An evaluation of expendable bathythermograph and Tropical Atmosphere‐Ocean Array data for monitoring tropical ocean variability

Abstract

Two important sources of information for monitoring and predicting the El Niño‐Southern Oscillation are the Tropical Atmosphere Ocean (TAO) buoy array and the volunteer observing ship expendable bathythermograph (VOS XBT) network. The subsurface ocean temperature measurements of these two networks are evaluated in the context of analysis and monitoring of medium‐ to low‐frequency variability in the tropical Pacific Ocean. It is shown that both systems capture the principal features that characterize variability in the equatorial central and eastern Pacific Ocean but only TAO can capture the higher temporal fluctuations in detail. In the western Pacific, the VOS XBT network is able to capture the slowly moving, westward propagating Rossby waves and their reflection and interaction with the western boundary. TAO is limited in this respect because of its equatorial focus. The accuracy of temperature maps is estimated using the rms analysis error variance. The accuracy of the VOS XBT network fluctuates in space and time but is relatively steady averaged over the tropical region, while that of TAO has gradually improved as the basin wide array has been implemented. Case studies using VOS XBT data only, TAO data only, and the combined full data set indicate the networks are complementary and supportive, evaluated basin wide and over the tropical region. The high temporal resolution and regularity of the equatorially focused TAO network are complemented by the broad‐scale, irregular tropical VOS XBT sampling in the equatorial region and by VOS coverage outside the domain of TAO. A proxy for net information content is derived from the estimated rms error variance of the analysis. For the equatorial region, TAO now provides in excess of 70% of the information, its dominance beginning in 1992. For the tropical region (20°S–20°N) the net information content of the respective systems is of comparable magnitude, each equivalent to in excess of 100 independent samples per 10‐day period in 1994, reinforcing the notion that both systems are valuable sources of subsurface ocean information for monitoring medium‐ to low‐frequency tropical variability.