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Boreal Summer Oscillation Dynamics and Mechanisms in Equatorial Pacific

Study on initiation mechanisms and energy transfer of Boreal Summer Intraseasonal Oscillation. Analyzing NCEP/NCAR Reanalysis data from 1980-2001 to understand the influence of local and global triggers on ISO behavior. Investigates the role of equatorial Rossby wave response, Kelvin wave propagation, and orographic effects in generating intraseasonal variability. Offers insights into the establishment of anomalous meridional temperature gradients and crossing-equatorial flows in the western equatorial Indian Ocean.

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Boreal Summer Oscillation Dynamics and Mechanisms in Equatorial Pacific

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  1. Re-initiation of the Boreal SummerIntraseasonal OscillationSimon Jiang and Tim LiDepartment of Meteorology, University of Hawaii

  2. INTRODUCTION Extropical triggering: Rossby wave propagates to equatorial region (Hsu et al, 1993 for Winter ISO) Internal triggering mechanism: Hu and Randall (1994), Blade and Hartmann (1993) **Eastward propagating Kelvin Wave Lau and Peng (1987); Chang and Lim (1988) (Matthews 2000) First is a local mechanism operating over the warm-pool region. Over the equatorial western Indian Ocean, the equatorial Rossby wave response to the west of enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge. This tends to suppress ascent in the boundary layer and shuts off the deep convection, eventually leading to a convective anomaly of the opposite sign. The second mechanism is global, involving completing a circuit of the equator. **Energy transfer from westward propagating Rossby waves to equatorial Kelvin wave (Wang and Xie, 1997)

  3. 2. DATA NCEP/NCAR Reanalysis • 1980-2001 daily averaged • Horizontal resolution: 2.5 by 2.5 degree • 12 vertical levels • u, v, w, q NOAA OLR

  4. EOF1 (12.1%) EOF2 (8.4%) Time series of EOF 1 59 cases 67 cases

  5. Evolution of composite OLR

  6. “south”-type initiation “west”-type initiation

  7. Evolution of composite OLR

  8. Evolution of specific humidity (-2.5oS-2.5oN ) initiation date

  9. Evolution of divergence (-2.5oS-2.5oN ) initiation date

  10. Evolution of divergence at 925mb (10-7s-1) Orography (m)

  11. What cause the PBL divergence in far western IO? • Simple Gill response to heating in eastern IO • 3D basic flow • Orographic effect

  12. AGCM simulation: Princeton AGCM Horizontal resolution: T42 Vertical: 5 levels 1. Based on the observed mean state of , obtain the tendency term in each equation by the mean state. 2. Include heating term in the thermal equation, integrate the model and reach a steady state. 3.The anomalous circulation is obtained by the deviation between the new state and mean state. And the anomalous tendency terms can also be obtained by the difference between the total tendency and the one by mean state only.

  13. Observed at day 0 Model simulated

  14. Vertical heating profile Horizontal heating distribution K/day

  15. Full summer mean state Topography removed (by smoothing PS and T over Africa continent)

  16. Divergence averaged over: 45-55oE, -5oS-2.5oN on SIGMA=0.9 (unit: 10-8S-1) PS, T smoothed over Africa No mean flow Full summer mean flow

  17. Temperature budget Time (day)

  18. Control experiment Topography removed No momentum advection No temperature Advection

  19. Budget of temperature advection terms in the control experiment: Averaged over: 45-55oE, -5oS-5oN. (unit: 10-7KS-1) Horizontal Advection Vertical Advection Total

  20. Budget of horizontal temperature advection terms: Averaged over: 45-55oE, -5oS-5oN. mean PBL wind and temperature (unit: 10-7KS-1) Anomalous PBL wind and temperature

  21. How are anomalous crossing-equatorial flows established over western equatorial IO? A model with the zonal mean basic state: Zonal mean environmental flow No environmental flow

  22. Without mean flow Zonal mean T, U Zonal mean T, U, and V Zonal mean U, Area mean T

  23. How is the anomalous meridional temp gradient established? Temperature and wind on sigma=0.9 T-tendency evolution difference between (45-60E,-12.5~-7.5S) and (45-60E, 7.5~12.5N) Temperature and wind on sigma=0.9 Temperature and wind on sigma=0.9

  24. T-tendency evolution difference between (50-60E,-12.5~-7.5S) and (50-60E, 7.5~12.5N)

  25. Temperature advection and divergence at 925mb (NCEP) (40-60oE, -2.5oS-2.5oN) normalized day Budget of horizontal temperature advection Unit: KS-1 day

  26. Summary BSISO initiation in the western IO is originated in the boundary layer, which is clearly seen from the PBL divergence and moisture fields. A thermal advection mechanism is proposed, in which the equatorial asymmetric summer mean flow is essential to cause the asymmetry of perturbation wind and temperature in response to a symmetric ISO heating. The so-induced cold advection in PBL leads to subsidence in the top of boundary layer and thus divergence in PBL. In the absence of the 3D mean flow, the ISO heating in the eastern IO may cause a zonal wind divergence in western IO, but its magnitude is one-order-of-magnitude smaller. Topography in the tropical Africa may enhance the PBL divergence induced by the mean flow.

