The eastern equatorial Pacific and Atlantic Oceans are marked by relatively cool surface water, referred to as the Pacific and Equatorial cold tongues. Although smaller than the Pacific cold tongue, the Atlantic cold tongue is expansive, spanning several degrees latitude across the Equator and tens of degrees longitude. These cold tongues act as sinks for atmospheric heat and are critical to the global heat balance. The seasonal cycle in sea surface temperature varies in magnitude decreasing from east to west along the equator. Peak cooling occurs in boreal summer, first in the east and progressing westward. The Atlantic's cold tongue exerts considerable regional influence on both sides of the ocean, contributing to variable rainfall over Northeast Brazil and to weather patterns in densely populated areas of western Africa. However, measurements in the Atlantic cold tongue comparable to those made in the Pacific cold tongue do not presently exist. This project will close this gap and discern primary differences between the equatorial mixing regimes in the Pacific and Atlantic basins. A post-doctoral associate will participate in a broad analysis of the new observations and be trained in equatorial dynamics. Undergraduate engineering students will be employed in summer months to help with construction and testing of instrumentation. The Mary's Peak webcam will continue to be supported. This is a webcam installed on the roof of the Oceanography building at the Oregon State University that provides a visualization of natural fluid flows traced by clouds, providing an educational glimpse into fluid motions in the atmosphere. This project will also complement ongoing measurements in the Pacific cold tongue, theoretical analyses of Tropical Instability Waves, and ongoing international studies in the tropical Atlantic. The data set will be archived and shared with the community.

Away from the equator, the annual cycle in sea surface temperature (SST) is related to seasonally-varying solar heating. Despite the fact that the equatorial oceans are heated year round by the atmosphere, they also exhibit an annual cycle in SST. Recent, yet distinct, measurements in the equatorial cold tongues of both Pacific and Atlantic Oceans establish the dominant role that mixing plays in boreal summer cooling of the sea surface to produce this annual cycle in SST. At least two significant, and puzzling, differences exist between the two cold tongues. Firstly, measurements from the Atlantic show that the deep diurnal mixing cycle (below the mixed layer but above the core of the Equatorial Undercurrent), which is so critical to mixing in the upper equatorial Pacific, is not present in the Atlantic. Secondly, climate models predict a Pacific cold tongue that is too cool and an Atlantic cold tongue that is too warm. The now well-established PIRATA (Pilot Research Moored Array in the Tropical Atlantic) array is providing new insight into equatorial Atlantic dynamics and its role in critical weather patterns on both sides of that ocean. This project will use it as the backbone to measure the vertical structure and short-time variability of cold tongue mixing in the Atlantic, for comparison to what has been learned from the Pacific cold tongue. Five newly-developed mixing meters (chi-pods) will each be deployed on two equatorial PIRATA moorings at 10W and 23W across the Atlantic cold tongue for a period of four years. These two moorings have dense upper ocean sampling, surface meteorology, and vertical profiles of ocean currents to complement the mixing measurements. These new observations will establish seasonal and inter-annual variations of cold tongue mixing and how it influences SST and the observed weakening of the Atlantic cold tongue, and also define the mechanisms for differences in Pacific and Atlantic mixing that creates the bipolar climate model responses in SST trends.

Agency
National Science Foundation (NSF)
Institute
Division of Ocean Sciences (OCE)
Type
Standard Grant (Standard)
Application #
1431518
Program Officer
Baris Uz
Project Start
Project End
Budget Start
2014-09-01
Budget End
2020-09-30
Support Year
Fiscal Year
2014
Total Cost
$1,500,000
Indirect Cost
Name
Oregon State University
Department
Type
DUNS #
City
Corvallis
State
OR
Country
United States
Zip Code
97331