Influence of PDO on South Asian summer monsoon and monsoon–ENSO relation

This study has investigated the possible relation between the Indian summer monsoon and the Pacific Decadal Oscillation (PDO) observed in the sea surface temperature (SST) of the North Pacific Ocean. Using long records of observations and coupled model (NCAR CCSM4) simulation, this study has found that the warm (cold) phase of the PDO is associated with deficit (excess) rainfall over India. The PDO extends its influence to the tropical Pacific and modifies the relation between the monsoon rainfall and El Niño-Southern Oscillation (ENSO). During the warm PDO period, the impact of El Niño (La Niña) on the monsoon rainfall is enhanced (reduced). A hypothesis put forward for the mechanism by which PDO affects the monsoon starts with the seasonal footprinting of SST from the North Pacific to the subtropical Pacific. This condition affects the trade winds, and either strengthens or weakens the Walker circulation over the Pacific and Indian Oceans depending on the phase of the PDO. The associated Hadley circulation in the monsoon region determines the impact of PDO on the monsoon rainfall. We suggest that knowing the phase of PDO may lead to better long-term prediction of the seasonal monsoon rainfall and the impact of ENSO on monsoon.     

Droughts near the northern fringe of the East Asian summer monsoon in China during 1470–2003

Historical annual dry–wet index for 1470–2003 combined with instrumental precipitation since 1951 were used to identify extremely dry years and events near the northern fringe of the East Asian summer monsoon in China—the Great Bend of the Yellow River (GBYR) region. In total, 49 drought years, of which 26 were severe, were identified. Composites of the dry–wet index under the drought years show an opposite wet pattern over the Southeast China. The longest drought event lasted for 6?years (1528–1533), the second longest one 4?years (1637–1640). The most severe 2-year-long drought occurred in 1928–1929, and the two driest single years were 1900 and 1965. These persistent and extreme drought events caused severe famines and huge losses of human lives. Wavelet transform applied to the dry–wet index indicates that the severe drought years are nested in several significant dry–wet variations across multiple timescales, i.e., the 65–85?year timescale during 1600– 1800, 40–55?year timescale before 1640 and 20–35?year timescale mainly from 1550 to 1640. These timescales of dry–wet variations are discussed in relation to those forcing such as cycles of solar radiation, oscillation in the thermohaline circulation and the Pacific Decadal Oscillation (PDO). Comparing 850?hPa winds in Asia in extremely dry and wet years, it was concluded that dry–wet variability in the GBYR region strongly depends upon whether the southerly monsoon flow can reach northern China.     

Intraseasonal variation of the East Asian summer monsoon in La Ni?a years

合成分析了La Ni?a年东亚夏季风和东亚夏季降水的季节内变化。结果表明,La Ni?a年夏季暖池对流偏强,西太平洋出现异常气旋,西太平洋副高偏向东北。这种异常型随季节进程有明显变化,7月异常达到最大。La Ni?a年东亚降水呈纬向型分布,中国东部降水偏少,西太平洋海面降水偏多。与此相比,El Ni?o年降水呈经向型分布,热带降水偏少,副热带和中纬度降水偏多。因此,不能简单将La Ni?a的影响认为是El Ni?o的反对称。     

Impact of ice sheet induced North Atlantic oscillation on East Asian summer monsoon during an interglacial 500,000?years ago

