Oceanic Water Vapor Derived from TOPEX Microwave Radiometer: Climatology and Variability
Ocean University of China,
5 Yushan Road,
Using the newly available TOPEX microwave radiometer (TMR) data spanning 1993 through 2003, a 11-year 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 trans-ocean 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 dipole-like 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 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 in the tropical oceans of the Southern Hemisphere (SH).
4) Our results suggest that El Niños (La Niñas) can weaken (strengthen) the seasonality of OWV by decreasing (increasing) the annual amplitude. The change of amplitude is usually slight but significant, especially for the six most dynamic seasonal belts across the major continents at midlatitudes. The ENSO impact on the annual phase of OWV is seen to be highly systematic and geographically correlated. The most striking feature is a large-scale advancing/delay of about 10 days for the midlatitude oceans of the northern hemisphere in reaching their summer maxima during the El Niño/La Niña years.
5) In addition, an alternative scheme for estimating the mean position of the ITCZ based on the annual phase map of OWV is proposed. The so-obtained ITCZ climatology agrees with existing results in that the mean position meanders from 2°S to 8°N oceanwide, and stays constantly north of the equator over the Atlantic and eastern Pacific, but differs in that it favors 4°N in latitude rather than 6°N as previously concluded.
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.