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The Oceanic Role in Tropical Atlantic Climate Variability on Interannual to Decadal Time Scales

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July 1, 2005 through June 30, 2006

Drs. Jiayan Yang and Terrence M. Joyce
Woods Hole Oceanographic Institution, Woods Hole, MA 02543

Program Manager: Dr. Jim Todd, NOAA Office of Climate, CLIVAR-Atlantic Program

Related NOAA Strategic Plan Goal:
Goal 2. Understand climate variability and change to enhance society’s ability to plan and respond.
Goal 3. Serve society’s needs for weather and water information.

The project is to investigate the role of oceanic dynamics in the seasonal to interannual climate variability in the tropical Atlantic Ocean. In the first year of the project, we have analyzed the SST dipole mode and identified that the cross-equatorial transport, forced by the wind-stress curl, and the North Equatorial Countercurrent (NECC) play important role in the development of the SST dipole mode. So we have emphasized studying these two processes in the last one year. It was found that the NECC transport is forced both remotely by equatorial wind and locally by local wind stress. The contribution from the equatorial forcing is much greater for the seasonal time scales. The finding leads to a paper published in Journal of Physical Oceanography in 2006. Currently, we are investigating both local and remote forcing mechanisms for the interannual and decadal time scales. We have also studied the South Equatorial Countercurrent (SECC) variability.

Yang, J. and T. Joyce, 2006: Local and equatorial forcing of seasonal variations of the North Equatorial Countercurrent in the Atlantic Ocean. Journal of Physical Oceanography, Vol. 36, 238–254.

Abstract: The seasonal variation of the North Equatorial Countercurrent (NECC) in the tropical Atlantic Ocean is investigated by using a linear, one-layer reduced gravity ocean  model and by analyzing sea level height (SSH) data from TOPEX/POSEIDON (T/P) altimeters. The T/P data indicate that the seasonal variability of the NECC geostrophic transport, between 3°N and 10°N, is dominated by SSH changes in the southern flank of the current. Since the southern boundary of the NECC is located partially within the equatorial waveguide, the SSH variation there can be influenced considerably by the equatorial dynamics. We hypothesize therefore that the wind stress forcing along the equator is the leading driver for the seasonal cycle of the NECC transport. The wind stress curl in the NECC region is an important but smaller contributor. This hypothesis is tested by several sensitivity experiments that are designed toseparate the two forcing mechanisms. In the first sensitivity run, a wind stress field that has a zero curl is used to force the ocean model. The result shows that the NECC geostrophic transport retains most of its seasonal variability. The same happens to another experiment in which the seasonal wind stress is applied only within a narrow band along the equator outside the NECC range. To further demonstrate the role of equatorial waves, we ran another experiment in which the wind stress in the southern hemisphere is altered so that the model excludes hemispherically symmetrical waves (Kelvin waves and odd-numbered meridional modes of equatorial Rossby waves), and instead, excites only the anti-symmetrical equatorial Rossby modes. The circulation in the northern tropical ocean, including the NECC, is affected considerably even though the local wind stress there remains unchanged. All these appear to support our hypothesis.

Yang, J. and T. Joyce, 2006: Local and equatorial forcing of seasonal variations of the North Equatorial Countercurrent in the Atlantic Ocean. Journal of Physical Oceanography, Vol. 36, 238–254.

Our interactions with the NOAA have been in the form of informal science discussions with AOML and PMEL scientists.

Jiayan Yang has supervised a MIT-WHOI Joint Program student, Mr. Shin Kida, who defended his Ph.D dissertation successfully in July, 2006. Yang has also served as a committee member in the WHOI Summer and Minority Student Committee in the last three years. Through this role he has interacted with some undergraduate students who have been interested in marine research.

Last updated: August 19, 2008

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