The Sea Ice Extent Anomaly in the North Pacific and Its Impact on the East Asian Summer Monsoon Rainfall

Transcription

1 3434 JOURNAL OF CLIMATE VOLUME 17 The Sea Ice Extent Anomaly in the North Pacific and Its Impact on the East Asian Summer Monsoon Rainfall PING ZHAO * International Arctic Research Center, University of Alaska, Fairbanks, Fairbanks, Alaska, and Chinese Academy of Meteorological Sciences, Beijing, China XIANGDONG ZHANG International Arctic Research Center, University of Alaska, Fairbanks, Fairbanks, Alaska XIUJI ZHOU Chinese Academy of Meteorological Sciences, Beijing, China MOTO IKEDA Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan YONGHONG YIN Chinese Academy of Meteorological Sciences, Beijing, China (Manuscript received 21 January 2003, in final form 15 March 2004) ABSTRACT The relationship between extreme anomalies of the spring sea ice extent over the Bering Sea and the Sea of Okhotsk and rainfall variability in the east Asian summer monsoon was examined through an analysis of observed data and modeling experiments. The results show that reduced sea ice extent leads to an enhanced summer monsoon rainfall in southeastern China. This relationship is well supported by the background atmospheric circulation changes and the stationary wave dynamics. A difference in the 500-hPa geopotential height composed from the NCEP NCAR reanalysis data and model output between the light and heavy sea ice cases shows an anomalous high in the east Asian summer, which favors the invasion of a cold air mass into southern China and prevents the east Asian summer monsoon from advancing northward. Hence, the mei-yu front and its associated rainfall intensify and stay in southeastern China. The generation of the summer anomalous high and its interseasonal link to the spring sea ice extent anomalies can be accounted for by the stationary wave dynamics and the land surface process. In spring, the decrease in sea ice extent forces eastward-propagating wave activity flux and causes an anomalous high in Europe along with a decrease in precipitation. The decreased soil water content results in a higher land surface temperature and more sensible heat flux in summer, and this strengthens summer stationary wave activities in Europe. The eastward propagation of the wave energy and its intensification in east Asia are responsible for the anomalous high in the east Asian summer. In this process, the European land surface acts as a bridge linking the spring sea ice extent anomalies with the east Asian summer monsoon. 1. Introduction Sea ice seasonally covers the northern part of the Pacific, mainly in the Bering Sea and the Sea of Okhotsk * Visiting Scientist at the International Arctic Research Center, University of Alaska, Fairbanks, Fairbanks, Alaska. Corresponding author address: Dr. Xiandong Zhang, International Arctic Research Center, University of Alaska, Fairbanks, 930 Koyukuk Dr., Fairbanks, AK xdz@iarc.uaf.edu (BOS). It develops in late fall, starting from the northern and northwestern parts of the Bering Sea and the Sea of Okhotsk, respectively, and retreats in late spring. The sea ice extents in these seas are apparently affected by interannual variations of the large-scale atmospheric circulation (Cavalier and Parkinson 1987; Parkinson 1990; Fang and Wallace 1994). Specifically, Niebauer (1988, 1998) found from his study of the relationship between the El Niño Southern Oscillation (ENSO) and the northern Pacific weather pattern that changes in the position of the Aleutian low result in noticeable sea ice extent anomalies in the Bering Sea. On the other hand, sea ice 2004 American Meteorological Society

