1.State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 2.University of Chinese Academy of Sciences, Beijing 100049, China Manuscript received: 2018-10-17 Manuscript revised: 2019-01-30 Manuscript accepted: 2019-02-25 Abstract:Using GFDL CM2p1 (Geophysical Fluid Dynamics Laboratory Climate Model, version 2p1), the effects of initial sea temperature errors on the predictability of the Indian Ocean Dipole (IOD) are explored. When initial temperature errors are superimposed on the tropical Indian Ocean, a winter predictability barrier (WPB) and a summer predictability barrier (SPB) exist in IOD predictions. The existence of the WPB has a close relation with El Ni?o-Southern Oscillation (ENSO) in the winter of the growing phase of positive IOD events. That is, when ENSO exists in winter, no WPB appears in IOD predictions, and vice versa. In contrast, there is no inherent connection between the existence of the SPB and ENSO. Only the dominant spatial pattern of SPB-related initial errors is studied in this paper, which presents a significant west-east dipole pattern in the tropical Indian Ocean and is similar to that of WPB-related initial errors in previous studies. The SPB-related initial errors superimposed on the tropical Indian Ocean induce the sea surface temperature (SST) and wind anomalies in the tropical Pacific Ocean. Then, under the interaction between the Indian and Pacific oceans through the atmospheric bridge and Indonesian Throughflow, a west-east dipole pattern of SST errors appears in summer, which is further strengthened under the Bjerknes feedback and yields a significant SPB. Keywords: predictability barrier, initial errors, Indian Ocean Dipole, Indian Ocean 摘要:利用GFDL CM2p1模式, 本文探讨了初始海温误差对印度洋偶极子(IOD)事件可预报性的影响. 当热带印度洋存在初始海温误差时, IOD预报发生了冬季预报障碍(WPB)现象和夏季预报障碍(SPB)现象. WPB发生与否与正IOD事件发展位相冬季的厄尔尼诺-南方涛动(ENSO)有关. 即当冬季存在ENSO时, IOD预测不发生WPB现象, 反之亦然. 相比之下, SPB发生与否和ENSO没有必然联系. 此外, 进一步探讨了最容易导致SPB现象的初始海温误差的主要模态, 指出该模态在热带印度洋上表现为东-西偶极子型, 这和前人研究中最容易导致WPB现象的初始海温误差模态相似. 当在热带印度洋上叠加这些初始海温误差后, 热带太平洋上出现了海表温度异常和风场异常, 进而通过大气桥和印尼贯穿流的作用影响热带印度洋, 使之在夏季出现了东-西偶极子型的海表温度异常, 该异常在Bjerknes作用下快速发展, 加强, 最终导致SPB现象的发生. 关键词:预报障碍, 初始误差, 印度洋偶极子, 印度洋
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4.1. Spatial patterns of initial errors that induce a significant SPB
As stated above, 31 SPB-related initial errors were selected in section 3. We put these initial errors together and assume them as time-varying fields corresponding to 31 different time points. Then, empirical orthogonal function (EOF) analysis is applied to these initial errors and the leading EOF mode (i.e., EOF1) is obtained, which describes the dominant spatial patterns of SPB-related initial errors. Corresponding to the EOF1 mode, the time series (i.e., PC1) has positive and negative values, indicating that some SPB-related initial errors have similar spatial patterns to the EOF1 mode and others have opposite ones. Therefore, the SPB-related initial errors are classified into two categories. The SPB-related initial errors that correspond to positive values of PC1 comprise the first category, and their composite is defined as type-1 initial error. Similarly, the composite of the SPB-related initial errors corresponding to the negative values of PC1 is defined as type-2 initial error. Type-1 initial error shows a significant west-east dipole pattern, both in the surface and subsurface components, with negative values in the western Indian Ocean and positive values in the eastern Indian Ocean (Fig. 5). The largest absolute values of type-1 initial error are concentrated in the subsurface component of the eastern tropical Indian Ocean. Type-2 initial error is almost opposite to type-1 initial error. Interestingly, the dominant spatial patterns of SPB-related initial errors in the tropical Indian Ocean are similar to those of the WPB-related initial errors analyzed in (Feng et al., 2017). The absolute values of spatial correlation coefficients between them are larger than 0.89. Therefore, initial errors with a west-east dipole pattern in the tropical Indian Ocean are inclined to yield a significant WPB and SPB, which results in large prediction uncertainties in winter and summer, and greatly limits the forecasting skill of the occurrence and the intensity of IOD events. (Feng et al., 2017) demonstrated that the subsurface eastern tropical Indian Ocean is the potential observing location (i.e., sensitive area) for advancing beyond the WPB for positive IOD events. That is, if intensive observations are carried out over this area, it will reduce the prediction errors in winter and weaken the WPB, improving the skill of wintertime IOD-event forecasts. Notably, the large values of SPB-related initial errors are also concentrated in the subsurface eastern Indian Ocean. That is, the initial errors in this area contribute greatly to the prediction uncertainties of positive IOD events. Therefore, if intensive observations are carried out over this area, it will not only weaken the WPB, but also reduce the prediction uncertainties in summer, and further weaken the SPB, ultimately greatly improving the forecasting skill with respect to the occurrence and intensity of positive IOD events. Figure5. Spatial patterns of two types of SPB-related initial errors (units: °C). Panels (a, b) denote the surface and subsurface components for type-1 initial error; panels (c, d) denote the surface and subsurface components for type-2 initial error. Dotted areas indicate that the composites of temperature errors exceed the 95% confidence level, as determined by the t-test.
