Tropical Instability Waves (TIWs) are westward-traveling waves associated with shear instabilities of the equatorial current system, and are observed at the edges of the equatorial cold tongues in the Pacific and Atlantic Oceans. TIWs have average wavelengths of 1000-2000 kilometers, periods of 20-40 days, and phase speeds of 0.3-0.5 m/s. These waves redistribute various ocean properties, including temperature, salinity, and nutrients. They interact with ocean currents and large-scale climate variability, such as the El Niño-Southern Oscillation, and influence marine ecosystems and the carbon cycle.
TIWs have previously been observed by satellite observations of sea surface temperature, sea level, ocean surface wind, and ocean surface chlorophyll-a (Figure 1) and sparse direct ocean measurements. Salinity has been found to play an important role in the physics of these waves, and observations of the salinity structure are important to understanding TIWs and their impact on climate variability and biogeochemistry. However, until now salinity observations of TIWs have been limited to very sparse direct ocean measurements. The Aquarius instrument onboard the Aquarius/Satélite de Aplicaciones Científicas (SAC)-D satellite provides an unprecedented opportunity to observe the salinity response to these waves.
(upper) Sea Surface Temperature (SST) and (lower) chlorophyll-a (chl-a) in the eastern Pacific on July 10, 2012. Images created using PO.DAAC’s State of the Ocean (SOTO) Visualization Tool.
A recent Bulletin of the American Meteorological Society article lead-authored by Gary Lagerloef of Earth and Space Research, Aquarius Principal Investigator, noted a wave-like pattern in salinity in the tropical Pacific Ocean (Lagerloef et al., 2012). The salinity signature was revealed to be associated with TIWs in a recent Geophysical Research Letters article lead-authored by Tong Lee of NASA's Jet Propulsion Laboratory (Lee et al., 2012).
As an example, Figure 2 shows sea surface salinity (color shading in panels a and d) on December 18, 2011, derived from Aquarius measurements, and resolves peaks and valleys of TIWs in the eastern to central equatorial Pacific Ocean. The salinity structure of TIWs aligns with satellite-derived SST (contour lines in panel a), ocean surface currents (arrows in panel b), sea level (contours in panel c), wind speed (contours in panel d), and chl-a (panel e). Sea surface salinity (SSS; http://podaac.jpl.nasa.gov/dataset/AQUARIUS_L2_SSS), SST (http://podaac.jpl.nasa.gov/dataset/NCDC-L4LRblend-GLOB-AVHRR_OI), ocean current (http://podaac.jpl.nasa.gov/dataset/OSCAR_L4_OC_third-deg), and wind speed (http://podaac.jpl.nasa.gov/dataset/ASCAT-L2-25km) data were obtained from the PO.DAAC. Sea surface height anomaly (SSHA or sea level) data was obtained from AVISO (http:/www.aviso.oceanobs.com) and chl-a data from the Ocean Biology Processing Group (http://oceancolor.gsfc.nasa.gov).
Aquarius' salinity observations show a clear signature of TIWs near the equator in the Pacific Ocean where large contrasts in salinity occur between the saltier waters of the South Pacific and fresher waters of the North Pacific. Salinity variability associated with TIWs is larger near the equator, while sea surface temperature and sea level variability associated with the waves is larger a few degrees north of the equator. The Aquarius data reveal that TIWs travel faster at the equator than they do a few degrees to the north, a feature that had not been previously captured by TIW studies using SST and SSHA. In association with a faster phase speed at the equator, Aquarius observations showed that the waves have a dominant period of approximately 17 days compared with 33 days a few degrees north of the equator. Aquarius' ability to reveal such detail of surface dynamical structures and resolve oceanic features on such short timescales were unexpected, as the mission was designed to study salinity changes on longer time scales. Salinity observations from Aquarius fill a major gap in observing and understanding TIWs, and provide additional constraints to models and assimilation products in simulating TIWs and their consequences.
SSS (color shading in a-d), SST (contours in a), surface current (contours in b), SSHA (contours in c), wind speed (contours in d), and chl-a (e) centered on December 18, 2011. The SSS, SST, SSHA, wind speed, and chl-a are 7-day averages. Surface currents are 10-day average centered on December 18. The units of SSS, SST, SSHA, wind speed, and log chl-a are psu, °C, cm, and m/s, and mg/m3. Image from Lee et al. (2012).
PO.DAAC Science Team