth in the Pacific Ocean. (The image includes the Arctic Ocean on the extreme right.) Note the uniformity of both properties below 1000 m depth; the temperature is in the range 0 - 4°C, the salinity is near 34.6 - 34.7.

11. Temperature and Salinity Distributions Enlarged T(top) and S(bottom) sections in the upper 1500 m of the Pacific Ocean. The highest temperature in the tropics is above 28°C. The permanent thermocline is the depth range of rapid temperature change, which in the tropics is found at 150 - 600 m. Salinity also displays large changes in the upper 500 m, mainly in response to the precipitation - evaporation balance. In the depth range 800 - 1500 m the salinity is rather uniform at about 34.5 over most of the Pacific Ocean.

12. Temperature profiles • In many ocean regions, temperature decreases with depth, results in an increase of density with depth and produces a stable density stratification. • Vertically, there is an upper zone (mixed layer + thermocline) and a deep zone (with little change). • Mixed layer: depth 50-200 m. Temperature is close to SST. Bottom at depth with T=SST-0.5oC or, more accurately, 0.125σθ denser than the surface value. • Permanent thermocline: depth 200-1000 m, temperature changes rapidly with depth. It is located at 150 - 400 m depth in the tropics and at 400 - 1000 m depth in the subtropics and mid-latitudes. The temperature range is 8 - 15°C in mid-latitudes. Hard to define depth limits, especially lower limit. Present all the time in low and mid-latitudes. Also called main thermocline. There is often no permanent thermocline at high latitudes, but a permanent “pycnocline” associated with a permanent “halocline”. (From Pickard and Emery: Descriptive Physical Oceanography: An Introduction)

13. Equatorial Thermocline Note the high vertical temperature gradient around 20oC isotherm.

14. Note the high vertical temperature gradient around 20oC isotherm. 

15. Mean 20oC isotherm (unit: meter) Climatology from ocean data assimilation (1958-1998) The thermocline zone is sometimes characterized by the depth at which the temperature gradient is a maximum (the “thermocline depth”).In the equatorial ocean, 20oC isotherm is a good indicator of the themocline location.The maxima of the 20oC isotherm (the major warm waters in the upper ocean) are located differently from those of SST (The former is mainly determined by ocean dynamics while the latter by surface heat flux)

16. Salinity Profiles • Salinity increases at 100-200 m in the tropics (sinking heavy waters and flowing equatorward) • In high latitude: salinity increases with depth to 2000 m. • At low and mid-latitudes, salinity decreases vertically and shows intermediate minima at 600 to 1000 m and then increases to 2000 m. They are linked with water mass formation at the Polar Fronts where precipitation is high. At very great depth, salinity increases again because the water near the ocean bottom originates from polar regions where it sinks during the winter; freezing during the process increases its salinity. • A decrease in salinity produces a density decrease. Taken on its own, the salinity stratification would therefore produce an unstable density stratification. In the ocean the effect of the temperature decrease is stronger than the effect of the salinity decrease, so the ocean is stably stratified. (From Pickard and Emery: Descriptive Physical Oceanography: An Introduction)

17. Dissolved Oxygen Distribution • High values close to the surface (saturated or supersaturated). • Minimum in the upper 1000 m between the tropics. • Relatively high values below 1000 m in the Atlantic (North Atlantic Deep Water). • Low value in the North Pacific. • Similar distributions in the Southern latitudes in both oceans. Range: 0-8 mL/L (Large proportion within 1-6 mL/L)

18. Atlantic Pacific

20. Seasonal Change in the Upper Ocean (The Annual Cycle)

21. ● Amplitude Equator: 1o-2oC Mid-latitudes (40o): 5o-10oC Poleward: Reduced (melting and freezing of sea ice) Coastal region: 10o-20oC ● Evolution Northern hemisphere Maximum SST: August/September Minimum SST: February/March Sub surface: Delayed up to 2 months. ● Annual cycle decreases rapidly with depth: confined mostly above 100 little change below 200 meters SST Annual Cycle Departure of monthly climatology from climatological annual mean (based on CPC data, 1982-2001)

22. Annual cycle of temperature in the upper layer Mixed layer: In winter, SST is low, wind waves are large), mixed layer is deep (extending to the main thermocline). In summer, (SST highwater stable), misxed layer is shallow. Seasonal thermocline: seasonal thermocline develops in the upper zone in summer. high stability within the seasonal thermocline separate the water from upper to deeper zones causing a “fossilized mixing zone” (water from remaining winter mixed layer) Example: Seasonal thermocline at Ocean Weather Station “P” (50oN, 145oW) March is nearly isothermal in upper 100 meters. March-August, SST increases, (absorption of solar radiation). Mixed layer  30 m. August-March, net loss of heat, seasonal thermocline eroding due to mixing.

23. Seasonal change of temperature profiles at different latitudes There could be no permanent thermocline at high latitudes.

24. Temperature diurnal cycle • SST diurnal cycle: usually small(<0.4oC) • Diurnal cycle is mainly in upper 10 meters • Produce a “diurnal thermocline” • Localized higher amplitude: 1oC (occasionally 3o-4oC) in regions of high isolation + low wind, 2~3oC in shallow water along coast.

25. Seasonal Cycle of the Thermocline at Equatorial Atlantic Ocean

26. The Interannual Variability

27. Standard deviation of SST anomalies

29. Fluctuations of the equatorial undercurrent and thermocline depth during 1981-1983

32. An example of El Niño evolution

33. December,1997-February, 1998 20oC isotherm anomalies (m) COLA ODA SSH anomalies (cm) TOPEX/Poseidon

35. Tropical Atlantic SST Anomalies

38. ENSO-like phenomenon is not just found in the Pacific

39. Anomalous event in the Indian Ocean

42. And not just the tropics Image courtesy of Stepen Hare and Nathan Mantua, University of Washington, units are degrees Celsius The Pacific Decadal Oscillation (PDO) is a long-term ocean fluctuation of the Pacific Ocean. The PDO waxes and wanes approximately every 20 to 30 years.

44. And it is not just ENSO!