Archive of Spotlight Feature Articles

Where the trade winds meet: air-sea coupling in the inter-tropical convergence zone

by Tom Farrar, WHOI Joint Program Student
and Robert Weller, Senior Scientist, WHOI/CICOR

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.

Monthly composite satellite images of clouds show the ITCZ as a band of clouds several degrees in latitude across. In tropical oceans, the low winds make the upper ocean sensitive to surface heat and freshwater exchanges; and one can hypothesize that the lack of solar heating indicated by the satellite composites may lead to cooling of sea surface temperature under the ITCZ. Yet, as the ITCZ moves north and south annually, from about 10°N to about 3°N, the ocean underneath is typically warm. The sea surface temperature field and the location of warm surface water can influence atmospheric convection, and a central goal of the NOAA Pan American Climate Study (PACS) is to improve understanding of the coupling of the ITCZ to the underlying ocean. Because the strength and position of the ITCZ influence tropical and global weather patterns, it is anticipated that better knowledge of links between the ITCZ and the ocean can be used to improve the skill of seasonal-to-interannual prediction in the Americas.

PACS Surface Mooring buoy with a research ship in the background

Figure 1. PACS Surface Mooring with redundant meteorological instrumentation on the buoy tower-top. A Scripps Institution of Oceanography research ship is in the background.

From April 1997 to October 1998 Robert Weller and Steven Anderson of Woods Hole Oceanographic Institution (WHOI) deployed heavily instrumented surface moorings in the eastern tropical Pacific (Figure 1). One mooring was at 10°N, 125°W, near the northernmost climatological position of the ITCZ. Surface meteorology and air-sea fluxes were measured with temporal resolution of 15 minutes or better; ocean temperature, salinity, and velocity observations were also collected every 15 minutes from oceanographic instruments spaced close to each other along the mooring line in the upper 200 meters. These unique data have allowed a new understanding of atmosphere-ocean coupling in the eastern tropical Pacific. The high temporal resolution of the data shows that while mean winds are low and averaged cloud cover is complete, that the surface meteorology under the ITCZ is marked by the passage of squalls characterized by episodes of rain and winds that can briefly exceed 10 m s^-1 and that in between the squalls, the skies are clear and the winds are light. Outside the ITCZ, in contrast, much less rain and steady, moderate trade winds are seen outside the ITCZ. Despite the fact that solar radiation is blocked by the clouds in the ITCZ, the measurements reveal that the net heating of the sea surface is actually larger under the ITCZ because weak winds and high relative humidity cause reduced evaporative cooling of the sea surface.

infrared satellite image of the earth shows the ITCZ as a band of clouds

Figure 2. Infrared satellite image of the earth, which gives a measure of convective activity. The ITCZ is recognizable as a band of clouds near 10°N, and easterly waves are visible as patches of clouds. Larger view

graph showing the sea surface signature of waves at the mooring

Figure 3. The surface signature of these waves at the mooring. The intermittent periods of high winds (with a typical period of 3-5 days) at the mooring are associated with the passing easterly waves. Also, the sea surface temperature, net heat flux into the ocean and precipitation rate are shown. Larger view

The data analysis also shows enhanced energy in the surface winds at 3-5 day periods. This variability is associated with the passage of westward propagating atmospheric easterly waves over the mooring. Figures 2 and 3 provide two perspectives of the passing easterly waves during ITCZ conditions. Figure 2 shows an infrared satellite image of the earth, which gives a measure of convective activity; the ITCZ is recognizable as a band of clouds near 10°N, and easterly waves are visible as patches of clouds within the ITCZ. Figure 3 illustrates the surface signature of these waves at the mooring. The intermittent periods of high winds (with a typical period of 3-5 days) at the mooring are associated with the passing easterly waves.

Also shown in Figure 3 are the sea surface temperature, net heat flux into the ocean and precipitation rate. It can be seen that SST sometimes has a large diurnal cycle during ITCZ conditions. In fact, SST increased by more than 2°C (almost 4°F) over a few hours on the morning of June 18. This large change in SST coincided with a period of weak winds, but the heat flux into the ocean was not unusually large at this time. The principal reason for the rapid warming of the sea surface is associated with the weak winds, which are less effective at “stirring” the ocean to entrain cooler water from depth. Weller and Farrar are actively engaged in analysis of the PACS mooring data to assess the role of local air-sea coupling inside and outside of the ITCZ, using both satellite and in situ observations from the surrounding region to set the large scale context for interpretation of the high quality mooring data. Together with other PACS investigators, they hope to increase understanding of the processes that contribute to coupling of the ocean and atmosphere in the ITCZ. The project was funded by the Office of Global Programs (OGP) through the Cooperative Institute for Climate and Ocean Research (CICOR), a NOAA/WHOI Joint Institute.

More Information:

PanAmerican Climate Studies (PACS) Web site
http://tao.atmos.washington.edu/pacs

Upper Ocean Processes Group Web site at Woods Hole Oceanographic Institution
http://uop.whoi.edu/

Cooperative Institute for Climate and Ocean Research (CICOR)
http://www.whoi.edu/science/cicor

CLIVAR PACS project abstracts:
http://www.ogp.noaa.gov/mpe/clivar/pacs/fy99/fy99main.htm

Abstract of Enhanced TAO Monitoring of Ocean-Atmosphere Interactions in the Cold Tongue / ITCZ Complex
http://www.ogp.noaa.gov/mpe/clivar/pacs/fy99/cronin/cronin99.htm

National Climate Data Center, satellite images:
http://www.ncdc.noaa.gov/oa/satellite/satelliteresources.html

The Cooperative Institute for Climate and Ocean Research (CICOR) is a cooperative institute between NOAA and the Woods Hole Oceanographic Institution. The research activities of CICOR are organized around three themes: the coastal ocean and near-shore processes, the ocean's participation in climate and climate variability, and marine ecosystem processes analysis. These theme areas are interrelated, and scientific progress requires collaborations by scientists within and between disciplines. Progress depends on a combination of fundamental process studies, the development and deployment of technological systems for sustained observation, and the development of predictive models that are based on an understanding of the underlying processes and that assimilate information from observational systems.

[7/14/03]

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