A unique dataset of 53-year (1961–2013) rainfall measurements from 104 stations uniformly distributed in the Northeast China, combined with the observation-based NCEP/NCAR atmospheric reanalysis, is ...used to analyze the precipitation anomalies in Northeast China during late boreal summer (July–August) and their relationship with the anomalous moisture transport associated with the fluctuations of the East Asian Summer Monsoon (EASM) circulation. Based on this analysis, a new EASM influence index (
I
EASM
) is proposed to quantify the EASM effects on the Northeast China summer precipitation. The relationship between the I
EASM
variations and patterns of the anomalous regional atmospheric circulation is demonstrated. The characteristics of several precursors that lead to the major fluctuations of the
I
EASM
index are also explored. The results show that the EASM influence index is closely linked to the anomalous rainfall in Northeast China and can be used as a major factor to measure the physical processes that affect the regional dry and wet conditions. The
I
EASM
index responds to the large-scale anomalies of the atmospheric circulation sensitively. Specifically, the high
I
EASM
values are associated with the intensified Mongolia cyclone, blocking developing near the Ural Mountains and a northwestward shift of subtropical high over the western Pacific. The low
I
EASM
values are associated with a reversed pattern of these features. The
I
EASM
anomalous fluctuation has some precursors. A major high (low) index during the summer is likely preceded with the pattern of the sea surface temperature anomalies of an El Niño (La Niña) event in the Pacific from the previous early fall to early winter.
In this work, the evolution and prediction of the persistent and remarkable warm sea surface temperature anomaly (SSTA) in the northeastern Pacific during October 2013–June 2016 are examined. Based ...on experiments with an atmospheric model, the possible contribution of SSTAs in different ocean basins to the atmospheric circulation anomalies is identified. Further, through verifying the real-time forecasts, current capabilities in predicting such an extreme warm event with a state-of-the-art coupled general circulation model are assessed.
During the long-lasting warm event, there were two warm maxima in the area-averaged SSTA around January 2014 and July 2015, respectively. The warm anomaly originated at the oceanic surface and propagated downward and reached about 300 m. Model experiments forced by observed SST suggest that the long persistence of the atmospheric anomalies in the northeastern Pacific as a whole may be partially explained by SST forcing, particularly in the tropical Pacific Ocean associated with a persistent warm SSTA in 2014/15 and an extremely strong El Niño in 2015/16, via its influence on atmospheric circulation over the North Pacific. Nevertheless, it was a challenge to predict the evolution of this warm event, especially for its growth. That is consistent with the fact that the SSTAs in extratropical oceans are largely a consequence of unpredictable atmospheric variability.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Using seasonal hindcasts of six different models participating in the North American Multimodel Ensemble project, the trend of the predicted sea surface temperature (SST) in the tropical Pacific for ...1982–2014 at each lead month and its temporal evolution with respect to the lead month are investigated for all individual models. Since the coupled models are initialized with the observed ocean, atmosphere, land states from observation-based reanalysis, some of them using their own data assimilation process, one would expect that the observed SST trend is reasonably well captured in their seasonal predictions. However, although the observed SST features a weak-cooling trend for the 33-year period with La Niña-like spatial pattern in the tropical central-eastern Pacific all year round, it is demonstrated that all models having a time-dependent realistic concentration of greenhouse gases (GHG) display a warming trend in the equatorial Pacific that amplifies as the lead-time increases. In addition, these models’ behaviors are nearly independent of the starting month of the hindcasts although the growth rates of the trend vary with the lead month. This key characteristic of the forecasted SST trend in the equatorial Pacific is also identified in the NCAR CCSM3 hindcasts that have the GHG concentration for a fixed year. This suggests that a global warming forcing may not play a significant role in generating the spurious warming trend of the coupled models’ SST hindcasts in the tropical Pacific. This model SST trend in the tropical central-eastern Pacific, which is opposite to the observed one, causes a developing El Niño-like warming bias in the forecasted SST with its peak in boreal winter. Its implications for seasonal prediction are discussed.
The land climate predictability at seasonal and interannual time scales is largely due to the influence of the ocean. The connections between global sea surface temperature anomaly (SSTA) and ...precipitation anomaly over land as a whole are assessed using observations and Atmospheric Model Intercomparison Project (AMIP) simulations for 1957–2018 in this work. With a novel bulk connectivity matrix, the regions of SSTA having the most significant connections with global land precipitation anomaly are identified and the monthly evolution is evaluated. The similarities and differences between the observations and AMIP simulations are examined. In both the observations and AMIP simulations, SSTA in the tropical central and eastern Pacific connects strongly with the global land precipitation anomaly. Compared with that in the tropical Pacific, the connections with SSTA along the equatorial Indian and Atlantic Oceans are weaker. The global connection between SSTA and land precipitation anomaly is the strongest (weakest) in October (June) in the observations and the average of 17-individual members of the AMIP simulations, and in February (June) in the 17-member ensemble mean of the AMIP simulations. Compared with the observation, the strength of the connectivity is overestimated in the ensemble mean, and underestimated in an individual member of the AMIP simulations. The results of the bulk connectivity matrix in this work can serve as a benchmark to evaluate the connection of SSTA with global land precipitation variation in climate models.