  27. Cause of Seasonal Dependence of Intraseasonal Variability in the Tropical Atmosphere and Ocean Tim Li ( Department of Meteorology and IPRC University of Hawaii ) Acknowledgement: Xian Jiang, Francis Tam, Chunhua Zhou

  28. Outline • Observed seasonal dependence of ISO in the tropical atmosphere • Physical interpretation – A thermal equator hypothesis • Observed seasonal dependence of intraseasonal SST variability • Physical interpretation – A background wind-ocean MLD control hypothesis

  29. Wang and Rui (1990) 1.Introduction: Madden and Julia(1971,1972) NE SE EN EN EE Winter ISO  eastward propagating MJO along the equator Summer ISO: more complex propagation characteristics: northward propagation in IO and westward propagation off the equator in WP (Yasunari 1979,1980;, Murakami 1980; Krishnamurti and Subrahmanyam 1982) • 3 categories: eastward (65%), independent northward (20%) and westward (15%) propagation.

  30. Observed OLR Composite in northern summer

  31. Effects of the thermal equator on the seasonality of ISO behavior What is the effect of the maritime continent? • diurnal cycle • moisture sink • friction Winter: continuous eastward propagation across the maritime continent Summer: northward bifurcation over the eastern IO/maritime continent

  32. Rainfall evolution along 70~95oE (15~90 day filtered) from ECHAM4 15 10 5 14 9 4 13 8 3 12 7 2 11 6 1

  33. Composite rainfall evolution from ECHAM4

  34. Control Experiment (75-95oE) Phase Speed (m/s) Southward Northward Wave Length (1000km) Wave number Period (days) EXP_MC Phase Speed (m/s) Northward Southward Wave number Wave Length (1000km) Period (days)

  35. Surface Moisture Winter DJF Hypothesis: ISO seasonality is caused by the distinctive asymmetry of the thermal equator between northern winter and summer. Summer JJA

  36. Precipitation PBL divergence (1000-850mb) Winter Winter DJF DJF JJA JJA

  37. PBL divergence (1000-850mb) Hypothesis(cont’d): Winter DJF • boreal winter: Atmospheric moist Kelvin waves are unstable while Rossby waves are stable  Maximum perturbation is confined near the equator and ISO moves eastward • boreal summer: Atmospheric Kelvin Waves stabilize due to low-level divergence over MC  ISO Wave packet decoupled, and Rossby waves emanated from the wave packet. Summer JJA

  38. A 21/2 layer dynamic framework (Wang and Li 1994, 1994) 0 u1,v1, 1 2 u3,v3, 3 4 PBL uB,vB Convective heating is proportional to moisture convergence in PBL: and background low-level convergence or background low-leveldivergence

  39. frequency SST/q Profile-- symmetric SST q K 0.0174 R 0.015 Kelvin Wave Growth rate R K

  40. SST/q Profile -- asymmetric q SST frequency 0.0174 K 0.015 R Rossby Wave K R Growth rate

  41. Specific humidity (900mb) PBL divergence (110-140oE) summer winter summer winter K R Growth rate K R Winter Summer

  42. Summary: Seasonality of atmospheric ISO behavior • The maritime continent is not an essential factor leading to the winter-summer asymmetry in the ISO behavior. • An eigenvalue analysis indicates that the equatorial asymmetry of the summer mean state over the maritime continent region leads to the decay of equatorial Kelvin waves but favors the growth of Rossby waves, whereas the winter mean state favors the growth of the Kelvin waves. • The significant northward shift of the thermal equator is the essential cause of meridional bifurcation of ISO over eastern IO off Sumatra in boreal summer.

  43. Intraseasonal SST Variability over the Indian Ocean

  44. (Saji et al. 2004)

  45. Phase composites for the winter intraseasonal SST variability N.H. Saji, and co-authors, submitted to J. Climate Composite anomaly wind vectors (QuikSCAT), SST (TMI, shaded) and OLR (NOAA, dashed line: reduced OLR; solid line: increased OLR) during intraseasonal cooling (left panel) and warming (right panel) events in Indian Ocean SST.

  46. Li et al. 2002, GRL

  47. Hypothesis Atmospheric ISO convection z Seasonal dependence of intraseasonal SST variability is caused by • Summer-winter difference in the background zonal wind • Summer-winter difference in the ISO forcing • Summer-winter difference in the mixed layer depth EQ W E Seasonal mean wind in DJF • SST Tendency is affected by • Latent heat flux • Short wave cloud forcing

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