Mean 20oC isotherm (unit: meter)

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)

Climatology from ocean data assimilation (1958-1998)

Seasonal Change

in the Upper Ocean

(The Annual Cycle)

SST Annual Cycle● Amplitude Equator: 1o-2oCMid-latitudes (40o): 5o-10oCPoleward: Reduced (melting and freezing of sea ice)Coastal region: 10o-20oC● EvolutionNorthern hemisphereMaximum SST: August/SeptemberMinimum SST: February/MarchSub 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 climatologicalannual mean (based on CPC data, 1982-2001)

Annual cycle of temperature in the upper layerMixed layer:In winter, SST is low, wind waves are large), mixed layer is deep (extending to the main thermocline).In summer, (SST highwater stable), misxed layer is shallow. Seasonal thermocline:seasonal thermocline develops in the upper zone in summer.high stability within the seasonal thermoclineseparate the water from upper to deeper zonescausing 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

December,1997-February, 199820

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Tropical Atlantic

SST Anomalies

ENSO-like phenomenon is not just found in the Pacific

Anomalous event in the Indian Ocean

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 not just the tropics

And it is not just ENSO!