Definitions in-situ density anomaly: σs,t,p = ρ – 1000 kg/m3 Atmospheric-pressure density anomaly : σt = σs,t,0= ρs,t,0 – 1000 kg/m3 Specific volume anomaly: δ= αs, t, p – α35, 0, p δ = δs + δt + δs,t + δs,p + δt,p + δs,t,p Thermosteric anomaly: Δs,t = δs + δt + δs, t Potential Temperature: Potential density: σθ=ρs,θ,0 – 1000

Static Stability: A Summary If the ocean vertical structure is stable, heavy water must be below light water The compressibility of water parcel should also be taken into consideration Stability Stable: E>0 Unstable: E<0 Neatral: E=0 A first approximation, A better approximation, Where C is sound speed appropriate up to 1000 meters

An example of vertical profiles of temperature, salinity and density

θ and σθ in deep ocean Note that temperature increases in very deep ocean due to high compressibility

σθ and σ4

In practice, E≈ -25 ~ -50 x 10-8 m-1 in upper 50 m. a) mixed layer is slightly unstable subtropics, increase of salinity due to evaporation, vertical overturning occurs when E ~ -16 to -64x10-8 m-1, because of the effects of heat conduction, friction, eddy diffusion, etc. b) observational errors Error of σt ~ 5 x 10-3 The error of σt at two depths Δσt ~ 10-2 for Δz=z1-z2=20 m.

(Brunt-Väisälä frequency, N) Buoyancy frequency (Brunt-Väisälä frequency, N) We have known that , (depending on δz, restoring force) Since δz is the vertical displacement of the parcel, then or Its solution has the form where (radians/s)2 N is the maximum frequency of internal waves in water of stability E. Period: E=1000 x 10-8 m-1, τ=10 min E=100 x 10-8 m-1, τ=33 min E=1 x 10-8 m-1, τ≈6hr

Buoyancy Frequency and ocean stratification

Buoyancy frequency, an example

Ocean Mean State Sea Surface Properties Vertical Structure Thermocline

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)

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 (near -2oC) CPC Analysis, January 1982-December 2001

Mean Sea Surface Salinity (SSS) NODC WOA98, levitus et al • 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 ) Mean Sea Surface Salinity (SSS)

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

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

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.

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)

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.

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.

Temperature profiles (From Pickard and Emery: Descriptive Physical Oceanography: An Introduction) 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. 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.

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

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

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)

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. (From Pickard and Emery: Descriptive Physical Oceanography: An Introduction) 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.

Seasonal Change in the Upper Ocean (The Annual Cycle)

SST Annual Cycle ● 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 Departure of monthly climatology from climatological annual mean (based on CPC data, 1982-2001)

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.

Seasonal change of temperature profiles at different latitudes

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.

Seasonal Cycle of the Thermocline at Equatorial Atlantic Ocean

The Interannual Variability

Standard deviation of SST anomalies

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

An example of El Niño evolution

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

Tropical Atlantic SST Anomalies

ENSO-like phenomenon is not just found in the Pacific

Anomalous event in the Indian Ocean

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.

And it is not just ENSO!