15A.5 Drastic Thickening of the Barrier Layer Off the Western Coast of Sumatra Due to the Madden Julian Oscillation Passage during the Pre-Years of the Maritime Continent Campaign
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Qoosaku Moteki, Japan Agency for Marine Earth Science and Technology, Yokosuka city, Japan; and K. Yoneyama, M. Katsumata, K. Ando, and T. Hasegawa The drastic thickening of the barrier layer in the marginal sea off the western coast of Sumatra during the passage of the Madden Julian Oscillation (MJO) observed during December 2015 is investigated. Before the MJO arrival, the halocline above 20 m depth was very strong and the barrier layer thickness was 5-10 m from based on R/V Mirai observations. During the MJO forcing of 13-16 December, the isothermal layer was drastically deepened from 20 m to 100 m. Meanwhile, the mixed layer deepening was lagged behind the isothermal layer deepening by 1 day, and the barrier layer underwent dramatic thickening to 60 m within 24 hours. An evaluation of the vertical salinity gradient tendency showed that the dramatic thickening of the barrier layer was due to the vertical oceanic mixing by the atmospheric MJO forcing and the vertical stretching by the oceanic downwelling coastal Kelvin wave intruding from the open ocean. One of the important factors in the drastic barrier layer thickening was concluded to be the atmospheric external forcing and the oceanic internal wave being in-phase. The downwelling oceanic Kelvin wave continuously lowered the thermocline from the middle of November to the end of December, and the salinity stratification in the vicinity of the thermocline was continuously mitigated by the vertical stretching. Under such conditions, the MJO forcing caused vertical mixing of the freshwater with the strong salinity stratification and temperature stratification near the surface. The combination of the two distinct processes caused the drastic thickening of the barrier layer, and the barrier layer thickness reached a maximum of 85 m 5 days after the MJO arrival.
![depth(m) Salinity(psu)
temperature ( )℃
MLD:10m
ILD:30m
barrier layer thickness
BLT=ILD-MLD:20m
ILD( T=0.2 )⊿ ℃
MLD( [ T=0.2 ] 0.07kg/m⊿σ ⊿ ℃ ≒ 3
)
reference depth:10m
BL prevents upwellingBL prevents upwelling
colder watercolder water](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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15A.5 Drastic Thickening of the Barrier Layer Off the Western Coast of Sumatra Due to the Madden Julian Oscillation Passage during the Pre-Years of the Maritime Continent Campaign
- 1. JAMSTEC/DCOP Qoosaku MOTEKIQoosaku MOTEKI Masaki KATSUMATA Kunio YONEYAMA Kentaro ANDO Takuya HASEGAWA Drastic thickening of the barrier layerbarrier layer off the western coast of Sumatra due to the MJO passage during the Pre-YMC Moteki et al. 2018,. under revision, Progress in Earth and Planetary Sciences
- 2. depth(m) Salinity(psu) temperature ( )℃ MLD:10m ILD:30m barrier layer thickness BLT=ILD-MLD:20m ILD( T=0.2 )⊿ ℃ MLD( [ T=0.2 ] 0.07kg/m⊿σ ⊿ ℃ ≒ 3 ) reference depth:10m BL prevents upwellingBL prevents upwelling colder watercolder water
- 3. Sato, K., T. Suga, and K. Hanawa, 2004: Barrier layer in the North Pacific subtropical gyre. Geophysical Research Letters. Seasonal distribution of BLT Boreal summer Boreal winter large seasonality of BLlarge seasonality of BLlarge seasonality of BLT variationlarge seasonality of BLT variation
- 4. R/V MiraiR/V Mirai 2015/12/13 00Z MJO observed during cruise Global-IR strong sfc. westerlies
- 5. Drastic thickening of BL BLT (barrier layer thickness) increased from 5m to 60m within 1-day, up to 85m during 5-days. How did the BL drastically thicken during the MJO passage?
