Equatorial Rossby Waves and Twin Tropical Cyclogenesis Carl J. Schreck, III John Molinari Department of Earth and Atmospheric Sciences University at Albany State University of New York

Twin Tropical Cyclones Hawaii Paka Pam Pre-IvanPre-Joan Lusi

Convectively Coupled ER waves Shading indicates divergence, hatching indicates convergence Create Symmetric regions of convergence and cyclonic vorticity Propagate westward

Composite ER wave from Molinari et al. (2006) Composited about maximum vorticity at time of tropical cyclogenesis Cyclonic vorticity contoured every 0.5 × s -1 OLR anomalies shaded every 10 W m -2 Convectively coupled ER waves provide favorable regions for tropical cyclogenesis –See also Bessafi & Wheeler (2006) and Frank & Roundy (2006) –None of them mention twins

Convectively Forced ER Waves (Gill 1980) Cyclonic regions develop on both sides of the equator in response to near- equatorial heating Heckley & Gill (1984) found that this steady state solution could be reached within 3 days of the sudden “switch-on” of heating Vertical Motion Height

Liebmann et al. (1994): MJO Composite based on peak day band-pass OLR anomalies at 12°N and 12°S rotated to 0° longitude –35-95 day 850-hPa relative vorticity (contours) –35-95 day OLR (shaded) –Tropical cyclogenesis (*) About half the tropical cyclones form within 45° to the east of the heating No mention of twin tropical cyclogenesis, but both composites show cyclogenesis in both hemispheres Northern Hemisphere Composite Southern Hemisphere Composite

Keen (1982): Mid-latitude interactions

Lander (1990): Convectively Forced ER waves 12 34

Twin Tropical Cyclone Definitions Keen (1982)Lander (1990)Harrison & Giese (1991) N.H. storm is within 9° to the east or 17° to the west of the S.H. storm Along the same longitude Both form between 160°E and 160°W Storms form within 22° latitude of each other Both form at about 5° latitude Both from between 20°S and 20°N Form within 9 days of each other Form nearly simultaneously Named within 8 days of each other (most were within 5 days) 22 sets happen from 1971 to 1979 Twins with Typhoon intensity happen “Once every two or three years” 5 sets happen from 1955 to 1979

3.25 days 1.4° 24 h 2.8° 1.5° 42 h 8.6° 6.75 days Red-filled symbols indicate N.H. storms Blue-filled symbols indicate S.H. storms All equatorward of 10.5° latitude Four Sets of twin tropical cyclones form in during a two-month period from 4 October to 2 December –All occur in the central Pacific –Consistent with the intense El Nino of Fall 1997 Pacific Tropical Cyclogenesis

Data & Methods ECMWF operational analyses –1.125° grid –12-hour temporal resolution CLAUS Brightness Temperature data –0.5° grid –3-hour temporal resolution Combined JTWC and NHC global best track data –Tropical cyclogenesis is considered to occur when a storm first appears in the best track

Data & Methods Unfiltered Data Time-filtered data –15-40 day band-pass –40-day low-pass Space-time filtered data (Wheeler & Kiladis 1999) –ER-band –MJO-band

Wheeler & Kiladis (1999) Space-time filters Shading indicates OLR power above a red background Thin lines are shallow water dispersion curves Heavy lines outline the space-time filters

Unfiltered map on 26 September 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 27 September 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 28 September 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 29 September 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 30 September 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 1 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 2 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 3 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 4 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 5 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 6 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 7 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 8 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 9 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 10 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered map on 11 October 850-hPa winds 850-hPa heights (contours every 10 m) Brightness temperature – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Tropical cyclone locations

Unfiltered Twin tropical cyclogenesis –Red-filled symbols indicate N.H. storms –Blue-filled symbols indicate S.H. storms 850-hPa u –Contours every 3 m s -1 –Westerlies in red –Easterlies in blue –Averaged 4.5°S-4.5°N Brightness temp. – K shaded with cyan –Less than 265 K shaded with warm colors in 25 K intervals –Averaged 10°S-10°N

40-day low- pass filtered Twin tropical cyclogenesis –Red-filled symbols indicate N.H. storms –Blue-filled symbols indicate S.H. storms 850-hPa u –Contours every 2 m s -1 –Westerlies in red –Easterlies in blue –Averaged 4.5°S-4.5°N Brightness temp. – K shaded with cyan –Less than 270 K shaded with warm colors in 10 K intervals –Averaged 10°S-10°N

MJO-band space- time filtered Twin tropical cyclogenesis –Red-filled symbols indicate N.H. storms –Blue-filled symbols indicate S.H. storms 850-hPa u –Contours every 1 m s -1 –Westerlies in red –Easterlies in blue –Averaged 4.5°S-4.5°N Brightness temp. –Shaded every 2.5 K –Negative anomalies shaded with warm colors –Positive anomalies shaded with cool colors –Averaged 10°S-10°N

Low-pass filtered map on 20 September 850-hPa winds Brightness temperature – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis within 4.5 days before or after plot

Low-pass filtered map on 29 September 850-hPa winds Brightness temperature – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis within 4.5 days before or after plot

Low-pass filtered map on 8 October 850-hPa winds Brightness temperature – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis within 4.5 days before or after plot

