Madden Julian Oscillation Causes Severe Intensity of Italy Storm

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An intra-seasonal migrating tropical feature known as the Madden Julian oscillation representative of a massive area of tropical thunderstorm activity was tapped by a low pressure system south of France on November 2 making the attendant storm much stronger and causing widespread damage including 17 deaths across Italy.

Madden Julian Oscillation & Historic Early November 2018 Italy Storm

Discussion: According to CNN-International the death toll reached 17 due to flooding rain and high wind in an early weekend storm across Italy. Most of the fatalities were caused by fallen trees. Millions of trees were toppled by wind gusts estimates to approach 120 mph. High wind lead to 5 foot storm surges in some areas along the coast. What caused the unusual intensity of this storm?

Presence of the Madden Julian oscillation (MJO) across the tropical Atlantic stretching east across equatorial Africa caused the intensity of this storm to produce hurricane-like weather. On Friday at 1600 GMT vast regions of intense convection – massive lines of thunderstorms/squalls producing violent and intense wind – surrounded Italy to the east of a low pressure area south of France. Each squall line identified in Fig. 1 contained violent weather much of it drifting northeastward across Italy.

Thunderstorm activity and attendant squalls producing the type of wind reported (and damaged observed) and associated flash flooding could only occur if this storm had entrained moisture from the deep tropics. Notice in Fig. 2 the tail of convection this storm has to a massive area of tropical thunderstorm activity associated with the Madden Julian oscillation (MJO).

Fig. 1: Satellite view of Europe/North Africa at 1600 GMT on November 2, 2018 identifying heavy convection/squalls in the vicinity of and across Italy.

Fig. 2: Classic Madden Julian oscillation signature identifies convection across tropical Africa and the attendant attachment of that energy to the Italy storm.

The Madden Julian oscillation is a large area of thunderstorm activity originating in the Indian Ocean and caused by differential heating between the warm tropical environment near the equator and the rising Asian Continent to the north. The MJO is an intra-seasonal migrating tropical oscillation which shifts eastward from its home in the Indian Ocean circumnavigating the global equatorial region usually in 45-60 days. The MJO enhances tropical cyclone activity and also releases heat from its massive convection profile in the deep tropics pole ward amplifying the jet stream (i.e. creating an increase in trough and ridge orientation). The MJO is frequently associated with extreme weather events and the Italy severe storm is the latest example.

Fig. 3: Tracking the Madden Julian oscillation where the blue streaks are transient convection phase the most recent shifting east into equatorial Africa helping to intensify the Italy storm.

Presence of equatorial MJO convection has streaked eastward across the Pacific and Atlantic tropics frequently since last July (Fig. 3). The most recent easterly surge reaching equatorial Africa occurred in late OCT/early NOV and helped make the low pressure area south of France much more intense than normal leading to the historic severe storm across Italy.

MJO episodes are stronger when the ocean temperatures in the already warm tropics are warmer than normal. The ocean surface off the west coast of Africa in the equatorial region is much warmer than normal (Fig. 4) certainly enhancing the convection associated with the MJO stretched across this region and contributing to the intensity of the stormy affecting Italy.

Fig. 4: MJO episodes moving across warmer than normal waters in the equatorial region as observed in early November are stronger as thunderstorm activity over warmer water is stronger and more widespread.