Marine Isotope Stage (MIS) 13, an interglacial about 500,000?years ago, is unique due to an exceptionally strong East Asia summer monsoon (EASM) occurring in a relatively cool climate with low greenhouse gas concentrations (GHG). This paper attempts to find one of the possible mechanisms for this seeming paradox. Simulations with an Earth System model LOVECLIM show that the presence of ice sheets over North America and Eurasia during MIS-13 induces a positive phase of the winter North Atlantic Oscillation (NAO) like feature. The ocean having a longer memory than the atmosphere, the oceanic anomalies associated with NAO persists until summer. The signals of summer NAO are transmitted to East Asia to reinforce the monsoon there through the stationary waves excited at the Asian Jet entrance. The geopotential height shows clearly a mid-latitude wave train with positive anomalies over the eastern Mediterranean/Caspian Sea and the Okhotsk Sea and a negative anomaly over Lake Baikal. This reinforces the effect of the high-latitude wave train induced independently by the Eurasian ice sheet topography as shown in previous study. These features reinforce the Meiyu front and enhance the precipitation over East Asia. The results obtained from LOVECLIM are further confirmed by an atmospheric general circulation model, ARPEGE.     

Interdecadal variation of the West African summer monsoon during 1979–2010 and associated variability

This paper addresses the interdecadal variation of the West African summer monsoon (WASM) along with its background of atmospheric circulation and possible physical mechanism over the past 32?years (1979–2010). It is indicated that the WASM starts to strengthen from 1998 as the rainfall begins to increase over western West Africa on the whole, which shows a new interdecadal variation. In this interdecadal variation, the strengthened ascending motion corresponding to enhanced divergence (convergence) movement on the upper (lower) troposphere is prone to develop the local circulation of the monsoon. Moreover, the strengthened southwestern (eastern) wind on the lower (upper) level leads to more moisture from the Atlantic and the Gulf of Guinea transported to the West African continent. In addition, the summer subtropical high over the north Atlantic and western West Africa is strong and northward, and the tropical east wind is also strong. Statistically, the weaker (stronger) the spring North Atlantic Oscillation (NAO) is, the stronger (weaker) the tropical easterly is, and then the WASM is also stronger. But the effect of the NAO on the decadal variation of the WASM is not so significant from the north Atlantic anomaly sensitivity simulation with a single model. This is also an indication that the relationship between the WASM and NAO is complicated in an interdecadal time scale and is needed further study. In terms of sea surface temperature (SST) variation, the tendency is toward warming in the subtropical north Pacific, the south Pacific and north Atlantic. Numerical simulation experiments and data analysis show that the SST variation in the north Pacific plays an important role in the latest interdecadal strengthening of the WASM during the past 32?years, while the influences of the south Pacific and the north Atlantic SST anomalies are not so significant to the associated atmospheric circulation changes.     

Air–sea interaction and formation of the Asian summer monsoon onset vortex over the Bay of Bengal

In spring over the southern Bay of Bengal (BOB), a vortex commonly develops, followed by the Asian summer monsoon onset. An analysis of relevant data and a case study reveals that the BOB monsoon onset vortex is formed as a consequence of air–sea interaction over BOB, which is modulated by Tibetan Plateau forcing and the land–sea thermal contrast over the South Asian area during the spring season. Tibetan Plateau forcing in spring generates a prevailing cold northwesterly over India in the lower troposphere. Strong surface sensible heating is then released, forming a prominent surface cyclone with a strong southwesterly along the coastal ocean in northwestern BOB. This southwesterly induces a local offshore current and upwelling, resulting in cold sea surface temperatures (SSTs). The southwesterly, together with the near-equatorial westerly, also results in a surface anticyclone with descending air over most of BOB and a cyclone with ascending air over the southern part of BOB. In the eastern part of central BOB, where sky is clear, surface wind is weak, and ocean mixed layer is shallow, intense solar radiation and low energy loss due to weak surface latent and sensible heat fluxes act onto a thin ocean layer, resulting in the development of a unique BOB warm pool in spring. Near the surface, water vapor is transferred from northern BOB and other regions to southeastern BOB, where surface sensible heating is relatively high. The atmospheric available potential energy is generated and converted to kinetic energy, thereby resulting in vortex formation. The vortex then intensifies and moves northward, where SST is higher and surface sensible heating is stronger. Meanwhile, the zonal-mean kinetic energy is converted to eddy kinetic energy in the area east of the vortex, and the vortex turns eastward. Eventually, southwesterly sweeps over eastern BOB and merges with the subtropical westerly, leading to the onset of the Asian summer monsoon.     

