The SST pattern across the tropical central and eastern Pacific Ocean remains on the La Nina side in December with negative surface temperature anomalies reducing the available moisture for rainfall in the region. The Nino 3.4 Index was -0.6K for December and the Relative 3.4 Index was -1.0K, both about the same as last month. This SST pattern helps to shift the Walker Circulation so that anomalous descent is concentrated around the Equator at ~180° longitude.

The December mean precipitation map (Fig. 1, top panel) shows the southward seasonal shift of the tropical rain areas over the continents and more subtle shifts in the oceans.  The Northern Hemisphere ocean storm tracks are also evident, extending into the western ends of the continents, and in the Southern Hemisphere the sub-tropical to mid-latitude streaks off of southern Africa, Australia to Mid-Pacific, and South America are evident.  And far to the south the circum-polar feature at ~50° S is continuous with variations in intensity, width, etc.

The December precipitation anomaly map (Fig.1, middle panel) has the typical La Nina features of a strong negative feature at around 180° longitude with a negative extension in a shallow arc to the east to the South American coast.  To the west there is a broad, very variable positive feature over the Maritime Continent and extending into northern Australia.  Both of these features are obvious in the La Nina composite for December (Fig. 1, bottom panel).  The positive December anomaly area was also related to some heavy rain events and flooding in Indonesia and Brunei, with also a little flooding occurring in northeast Australia.

To the west the eastern Indian Ocean is mostly positive with tropical cyclones traversing parts of the southern Indian Ocean, with TC Grant leaving a narrow trail across the anomaly map headed westward from northwest of Australia.  Southern Africa is covered mostly with a positive anomalies with associated flooding in Mozambique and Tanzania, with an extension to the southeastward far into the ocean.  These features also appear in the La Nina composite.

Across the mid-latitude Pacific an extension of the Maritime Continent anomaly feature weaves northeastward in a very variable fashion and intersects the west coast of North America.  This feature was associated with multiple atmospheric rivers transporting tropical moisture into the west coast of the U.S. with significant floods in Washington and Oregon and mudslides burying cars in southern California. The December precipitation added to previous months so that the long-term drought over California is essentially over.  And much further up the coast very heavy, anomalous snows in Juneau, Alaska collapsed roofs and sank boats in the harbor. This pattern also fits with the La Nina composite as does the this month’s negative anomaly from Mexico to the eastern U.S.  In South America the La Nina link is weaker, although the Atlantic ITCZ has a positive anomaly in both maps.