Equatorial Wave Activity during NOAA's 2016 El Niño Rapid Response Campaign

The El Niño Rapid Response (ENRR) field campaign targeted equatorial Pacific atmospheric convective activity during January-March 2016 through enhanced observations using dropsondes from the NOAA G-IV aircraft and radiosonde observations from Kiritimati (Christmas) Island and the NOAA research ship the Ronald H. Brown. This presentation examines the equatorial wave activity observed during ENRR and its relationship to tropical convection, and compares this activity to observations of past large El Niño events. The 2015-16 El Niño had much in common with the events having similar amplitude sea surface temperature (SST) anomalies during 1982-83 and 1997-98, but also differed in several key aspects. All of these episodes featured enhanced convectively coupled Kelvin wave activity crossing the entire Pacific basin, which is generally absent during the northern winter seasons of near normal or La Niña SSTs. Prior to the ENRR period during December 2015 a large amplitude Madden-Julian Oscillation (MJO) was observed, with a convective signal that propagated unusually far to the east (~150W). This was associated with an eastward displacement of the North Pacific storm track and heavy precipitation along the west coast of North America, broadly matching the large scale behavior of MJO evolution in statistical composites during El Niño. A second MJO-like event occurred during the latter part of February, 2016, but despite a similar convective heating field, the basic state flow was much different than during December, with a well-developed “westerly duct”, favoring the intrusion of extratropical Rossby wave energy into the equatorial eastern Pacific region. This interaction is diagnosed through upper tropospheric E Vector fields and other metrics. The second MJO event was accompanied by a distinct lack of storm activity and associated precipitation along the west coast of North America. Based on the preliminary results of AMIP simulations using observed SSTs, these differences are hypothesized to be due to a certain level of “internal variability” within the storm track itself that may have been overriding the large scale forcing by the tropical diabatic heating field. Finally, the performance of the NCEP GFS operational model for quantitative precipitation forecasts is assessed for the tropical and extratropical Pacific. While model skill for is substantially better in the extratropics, tropical QPF skill improves significantly when there are enhanced extratropical impacts on the low latitudes.