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Hai Wang
,
Shang-Ping Xie
,
Yu Kosaka
,
Qinyu Liu
, and
Yan Du

Abstract

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20° and 40°N and weakens the EASM north of 20°N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north–south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean–atmosphere coupling.

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Zhen-Qiang Zhou
,
Renhe Zhang
, and
Shang-Ping Xie

Abstract

Year-to-year variability of surface air temperature (SAT) over central India is most pronounced in June. Climatologically over central India, SAT peaks in May, and the transition from the hot premonsoon to the cooler monsoon period takes place around 9 June, associated with the northeastward propagation of intraseasonal convective anomalies from the western equatorial Indian Ocean. Positive (negative) SAT anomalies during June correspond to a delayed (early) Indian summer monsoon onset and tend to occur during post–El Niño summers. On the interannual time scale, positive SAT anomalies of June over central India are associated with positive SST anomalies over both the equatorial eastern–central Pacific and Indian Oceans, representing El Niño effects in developing and decay years, respectively. Although El Niño peaks in winter, the correlations between winter El Niño and Indian SAT peak in the subsequent June, representing a post–El Niño summer capacitor effect associated with positive SST anomalies over the north Indian Ocean. These results have important implications for the prediction of Indian summer climate including both SAT and summer monsoon onset over central India.

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Xudong Wang
,
Shang-Ping Xie
,
Zhaoyong Guan
, and
Minyang Wang

Abstract

The summer intraseasonal oscillation (ISO) is characterized by a northward-moving rainband in the Indo–western Pacific warm pool region. The physical origin of the ISO is not fully understood, as it is masked by strong interaction of convection and circulation. This study examines intraseasonal to interannual variability during June–August over the Indo–western Pacific warm pool region. The results show that the tropical northwest Pacific anomalous anticyclone (NWP-AAC) is a fundamental mode on both intraseasonal and interannual time scales, destabilized by the monsoon mean state, specifically through barotropic energy conversion and convective feedback in the low-level confluence between the monsoon westerlies and easterly trade winds. On the interannual time scale, the NWP-AAC shows a biennial tendency, reversing phase from the summer of El Niño to the summer that follows; the AAC in post–El Niño summer is excited indirectly through sea surface temperature anomalies in the Indo–NWP. On the intraseasonal time scale, the column-integrated moisture advection causes the NWP-AAC-related convection to propagate northward. Our results provide a unifying view of multiscale Asian summer monsoon variability, with important implications for subseasonal to seasonal prediction.

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Hiroyuki Tomita
,
Shang-Ping Xie
,
Hiroki Tokinaga
, and
Yoshimi Kawai

Abstract

A unique set of observations on board research vessel (R/V) Mirai in April 2010 captured a striking cloud hole over a cold meander of the Kuroshio Extension (KE) east of Japan as corroborated by atmospheric soundings, ceilometer, shipboard radiation data, and satellite cloud images. Distinct differences were also observed between the warm meander farther to the north and warm water south of the KE. The atmosphere is highly unstable over the warm meander, promoting a well-mixed marine atmospheric boundary layer (MABL) and a layer of solid stratocumulus clouds capped by a strong inversion. Over the warm water south of the KE, MABL deepens and is decoupled from the ocean surface. Scattered cumulus clouds develop as captured by rapid variations in ceilometer-derived cloud base. The results show that the meandering KE front affects the entire MABL and the clouds. Such atmospheric response can potentially intensify the baroclinicity in the lower atmosphere.

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Tomomichi Ogata
,
Shang-Ping Xie
,
Jian Lan
, and
Xiaotong Zheng

Abstract

Interannual anomalies of sea surface temperature (SST), wind, and cloudiness in the southeastern tropical Indian Ocean (SE-TIO) show negative skewness. In this research, asymmetry between warm and cold episodes in the SE-TIO and the importance of ocean dynamics are investigated. A coupled model simulation and observations show an asymmetric relationship between SST and the thermocline depth in the SE-TIO where SST is more sensitive to an anomalous shoaling than to deepening of the thermocline. This asymmetric thermocline feedback on SST is a result of a deep mean thermocline. Sensitivity experiments with an ocean general circulation model (OGCM) show that a negative SST skewness arises in response to sinusoidal zonal wind variations that are symmetric between the westerly and easterly phases. Heat budget analysis with an OGCM hindcast also supports the importance of ocean dynamics for SST skewness off Sumatra and Java.