Land–sea heating contrast in an idealized Asian summer monsoon

Mechanisms determining the tropospheric temperature gradient that is related to the intensity of the Asian summer monsoon are examined in an intermediate atmospheric model coupled with a mixed-layer ocean and a simple land surface model with an idealized Afro–Eurasian continent and no physical topography. These include processes involving in the influence of the Eurasian continent, thermal effects of the Tibetan Plateau and effects of sea surface temperature. The mechanical effect on the large-scale flow induced by the Plateau is not included in this study. The idealized land–sea geometry without topography induces a positive meridional tropospheric temperature gradient thus a weak Asian summer monsoon circulation. Higher prescribed heating and weaker surface albedo over Eurasia and the Tibetan Plateau, which mimic effects of different land surface processes and the thermal effect of the uplift of the Tibetan Plateau, strengthens the meridional temperature gradient, and so as cold tropical SST anomalies. The strengthened meridional temperature gradient enhances the Asian summer monsoon circulation and favors the strong convection. The corresponding monsoon rainbelt extends northward and northeastward and creates variations of the monsoon rainfall anomalies in different subregions. The surface albedo over the Tibetan Plateau has a relatively weak inverse relation with the intensity of the Asian summer monsoon. The longitudinal gradient of ENSO-like SST anomalies induces a more complicated pattern of the tropospheric temperature anomalies. First, the positive (negative) longitudinal gradient induced by the El Niño (La Niña)-like SST anomalies weakens (strengthens) the Walker circulation and the circulation between South Asia and northern Africa and therefore the intensity of the Asian summer monsoon, while the corresponding monsoon rainbelt extends northward (southward). The El Niño (La Niña)-like SST anomalies also induces colder (warmer) tropospheric temperature over Eurasia and warmer (colder) tropospheric temperature over the Indian Ocean. The associated negative (positive) meridional gradient of the tropospheric temperature anomalies is consistent with the existence of the weak (strong) Asian summer monsoon.     

Role of TBO and ENSO scale ocean–atmosphere interaction in the Indo-Pacific region on Asian summer monsoon variability

Interannual variability of the Indian summer monsoon rainfall has two dominant periodicities, one on the quasi-biennial (2–3 year) time scale corresponding to tropospheric biennial oscillation (TBO) and the other on low frequency (3–7 year) corresponding to El Niño Southern Oscillation (ENSO). In the present study, the spatial and temporal patterns of various atmospheric and oceanic parameters associated with the Indian summer monsoon on the above two periodicities were investigated using NCEP/NCAR reanalysis data sets for the period 1950–2005. Influences of Indian and Pacific Ocean SSTs on the monsoon season rainfall are different for both of the time scales. Seasonal evolution and movement of SST and Walker circulation are also different. SST and velocity potential anomalies are southeast propagating on the TBO scale, while they are stationary on the ENSO scale. Latent heat flux and relative humidity anomalies over the Indian Ocean and local Hadley circulation between the Indian monsoon region and adjacent oceans have interannual variability only on the TBO time scale. Local processes over the Indian Ocean determine the Indian Ocean SST in biennial periodicity, while the effect of equatorial east Pacific SST is significant in the ENSO periodicity. TBO scale variability is dependent on the local factors of the Indian Ocean and the Indian summer monsoon, while the ENSO scale processes are remotely controlled by the Pacific Ocean.     