- 6. IL deepeningby vertical mixingwith MJO forcing MLDMLD ILDILD BLTBLT MJO depth(m) shallow ML/IL/BLshallow ML/IL/BL before MJObefore MJO IL deepeningIL deepening after MJOafter MJO ML remainsML remains shallowshallow
- 7. depth(m) IL deepeningby vertical mixingwith MJO forcing (increasing turbulence under strong winds) vertical mixingvertical mixing by windsby winds vertical mixingvertical mixing by windsby winds
- 8. BL thickeningby vertical mixingwith MJO forcingand stretchingwith downwelling thermocline stretched haloclinestretched halocline makes difference btw.makes difference btw. MLD/ILDMLD/ILD salinity stratification is stretched bysalinity stratification is stretched by downwelling thermoclinedownwelling thermocline MJO depth(m) strong salinitystrong salinity stratification bystratification by continuous rainfallcontinuous rainfall
- 9. Argo fortunately trapped in the marginal sea from Nov. 2015 track of
- 10. MJO Pre-YMC Argo Argo captured thinning process of BL after the MJO 10 days after MJO,10 days after MJO, high SST can behigh SST can be recovered throughrecovered through thinning process of BLthinning process of BL thickening BL wasthickening BL was captured wellcaptured well
- 11. MJO Pre-YMC high SST > 30℃ can be recovered in the next 10 days air-sea couplingair-sea coupling simulation should usesimulation should use higher V. reso. to exactlyhigher V. reso. to exactly represent BLrepresent BL If thereIf there’’s not BL, colder waters not BL, colder water upwelling could decreaseupwelling could decrease SSTSST cooled ocean sfc. withcooled ocean sfc. with thick BL can be easilythick BL can be easily recovered as beforerecovered as before Argo
- 12. Drastic thickening of BL was due to the vertical mixing under the MJO forcing. High SST can be recovered in the next 10 days because the colder water upwelling is prevented. Higher vertical resolution could improve the SST variability.
- 13. downwelling Kelvin wave arrives Sumatra R/V MiraiR/V Mirai ECCO2(sea surface hight anomaly) JRA55(surface westerly wind)
- 14. low SSS(< 33psu)low SSS(< 33psu) 2015/12/13 00Z surface salinity ECCO2
- 15. BLT representation by ECCO2 (vertical resolution of 10 m is NOT enough) MLDMLD ILDILD BLTBLT MJO depth(m)
- 16. V. mixing simulation 1D diffusion simulation by Gasper et al. (1990) model Integration: 6 days Initial: CTD 2015/12/12 14Z Forcing: Surface Met. Obs. SR ・ LR ・ Wind Stress
- 17. S0(P-E)=100 〜 10-2 m/day Mixing term: +10-2 ∼10-4 /m/day
- 21. ECCO2ECCO2 IL: T=0.2⊿ ℃IL: T=0.2⊿ ℃ ref. lev. 15mref. lev. 15m CTD Obs.CTD Obs. IL: T=0.2⊿ ℃IL: T=0.2⊿ ℃ ref. lev. 20mref. lev. 20m
- 23. BL formation: Salinity stratification tendency H. adv. V. adv. tilting stretchin g vertical mixing Cronin, M. F., and M. J. McPhaden, 2002: Barrier layer formation during westerly wind bursts. J Geophys Res-Oceans, 107. tendency H. adv. tilting stretching vertical mixing
- 24. Factors for drastic thickening BLT increased by vertical mixing by MJO forcing and downwelling thermocline by oceanic Kelvin wave. in-phase of MJO and oceanic Kelvin wave
- 25. BL thickeningby vertical mixingwith MJO forcingand stretchingwith downwelling Kelvin wave mitigating haloclinemitigating halocline by vertical mixing afterby vertical mixing after MJO forcingMJO forcing salinity stratification is stretching bysalinity stratification is stretching by downwelling Kelvin wavedownwelling Kelvin wave MJO depth(m) strong salinitystrong salinity stratification bystratification by continuous rainfallcontinuous rainfall
- 26. Estimation of BL formation tendency ： +10-1 〜 10-2 /m/day || H. advection ： +10-3 〜 10-4 /m/day V. advection ： +10-2 〜 10-3 /m/day tilting ： -10-2 〜 10-3 /m/day stretching ： +10-2 〜 10-3 /m/day V. mixing ： +10-2 〜 10-4 /m/day
- 27. Pair (+Szt:−Szt) means BL thickening Sal. strat. tendency: ±1×10-1 /m/day
- 28. U: +103 m/day ∇Sz: -10-6 〜 10-7 /m2 Advection term: +10-3 ∼10-4 /m/day
- 29. ∇S: -10-5 〜 10-6 /m Uz: -103 /day Tilting term: −10-2 ∼10-3 /m/day
- 30. wz: -10-1 〜 10-2 /day Sz: +10-1 /m Stretching term: +10-2 ∼10-3 /m/day
- 34. MJO passagedisturbance passage 35m 35m 20m 25m 25m 10m 60m 70m 85m 100m
- 35. review on previous studies Cronin and McPhaden 2002, JC “Barrier layer formation during westerly wind bursts” Tilting is critical for the BL formation.
- 36. review on previous studies Vialard, J., and P. Delecluse, 1998, JPO “An OGCM study for the TOGA decade. Part II: Barrier-layer formation and variability“ Stretching by subduction from convergence is critical.