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Kelvin J. Richards
,
Shang-Ping Xie
, and
Toru Miyama

Abstract

The zonal and meridional asymmetries in the eastern tropical Pacific (the eastern equatorial cold tongue and the northern intertropical convergence zone) are key aspects of the region that are strongly influenced by ocean–atmosphere interactions. Here the authors investigate the impact of vertical mixing in the ocean on these asymmetries, employing a coupled ocean–atmosphere regional model. Results highlight the need to study the impact of processes such as vertical mixing in the context of the coupled system.

Changes to the vertical mixing in the ocean are found to produce large changes in the state of the system, which include changes to the surface properties of the ocean, the ocean currents, the surface wind field, and clouds and precipitation in the atmosphere. Much of the strength of the impact is through interactions between the ocean and atmosphere. Increasing ocean mixing has an opposite effect on the zonal and meridional asymmetries. The zonal asymmetry is increased (i.e., a colder eastern equatorial cold tongue and increased easterly winds), whereas the meridional asymmetry is decreased (a reduced north–south temperature difference and reduced southerlies), with the impact being enhanced by the Bjerknes and wind–evaporation–sea surface temperature feedbacks.

Water mass transformations are analyzed by consideration of the diapynic fluxes. Although the general character of the diapycnic transport remains relatively unchanged with a change in ocean mixing, there are changes to the magnitude and location of the transport in density space. Oceanic vertical mixing impacts the balance of terms contributing to the heating of the ocean surface mixed layer. With reduced mixing the advection of heat plays an increased role in areas such as the far eastern tropical Pacific and under the intertropical convergence zone.

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Lei Wang
,
Yuqing Wang
,
Axel Lauer
, and
Shang-Ping Xie

Abstract

The seasonal cycle of marine boundary layer (MBL) clouds over the eastern Pacific Ocean is studied with the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM). The results show that the model is capable of simulating not only the overall seasonal cycle but also the spatial distribution, cloud regime transition, and vertical structure of MBL clouds over the eastern Pacific. Although the modeled MBL cloud layer is generally too high in altitude over the open ocean when compared with available satellite observations, the model simulated well the westward deepening and decoupling of the MBL, the rise in cloud base and cloud top of the low cloud decks off the Peru and California coasts, and the cloud regime transition from stratocumulus near the coast to trade cumulus farther to the west in both the southeast and northeast Pacific. In particular, the model reproduced major features of the seasonal variations in stratocumulus decks off the Peru and California coasts, including cloud amount, surface latent heat flux, subcloud-layer mixing, and the degree of MBL decoupling. In both observations and the model simulation, in the season with small low-level cloudiness, surface latent heat flux is large and the cloud base is high. This coincides with weak subcloud-layer mixing and strong entrainment at cloud top, characterized by a high degree of MBL decoupling, while the opposite is true for the season with large low-level cloudiness. This seasonal cycle in low-cloud properties resembles the downstream stratocumulus-to-cumulus transition of marine low clouds and can be explained by the “deepening–decoupling” mechanism proposed in previous studies. It is found that the seasonal variations of low-level clouds off the Peru coast are mainly caused by a large seasonal variability in sea surface temperature, whereas those off the California coast are largely attributed to the seasonal cycle in lower-tropospheric temperature.