An ocean–atmosphere interaction mechanism for the active break cycle of the Asian summer monsoon

A typical active–break cycle of the Asian summer monsoon is taken as beginning with maximum SST (pentad 0) over the north Bay of Bengal when the oceans to its west and east from longitude 40°–160°E, and between latitudes 10° and 25°N (area A) also has maximum SST. During this pentad the recently found “Cold Pool” of the Bay of Bengal (between latitudes 3°N and 10°N) has its minimum SST. An area of convection takes genesis over the Bay of Bengal immediately after pentad 0 in the zone of large SST gradient north of the Cold Pool and it pulls the monsoon Low Level Jetstream (LLJ) through peninsular India. Convection and the LLJ westerlies then spread to the western Pacific Ocean during pentads 1–4 taken as the active phase of the monsoon during which convection and LLJ have grown in a positive feed back process. The cyclonic vorticity to the north of the LLJ axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating effect of atmospheric stabilization by each short spell of deep convection. By the end of pentad 4 the SST over area A has cooled and the convection weakens there, when the LLJ turns clockwise over the Arabian Sea and flows close to the equator in the Indian ocean. A band of convection develops at pentad 5 between the equator and latitude 10°S over the Indian ocean and it is nourished by the cyclonic vorticity of the LLJ now near the equator and the moisture supply through it. This is taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active–break cycle of 40 day duration. With reduced wind and convection over the area A during the break phase, solar radiation and light winds make the SST there warm rapidly and a new active–break cycle begins. SST, convection, LLJ and the net heat flux at the ocean surface have important roles in this new way of looking at the active–break cycle as a coupled ocean–atmosphere phenomenon.     

Discrepancies in boreal summer monsoon rainfall between GPCP and CMAP products during 1979–2014

GPCP和CMAP资料均广泛应用于降水变率的研究中,然而两者在描述北半球季风区降水年际变率上的差异较少受到关注。本文研究了1979–2014年北半球5大季风区降水因资料选择造成的研究结果差异。研究发现:在气候态上,西北太平洋季风区夏季降水的空间分布型和降水量差异较大;在年际变率上,北非季风区和印度季风区夏季降水存在较大差异。两者的差异在90年代后明显减小。一般来说,两者的算数平均可以减少资料不确定性带来的影响。     

Impacts of Coastal SST Variability on the East Asian Summer Monsoon

The impacts of the seasonal and interannual SST variability in the East Asia coastal regions （EACRSST） on the East Asian summer monsoon （EASM） have been examined using a regional climate model （PδRCM9） in this paper. The simulation results show that the correlation between the EACRSST and the EASM is strengthened after the mid-1970s and also the variability of the EACRSST forcing becomes much more important to the EASM interannual variability after the mid-1970s. The impacts of the EACRSST on the summer precipitation over each sub-region in the EASM region become weak gradually from south to north, and the temporal evolution features of the summer precipitation differences over North and Northeast China agree well with those of the index of EASM （IEASM） differences.
The mechanism analyses show that different EACRSST forcings result in the differences of sensible and latent heat flux exchanges at the air-sea interface, which alter the heating rate of the atmosphere. The heating rate differences induce low level air temperature differences over East Asia, resulting in the differences of the land-sea thermal contrast （LSTC） which lead to 850 hPa geopotential height changes. When the 850 hPa geopotential height increases over the East Asian continent and decreases over the coast of East China and the adjacent oceans during the weakening period of weakens consequently. On the contrary, the EASM enhances during the strengthening period of the LSTC.     

Estimating the trends and variability of atmospheric action centers over the Asian-Pacific region in summer in 1950–1979 and 1980–2012

The trend significance and the residual variability of integral atmospheric characteristics in the atmospheric action centers in the Asian-Pacific region in summer in 1950-1979 and 1980-2012 are computed. Basic differences are revealed between trends in circulation and residual variability in the atmo spheric action centers in the surface pressure field and in the field of geopotential H ₅₀₀ for these time periods. Increase in significant trends for the whole period and decrease in residual variability were found in the area of the Asian low in 1980-2012. A significant trend was observed in June and September in the area of the Hawaiian high. The summer Far Eastern low has intensified in recent years. The Okhotsk high strengthened in May and weakened in June, August, and September in the 2000s.     