- 37. review on previous studies Girishkumar, et al. 1998, JGR-O “Intraseasonal variability in barrier layer thickness in the south central Bay of Bengal“ Stretching by Rossby wave is critical. intraseasonal time scale
- 40. 80km80km100km
- 41. 80km80km100km
- 42. depth(m) Salinity(psu) temperature ( )℃ fresh lens fresh lens index: (σ10m−σ6m)/4 (kg/m4 ) > 0 (fresh lens)
- 43. depth(m) Salinity(psu) temperature ( )℃ reference depth: 10m fresh lens index: (σ10m−σ6m)/4 (kg/m4 ) < 0 (mixed above 10 m)
- 44. 80km80km100km
- 45. MJO MLDMLD ILDILD BLTBLT depth(m) ILD( T=0.2 )⊿ ℃ MLD ( [ T=0.2 ])⊿σ ⊿ ℃
- 46. ARGO
Hinweis der Redaktion
- My name is Qoosaku MOTEKI from JAMSTEC in Japan. I was on the R/V Mirai cruise during Pre-YMC in 2015 in charge of TurboMAP operation to measure the ocean turbulence. I would like to talk about the situation in the ocean under the MJO passage that was observed during Pre-YMC campaign. During the R/V Mirai cruise off the western coast of Sumatra, we observed not only the MJO itself but also the drastic change of the ocean surface structure by the MJO forcing.
- First of all, generally looking at TS profile in the ocean, the barrier layer is defined by the difference between the density mixed layer depth and isothermal layer depth. Usually, the reference depth is 10 m, and temperature difference of 0.2 degC is used here. The barrier layer prevents to vertically mix the colder water from the bottom of ILD due to the density stratification originated from salinity stratification. If we have a thick barrier layer, the SST decrease can be mainly by the surface heat flux but is difficult to be affected by upwelling colder water. So we should take account of the barrier layer thickness because heat budget process btw A&O should be significantly changed btw. cases with and without thick BL.
- Generally, thick BLs more than 40 m indicated by the red circles distribute in the central Pacific and Bay of Bengal. Sato 2004 statistically showed the seasonal distribution of BLT from the Argo profiles. Over the eastern Indian Ocean, BLT was variated between winter and summer. So this is a motivation to study the formation process of BL from high-frequency high vertical resolution in-situ observation at a fixed point.
- Yoneyama-san has already introduced that the MJO was successfully observed during Pre-YMC. Under the MJO forcing of strong surface westerlies starting from 13 December over the R/V Mirai,
- BLT drastically increased from 5m to 60m within only 24 hours up to 85 m during 5 days. So our question is “How did the barrier layer drastically thicken during the MJO passage?”
- This is the time-depth cross section of temperature from R/V Mirai CTD observation. Density defined mixed layer depth is shown by dashed line, and temperature defined isothermal layer depth is shown by dotted line. The blue line of the 29degC shows continuous downwelling thermocline from 23 November to 17 December. before the MJO arriving on 13 Dec., you can see MLD and ILD are quite shallow and BLT is less than 20 m. After the MJO arriving, ILD is rapidly increasing from 20 m to 80 m, but MLD remains very shallow. Then, BLT is drastically thickening during the MJO passage due to the vertical mixing, in fact,
- increasing turbulence is clearly observed like this. The bold solid line shows sea surface wind speed with the right axis. And you can see that wind bursts induce the turbulence. So this increasing turbulence could be mixing temperature well and ILD rapidly increased. If the salinity was also mixed well, MLD and ILD should be the same and BLT could not increase. So, how about the salinity?
- There is a very strong salinity stratification near the surface before the MJO like this. The salinity above 10 m before the MJO is less than 33 psu that is much smaller than usual values over the open oceans because of continuous freshwater flux from diurnal rainfall around Sumatra. So the salinity stratification was actually mitigated by the vertical mixing but not mixed completely and mitigated salinity stratification was stretched in the sub-surface. This makes difference between MLD and ILD and drastic thickening of BL. Also, continuous downwelling thermocline is favorable condition for IL deepening and it can also stretch the salinity stratification around the bottom of IL.
- One more thing, one of the Argo floats fortunately trapped in the marginal sea for longer period including Pre-YMC period.
- Argo profiles once per 10 days also well capture the drastic thickening process of BL. In addition, 10 days after the MJO, high SST more than 30degC can be recovered through the thinning process of BL.
- This means that the cooled ocean surface with the thick BL can be easily recovered as before high SST by 10 days. If there is not thick BL, it is more difficult to recover high SST because of upwelling colder water. So thinking about air-sea coupling simulation, we should use higher vertical resolution near the surface to improve the representation of the barrier layer process like this. This is the most important message from this observation to the MJO simulation.
