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Madden Julian Oscillation Shift Causes U.S. Pattern Change, Likely to Activate Tropics

Fig. 1: Meteorological summer ends soon but summertime warmth continues through the first third of September across the Central and East U.S.

Discussion: The influence of the tropics lead by Madden Julian oscillation (MJO) on mid-latitude climate is forecast to change over the next 1-2 weeks. During the past 90 days the tropical heavy rainfall belt has been centered in the West Pacific extending westward through Southeast Asia to India and eastward to the waters south of Hawaii (Fig. 2). In this location excessive rainfall episodes whether tropical cyclone activity or wet monsoon have persisted and caused by the wet phase of the MJO (Fig. 3). Conversely, the dry phase of the MJO has prevented any significant rainfall throughout the tropical North Atlantic basin.

The latest NCEP and ECMWF 14-day MJO forecasts indicate the MJO is likely to depart the West Pacific and shift toward the tropical North Atlantic in early September (Fig. 4-5). The arrival of the MJO in the tropical North Atlantic shifts the previously dry regime to much wetter than normal in both the subtropics and outer tropical North Atlantic regions (Fig. 6). The wet weather is caused by a dramatic increase in tropical waves and potential tropical cyclone activity as the most active part of the season (Sep. 10) approaches.

As the subtropical/tropical North Atlantic becomes more convection-active the heat release pole ward from the convection will amplify the subtropical ridge pattern across the eastern U.S. (Fig. 7) causing much warmer than normal temperatures into the arrival of meteorological autumn.

Operational models vary on details but tropical cyclones should emerge in the 11-15 day period (and beyond) in the North Atlantic.

During early tropical cyclone season forecasters noted the unusual cool signature in the outer North Atlantic tropics as a leading diagnostic to lower seasonal activity forecasts. However, the tropical North Atlantic has warmed significantly during the past 2-3 weeks (Fig. 8). The warming trend now supports development and intensification of tropical cyclone activity if the atmospheric environment becomes more supportive. Influence of the MJO will enhance that needed support.

September is likely (very) tropically active!

Fig. 2: During the past 90 days the tropical wet belt has been biased toward Southeast Asia, the West Pacific and east to waters south of Hawaii. The North Atlantic tropics have been dry.

Fig. 3: During northern hemisphere meteorological summer the MJO has been stationary from Indonesia to the Dateline enhancing the Southeast Asia wet monsoon and West/Central Pacific tropical cyclone activity.

Fig. 4-5: Both the NCEP and ECMWF 14-day forecasts agree that MJO shifts toward into the tropical Atlantic/Africa sectors strongest Sep. 4-8 enhancing North Atlantic tropics convection/thunderstorm activity.  

Fig. 6: The GFS ENS percent of normal rainfall 15-day forecast across the North Atlantic basin indicates a much wetter than normal regime implying increased risk of tropical cyclone activity.

Fig. 7: The GFS OP 500 MB anomaly 15-day forecast projects an amplified upper high pressure ridge across the eastern U.S.

Fig. 8: The North Atlantic SSTA analysis identifies a warming regime!