Retrieval the statistical–dynamical model of western Pacific subtropical high ridge line index and key members of Asian summer monsoon system

The western Pacific subtropical high (WPSH) is closely correlated with the East Asian climate. To date, the underlying mechanisms and sustaining factors have not been positively elucidated. Based on the concept of dynamical system model reconstruction, this paper presents a nonlinear statistical–dynamical model of the subtropical high ridge line (SHRL) in concurrence with four summer monsoon factors. SHRL variations from 1990 to 2011 are subdivided into three categories, while parameter differences relating to three differing models are examined. Dynamical characteristics of SHRL are analyzed and an aberrance mechanism subsequently developed. Modeling suggests that different parameters may lead to significant variance pertaining to monsoon variables corresponding with numerous WPSH activities. Dynamical system bifurcation and mutation indicates that the South China Sea monsoon trough is a significant factor with respect to the occurrence and maintenance of the ‘double-ridge’ phenomenon. Moreover, the occurrence of the Mascarene cold high is predicted to cause an abnormal northward location of WPSH, resulting in the “empty plum” phenomenon.     

Intraseasonal variability of western North Pacific subtropical high based on the El Ni?o influence and its relationship with East Asian summer monsoon

The boreal summer season could be divided into two periods in terms of the variability of western North Pacific subtropical high (WNPSH) based on the El Ni?o influence. The correlation analysis indicates that the WNPSH in the period of pentad 32?C37 (June 5 to July 4, first period) is not affected by El Ni?o, while that in the period of pentad 40?C45 (July 15 to August 13, second period) is strongly affected by sea surface temperature in the equatorial eastern Pacific in the previous winter. The different response of low-level circulation over the western North Pacific (WNP) to the El Ni?o forcing between two periods seems to be due to the difference of mean climatological fields over the WNP and the East Asian regions. The WNPSH in the first period is closely connected to the variability of North Pacific subtropical High. In the second period, on the other hand, the WNPSH variability is dominantly controlled by the convective activity over the WNPSH region and it is related with the El Ni?o forcing. The composite analysis on the relationship between the WNPSH and the East Asian summer monsoon exhibits distinct contrasts between two periods. In the first period, the East Asian stationary front exists all the time regardless of the strength of the WNPSH. On the other hand, in the second period the East Asian stationary front appears only when the WNPSH is strong, while there is no obvious East Asian frontal zone when it is weak.     

The asymmetric effects of El Niño and La Niña on the East Asian winter monsoon and their simulation by CMIP5 atmospheric models

El Niño–Southern Oscillation (ENSO) events significantly affect the year-by-year variations of the East Asian winter monsoon (EAWM). However, the effect of La Niña events on the EAWM is not a mirror image of that of El Niño events. Although the EAWM becomes generally weaker during El Niño events and stronger during La Niña winters, the enhanced precipitation over the southeastern China and warmer surface air temperature along the East Asian coastline during El Niño years are more significant. These asymmetric effects are caused by the asymmetric longitudinal positions of the western North Pacific (WNP) anticyclone during El Niño events and the WNP cyclone during La Niña events; specifically, the center of the WNP cyclone during La Niña events is westward-shifted relative to its El Niño counterpart. This central-position shift results from the longitudinal shift of remote El Niño and La Niña anomalous heating, and asymmetry in the amplitude of local sea surface temperature anomalies over the WNP. However, such asymmetric effects of ENSO on the EAWM are barely reproduced by the atmospheric models of Phase 5 of the Coupled Model Intercomparison Project (CMIP5), although the spatial patterns of anomalous circulations are reasonably reproduced. The major limitation of the CMIP5 models is an overestimation of the anomalous WNP anticyclone/cyclone, which leads to stronger EAWM rainfall responses. The overestimated latent heat flux anomalies near the South China Sea and the northern WNP might be a key factor behind the overestimated anomalous circulations.