Mean 20 o C 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, 20 o C isotherm is a good indicator of the themocline location. The maxima of the 20 o C 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 ( )

Seasonal Change in the Upper Ocean (The Annual Cycle)

SST Annual Cycle ● Amplitude Equator: 1 o -2 o C Mid-latitudes (40 o ): 5 o -10 o C Poleward: Reduced (melting and freezing of sea ice) Coastal region: 10 o -20 o C ● 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, )

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” (50 o N, 145 o W) 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(<0.4 o C) Diurnal cycle is mainly in upper 10 meters Produce a “diurnal thermocline” Localized higher amplitude: 1 o C (occasionally 3 o -4 o C) in regions of high isolation + low wind, 2~3 o C 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

An example of El Niño evolution

December,1997-February, o C 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

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!