Paper Describing Long-Term Record of Water Vapor over the Oceans Published Online

Date Added: 
Monday, April 23, 2018

RSS produces a merged monthly dataset of total precipitable water (TPW) over the world’s ice-free oceans. A paper describing this dataset has been accepted by Earth and Space Science, a peer-reviewed open-access journal published by the AGU. The online version can be found here.

The plain language summary from the paper is

Water vapor in the atmosphere is important for studying our changing climate because it is a contributor to the greenhouse effect and the amount of vapor is expected to increase as the planet warms. Measurements made by polar orbiting satellites cover the entire globe, making them especially useful for global climate studies. A certain type of satellite sensor, a microwave imaging radiometer, can accurately measure the total amount of vapor in the atmosphere over the ocean. Since 1988, more than a dozen such instruments have been operated by different agencies in the U.S., Europe, and Japan. In this work, we combine the results from 11 of these sensors into a single long record that can be used to investigate climate change, taking care to remove any small differences in calibration between the different sensors. The record clearly shows that the amount of vapor in the atmosphere over ocean regions has been increasing at a rate of about 1.5% per decade over the last 30 years as the planet warms. This is not surprising since warmer air tends to “hold” more water vapor but is a nice confirmation of estimates of temperature rise made by other instruments and methods.

The two time-series plots below show time series of TPW for near-global and tropical ocean-only averages for 1988-2017.

Both regions show a clear increase in TPW, with local maxima coinciding with large El Nino events in 1998-1999 and 2015-2016. The different colored lines show the results for representative members of a 50-member error ensemble. The error ensemble is a simple way to present our estimates of uncertainty in the measurement and dataset construction process. The increase in vapor is robust to measurement and construction error.

The map below shows the trend in TPW over the 30-year 1988-2017 period

In most regions of the world’s oceans, water vapor increased over the last 30 years. The largest increases are over the Western Pacific warm pool, the Indian Ocean, and the Intertropical Convergence Zone (ITCZ). Regions of decreased TPW in the subtropical Pacific may be related to circulation changes associated with the Pacific Decadal Oscillation (PDO) or the Interdecadal Pacific Oscillation (IPO). Note that the largest increase are roughly 3 times larger in magnitude than the largest decreases.

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