Precipitable water structure in tropical systems as estimated from TOVS

Abstract

The spatial and temporal structures of precipitable water (pW) in tropical systems are estimated using TIROS Operational Vertical Sounder (TOVS) radiances over ten cool seasons and two entire years. Statistical regression models account for approximately 75% of the variance of PW compared to radiosonde data. Large scale features that readily appear in daily and mean PW fields and synoptic scale features found on daily maps coincide with those seen in microwave data. Daily PW values are used to determine descriptive statistics. Maximum values associate with the Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ). These extrema migrate annually with the most dramatic shift associated with the monsoon. The largest daily variations reside in the PW gradients poleward of the ITCZ while the lowest stay in the ITCZ and subtropical highs. El Nino/Southern Oscillation (ENSO) phases affect the PW patterns, but not the regionally averaged amount. The warmer the phase, the moister the equatorial eastern Pacific. This moistening can reach 2 cm and is over twice the amount expected from SST changes alone. PW signals associated with several cycles longer than synoptic scale are quantified. The annual cycle dominates over the semi-annual cycle, particularly away from the ITCZ and over ocean areas dominated by subtropical highs. It accounts for up to 80% of the local variance of PW. The semi-annual cycle mostly appears near the equator. Madden-Julian Oscillation (MJO) phases affect the PW pattern with amplitudes up to 2 mm (rms) and accounts for up to 15% of the variance. The pattern moves eastward as an east-west oriented wave that adds a north-south dipole as it approaches the dateline. The dipole fades before the wave reaches the American coast. MJO PW maxima coincide with Outgoing Longwave Radiation (OLR) negative anomalies. The similarities between the 10-15 d, 15-20 d and 20-25 d cycles suggest they are one feature operating in the 1025 d time scale. They have PW patterns similar to MJO'S, except for the north-south dipole. Each fluctuation accounts for up to 10% of the variance.