Congwen Zhu

The 30-60 day intraseasonal oscillation over the western North Pacific Ocean and its impacts on summer flooding in China during 1998

[i] Eastern China experienced a series of severe floods during the summer of 1998. These floods are shown to be consistent with the propagation and activity of the 30 - 60 day intraseasonal oscillation (ISO) over the western North Pacific (WNP), where the monsoon trough and the subtropical anti-cyclone appear as an anti-clockwise propagation with the enhanced and suppressed convective anomalies in a 30-60 day period. The Meiyu circulation pattern in the lower atmosphere successively dominates the WNP, which results in southern China and the Yangtze River valley suffering much more rainfall and floods than normal. There are remarkable signals of 30-60-day ISO in convective anomalies over the Bay of Bengal and east of Philippine Sea. During the cycle ofthe 30-60 day ISO, these convective anomalies move towards the South China Sea (SCS), where they affect the convective activity of the SCS summer monsoon, therefore maintaining the cycle of 30-60 day ISO over the WNP.

Two types of El Niño-related Southern Oscillation and their different impacts on global land precipitation

The contrast between the eastern and central Pacific (EP- and CP-) El Niño is observed in the different responses of zonal and vertical circulation in the tropics. To measure the different responses of the atmospheric circulation to the two types of El Niño, an eastern and a central Pacific southern oscillation index (EP- and CP-SOI) are defined based on the air-sea coupled relationship between eddy sea level pressure and sea surface temperature. Analyses suggest that while the EP-SOI exhibits variability on an interannual (2–7-yr) time scale, decadal (10–15-yr) variations in the CP-SOI are more dominant; both are strongly coupled with their respective EP- and CP-El Niño patterns. Composite analysis suggests that, during EP-ENSO, the Walker circulation exhibits a dipole structure in the lower-level (850 hPa) and upper-level (200 hPa) velocity potential anomalies and exhibits a signal cell over the Pacific. In the case of CP-ENSO, however, the Walker circulation shows a tripole structure and exhibits double cells over the Pacific. In addition, the two types of ENSO events show opposite impacts on global land precipitation in the boreal winter and spring seasons. For example, seasonal precipitation across mainland China exhibits an opposite relationship with the EP- and CP-ENSO during winter and spring, but the rainfall over the lower reaches of the Yangtze River and South China shows an opposite relationship during the rest of the seasons. Therefore, the different relationships between rainfall and EP- and CP-ENSO should be carefully considered when predicting seasonal rainfall in the East Asian monsoon regions.

The natural oscillation of two types of ENSO events based on analyses of CMIP5 model control runs

The eastern- and central-Pacific El Niño-Southern Oscillation (EP- and CP-ENSO) have been found to be dominant in the tropical Pacific Ocean, and are characterized by interannual and decadal oscillation, respectively. In the present study, we defined the EP- and CP-ENSO modes by singular value decomposition (SVD) between SST and sea level pressure (SLP) anomalous fields. We evaluated the natural features of these two types of ENSO modes as simulated by the pre-industrial control runs of 20 models involved in phase five of the Coupled Model Intercomparison Project (CMIP5). The results suggested that all the models show good skill in simulating the SST and SLP anomaly dipolar structures for the EP-ENSO mode, but only 12 exhibit good performance in simulating the tripolar CP-ENSO modes. Wavelet analysis suggested that the ensemble principal components in these 12 models exhibit an interannual and multi-decadal oscillation related to the EP- and CP-ENSO, respectively. Since there are no changes in external forcing in the pre-industrial control runs, such a result implies that the decadal oscillation of CP-ENSO is possibly a result of natural climate variability rather than external forcing.

CMIP5 Projections of Two Types of El Niño and Their Related Tropical Precipitation in the Twenty-First Century

AbstractFuture projections of the eastern-Pacific (EP) and central-Pacific (CP) types of El Nino in the twenty-first century, as well as their associated tropical circulation and precipitation variability, are investigated using historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). As inferred from CMIP5 models that best capture both El Nino flavors, EP El Nino sea surface temperature (SST) variability will become weaker in the future climate, while no robust change of CP El Nino SST is found. Models also reach no consensus on the future change of relative frequency from CP to EP El Nino. However, there are robust changes in the tropical overturning circulation and precipitation associated with both types of El Nino. Under a warmer climate, magnitudes of precipitation anomalies during EP El Nino are projected to increase, presenting significant enhancement of the dry (wet) signal over the we...

A novel way to detect correlations on multi-time scales, with temporal evolution and for multi-variables.

