Xiouhua Fu

Pacific–East Asian Teleconnection: How Does ENSO Affect East Asian Climate?

Observational evidence is presented to show a teleconnection between the central Pacific and East Asia during the extreme phases of ENSO cycles. This Pacific‐East Asian teleconnection is confined to the lower troposphere. The key system that bridges the warm (cold) events in the eastern Pacific and the weak (strong) East Asian winter monsoons is an anomalous lower-tropospheric anticyclone (cyclone) located in the western North Pacific. The western North Pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures. The anomalies persist until the following spring or early summer, causing anomalously wet (dry) conditions along the East Asian polar front stretching from southern China northeastward to the east of Japan (Kuroshio extension). Using atmospheric general circulation and intermediate models, the authors show that the anomalous Philippine Sea anticyclone results from a Rossby-wave response to suppressed convective heating, which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central Pacific warming. The development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling. Both the anticyclone and the cooling region propagate slowly eastward. The development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades. The rapid establishment of the Philippine Sea wind and SST anomalies implies the occurrence of extratropical‐tropical interactions through cold surge‐induced exchanges of surface buoyancy flux. The central Pacific warming plays an essential role in the development of the western Pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone‐ SST interaction and midlatitude‐tropical interaction in the western North Pacific.

Coupling between northward-propagating, intraseasonal oscillations and sea surface temperature in the Indian Ocean

Using a hybrid atmosphere‐ocean coupled model, it is shown that during the boreal summer northwardpropagating, intraseasonal oscillations (NPISOs) are strongly coupled to the underlying sea surface temperature (SST) in the Indian Ocean sector. On the one hand, the intraseasonal atmospheric convection changes the SST through solar radiation, latent heat flux, and mixed-layer entrainment; on the other, the induced SST fluctuations feed back to affect the intraseasonal convection. The preferential northward, rather than southward, propagation of boreal summer ISOs in the Indian Ocean is partially explained by an interaction among the summer-mean climate state, the atmospheric disturbances, and the ocean surface temperature. A solution to an atmosphere-only model forced with daily SST produces much stronger NPISOs than a similar solution forced with monthly mean SST (AMIP-type run). The atmosphere-only model, however, even when it is forced by daily SST from the coupled model (with a small amount of noise in the initial and/or boundary conditions), is unable to reproduce the NPISOs in the coupled case. In the coupled system, intraseasonal SST anomalies are forced by intraseasonal atmospheric convection, and hence are in quadrature with the convection. In the stand-alone atmospheric model, however, SST acts only as a boundary forcing, and the resultant atmospheric convection has almost the same phase with the underlying SST. One consequence is that the intensity of the SST-forced intraseasonal convection in the stand-alone atmospheric model is considerably weaker than in the coupled model. Finally, solutions indicate that the northward movement of the off-equatorial convection in the northern Indian Ocean is more closely related to local intraseasonal SST anomalies than to the equatorial eastward-moving Madden‐Julian oscillation: Positive (negative) SST anomalies in the northern Indian Ocean lead the active (break) phases of the intraseasonal convection by about 2 pentads (10 days). Therefore, intraseasonal SST anomalies in the northern Indian Ocean are potentially a useful index to forecast active (break) spells of the south Asian summer monsoon.

Simulation of the Intraseasonal Oscillation in the ECHAM-4 Model: The Impact of Coupling with an Ocean Model*

Abstract Three 15-yr integrations were made with the ECHAM-4 atmospheric GCM (AGCM); in the first integration, the model lower boundary conditions were the observed monthly mean sea surface temperatures, and, in the second, the AGCM was coupled to the University of Hawaii 2.5-layer intermediate ocean model. In the third simulation, the SST climatology generated in the coupled run was used to create monthly mean SSTs, which were then used to drive the AGCM in an uncoupled configuration similar to the first run. The simulation of the intraseasonal oscillation (ISO) in these three runs was compared with data from the NCEP reanalysis and outgoing longwave radiation from NOAA polar-orbiting satellites, with particular emphasis on the boreal summer ISO. The overall effect of coupling the AGCM to the ocean model is to improve the intraseasonal variability of the model. Upon coupling, the simulated boreal winter ISO becomes more spatially coherent and has a more realistic phase speed. In the May–June Asian monsoo...