Low-pass filtered map on 17 October 850-hPa winds Brightness temperature – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis within 4.5 days before or after plot

Low-pass filtered map on 26 October 850-hPa winds Brightness temperature – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis within 4.5 days before or after plot

Low-pass filtered map on 4 November 850-hPa winds Brightness temperature – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis within 4.5 days before or after plot

Summary: Low-frequency convectively generated ER waves The MJO could provide a favorable environment for the first three sets of twin tropical cyclones, as in Liebmann et al. (1994) No active MJO present for the final set of twins –But a broad area of convection was associated with the development of equatorial westerlies

Unfiltered Twin tropical cyclogenesis –Red-filled symbols indicate N.H. storms –Blue-filled symbols indicate S.H. storms 850-hPa u –Contours every 3 m s -1 –Westerlies in red –Easterlies in blue –Averaged 4.5°S-4.5°N Brightness temp. – K shaded with cyan –Less than 265 K shaded with warm colors in 25 K intervals –Averaged 10°S-10°N

15-40 day band-pass filtered Twin tropical cyclogenesis –Red-filled symbols indicate N.H. storms –Blue-filled symbols indicate S.H. storms 850-hPa u –Contours every 1 m s -1 –Westerlies in red –Easterlies in blue –Averaged 4.5°S-4.5°N Brightness temp. –Shaded every 5 K –Negative anomalies shaded with warm colors –Positive anomalies shaded with cool colors –Averaged 10°S-10°N

Twin tropical cyclogenesis –Red-filled symbols indicate N.H. storms –Blue-filled symbols indicate S.H. storms 850-hPa u –Contours every 1 m s -1 –Westerlies in red –Easterlies in blue –Averaged 4.5°S-4.5°N Brightness temp. –Shaded every 2.5 K –Negative anomalies shaded with warm colors –Positive anomalies shaded with cool colors –Averaged 10°S-10°N ER-band space- time filtered

Summary: Convectively Coupled Wave Packets First two sets of twins appear to be associated with a convectively coupled ER wave packet during an active MJO –Evidence even exists of the anticyclonic phase in the unfiltered data Time-filtered anomalies actually propagate eastward leading up to the third set of twins Convectively coupled ER wave signature associated with the final set of twins is probably just a reflection of the tropical cyclones in the filter

Convectively Forced ER waves The MJO and convectively coupled ER waves provide favorable regions for twin tropical cyclogenesis But what determines when and where the storms actually form within these broad regions? Convectively forced ER waves could be one explanation Averaged maps of winds and brightness temp. before and after the development of equatorial westerlies may show the influence of convectively forced ER waves

Unfiltered map averaged September 850-hPa winds Brightness Temp. – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals

Unfiltered map averaged 29 September to 4 October 850-hPa winds Brightness Temp. – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis locations shown

23 September Unfiltered Brightness Temp. – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Unfiltered 1000-hPa: –Winds –Virtual Temp. Countoured every 2°C –Convergence Shaded in s -1 intervals

26 September Unfiltered Brightness Temp. – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Unfiltered 1000-hPa: –Winds –Virtual Temp. Countoured every 2°C –Convergence Shaded in s -1 intervals

29 September Unfiltered Brightness Temp. – K is shaded with cyan –Less than 265 K is shaded with warm colors in 25 K intervals Unfiltered 1000-hPa: –Winds –Virtual Temp. Countoured every 2°C –Convergence Shaded in s -1 intervals

Unfiltered map averaged October 850-hPa winds Brightness Temp. – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals

Unfiltered map averaged October 850-hPa winds Brightness Temp. – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis locations shown

Unfiltered map averaged November 850-hPa winds Brightness Temp. – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals

Unfiltered map averaged November 850-hPa winds Brightness Temp. – K is shaded with cyan –Less than 280 K is shaded with warm colors in 10 K intervals Twin tropical cyclogenesis locations shown

Summary: Convectively forced ER waves In each case, convection sustains for 6 days near 10°S leading up to the development of equatorial westerlies After westerlies develop, convection intensifies and spreads across the equator The first tropical cyclone forms 2-6 days after the development of the equatorial westerlies Convection seems to be triggered by a frontal zone before the first ER wave –Other two waves lack obvious triggers

Conclusions The MJO created cyclonic regions that were favorable for the first three sets of twin tropical cyclogenesis Final set of twins had similar low-frequency convection, but probably not the MJO A convectively coupled ER wave packet may have contributed to the first two sets of twins, but probably not the last two Convectively forced ER waves might determine when the twin tropical cyclones formed

Future Work More complete climatology of twin tropical cyclones is needed to determine common preconditions Apply other filters to convectively coupled ER waves Vertical structures of the convectively forced ER waves need to be examined Use idealized modeling to determine how ER waves are influenced by –Surface friction –Surface heat fluxes –Convective heating –Various background conditions

Acknowledgements John Molinari Dave Vollaro, Anantha Aiyyer, Kristen Corbosiero, Kelly Canavan, Kay Shelton, and Jackie Frank All the grad students Ron McTaggart-Cowan Chris Thorncroft, Paul Roundy, and all the faculty Kevin Tyle & David Knight Celeste Iovinella, Lynn Hughes, & Sharon Baumgardner Mary & My Parents