In this paper, two new methods, Temporal evolution of Detrended Cross-Correlation Analysis (TDCCA) and Temporal evolution of Detrended Partial-Cross-Correlation Analysis (TDPCCA), are proposed by generalizing DCCA and DPCCA. Applying TDCCA/TDPCCA, it is possible to study correlations on multi-time scales and over different periods. To illustrate their properties, we used two climatological examples: i) Global Sea Level (GSL) versus North Atlantic Oscillation (NAO); and ii) Summer Rainfall over Yangtze River (SRYR) versus previous winter Pacific Decadal Oscillation (PDO). We find significant correlations between GSL and NAO on time scales of 60 to 140 years, but the correlations are non-significant between 1865–1875. As for SRYR and PDO, significant correlations are found on time scales of 30 to 35 years, but the correlations are more pronounced during the recent 30 years. By combining TDCCA/TDPCCA and DCCA/DPCCA, we proposed a new correlation-detection system, which compared to traditional methods, can objectively show how two time series are related (on which time scale, during which time period). These are important not only for diagnosis of complex system, but also for better designs of prediction models. Therefore, the new methods offer new opportunities for applications in natural sciences, such as ecology, economy, sociology and other research fields.

Two Types of Interannual Variability of South China Sea Summer Monsoon Onset Related to the SST Anomalies before and after 1993/94

AbstractAn advance in the timing of the onset of the South China Sea (SCS) summer monsoon (SCSSM) during the period 1980–2014 can be detected after 1993/94. In the present study, the interannual variability of the SCSSM onset is classified into two types for the periods before and after 1993/94, based on their different characteristics of vertical coupling between the upper- and lower-tropospheric circulation and the differences in their related sea surface temperature anomalies (SSTAs). On the interannual time scale, type-I SCSSM onset is characterized by anomalous low-level circulation over the northern SCS during 1980–93, whereas type-II SCSSM onset is associated with anomalies of upper-level circulation in the tropics during 1994–2014. The upper-tropospheric thermodynamic field and circulation structures over the SCS are distinct between the two types of SCSSM onset, and this investigation shows the importance of the role played by the spring SSTAs in the southern Indian Ocean (SIO) and that of ENSO e...

Thermocline Fluctuations in the Equatorial Pacific Related to the Two Types of El Niño Events

AbstractThe interannual fluctuations of the equatorial thermocline are usually associated with El Nino activity, but the linkage between the thermocline modes and El Nino is still under debate. In the present study, a mode function decomposition method is applied to the equatorial Pacific thermocline, and the results show that the first two dominant modes (M1 and M2) identify two distinct characteristics of the equatorial Pacific thermocline. The M1 reflects a basinwide zonally tilted thermocline related to the eastern Pacific (EP) El Nino, with shoaling (deepening) in the western (eastern) equatorial Pacific. The M2 represents the central Pacific (CP) El Nino, characterized by a V-shaped equatorial Pacific thermocline (i.e., deep in the central equatorial Pacific and shallow on both the western and eastern boundaries). Furthermore, both modes are stable and significant on the interannual time scale, and manifest as the major feature of the thermocline fluctuations associated with the two types of El Nino...

A possible precursor of the South China Sea summer monsoon onset: Effect of the South Asian High

In climatology, the South China Sea (SCS) summer monsoon (SCSSM) generally onsets in Pentad 28 (16–20 May). The eastward extension of the South Asian High into the SCS in Pentad 27 is a possible precursor to the SCSSM onset. In the upper troposphere, the South Asian High (SAH) warms the air by inducing the positive potential vorticity advection, which strengthens precipitation over the southern SCS. When local convection becomes strong enough to decrease the vertical temperature gradient in Pentads 28–29, the upper level warming center overlies the subcloud warm region over the SCS to satisfy the requirement of the angular momentum conservation. A cross-equatorial flow then forms, together with strong vertical easterly wind shear and deep monsoon convection over the SCS, marking the full establishment of the SCSSM. The abrupt change in the SAH in May could therefore be an early indicator of the onset of the SCSSM.

Sea Surface Temperature in the Subtropical Pacific Boosted the 2015 El Niño and Hindered the 2016 La Niña

AbstractAfter the quick decaying of the 2015 super El Nino, the predicted La Nina unexpectedly failed to materialize to the anticipated standard in 2016. Diagnostic analyses, as well as numerical experiments, showed that this ENSO evolution of the 2015 super El Nino and the hindered 2016 La Nina may be essentially caused by sea surface temperature anomalies (SSTAs) in the subtropical Pacific. The self-sustaining SSTAs in the subtropical Pacific tend to weaken the trade winds during boreal spring–summer, leading to anomalous westerlies along the equatorial region over a period of more than one season. Such long-lasting wind anomalies provide an essential requirement for ENSO formation, particularly before a positive Bjerknes feedback is thoroughly built up between the oceanic and atmospheric states. Besides the 2015 super El Nino and the hindered La Nina in 2016, there were several other El Nino and La Nina events that cannot be explained only by the oceanic heat content in the equatorial Pacific. However,...

On climate prediction: how much can we expect from climate memory?

Slowing variability in climate system is an important source of climate predictability. However, it is still challenging for current dynamical models to fully capture the variability as well as its impacts on future climate. In this study, instead of simulating the internal multi-scale oscillations in dynamical models, we discussed the effects of internal variability in terms of climate memory. By decomposing climate state x(t) at a certain time point t into memory part M(t) and non-memory part