The interannual relationship between the East Asian summer monsoon and the tropical Pacific SSTs is studied using rainfall data in the Yangtze River Valley and the NCEP reanalysis for 1951‐96. The datasets are also partitioned into two periods, 1951‐77 and 1978‐96, to study the interdecadal variations of this relationship. A wet summer monsoon is preceded by a warm equatorial eastern Pacific in the previous winter and followed by a cold equatorial eastern Pacific in the following fall. This relationship involves primarily the rainfall during the pre-Mei-yu/Mei-yu season (May‐June) but not the post-Mei-yu season (July‐August). In a wet monsoon year, the western North Pacific subtropical ridge is stronger as a result of positive feedback that involves the anomalous Hadley and Walker circulations, an atmospheric Rossby wave response to the western Pacific complementary cooling, and the evaporation‐wind feedback. This ridge extends farther to the west from the previous winter to the following fall, resulting in an 850-hPa anomalous anticyclone near the southeast coast of China. This anticyclone 1) blocks the pre-Mei-yu and Mei-yu fronts from moving southward thereby extending the time that the fronts produce stationary rainfall; 2) enhances the pressure gradient to its northwest resulting in a more intense front; and 3) induces anomalous warming of the South China Sea surface through increased downwelling, which leads to a higher moisture supply to the rain area. A positive feedback from the strong monsoon rainfall also appears to occur, leading to an intensified anomalous anticyclone near the monsoon region. This SST‐subtropical ridge‐monsoon rainfall relationship is observed in both the interannual timescale within each interdecadal period and in the interdecadal scale. The SST anomalies (SSTAs) change sign in northern spring and resemble a tropospheric biennial oscillation (TBO) pattern during the first interdecadal period (1951‐77). In the second interdecadal period (1978‐96) the sign change occurs in northern fall and the TBO pattern in the equatorial eastern Pacific SST is replaced by longer timescales. This interdecadal variation of the monsoon‐SST relationship results from the interdecadal change of the background state of the coupled ocean‐atmosphere system. This difference gives rise to the different degrees of importance of the feedback from the anomalous circulations near the monsoon region to the equatorial eastern Pacific. In a wet monsoon year, the anomalous easterly winds south of the monsoon-enhanced anomalous anticyclone start to propagate slowly eastward toward the eastern Pacific in May and June, apparently as a result of an atmosphere‐ocean coupled wave motion. These anomalous easterlies carry with them a cooling effect on the ocean surface. In 1951‐77 this effect is insignificant as the equatorial eastern Pacific SSTAs, already change from warm to cold in northern spring, probably as a result of negative feedback processes discussed in ENSO mechanisms. In 1978‐96 the equatorial eastern Pacific has a warmer mean SST. A stronger positive feedback between SSTA and the Walker circulation during a warm phase tends to keep the SSTA warm until northern fall, when the eastward-propagating anomalous easterly winds reach the eastern Pacific and reverse the SSTA.

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Interannual and Interdecadal Variations of the East Asian Summer Monsoon and Tropical Pacific SSTs. Part I: Roles of the Subtropical Ridge

El Nino events are characterized by surface warming of the tropical Pacific Ocean and weakening of equatorial trade winds that occur every few years Such conditions are accompanied by changes in atmospheric and oceanic circulation, affecting global climate, marine and terrestrial ecosystems, fisheries and human activities The alternation of warm El Nino and cold La Nina conditions, referred to as the El Nino–Southern Oscillation (ENSO), represents the strongest year-to-year fluctuation of the global climate system Here we provide a synopsis of our current understanding of the spatio-temporal complexity of this important climate mode and its influence on the Earth system

/pdf/el-nino-southern-oscillation-complexity-5alyl3svf7.pdf

El Niño–Southern Oscillation complexity

In the late 1970s, the ENSO cycle exhibited frequency change. The oscillation period increased from 2–4 yr (high frequency) during 1962–75 to 4–6 yr (low frequency) during 1980–93. Observations suggest that this frequency change was accompanied by a significant change in the structure of the coupled ENSO mode. In comparison with the high-frequency regime, the structure of the coupled mode in the low-frequency regime shows three distinctive features during the warm phase of ENSO: the eastward shift of the westerly anomalies, the meridional expansion of the westerly anomalies, and the weaker intensity of the easterly anomalies in the eastern Pacific. To test the robustness of the relationship between the oscillation period and the structure of the coupled mode, the authors designed empirical atmospheric models based on observations and coupled them with the ocean model of Zebiak and Cane. Numerical experiments demonstrate that the ENSO period is sensitive to changes in the wind anomaly pattern in a...