This study focuses on improving prediction of the two types of ENSO by combining multi-model ensemble (MME) with a statistical error correction method that is based on a stepwise pattern projection ...and applied to all models before doing the MME. We evaluate such a combinational approach using five dynamical model datasets from the North American Multi-model Ensemble (NMME) project for the period of 1982–2010. The prediction skills of the proposed MME show an improvement over most tropical Pacific regions. With regard to the two ENSO types, improvements in prediction skills of the proposed MME are particularly evident for the Niño indices for short lead time. The differences between the Eastern Pacific and Central Pacific ENSO types are more pronounced in the corrected forecasts compared with the uncorrected ones. The zonal center position of sea surface temperature anomalies for the corrected MME is closer to the observed than that for the uncorrected MME. The results indicate that reducing prediction errors of each model member by a good correction method before applying the MME method can provide an effective way for empirically improving forecasts of the two ENSO types.
The influence of the El Niño–Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO) interference on the dry and wet conditions in the Great Plains of the United States has been examined ...using monthly observational datasets. It is shown that both ENSO and PDO can generate a similar pattern of atmospheric and oceanic anomalies over the eastern part of the North Pacific and western North America that has significant impact on the climate over the Great Plains. Furthermore, the relationship between ENSO–PDO and climate anomalies in the Great Plains is intensified when ENSO and PDO are in phase (El Niño and warm PDO or La Niña and cold PDO). On average, anomalies over the Great Plains favor wet (dry) conditions when both ENSO and PDO are in the positive (negative) phase. However, when ENSO and PDO are out of phase, the relationship is weakened and the anomalies over the Great Plains tend toward neutral. Without ENSO, PDO alone does not affect the North American climate significantly. The relationship is quite robust for different seasons, with the strongest effects for the months of spring and the weakest effects for the months of autumn, whereas the months of winter and summer fall in between. The seasonality of the relationship may be associated with the seasonal dependence of the anomalies of general circulation and the pattern of mean seasonal cycle in the North Pacific.
The contrasting impact of the interference of ENSO and PDO on the climate anomalies in the Great Plains is associated with differences in the ocean–atmosphere anomalies. When ENSO and PDO are in phase, the sea surface temperature (SST) anomalies extend from the equatorial Pacific to the higher latitudes of the North Pacific via the eastern ocean. The distribution of the corresponding anomalies of sea level pressure (SLP) and the wind at 1000 hPa form an ellipse with a southeast–northwest orientation of the long axis because the SST anomalies promote coherent changes in SLP in the central North Pacific. In the upper troposphere, a similar teleconnection pattern with the same sign generated by ENSO and PDO is overlapped and enhanced, which favors anomaly (dry and wet) conditions in the Great Plains. However, when ENSO and PDO are out of phase, the SST anomalies have the same sign in the tropical and central North Pacific, which is opposite to the anomalies near the west coast of North America. The anomalously cyclonic circulation over the North Pacific is weaker in the out-of-phase situation than in the in-phase situation. The distribution of the anomalies of SLP and the wind at 1000 hPa resembles a circle. Meanwhile, in the upper troposphere, ENSO and PDO generate a similar teleconnection pattern with opposite sign, causing cancellation of the anomalous circulation and favoring neutral climate in the Great Plains.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Seasonal prediction of extratropical climate (e.g., the East Asian climate) is partly dependent upon the prediction skill for rainfall over the Maritime Continent (MC). Aprevious study by the authors ...found that the NCEP Climate Forecast System, version 2 (CFSv2), had difference in skill between predicting rainfall over the western MC (WMC) and the eastern MC (EMC), especially in the wet season. In this study, the potential mechanisms for this phenomenon are examined. It is shown that observationally in the wet season (from boreal winter to early spring) the EMC rainfall is closely linked to both ENSO and local sea surface temperature (SST) anomalies, whereas the WMC rainfall is only moderately correlated with ENSO. The model hindcast unrealistically predicts the relationship of the WMC rainfall with local SST and ENSO (even opposite to the observed feature), which contributes to lower prediction skill for the WMC rainfall. In the dry season (from boreal late summer to fall), the rainfall over the entire MC is significantly influenced by both ENSO and local SST in observations and this feature is well captured by the CFSv2. Therefore, the hindcasts do not show apparently different skill in rainfall prediction for EM Cand WMC in the dry season. The possible roles of atmospheric internal processes are also discussed.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In this study, we examined the temporal variations of the El Niño-Southern Oscillation (ENSO) prediction skill during 1958–2016 in the context of the evolution in the tropical Pacific subsurface ...ocean observing system. To examine the temporal variations of the seasonal prediction skill, spatial correlation skill (SCS) of the predicted SST anomalies (SSTA) in the tropical Pacific Ocean within 10
