MJO Modulation of Surface Air Temperature over Eastern North America during Northern Winter: Dependence on the Phase of the Stratospheric QBO

Previous observational studies have demonstrated a significant lagged correlation between the Madden-Julian oscillation (MJO) and the North Atlantic oscillation (NAO). The NAO, in turn, has a strong influence on weather conditions in both eastern North America and Europe. Here, composite analyses over the 1979-2016 period of ERA-Interim sea level pressure (SLP) and 2-meter surface air temperature (SAT) are performed to investigate the QBO influence on the MJO modulation of the NAO and resulting effects on intraseasonal near-surface climate in eastern North America. Only the extended northern winter season (Nov. to April) is considered and all data are filtered to accept variations with periods between about 20 and 100 days. We use the OLR-based MJO index (OMI) of Kiladis et al. [2014] to determine the amplitude and phase of the MJO on a given day. Statistical significance of the composites is assessed using a two-sided Student's t-test.

Previous work has shown that the NAO index (SLP over the Portuguese Azores minus that over Iceland) tends to be negative or neutral during the earliest MJO phases but becomes positive on average within 10-15 days after the occurrence of MJO phase 3 [e.g., Cassou, 2008; Lin et al., 2009]. A positive NAO index persists through MJO phase 6 before becoming weakly negative by phase 8. Our composite analyses confirm this evolution and show that the negative NAO index during MJO phases 1 and 2 is accompanied by negative SAT anomalies over eastern North America followed by positive anomalies peaking in phases 5 and 6 when the NAO index is most strongly positive. By phase 8, no significant SAT anomaly is present over North America south of 60^oN. During phases 1 and 2, the cooling anomaly is consistent with increased northerly geostrophic flow resulting from the negative SLP anomaly in the North Atlantic combined with a positive SLP anomaly over western Canada. During phases 3 to 6, as a positive SLP anomaly develops in the North Atlantic and a negative SLP anomaly strengthens over western Canada, increased southerly flow produces a warming anomaly in the same region.

Repeating the composite analyses for the easterly and westerly phases of the QBO (QBOE and QBOW) shows that this evolution is altered significantly depending on QBO phase. In general, the SLP anomalies associated with the MJO-induced Rossby wave train are more pronounced during QBOE than during QBOW. During MJO phases 1 and 2, the MJO-induced negative NAO index is much larger during QBOE and results in a strong cooling anomaly over most of North America, while, during QBOW, the NAO is nearly neutral and little or no cooling anomaly is present. The development of a positive NAO index is delayed during QBOE until phase 5 when it becomes strongly positive and a pronounced warming anomaly is produced over eastern North America during phases 5 and 6. In contrast, during QBOW, the warming anomaly begins in MJO phase 3 and persists with moderate amplitude through phase 6, becoming weaker or non-existent thereafter. Repetitions of the analysis for different time periods (1979-97 and 1998-2016) and different MJO amplitude ranges show that this differing evolution during the two QBO phases is robustly present for OMI amplitudes > 1.5.

As reviewed by Wang et al. [2018], a number of recent studies have found that the MJO propagates eastward more slowly and continuously during QBOE than during QBOW. This slower propagation speed would favor development of a stronger extratropical response [Bladé and Hartmann, 1995], thereby possibly explaining the more pronounced SLP anomalies and associated NAO response during QBOE found here.

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