Interdecadal Change of the Structure of the ENSO Mode and Its Impact on the ENSO Frequency

Four fundamental differences of air–sea interactions between the tropical Pacific and Indian Oceans are identified based on observational analyses and physical reasoning. The first difference is represented by the strong contrast of a zonal cloud–SST phase relationship between the warm and cool oceans. The in-phase cloud–SST relationship in the warm oceans leads to a strong negative feedback, while a significant phase difference in the cold tongue leads to a much weaker thermodynamic damping. The second difference arises from the reversal of the basic-state zonal wind and the tilting of the ocean thermocline, which leads to distinctive effects of ocean waves. The third difference lies in the existence of the Asian monsoon and its interaction with the adjacent oceans. The fourth difference is that the southeast Indian Ocean is a region where a positive atmosphere–ocean thermodynamic feedback exists in boreal summer. A conceptual coupled atmosphere–ocean model was constructed aimed to understand th...

A Theory for the Indian Ocean Dipole–Zonal Mode*

Canonical El Nino has a warming center in the eastern Pacific (EP), but in recent decades, El Nino warming center tends to occur more frequently in the central Pacific (CP). The definitions and names of this new type of El Nino, however, have been notoriously diverse, which makes it difficult to understand why the warming center shifts. Here, we show that the new type of El Nino events is characterized by: 1) the maximum warming standing and persisting in the CP and 2) the warming extending to the EP only briefly during its peak phase. For this reason, we refer to it as standing CP warming (CPW). Global warming has been blamed for the westward shift of maximum warming as well as more frequent occurrence of CPW. However, we find that since the late 1990s the standing CPW becomes a dominant mode in the Pacific; meanwhile, the epochal mean trade winds have strengthened and the equatorial thermocline slope has increased, contrary to the global warming-induced weakening trades and flattening thermocline. We propose that the recent predominance of standing CPW arises from a dramatic decadal change characterized by a grand La Nina-like background pattern and strong divergence in the CP atmospheric boundary layer. After the late 1990s, the anomalous mean CP wind divergence tends to weaken the anomalous convection and shift it westward from the underlying SST warming due to the suppressed low-level convergence feedback. This leads to a westward shift of anomalous westerly response and thus a zonally in-phase SST tendency, preventing eastward propagation of the SST anomaly. We anticipate more CPW events will occur in the coming decade provided the grand La Nina-like background state persists.

http://iprc.soest.hawaii.edu/users/li/paper/2012.xiang.wang.li.pdf

A new paradigm for the predominance of standing Central Pacific Warming after the late 1990s

A stationary SST mode is proposed to understand the physical mechanisms responsible for the phase transition of the El Nino-Southern Oscillation. This stationary SST mode differs from the original delayed oscillator mode and the slow SST mode in the sense that it considers both balanced and unbalanced thermocline depth variations and does not take into account the zonal propagation of SST. Within this mode, the Walker circulation acts as a positive feedback mechanism to amplify and maintain an existing interannual SST anomaly, whereas the Hadley circulation acts as a negative feedback mechanism that dismisses the original anomaly and causes the phase shift from a warm (cold) to a cold (warm) episode. The key to the cause of interannual oscillations in the stationary SST mode lies in the zonal-mean thermocline depth variation that is not in equilibrium with the winds. Because of the nonequilibrium, this part of the thermocline depth anomaly tends to have a phase lag with the wind (or SST) anomaly and therefore holds a key for the interannual oscillation. The zonally asymmetric part of the thermocline depth anomaly, on the other hand, is always in Sverdrup balance with the winds. Such a phase relationship agrees well with observations and with GCM simulations. The stationary SST mode strongly depends on the basin width, on the air-sea coupling strength, and on the seasonal-cycle basic state. For a reasonable parameter regime, it depicts an interannual oscillation with a period of 2-7 years. This stationary SST mode is also season dependent: it has a maximum growth rate during the later part of the year and a negative growth rate during the northern spring, which may explain the occurrence of the mature phases of the El Nino in the northern winter and a rapid drop of the lagged correlation of the Southern Oscillation index in the boreal spring.

http://www.soest.hawaii.edu/~li/www/li1997jas_phase.pdf

Tianming Li

Papers

Phase Transition of the El Niño–Southern Oscillation: A Stationary SST Mode