o
S-10
o
N and temporal correlation skill (TCS) of the area-averaged SSTA throughout the same basin for the four periods of 1958–1978, 1979–1994, 1995–2005 and 2006–2016 were evaluated. These periods correspond to low amount, first increase, medium amount and second increase of the subsurface ocean temperature observations. Our results show that the influence of the observing system is detectable in the skill increase—both in SCS and TCS metrics—during the period 1995–2005. However, the impact of the subsurface ocean observing system is difficult to quantify in the prediction skill metrics during 2006–2016. It is shown that SCS is determined to a large extent by the magnitude of the observed SSTA in the target month. There is visible skill increase in the TCS before and after 1979, but this appears to be the result of variations in the properties of the verifying SST. Thus, potential impacts of the observing system are masked by climate variations of SST at decadal time scales, which may be real or result of variations in the SST observing system. In particular, the multidecadal modulations of the tropical Pacific SST associated with the climate shifts in the late 1970s and the early 2000s have more significant influence on prediction skill than the changes in observing system.
Seasonality of sea surface temperature (SST) predictions in the tropical Indian Ocean (TIO) was investigated using hindcasts (1982–2009) made with the NCEP Climate Forecast System version 2 (CFSv2). ...CFSv2 produced useful predictions of the TIO SST with lead times up to several months. A substantial component of this skill was attributable to signals other than the Indian Ocean dipole (IOD). The prediction skill of the IOD index, defined as the difference between the SST anomaly (SSTA) averaged over 10°S–0°, 90°–110°E and 10°S–10°N, 50°–70°E, had strong seasonality, with high scores in the boreal autumn. In spite of skill in predicting its two poles with longer leads, CFSv2 did not have skill significantly better than persistence in predicting IOD. This was partly because the seasonal nature of IOD intrinsically limits its predictability.
The seasonality of the predictable patterns of the TIO SST was further explored by applying the maximum signal-to-noise (MSN) empirical orthogonal function (EOF) method to the predicted SSTA in March and October, respectively. The most predictable pattern in spring was the TIO basin warming, which is closely associated with El Niño. The basin mode, including its associated atmospheric anomalies, can be predicted at least 9 months ahead, even though some biases were evident. On the other hand, the most predictable pattern in fall was characterized by the IOD mode. This mode and its associated atmospheric variations can be skillfully predicted only 1–2 seasons ahead. Statistically, the predictable IOD mode coexists with El Niño; however, the 1994 event in a non-ENSO year (at least not a canonical ENSO year) can also be predicted at least 3 months ahead by CFSv2.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Observed oceanic and atmospheric anomalies in the tropical Pacific are analyzed to understand the symmetric and asymmetric characteristics between El Niño and La Niña evolutions. It is noted that the ...evolutions are largely symmetric for El Niño and La Niña prior to and in their mature phase, but become asymmetric afterwards. It is further demonstrated that this asymmetry is due to the fact that, on average, the discharge process associated with El Niño is stronger than the recharge counterpart associated with La Niña. The symmetric and asymmetric evolution for different phases of El Niño-Southern Oscillation (ENSO) is consistent with the associated recharge and discharge processes that are linked with the evolution of anomalous ocean surface current and gradient of sea surface height anomalies (SSHAs). It is suggested that the evolution of the meridional gradient of SSHAs in the central and eastern equatorial Pacific is different between El Niño and La Niña years due to relatively different rates of the SSHA changes on and off the equator. During the decay of a warm event, the equatorial SSHA quickly switches from positive to negative, meanwhile the off-equator areas are occupied by positive SSHAs. Such changes on the equator and in the off-equatorial regions make the concavity of the meridional SSHA favorable for growth and persistence of westward surface zonal current anomaly in the equatorial Pacific, then for the ENSO phase transition. During the decay of a cold event, the slow decrease of the negative SSHA on the equator combined with the disappearance of the negative SSHA in the off-equatorial regions makes the meridional concavity reverse easily and is unfavorable for the steady growth of the eastward surface zonal current anomaly and for the phase transition. Thus, the asymmetric evolution of ENSO is associated with both the atmosphere and ocean anomalies on the equator and off the equator.
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