Abstract
From January to March 2018, one of the strongest Madden–Julian Oscillation (MJO) events of the last 45 years progressed eastward along the equator from the Indian Ocean to the Pacific Ocean then back to the Indian Ocean. In response to strong tropospheric heating in the MJO's active convective envelope, several pronounced Rossby wave trains developed and extended from the equatorial tropics, across the extratropical Pacific and North America, and into the extratropical Atlantic. The origins of these Rossby wave trains evolved eastward with time, generally following the eastward progression of the MJO, but preferentially clustered in subtropical India and Southeast Asia and in two locations in the subtropical Pacific Ocean: along 160°E and 170°W. Over eastern North America, surface and lower‐tropospheric temperatures rose to more than 12 °C above normal when the MJO convective envelope was over the Indian Ocean (in mid‐January) and Western Hemisphere (in late February). In between those warm periods, temperatures cooled to below normal while the MJO convection was over the western Pacific. These temperature anomalies evolved in time with the pronounced Rossby wave trains that linked eastern North America with the Tropics in the Eastern Hemisphere: warm temperatures occurred when ridging was present over eastern North America and cooler temperatures occurred when troughing was present. This variability is discussed and placed in context of recent work showing the MJO's role in modulating temperature and circulation.
Highlights
Tropical heating in the convectively active region of the Madden–Julian Oscillation (MJO; Madden and Julian, 1971; 1972) has been shown to be an effective source of Rossby waves to the extratropics (Sardeshmukh and Hoskins, 1988; Jin and Hoskins, 1995)
A negative North Atlantic Oscillation (NAO) is associated with troughing over the eastern United States and adjacent North Atlantic and colder-than-average surface air temperatures warmer temperatures over the eastern United States can be expected after MJO convection is over the Indian Ocean (IO), and colder temperature can be expected after MJO convection is over the western Pacific Ocean (WP)
By early February, MJO convection had shifted into the WP (Figure 2c) with a wave train still extending from Southeast Asia to a very strong ridge over the Bering Strait to strong troughing over Hudson Bay to ridging again over the North Atlantic (Figure 2c)
Summary
Tropical heating in the convectively active region of the Madden–Julian Oscillation (MJO; Madden and Julian, 1971; 1972) has been shown to be an effective source of Rossby waves to the extratropics (Sardeshmukh and Hoskins, 1988; Jin and Hoskins, 1995). A positive NAO is associated with ridging over the eastern United States and adjacent North Atlantic and warmer-than-average surface air temperatures.
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