Executive summary: During the past week or so an excessive rainfall event emerged in the East-Central/Mid-South U.S. The rainfall amount occurring during this time is nearly 10 times normal near the Arkansas/Oklahoma border and southwest Illinois (Fig. 1 – above). This previously very dry zone suddenly has stream flows in the much above normal (dark blue) to high (black) range due to the heavy rainfall coupled with snow melt affecting the Mid-South to Midwest to Northeast U.S. (Fig. 2). The excessive rainfall event was caused by the combination of Madden Julian oscillation (MJO) and subsurface Kelvin Wave moving across the tropical East Pacific. There are concerns of another such event evolving for early calendar springtime.
Fig. 2: USGFS streamflow analysis across the U.S. The range is LOW stream flows for RED/ORANGE to HIGH stream flows for DARK BLUE/BLACK.
Discussion: During La Nina the subsurface equatorial East Pacific is a vast area of cooler than normal water brought to the surface by above normal trade wind speeds in the up-welling process producing the familiar cool SSTA signature of La Nina. The opposite regime exists in the equatorial West Pacific where surface and subsurface waters are warmer than normal.
Occasionally, pieces of the warm subsurface waters of the West Pacific are ejected eastward as a Kelvin Wave which can erode the cool waters of the subsurface East Pacific and weaken La Nina. Usually 2 to 3 Kelvin Waves can dissipate La Nina and if sufficiently strong cause a reversal in ENSO to El Nino.
The east-shifting Kelvin Waves often coincide with MJO. The MJO is an area of vast thunderstorm activity in the equatorial region born in the Indian Ocean and shifting east navigating the global tropics on average in about 7 weeks. The MJO varies in intensity depending on the equatorial ocean temperatures. The combination of an east-shifting Kelvin Wave with MJO will cause extreme weather in the U.S. and is to blame for the recent exceptional rainfall in the Mid-South/East-Central U.S.
Just-after mid-December the warm subsurface in the West Pacific, a classic La Nina signature was analyzed by NOAA/CPC (Fig. 3). A piece of that warmth was ejected eastward just-after mid-January initially moving beneath the remaining cool fuel to sustain La Nina in the equatorial East Pacific (Fig. 4). The eastern shift of the Kelvin Wave was driven in-part by the evolution of an immense MJO episode in the West Pacific (Fig. 5).
Fig. 3: Classic La Nina subsurface equatorial Pacific temperature regime with warmth near and west of the Dateline and cool fuel in the East Pacific on Dec. 19, 2018 as analyzed by NOAA/CPC.
Fig. 4: NOAA/CPC depiction of warmth in the West Pacific is ejected eastward via Kelvin Wave into the previously cool zone of the East Pacific Jan. 18, 2018.
Fig. 5: The 40-day Madden Julian oscillation daily core location analysis indicates a strong event in the West Pacific earlier in February. Near normal strength is represented by the inner circle. The farther out from the inner circle the stronger the (MJO) episode.
The MJO crossed the Dateline just-after mid-February enabling the Kelvin Wave to further warm the equatorial East Pacific subsurface as trade winds shut off eliminating the up-welling process(Fig. 6). The energy released to the atmosphere by the warming waters and entrained by prevailing westerly flow (Fig. 7) pole ward of the tropics lead to the heavy rains and warm temperatures causing snow melt that have flooded the Mid-South/East-Central U.S.
The new concern? A stronger Kelvin Wave has moved east of the Dateline (Fig. 8) the past 7-10 days and if the right MJO set-up develops (eastward shift across the Pacific) another major rainfall event would occur in the same area during early calendar spring. This set-up is not indicated yet as the ECMWF 30-day forecast indicates the MJO becomes weak.
Fig. 6: The Kelvin Wave sharply erodes the East Pacific subsurface cool fuel for La Nina on Feb. 12, 2018. Another more potent Kelvin Wave is forming near the Dateline.
Fig. 7: Satellite view of the fetch of tropical energy northeastward from the MJO-activated East Pacific tropics into the prevailing jet stream flow and carried toward the Mid-South U.S. where persistent heavy rains developed.
Fig. 8: The Kelvin Wave is clearly shifting eastward now east of the Dateline in the latest NOAA/CPC analysis.
Fig. 9: The coupling of the Madden Julian oscillation and the Kelvin Wave lead to a strong El Nino-like negative southern oscillation index in February leading into the U.S. heavy rainfall episode.
Conclusion: The Climate Impact Company concern is 1.) The MJO will be stronger than the ECMWF outlook and re-emerge across the Pacific later in March or 2.) The models are correct that MJO weakens but re-intensifies after the 30-day forecast expires (into early April). An eastward shift into the East Pacific of a Kelvin Wave the magnitude of the NOAA/CPC current analysis coupled with MJO would result in another hostile heavy rain and flooding episode and likely eliminate La Nina.