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Tom Farrar, 2005-2006 Graduate Student Researcher

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Tom Farrar

Ph.D. Conferred February 2007, MIT/WHOI Joint Program
S.M., MIT/WHOI Joint Program, 2003
B.S. in Physics, U. of Oklahoma, 2000
B.A. in Philosophy, U. of Oklahoma, 2000

Maximum solar heating occurs at latitudes near the equator, warming the sea surface and causing the surface air to rise. As the air ascends high into the atmosphere, it is replenished at the surface by the northeast trade winds from the north and the southeast trade winds from the south. This area of rising air is associated with deep atmospheric convection, heavy precipitation, and weak mean wind speeds. Early sailors were acutely aware of the tendency for weak winds in this region; they labeled this region the doldrums and carefully avoided it. Today, there is renewed scientific interest in the doldrums, also known as the Inter- Tropical Convergence Zone (ITCZ), because the strength and location of this band of convection exerts a profound influence on global weather patterns. The strength and location of atmospheric convection in the ITCZ is primarily determined by the sea surface temperature (SST) field.

Tom began his Ph.D. thesis research in October, 2004 under the supervision of Dr. Robert Weller. This research utilizes a unique data set of upper ocean and surface meteorology measurements from the eastern tropical Pacific to study the processes important in setting SST in the region. The data are from two WHOI surface moorings on 125ºW, one at 3° S in the equatorial cold tongue (a band of cold water that appears annually on the equator in the eastern Pacific) and one at 10° N near the northernmost climatological position of the ITCZ. This research is an outgrowth of Tom’s Master’s thesis research, which examined the evolution of the upper ocean thermal structure at the northern site.

This research has yielded insight into a mechanism by which the ocean influences the evolution of  SST on intraseasonal timescales at the 10ºN site (Farrar and Weller, submitted manuscript).
Prominent meridional current fluctuations with a period of about 2 months were observed in th mooring data, and these current fluctuations exerted a strong influence on the local SST, causing it to fluctuate with about a 2 month period from January-June of 1998. The SST fluctuations associated with this signal were substantial, with peak-to-peak amplitudes ranging from 0.5-0.8ºC. Farrar and Weller showed that the SST fluctuations were caused by horizontal advection along the meridional surface temperature gradient.

The two month signal in meridional currents was linked to a previously recognized sea surface height signal that is strongest in the latitude band 9-13ºN east of 120ºW. To resolve discrepancies in prior studies of the signal, Farrar and Weller also worked to characterize the signal observed at the mooring within its larger spatial and temporal context using satellite SST and sea surface height measurements. The signal was found to be associated with relatively short (5-15º wavelength) baroclinic Rossby waves. Farrar and Weller also found evidence that the intraseasonal velocity variability, and its annual cycle, are caused by instability of the westward flowing North Equatorial Current as it intensifies in the spring of each year. It is hoped that this improved understanding of mesoscale oceanic variability and its impact on SST in the region will allow for improved prediction of the oceanic mesocale SST field at monthly to seasonal timescales. This could be particularly important in the region of 9-13ºN in the eastern tropical Pacific, one of the world’s most prolific regions of tropical cyclogenesis.

Continuing progress will allow for the completion of the analysis of the data collected at 125°W during 1997-1998 in conjunction with preparation of Tom’s Ph. D. Thesis. The effort has shifted to examination of the surface forcing, upper ocean dynamics, and evolution of the thermal structure at 3°S. With that complete, Tom will use remote sensing and TAO data in conjunction with the upper ocean observations and air-sea fluxes from the two mooring sites to extend the effort to identify the important physical processes that drive the evolution of SST and upper ocean thermal structure over the broader region spanning the equatorial cold tongue and in the eastern Pacific warm pool.

Last updated: August 19, 2008

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