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 range of sea-surface temperature in °C calculated from the Reynolds and Smith (1995) mean sea-surface temperature data set. Contour interval is 1°C with heavy contours at 4°C and 8°C. Shaded areas exceed 8°C.

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), mixed 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

Growth and decay of the mixed layer and seasonal thermocline from November 1989 to September 1990 at the Bermuda Atlantic Time-series Station (BATS) at 31.8°N 64.1°W. Data were collected by the Bermuda Biological Station for Research, Inc. Note that pressure in decibars is nearly the same as depth in meters (see § 6.8 for a definition of decibars).

Figure 6.8 Typical temperature and salinity profiles in the open ocean. AAC: At 62.0°S, 170.0°E in the Antarctic Circumpolar Current on 16 January 1969 as measured by the R/V Hakuho Maru. Warm Pool: At 9.5°N, 176.3°E in the tropical west Pacific warm pool on 12 March 1989 as measured by Bryden and Hall on the R/V Moana Wave. BATS: At 31.8°N, 64.1°W near Bermuda on 17 April and 10 September 1990 as measured by the Bermuda Biological Station for Research, Inc. Data are included with Java Ocean Atlas.

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 of SST: 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

Some Examples of the Interannual Variability in Upper Tropical Ocean

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 Relation between fluctuations of SSH (  ) and thermocline depth (h)

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!