Role Of Air-sea Interaction In The Boreal Summer Intraseasonal Oscillation Over The Western North Pacific

The boreal summer intraseasonal oscillation(BSISO)is one of the dominant components of the tropical atmospheric low-frequency oscillations and mostly active in the Asian summer monsoon region with pronounced northward propagations.It is different from the MJO(Madden-Julian oscillation)which prevails in boreal winter with equatorial trapped eastward-propagating convective variability.The highest low-frequency variabilities of the convective activities over the entire Asian summer monsoon region fall in the tropical western North Pacific(WNP),in addition,the BSISO over the WNP strongly modulates the short-term climate variabilities of China.However,less attention has been paid to the BSISO over the WNP as compared with the BSISO over the Indian Ocean or with MJO.Particularly,researches aiming at the associated air-sea interaction are carried out relatively late.Due to the large discrepancies of the background flow and their inherent features,conclusions drawn from the studies on the BSISO over the Indian Ocean or MJO are not necessarily established in the BSISO over the WNP.It is no doubt that the role of air-sea interaction in the tropical atmospheric low-frequency oscillation continues to be a major research focus,hence more explorations of the air-sea interaction associated the BSISO over the WNP,especially the mechanisms and the role it plays,are needed.The uncertainties induced by the model dependence make it more necessary to conduct further observational studies.For the above-mentioned purposes and by diagnosing the OLR,CFSR and OAFlux datasets,following principal conclusions on the two dominant modes of the BSISO over the WNP with different frequency bands(30-60 day and 10-25 day)are drawn in this dissertation.1.The specific processes that how atmosphere and ocean interact with each other in the BSISO over the WNP are uncovered.The northward propagating convective anomalies associated with the BSISO over the WNP originate from the equatorial western Pacific,strengthen at around 15°N,and dissipate over the subtropical East Asia.The 30-60(10-25 day)day mode has a northward(northwestward)propagation with a principal period of roughly 40(15)days.By constructing time-lead/lag composites,it is revealed that significant sea surface temperature(SST)variabilities with organized structure are associated with the convective anomalies.The temporal evolutions and the meridional spatial distributions of these two variables are in near-quadrature,in which the enhanced(suppressed)convection leads the cold(warm)SST and the cold(warm)SST leads the suppressed(enhanced)convection,both by about a quarter of cycle.The near-quadrature phase relationship between the convective and SST anomalies in the BSISO over the WNP implies that the atmosphere and the ocean may influence each other.The specific processes of the atmospheric forcing on the ocean are generally the same in the two different BSISO modes.An enhanced convection reduces the amount of the shortwave radiation reaching the sea surface by increasing the cumulus cloud cover,and the associated cyclonic low-level wind anomaly appears to decelerate(accelerate)the near-surface wind speed in the northern(southern)portion of the convection where the background southwesterly monsoon prevails,thus reducing(enhancing)the upward latent heat and sensible heat fluxes.Through the above two major processes,an active convection induces a maximum warm(cold)SST anomaly in the northern(southern)portion of the convection in about a quarter of cycle,while an inactive convection has the opposite results.There are three major processes for the convection-generated SST anomalies to feedback to the convection.Firstly,the warm SST anomaly to the north of the active convection center increases the upward surface sensible and latent heat fluxes and results in positive anomalies,via amplifying the sea-air temperature and humidity differences.Secondly,the warm SST anomaly directly increases the low-level atmospheric instability by heating the surface air.The cold SST anomaly to the south of the active convection center has the opposite effects through the first two processes.Finally,the meridional SST anomaly gradient tends to offset the positive low-level convergence anomalies over the active convection center and its southern portion,but to promote positive low-level convergence anomalies over the warm SST anomalies,which spatially precede the active convection by about a quarter of cycle.These conditions created by the SST anomalies act to suppress the active convection over its central area/southern portion and the anomalous descending motions to the north,triggering the shallow convection that is the first stage or an indication of the deep convection developing over the warm SST anomalies.The above oceanic feedback processes are significant in the 30-60 day BSISO but relatively weaker in the 10-25 BSISO,since the anomalous low-level heating and moistening prior to the deep convection are virtually undetectable in the latter.2.The relative contributions of the atmospheric and oceanic effects to the low-level moisture budget anomaly are quantitatively diagnosed.A scheme for low-level moisture budget diagnosis with which the specific dynamical and diabatic processes can be distinguished is developed in combination with previous studies.With this scheme,the relative contribution to the low-level moisture budget anomaly induced by the atmosphere-related or ocean-related processes can be separated and quantitatively calculated.It is applied to the 30-60 day BSISO with stronger oceanic feedback.Result shows that the net effects of the atmosphere-related processes appear to consume the low-level moisture over the warm SST anomalies to the north of the active convection center,while the net effects of the ocean-related processes increase the low-level moisture and contributes to the positive local moistening tendency anomalies by enhancing the surface evaporation and low-level convergence.From the point of the low-level moisture budget anomaly,the oceanic feedback plays a leading role in the development of the convection over the warm SST anomalies which spatially precede the active convection by about a quarter of cycle,but the net atmospheric effect plays a negative role.3.The role of the air-sea interaction plays in the BSISO over the WNP is explained.In the cycle of the BSISO over the WNP,an enhanced convection results in a cold(warm)SST anomaly to the south(north)of the convection center,which in turn weakens the enhanced convection over its central area/southern portion,and provides favorable conditions for the convection development to its north.It is suggested that the above air-sea interaction processes provide a candidate mechanism for the northward propagation of the BSISO,and offer a negative feedback to the enhanced convection.Due to the large heat capacity of the ocean mixed layer,it takes time for the SST anomaly to change sign,hence such negative feedback is always delayed.It is thus speculated that the air-sea interaction is capable of providing an extra source of the low-frequency oscillation in which the persistency of the SST anomaly could affect the time scale.Based on the role of providing an extra source of the low-frequency oscillation,it is further deduced that the shorter period and weaker oceanic feedback of the 10-25 day BSISO can be both cause and effect of the other.On one hand,the time for the atmosphere to take effects on the ocean is also shorter due to the shorter period,which results in weaker SST variabilities and oceanic feedback.On the other hand,lacking of an effective delayed negative feedback offered by air-sea interaction leads to the absence of an extra source of the low-frequency oscillation with relatively longer period,hence the principal period of the 10-25 day BSISO is unable to be extended.