Differences of Boreal Summer Intraseasonal Oscillations Simulated in an Atmosphere–Ocean Coupled Model and an Atmosphere-Only Model*

A series of small-perturbation experiments has been conducted to demonstrate that an atmosphere‐ocean coupled model and an atmosphere-only model produce significantly different intensities of boreal summer intraseasonal oscillation (BSISO) and phase relationships between convection and underlying SST associated with BSISO. The coupled model not only simulates a stronger BSISO than the atmosphere-only model, but also generates a realistic phase relationship between intraseasonal convection and underlying SST. In the coupled model, positive (negative) SST fluctuations are highly correlated with more (less) precipitation with a time lead of 10 days as in the observations, suggesting that intraseasonal SST is a result of atmospheric convection, but at the same time, positively feeds back to increase the intensity of the convection. In the atmosphere-only model, however, SST is only a boundary forcing for the atmosphere. The intraseasonal convection in the atmosphereonly model is actually less correlated with underlying SST. The maximum correlation between convection and SST occurs when they are in phase with each other, which is in contrast to the observations. These results indicate that an atmosphere‐ocean coupled model produces a more realistic ISO compared to an atmosphereonly model.

Impact of Atmosphere-Ocean Coupling on the Predictability of Monsoon Intraseasonal Oscillations

The impact of air–sea coupling on the predictability of monsoon intraseasonal oscillations (MISO) has been investigated with an atmosphere–ocean coupled model and its atmospheric component. From a 15-yr coupled control run, 20 MISO events are selected. A series of twin perturbation experiments have been conducted for all the selected events using both the coupled model and the atmosphere-only model. Two complementary measures are used to quantify the MISO predictability: (i) the ratio of signal-to-forecast error and (ii) the spatial anomaly correlation coefficient (ACC). In the coupled model, the MISO predictability is generally higher over the Indian sector than that over the western Pacific with a maximum of 35 days in the eastern equatorial Indian Ocean. Air–sea coupling significantly improves the predictability in almost the entire Asian–western Pacific region. The mean predictability of the MISO-related rainfall over its active area (10°S–30°N, 60°–160°E) reaches about 24 days in the coupled model and is about 17 days in the atmosphere-only model. This result suggests that including an interactive ocean allows the MISO predictability of an atmosphere-only model to be extended by about a week. The extended predictability is primarily due to the coupled model capturing the two-way interactions between the MISO and underlying sea surface. The MISO forces a coherent intraseasonal SST response in underlying ocean that in return exerts an external control on the future evolutions of the MISO. The break phase of the MISO is more predictable than the active phase in both the atmosphere-only model and the coupled model as revealed in the observations. Air–sea coupling appears to extend the MISO predictability uniformly regardless of the active or break phases.

Critical Roles of the Stratiform Rainfall in Sustaining the Madden–Julian Oscillation: GCM Experiments*

Abstract This study assesses the impact of stratiform rainfall (i.e., large-scale rainfall) in the development and maintenance of the Madden–Julian oscillation (MJO) in a contemporary general circulation model: ECHAM4 AGCM and its coupled version. To examine how the model MJO would change as the stratiform proportion (the ratio of the stratiform versus total rainfall) varies, a suite of sensitivity experiments has been carried out under a weather forecast setting and with three 20-yr free integrations. In these experiments, the detrainment rates of deep/shallow convections that function as a water supply to stratiform clouds were modified, which results in significant changes of stratiform rainfall. Both the forecast experiments and long-term free integrations indicate that only when the model produces a significant proportion (≥30%) of stratiform rainfall can a robust MJO be sustained. When the stratiform rainfall proportion becomes small, the tropical rainfall in the model is dominated by drizzle-like r...

Impacts of Air–Sea Coupling on the Simulation of Mean Asian Summer Monsoon in the ECHAM4 Model*

Atmosphere‐ocean coupling was found to play a critical role in simulating the mean Asian summer monsoon and its climatological intraseasonal oscillation (CISO) in comparisons of the results from a stand-alone ECHAM4 atmospheric general circulation model (AGCM) and a coupled ECHAM4‐ocean [Wang‐Li‐Fu (WLF)] model. The stand-alone simulation considerably overestimates the equatorial Indian Ocean rainfall and underestimates monsoon rainfall near 158N, particularly over the eastern Arabian Sea and the Bay of Bengal. Upon coupling with an ocean model, the simulated monsoon rainfall becomes more realistic with the rainbelt near 158N (near the equator) intensified (reduced). These two rainbelts are connected by the northward-propagating CISOs that are significantly enhanced by the air‐sea interactions. Both local and remote air‐sea interactions in the tropical Indian and Pacific Oceans contribute to better simulation of the Asian summer monsoon. The local impact is primarily due to negative feedback between SST and convection. The excessive rainfall near the equatorial Indian Ocean reduces (increases) the downward solar radiation (upward latent heat flux). These changes of surface heat fluxes cool the sea surface upon coupling, thus reducing local rainfall. The cooling of the equatorial Indian Ocean drives an anticyclonic Rossby wave response and enhances the meridional land‐sea thermal contrast. Both strengthen the westerly monsoon flow and monsoon rainfall around 158N. The local negative feedback also diminishes the excessive CISO variability in the equatorial Indian Ocean that appeared in the stand-alone atmospheric run. The remote impact stems from the reduced rainfall in the western Pacific Ocean. The overestimated rainfall (easterly wind) in the western North (equatorial) Pacific cools the sea surface upon coupling, thus reducing rainfall in the tropical western Pacific. This reduced rainfall further enhances the Indian monsoon rainfall by strengthening the Indian‐Pacific Walker circulation. These results suggest that coupling an atmospheric model with an ocean model can better simulate Asian summer monsoon climatology.

