(Very intense rainfall over the past two weeks over the Yangtze River Valley region of China has resulted in severe flooding that has destroyed 54,000 buildings and displaced 1.4 million people. Image source: China’s Meteorological Agency.)
As China was reeling from the impacts of heavy rainfall, sea surface temperatures over the Northwestern Pacific were screaming hot. Surface waters over a region south and east of Taiwan on July 4 and 5 approached 31 degrees Celsius (88 degrees Fahrenheit) or 1-2 C above average. In addition, these hot waters extended to great depth beneath southeastern Pacific surface boundaries. As a result, the amount of ocean heat content available to fuel an intensifying Typhoon was at about the top range one would ever tend to see (near 150 KJ per square centimeter, according to Dr. Jeff Masters at Weather Underground).
Running through these deep, hot waters on July 4 was a 70 mph max wind intensity tropical storm Nepartak. But as wind shear lessened and Nepartak drew in a deep draught of that high energy Pacific Ocean heat and moisture, the storm exploded. In just one 24 hour period, the system added fully 80 mph of maximum wind speed intensity to leap from a mere tropical storm to a very strong category 4 Typhoon by Tuesday night.
The Typhoon is now expected to make landfall in Taiwan on Thursday as a Category 4 storm. Heavy storm surges, very powerful winds, and 5-15 inches of predicted rainfall over most of Taiwan with up to 43 inches in the higher elevations is expected to generate flooding, landslides, structural damage, and to disrupt crops.
The storm is then predicted to leap the Taiwan Strait and track north and west over mainland China. As the storm weakens, it is expected to be drawn northward into a trough that will ultimately funnel its moisture over the already flooded region between Shanghai and Wuhan. In this zone, between 4 and 24 inches of additional rainfall is possible as the storm winds down and unburdens its massive water load.
(NCEP’s predicted rainfall swath for Nepartak results in additional extreme rainfall expected for already flooded regions near the Yangtze River. Image source: NCEP and Weather Underground.)
In total, weather associated with Nepartak is expected to produce a 5 day long extreme wind, storm surge and rainfall event for Taiwan and China. Heaviest impacts will likely occur in Central Taiwan near the predicted point of landfall and in the region west of Shanghai where Nepartak’s predicted rains could result in even more severe flooding for the already hard hit Yangtze River region.
Conditions in Context — Ocean Heat Intensifying, Expanding in All Dimensions
In addition, extreme rains over China have been fueled by a record atmospheric moisture loading. As global temperatures increase, the rate of evaporation also increases. This results in more water held aloft in the Earth atmosphere. Overall, atmospheric water vapor content increases by about 7 percent for each 1 degree Celsius of warming. And the result is an increased likelihood for extreme droughts in some regions (due to added heat and related increasing evaporation rates) and extreme rainfall in others (due to the extra atmospheric moisture producing more intense storms with heavier downpours).
So while the La Nina to El Nino cycle is helping to drive the location of extreme rainfall over China and the location of tropical cyclone formation in the Pacific, human-forced warming is providing more atmospheric fuel to increase the top potential strength of these events.