- in summary, the observed drastic thickening of BL was due to the vertical mixing under the MJO forcing. Also, continuous downwelling thermocline was favorable condition for that. High SST can be recovered in the next 10 days because the colder water upwelling is prevented by the thick BL. Thinking about ocean coupling simulation, higher vertical resolution could improve the SST variability by exactly representing BL formation and modifying the feedback process from the ocean.
- The downwelling Kelvin wave could also affect the BL thickening. This is time longitude cross section of sea surface height anomaly from October to the end of December. The black contours indicate the surface westerly wind speed. You can easily see that positive anomaly of sea surface hight under the westerly winds propagates eastward and arrives Sumatra during the Pre-YMC period. This means that large scale downwelling should be there in addition to the MJO forcing.
- actually, looking at sea surface salinity distribution from objective analysis ECCO2, low salinity less than 33 psu shown by colder colors is distributed along the coastline like this. That is, the area off the western coast of Sumatra has the same potential condition for the drastic thickening of BL under the MJO forcing. In addition,
- This is the time-depth cross section of temperature from R/V Mirai CTD observation. Density defined mixed layer depth is shown by dashed line, and temperature defined isothermal layer depth is shown by dotted line. The blue line of the 29degC shows continuous downwelling thermocline from 23 November to 17 December. You can see MLD and ILD are quite shallow and BLT is less than 20 m before the MJO arriving on 13 Dec. After the MJO arriving, ILD is rapidly increasing from 20 m to 80 m, but very shallow MLD is keeping. Then, BLT is drastically thickening during the MJO passage due to the vertical mixing because
- On the basis of the Pre-YMC observations, BLT increased drastically within only 24 hours during the MJO passage over the R/V Mirai. Such drastic deepening has not observed ever. what I want to conclude here is,
- This time lag of deepening of ILD and MLD is due to
- それを示す非常に貴重なデータを得ることができました．
- それを示す非常に貴重なデータを得ることができました．
- それを示す非常に貴重なデータを得ることができました．
- それを示す非常に貴重なデータを得ることができました．
- それを示す非常に貴重なデータを得ることができました．
- In summary drastic thickening of BL is due to the vertical mixing by the MJO forcing and downwelling thermocline by oceanic Kelvin wave. The in-phase of these two factors in the atmosphere and ocean is very important.
- That’s all. thank you.
- Such drastic deepening is due to the vertical mixing by the MJO forcing and downwelling thermocline by oceanic Kelvin wave. The fact that these two factors in the atmosphere and ocean are in-phase is inducing drastic deepening.
- それを示す非常に貴重なデータを得ることができました．
- そこで、そうした取り組みによる直近の実績のポイント
- そこで、そうした取り組みによる直近の実績のポイント
- そこで、そうした取り組みによる直近の実績のポイント
- This is time series of mixed layer depth by black, isothermal layer depth by blue, and barrier layer thickness by black dashed line, from 23 Nov. to 17 Dec. based on the CTD observation.
- This is time series of mixed layer depth by black, isothermal layer depth by blue, and barrier layer thickness by black dashed line, from 23 Nov. to 17 Dec. based on the CTD observation.
- Such drastic deepening is due to the vertical mixing by the MJO forcing and downwelling thermocline by oceanic Kelvin wave. The fact that these two factors in the atmosphere and ocean are in-phase is inducing drastic deepening.
- Such drastic deepening is due to the vertical mixing by the MJO forcing and downwelling thermocline by oceanic Kelvin wave. The fact that these two factors in the atmosphere and ocean are in-phase is inducing drastic deepening.
- Such drastic deepening is due to the vertical mixing by the MJO forcing and downwelling thermocline by oceanic Kelvin wave. The fact that these two factors in the atmosphere and ocean are in-phase is inducing drastic deepening.
- That is, the vertical profile with fresh lens structure before the MJO is drastically changed
- to this one after the MJO because the fresh lens was broken by the vertical mixing due to the MJO forcing and ILD deepening by oceanic downwelling Kelvin wave. As a result, a thick barrier layer appears within 24 hours on 13 December.
- like this. This is a time-depth cross section of temperature from 3-hourly CTD observation. Mixed layer depth defined from density difference is shown by dashed line, and isothermal depth is shown by dotted line. The blue line of the 29degC shows continuous downwelling thermocline from 23 November to 17 December. You can see MLD and ILD are quite shallow and BLT is less than 20 m before the MJO arriving on 13 Dec. After the MJO arriving 13 Dec., ILD is rapidly increasing from 20 m to 80 m, but very shallow MLD is keeping less than 20 m. Then, BLT is drastically deepening during the MJO passage. This time lag of deepening of ILD and MLD is due to
- それを示す非常に貴重なデータを得ることができました．