Spatiotemporal Structures and Mechanisms of the Tropospheric Biennial Oscillation in the Indo-Pacific Warm Ocean Regions*

Abstract The observed structure and seasonal evolution characteristics of the tropospheric biennial oscillation (TBO) in the warm ocean areas of the Indo-Pacific region are explored using a seasonal-sequence EOF analysis approach. The major convective activity centers associated with the TBO appear in the southeast Indian Ocean (SEIO) and western North Pacific (WNP), accompanied by anticyclonic (or cyclonic) circulation patterns with a first-baroclinic-mode structure. The convection and circulation anomalies have distinctive life cycles in the SEIO and WNP: the former have a peak phase in northern fall and the latter persist from northern winter to subsequent summer. The mechanisms of the TBO in this region are investigated with a hybrid coupled GCM. Numerical results show that air–sea interaction in the warm ocean alone can support TBO variability that has many observed characteristics. Key processes involved in the TBO include the WNP monsoon variability and associated cross-equatorial flows, convective...

Sea Surface Temperature Feedback Extends the Predictability of Tropical Intraseasonal Oscillation

The possible impacts of different sea surface temperature (SST) configurations on the predictability of the boreal summer tropical intraseasonal oscillation (TISO) are assessed with a series of ensemble forecasts. The five different lower boundary conditions examined in this study are, respectively, (i) the fully interactive ocean–atmosphere coupling, (ii) “smoothed” SST, which excludes the intraseasonal signal from sea surface forcing, (iii) damped persistent SST, (iv) coupling to a slab mixed-layer ocean, and (v) daily SST from the coupled forecast. The full atmosphere–ocean coupling generates an interactive SST that results in the highest TISO predictability of about 30 days over Southeast Asia. The atmosphere-only model is capable of reaching this predictability if the ensemble mean daily SST forecast by the coupled model is used as the lower boundary condition, which suggests that, in principle, the so-called tier-one and tier-two systems have the same predictability for the boreal summer TISO. The atmosphere-only model driven by either smoothed or damped persistent SSTs, however, has the lowest predictability (20 days). The atmospheric model coupled to a slab mixed-layer ocean achieves a predictability of 25 days. The positive SST anomalies in the northern Indo–western Pacific Oceans trigger convective disturbances by moistening and warming up the atmospheric boundary layer. The seasonal mean easterly shear intensifies the anomalous convection by enhancing the surface convergence. An overturning meridional circulation driven by the off-equatorial anomalous convection suppresses the near-equatorial convection and enhances the northward flows, which further intensify the off-equatorial surface convergence and the TISO-related convection. Thus, the boreal summer mean easterly shear and the overturning meridional circulation in the northern Indo–western Pacific sector act as “amplifiers” for the SST feedback to the convection of the TISO.

An MJO Simulated by the NICAM at 14- and 7-km Resolutions

This study discloses detailed Madden‐Julian oscillation (MJO) characteristics in the two 30-day integrations of the global cloud-system-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) using the allseason real-time multivariate MJO index of Wheeler and Hendon. The model anomaly is derived by excluding the observed climatology because the simulation is sufficiently realistic. Results show that the MJO has a realistic evolution in amplitude pattern, geographical locations, eastward propagation, and baroclinic- and westwardtilted structures. In the central Indian Ocean, convection develops with the low-level easterly wind anomaly then matures where the low-level easterly and westerly anomalies meet. Anomalous moisture tilts slightly with height. In contrast, over the western Pacific, the convection grows with a low-level westerly anomaly. Moisture fluctuations, leading convection in eastward propagation, tilt clearly westward with height. The frictional moisture convergence mechanism operates to maintain the MJO. Such success can be attributed to the explicit representation of the interactions between convection and large-scale circulations. The simulated event, however, grows faster in phases 2 and 3, and peaks with 30% higher amplitude than that observed, although the 7-km version shows slight improvement. The fast-growth phases are induced by the fast-growing low-level convergence in the Indian Ocean and the strongly biased ITCZ in the west Pacific when the model undergoes a spinup. The simulated OLR has a substantial bias in the tropics. Possible solutions to the deficiencies are discussed.