A 10-Yr Climatology of Oceanic Water Vapor Derived from the TOPEX Microwave Radiometer

Abstract

Using the newly available TOPEX Microwave Radiometer (TMR) data spanning 1993 through 2002, a 10-yr climatology of oceanic water vapor (OWV) is constructed, of which the distribution and variation at various spatial?temporal scales are investigated. The new dataset confirms most of the well-known OWV features, and yields a number of interesting findings, due to its high quality, long duration, and unique orbit. 1) The TMR-derived climatology compares well, in both overall pattern and general statistics, with similar results based on radiosondes and other satellites. Climatological comparisons with sea surface temperature and oceanic precipitation suggest that the western Pacific warm pool is ?mirrored? in the atmosphere as a ?wet pool,? whereas the meteorological equator is reflected in OWV as a transocean equatorial wet belt. 2) It is found that El Niño (La Niña) events are accompanied by a significant increase (decrease) in the amount of OWV between 10°S and 10°N with a somewhat unexpected Southern Hemisphere dominance. This is particularly evident during the 1997/98 El Niño when the interannual variability of OWV reaches a record high. Composite maps of annual OWV anomalies disclose a dipolelike pattern in the western equatorial Pacific with a phase opposition between El Niño and La Niña years. 3) The annual amplitude of OWV is characterized by six cross-continent wet belts located largely in the subtropics of both hemispheres. The phase patterns of the annual and semiannual variations are hemispherically divided, and climatologically correlated, respectively. North (south) of the intertropical convergence zone (ITCZ), a majority of the oceanic areas have their water vapor maximum in August (February). Early peaks in July are found over a few continental shelf regions of the Northern Hemisphere (NH), while late peaks in March are found in the tropical oceans of the Southern Hemisphere (SH). Moreover, two delayed maximums in September are visible in the interior North Pacific and North Atlantic, respectively. 4) The daily cycle of OWV is strongly coupled with its seasonal cycle, and is therefore unstable in nature. But a double-peak structure with a general hemispheric phase reversal can still be identified. 5) The ratio of the NH versus SH OWV is roughly 1.17:1, and the relative importance of the interannual, annual, semiannual, diurnal, and semidiurnal variations in terms of mean amplitude is approximately 1.8:5:1.2:1:1. In view of these encouraging results, further exploration of present and future ?altimeter-borne? radiometer data will no doubt lead to an improved and complementary understanding of the OWV system in many aspects.