The September 2015 map of total precipitable water (or total column water vapor) anomalies created from RSS monthly water vapor products is shown below. Anomaly means the departure from an average September value, where the average water vapor is calculated over a period of 20 years, 1988 to 2007. The huge positive anomaly (wetter than normal) over the tropical Pacific Ocean clearly stands out.
The units for this map are in millimeters, where a “millimeter” is the depth of liquid water that would result if all the vapor were condensed to liquid and pooled at the surface. Given this, we can figure out how much extra water is being stored as water vapor over the tropical Pacific. We have calculated that the volume of just the extra vapor located inside the black box on the map, if all were condensed to liquid, would be about 220 cubic kilometers. This volume is more than that of Lake Tahoe (150 cubic kilometers, according to Wikipedia), a large and very deep lake on the California/Nevada border. In fact, the region in the black box holds more than four times the storage volume of all the reservoirs in California (about 40 million acre-feet, or 48 cubic kilometers).
Now, before Californians get all excited about the end of the drought, note that most of this vapor will probably be converted to rain right there in the tropical Pacific and never fall over land. But it does suggest that if and when Pacific storm systems become active and strong, they’ll have ample water vapor to tap into and hopefully produce large quantities of rain. The excess water vapor in the atmosphere over the tropical Pacific is mostly caused by the warmer than normal ocean temperatures that are part of El Niño, so we expect the floating lake to persist into the winter months if the El Niño persists as predicted.
It is interesting to compare this vapor anomaly map to that from September 1997, just before the last large El Niño event (shown below). In 1997, there was also a large positive vapor anomaly near the Equator, but it did not extend as far north as now, and only contained about 125 cubic kilometers of extra water. This is probably because the sea surface temperature in the Northeastern Pacific is much warmer than in 1997. The effect of this warm ocean water and the excess atmospheric water vapor on this coming winter’s weather is currently being studied at RSS and other research institutions.
The time series plot below shows how unprecedented the current anomaly is compared to the previous 28 years. The plot shows the amount of extra (or missing water vapor) for each month after January 1988 relative to the 20-year mean for that month. The calculation is done using the same box as is shown in the two maps above. Clearly the current amount of extra water vapor in the tropical Pacific is higher than it has been in the history of measuring water vapor by satellite.