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Youichi Tanimoto
,
Tomohisa Kanenari
,
Hiroki Tokinaga
, and
Shang-Ping Xie

Abstract

Atmospheric effects of sea surface temperature (SST) fronts along the Kuroshio and Kuroshio Extension (K-KE) are investigated by examining spatial characteristics of the climatological sea level pressure (SLP), surface winds and surface heat flux (Q) fields based on an in situ observation dataset. A hydrostatic effect of the SST front is observed during the northwesterly monsoon characterized by a westward-extending low-SLP wedge (trough) slightly south of the peak in Q along the K-KE. Ageostrophic surface westerlies crossing SLP isobars toward a trough center are found north of the low-SLP wedge apparently because of the eastward acceleration by the vertical mixing effect. This feature of the ageostrophic winds is less pronounced south of it, a north–south asymmetry arising as vertical mixing accelerates (decelerates) eastward ageostrophic winds north (south) of it. While the SLP trough near the SST front is found nearly year-round except for summer, its meridional location seasonally migrates probably due to the change in the mean surface flow.

Regional atmospheric model experiments are conducted to examine the effect of the SST front on the overlying atmosphere. The observed features in winter are adequately simulated when high-resolution SST is prescribed as the boundary condition. The strong Q along the K-KE and resultant SLP trough and ageostrophic surface winds are absent when the SST boundary condition is smoothed in space to weaken the SST front. These results illustrate that the cross-frontal change in Q is of great importance to leave the SST imprint on the overlying atmosphere via hydrostatic and vertical mixing adjustments.

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Wei Mei
,
Shang-Ping Xie
,
Ming Zhao
, and
Yuqing Wang

Abstract

Forced interannual-to-decadal variability of annual tropical cyclone (TC) track density in the western North Pacific between 1979 and 2008 is studied using TC tracks from observations and simulations by a 25-km-resolution version of the GFDL High-Resolution Atmospheric Model (HiRAM) that is forced by observed sea surface temperatures (SSTs). Two modes dominate the decadal variability: a nearly basinwide mode, and a dipole mode between the subtropics and lower latitudes. The former mode links to variations in TC number and is forced by SST variations over the off-equatorial tropical central North Pacific, whereas the latter might be associated with the Atlantic multidecadal oscillation. The interannual variability is also controlled by two modes: a basinwide mode driven by SST anomalies of opposite signs located in the tropical central Pacific and eastern Indian Ocean, and a southeast–northwest dipole mode connected to the conventional eastern Pacific ENSO. The seasonal evolution of the ENSO effect on TC activity is further explored via a joint empirical orthogonal function analysis using TC track density of consecutive seasons, and the analysis reveals that two types of ENSO are at work. Internal variability in TC track density is then examined using ensemble simulations from both HiRAM and a regional atmospheric model. It exhibits prominent spatial and seasonal patterns, and it is particularly strong in the South China Sea and along the coast of East Asia. This makes an accurate prediction and projection of TC landfall extremely challenging in these regions. In contrast, basin-integrated metrics (e.g., total TC counts and TC days) are more predictable.

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Yali Yang
,
Shang-Ping Xie
,
Yan Du
, and
Hiroki Tokinaga

Abstract

The correlation between sea surface temperature (SST) and El Niño–Southern Oscillation (ENSO) persists into post-ENSO September over the South China Sea (SCS), the longest correlation in the World Ocean. Slow modulations of this correlation are analyzed by using the International Comprehensive Ocean–Atmosphere Dataset (ICOADS). ENSO’s influence on SCS SST has experienced significant interdecadal changes over the past 138 years (1870–2007), with a double-peak structure correlation after the 1960s compared to a single-peak before the 1940s. According to the ENSO correlation character, the analysis period is divided into four epochs. In epoch 3, 1960–83, the SST warming and enhanced precipitation over the southeastern tropical Indian Ocean, rather than the Indian Ocean basinwide warming, induce easterly wind anomalies and warm up the SCS in the summer following El Niño. Besides the Indian Ocean effect, during epochs 2 (1930–40) and 4 (1984–2007), the Pacific–Japan (PJ) pattern of atmospheric circulation anomalies helps sustain the SCS SST warming through summer (June–August) with easterly wind anomalies. The associated increase in shortwave radiation and decrease in upward latent heat flux cause the SCS SST warming to persist into the summer. Meanwhile, the rainfall response around the SCS to ENSO shows interdecadal variability, with stronger variability after the 1980s. The results suggest that both the remote forcing from the tropical Indian Ocean and the PJ pattern are important for the ENSO teleconnection to the SCS and its interdecadal modulations.

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