Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Subseasonal variability of severe storms in the US: What we know and don’t know Bradford S. Barrett Fulbright Scholar, Universidad Nacional Autónoma de México Oceanography Department, U.S. Naval Academy 09 March 2016 PLR and AGS
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Outline Quick background on the MJO MJO modulation of severe weather –What we know Complexity of MJO modulation –What we don’t know
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Background: what is the MJO? MJO: a quasi-periodic oscillation of the near-equatorial troposphere –Most noticeable in low-troposphere zonal winds globally, and convection in the Eastern Hemisphere On the intraseasonal time scale (40-50 days), the dominant oscillation is itself a superposition of smaller features –But those smaller features tend to “bunch together” and persist, giving the oscillation its coherence Discovered in late 1960s and early 1970s by Roland Madden and Paul Julian –Noticed regular return periods (30 to 60 days) in daily surface pressure and upper-troposphere wind observations made in the tropics Adapted from Majda and Stechmann [2011]. MJO active MJO inactive
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department What is the MJO? Tropical atmospheric oscillation –Period of days Seen as anomalous cloud clusters km in horizontal scale –Clusters most vigorous in eastern hemisphere –Accompanying pressure and circulation perturbations also move east, circumnavigate planet along the equator Adapted from Madden and Julian [1972].
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Global teleconnection to the MJO In mid-1980s, MJO’s impact globally was confirmed by numerical simulations –A heat source over Indonesia (MJO-like) produces global response –Get wavy atmospheric response everywhere in upper-troposphere! –Longitudinal position, and intensity, of heat source can be categorized as phase (similar to Madden and Julian 1972) Adapted from Sardeshmukh and Hoskins [1988] anomalies
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department What is the “phase” of the MJO? Decomposition of the variable MJO convection and/or circulation into two principal components that determine longitude and amplitude of the convective maximum MJO now has predictability, up to 1 month –Lots of promise for medium-range weather (or short-range climate) prediction To take advantage of its predictability, need to understand how it affects weather around the world Wlink/MJO/CLIVAR/clivar_wh.shtml
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Characteristics of the MJO In the last 40 years: –60% of days are “active” (>1) –30% of days are “very active” (>1.5) –5% of days are “extremely active” (>2.5) Peak of MJO distribution occurs around amplitude of 1.0 –Long right tail of extreme amplitudes Fairly equal phase distribution –Nearly all 8 active phases occur about 12.5% of the time LaFleur, Barrett, and Henderson 2015 (Journal of Climate) Inactive (RMM 2 < 1.0) Active (1.0 < RMM 2 < 1.5 ) Very active (1.5 < RMM 2 < 2.5 ) Extremely active (RMM 2 > 2.5 )
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Characteristics of the MJO Pronounced seasonality in MJO activity: –Most VA and EA days occur in DJF and MAM –Most IA days occur in JJA and SON Within a season, certain phases occur more (or less) often: –Phase 8 in JJA vs Phase 1 in JJA –Phase 1 in JJA vs Phase 1 in DJF Questions: –What does a more amplified MJO mean for the extratropical response? –Does this response vary by season? By phase? LaFleur, Barrett, and Henderson 2015 (Journal of Climate) MJO amplitude anomalies Inactive (RMM 2 < 1.0) Active (1.0 < RMM 2 < 1.5 ) Very active (1.5 < RMM 2 < 2.5 ) Extremely active (RMM 2 > 2.5 )
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Characteristics of the MJO The greater the MJO amplitude, the larger the magnitude of streamfunction anomalies at 200 hPa –Because streamfunction is related to non-divergent wind field, larger correlations and larger correlation gradients imply stronger zonal winds Implications: –Active MJO affects global circulation more than inactive MJO does –Modulation of global circulation increases with amplitude of the MJO Inactive MJO Active MJO Very active MJO Extremely active MJO Correlation coefficient between MJO amplitude and streamfunction anomaly LaFleur, Barrett, and Henderson 2015 (Journal of Climate) Phase 2Phase 6
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department MJO modulation of severe weather Wavy atmospheric response to tropical MJO convective anomalies –Response is global: waves extend everywhere –Time scale for response is shorter than entire oscillation 8 phases per days gives 3-7 days per phase Synoptic scale Hence, other weather phenomena that respond to circulations on synoptic scale could be influenced by MJO
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department MJO and U.S. tornado days Main points: April and May tornado- day frequency in the US varies by MJO phase –Variability was supported by anomalies of CAPE and shear Variability is not consistent from month to month –Implies that either the MJO character is different between April and May, or the “recievership” of the atmosphere is different from April to May, or both Tornado-day (EF-1+) anomaly (U.S. between Rocky Mtns. & MS river) Source: Barrett and Gensini 2013 GRL
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department MJO and hail: April Notice differences in hail-day anomalies between Phases 3 and 5: –Below-normal hail anomalies in Phase 3 –Above-normal anomalies in Phase 5 Hail anomalies generally supported by buoyancy (CAPE) and circulation April anomalies: hail-day (shaded red=positive, blue=negative); CAPE x Shear (contoured) April anomalies: hail-day (shaded red=positive, blue=negative); 500-hPa height (contoured) Figures from Barrett and Henley (2015 Monthly Weather Review)
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department MJO and hail: May Notice differences in hail-day anomalies between Phases 3 and 4: –Below-normal hail anomalies in Phase 3 –Above-normal anomalies in Phase 4 Hail anomalies generally supported by buoyancy (CAPE) and circulation Figures from Barrett and Henley (2015 Monthly Weather Review) May anomalies: hail-day (shaded red=positive, blue=negative); CAPE x Shear (contoured) May anomalies: hail-day (shaded red=positive, blue=negative); 500-hPa height (contoured)
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Evolution of response to MJO Evolution of 200-hPa height anomalies (blue=trough), from 15 days to 00 days before MJO Phase 1 in April –Trough/ridge pattern seen over North America at day 00 largely absent 9 days prior –Height anomalies shift and deepen leading up to day 00 Because MJO is cyclic, day -09 could correspond to day 00 of Phase 8 Barrett and Henley (2015 Monthly Weather Review)
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Summary In April, days in MJO phases 3 and 4 are associated with below-normal severe activity. Days in phases 8, 1, 2, and 6 with above-normal activity In May, days in MJO phases 2 and 3 are associated with below-normal severe activity. Days in phases 4 and 5 are associated with above-normal activity. However, there is significant geographic variability across the U.S. (e.g., in May phase 4, above-normal activity only over Southern Plains and Southeast; below- normal over northern and central Plains)
Naval Oceanography Oceanography DepartmentB. Barrett Oceanography Department Future work Many questions remain! –Lags? Barrett and Gensini (2013) and Barrett and Henley (2015) looked at instantaneous response. What about time lags? –Does the extratropical response depend on MJO amplitude (e.g., does a MJO with RMM 2 of 1.1 behave differently from 2.7?) –What relationship exists for winter months? Jan-Mar? –Is there a better way of quantifying the MJO besides Wheeler Hendon 2004? Kiladis et al. 2014? Ventrice et al. 2013? –Do other measures of severe weather also exhibit modulation by MJO phase? Lightning, heavy rain? –“Receiver” state of the extratropical atmosphere? If the PNA is already positive, how does MJO convection in Indian Ocean (phases 2 and 3) in turn modulate the wave train? And, does this modulation vary seasonally?