Based on the above discussions, in addition to the initial errors in the tropical Pacific Ocean (Liu et al., 2018), the initial errors in the tropical Indian Ocean also play an important role in yielding a significant SPB. Therefore, to reduce the prediction uncertainties in summer and weaken the SPB, initial errors in the Indian Ocean should also be considered.
2 4.2. Developmental physical mechanisms of initial errors that induce a significant SPB -->
4.2. Developmental physical mechanisms of initial errors that induce a significant SPB
In this section, we calculate the sea temperature anomalies, which are defined as the difference in the sea temperature between the predictions and the "true state" of IOD events. Similarly, surface wind anomalies are also defined and calculated. Then, by analyzing the evolution of these temperature and wind anomalies, we identify how the SPB-related initial errors develop and cause large prediction uncertainties in summer, ultimately resulting in a significant SPB. Although type-1 and type-2 initial errors have opposite spatial patterns, the dominant characteristics of their evolution are only different in the first month and almost the same in the rest of the prediction year. Therefore, the evolution of the temperature and wind anomalies for two types of SPB-related initial errors is shown together in Fig. 6. Figure6. Evolutions of the SSTA (units: °C) and sea surface wind anomaly (units: m s-1) over the tropical Indian and Pacific oceans (left-hand column), and the equatorial (5°S-5°N) subsurface temperature anomaly (units: °C; right-hand column) for SPB-related initial errors. Dotted areas indicate that the composites of temperature anomalies exceed the 95% confidence level, as determined by the t-test.
When SPB-related initial errors are superimposed on the initial field of the reference-state IOD events, significant SST anomalies appear in the tropical Pacific Ocean in August, with positive values in the western Pacific Ocean and negative values in the central-eastern Pacific Ocean (Fig. 6). In the meantime, significant temperature anomalies appear in the subsurface Indian Ocean, with negative values in the western Indian Ocean and positive values in the eastern Indian Ocean. It is noted that the SST anomalies in the tropical Indian Ocean are relatively small in August and September. Therefore, the westerly wind anomalies at the equator in the tropical Indian Ocean may not be due to the local SST anomalies, but induced by the SST anomalies in the tropical Pacific Ocean. Specifically, the zonal SST gradient in the tropical Pacific Ocean causes easterly wind anomalies at the equator, which further modulate the Walker circulation in tropical oceans and induce anomalous westerly wind in the tropical Indian Ocean (Chen, 2011; Lian et al., 2014). Then, the anomalous westerly wind piles up warm water in the eastern Indian Ocean, and strengthens the subsurface warming there. In the first half of the prediction year, the positive temperature anomalies in the subsurface eastern Indian Ocean indicate a deepening of the thermocline depth and an increase in the sea surface height (SSH). This will reduce the translation of the warm water from the western Pacific Ocean to the eastern Indian Ocean, resulting in the deepening of the thermocline depth and the warming of the subsurface ocean in the western Pacific Ocean. The positive subsurface temperature anomalies further propagate eastward to the eastern Pacific Ocean, and cause the warming of the ocean and the weakening of the negative SST anomalies there. In January-March, in response to the zonal gradient of the surface temperature anomalies in the tropical Pacific Ocean, westerly wind anomalies appear at the equator in the central-eastern Pacific Ocean, which further induce easterly wind anomalies in the tropical Indian Ocean by modulating the Walker circulation in tropical oceans. On the one hand, the anomalous easterly wind favors upwelling to the coast of Sumatra and Java, resulting in an elevation of the thermocline depth and a decrease in the SSH, which advances the translation of the warm water from the western Pacific Ocean into the eastern Indian Ocean. This further results in an elevation of the thermocline depth and negative subsurface temperature anomalies in the western Pacific Ocean, which propagate eastward to the eastern Pacific Ocean to cool the ocean there. On the other hand, the southeast wind anomalies in the eastern Indian Ocean are opposite in direction to the climatological wind, which decreases the total wind speed, and thus the release of the latent heat flux from ocean to atmosphere, warming the sea surface water there. With the eastward propagation of the negative temperature anomalies in the Pacific Ocean and the warming of the sea surface water in the eastern Indian Ocean, northwest wind anomalies appear in the Indian Ocean in April-June. In summer, the northwest wind anomalies are opposite in direction to the climatological wind in the eastern Indian Ocean, which decreases the total wind speed and thus the release of latent heat flux from ocean to atmosphere, finally warming the sea surface water. Therefore, a significant west-east dipole pattern of SST anomalies appears, which is further strengthened under the effect of Bjerknes feedback, ultimately resulting in a significant SPB. Furthermore, we also analyze the evolution of sea temperature and surface wind anomalies for one sample of initial error that does not yield a significant SPB (Fig. 7). It is found that the SST anomalies grow fast and present a significant west-east dipole pattern in winter, which indicates large prediction errors in winter. In contrast, the SST anomalies show a basin-wide warming in summer, indicating small prediction errors in summer and thus no occurrence of an SPB. Based on the above discussions, although initial errors are superimposed on the tropical Indian Ocean only, they further cause temperature and wind anomalies in the tropical Pacific Ocean. The interaction between the Indian and Pacific oceans plays an important role in yielding a significant SPB. That is, if there is no Pacific Ocean, and only the Indian Ocean exists in models, no SPB is likely to occur in IOD predictions, even though dipole-pattern initial errors exist in the tropical Indian Ocean. Therefore, the Pacific Ocean is indispensable in yielding a significant SPB. Figure7. As in Fig. 6 but for one sample of initial error that does not yield a SPB.