2 1SEPTEMBER 2004 ZHAO ET AL insulates the atmosphere from the underlying warm water and reduces surface heat and moisture fluxes compared to open water. Sea ice anomalies change the local energy balance and consequently induce a feedback to large-scale atmospheric circulations. Honda et al. (1996, 1999) investigated the response of the atmosphere to sea ice extent anomalies in the Sea of Okhotsk and found that a sea ice anomaly triggers a wave train that propagates downstream to the Bering Sea and North America. Some statistical analyses have revealed relationships between the Arctic sea ice anomalies and the east Asian climate variability on an interannual time scale. A larger winter sea ice extent in the area of N, 60 E 180 could lead to a stronger Siberian high and lower air temperature in China in the same season (Wang et al. 1997). Positive winter sea ice extent anomalies in the Barents and Kara Seas may weaken northerlies in East Asia, hence reducing the cold air invasion into China (Wu and Huang 1999). Nevertheless, all these studies document the simultaneous correlations between sea ice anomalies and climatic variability in the Asian monsoon. The interseasonal relations, which have significant prediction implications, have not been well explored. The East Asian summer monsoon is a dominant climate system bringing rainfall to the East Asian areas. In climatology, the monsoon is established along the Yangtze River valley in June and July and produces a large amount of rainfall, which is characterized by a quasi-stationary front (named the mei-yu front) that reflects a contrast between the tropical warm air mass and the polar cold one. Accordingly, the monsoon activity has dramatic influences on the rainfall in these areas, which may result in either flooding or drought and has significant economic consequences. In this paper, we focus our attention on an exploration of the influence of the spring sea ice extent anomalies over the BOS on summer monsoon activities in China by analyzing existing data and an atmospheric general circulation model (AGCM). 2. Evidence of the relationship between the sea ice extent anomaly and monsoon rainfall a. The sea ice extent anomaly over the BOS FIG. 1. The time series of the observational sea ice extent anomalies averaged over the Bering Sea and in the Sea of Okhotsk, respectively, during spring (Mar May). For this study, we examined sea ice extent anomalies over the BOS. The sea ice extent data are derived from the Global Sea Ice and Sea Surface Temperature version 2.3b (GISST2.3b) dataset (Rayner et al. 1996) and provide monthly sea ice concentrations on global 1 1 grids from 1871 to Considering the availability and reliability of realistic measurements over the BOS, we chose the period and checked the variation of sea ice extents. The Bering Sea is confined to the area of N, 158 E 159 W and the Sea of Okhotsk is confined to the area of N, E. The time series of spring (March May) sea ice extent anomalies over these two seas, respectively, are depicted in Fig. 1. The amplitude of the sea ice extent anomaly over the Bering Sea is generally larger than that over the Sea of Okhotsk. It has been documented that there is an out-of-phase correlation of sea ice extent anomalies between the Bering Sea and the Sea of Okhotsk (Cavalieri et al. 1987; Deser et al. 2000). This correlation can be inferred from the two time series in Fig. 1 during the period from 1971 to Quantitatively the correlation for the period is 0.38, significant at the 90% confident level. However, this negative correlation does not appear in the entire period. The sea ice extent anomalies between the two seas show a consistent variation in the 1990s, which results in a correlation coefficient of 0.07 for the entire period that does not show a statistical significance. Given such a new correlation of sea ice extent anomalies, particularly synchronic decreases of sea ice extent in the 1990s, we may treat the sea ice extent anomalies over these two seas together. The seasonal cycle of monthly sea ice extent over the BOS and its standard deviation show that there are maximum values of km 2 in sea ice extent during March and values of km 2 in standard deviation during April (Fig. 2a). The latter implies a maximum variability in this season and presents the reason why we are using spring sea ice in this study (recall that spring sea ice extent anomalies are plotted in Fig. 1). The combined sea ice extent anomalies over the BOS show positive anomalies mainly occurring in the early and middle 1970s and negative anomalies in the 1990s (Fig. 2b). This is consistent with the shrinking trend of total sea ice extent over the whole Arctic Ocean and its shelf seas (Parkinson et al. 1999). The standard deviation of the spring sea ice extent anomalies over the BOS is km 2. In Fig. 2b, the variation of the sea ice extent anomaly

3 3436 JOURNAL OF CLIMATE VOLUME 17 FIG. 2. (a) The seasonal cycle of the observational sea ice extent (solid curve) and its std dev (dashed curve), and (b) the time series of the observational sea ice extent anomaly totally in the BOS during spring (Mar May). is more like a regime shift around 1992 than a longterm trend because the sea ice anomaly varies from a positive to negative phase alternatively before 1992, while it stays mostly in the negative phase after that. In this case, it should be better to use absolute sea ice extent anomalies than the artificially detrended data. Therefore, we chose the heaviest and lightest six years to make a composite analysis, respectively, based on the absolute sea ice extent anomaly. The selected years with more sea ice extent (collectively referred to as the heavy sea ice case) are 1971, 1973, 1975, 1976, 1977, and 1991; similarly, the selected years with less sea ice extent (collectively referred to as the light sea ice case) are 1979, 1994, 1996, 1997, 1998, and The mean sea ice extent anomaly in the heavy sea ice case is km 2 and km 2 in the light sea ice case. b. The relationship between the sea ice extent anomaly and monsoon rainfall We utilized the accumulated precipitation from June and July at the 160 meteorological stations in China to represent summer monsoon rainfall in east Asia. Summer in the following sections refers to June July. The climatologically averaged rainfall from 1971 to 1999 shows that a large area received over 400 mm of rain in eastern China between 26 and 31 N, with a maximum value of 500 mm (Fig. 3a). Correspondingly, a vertical cross-section of climatological pseudoequivalent potential temperature ( se ) and velocities along 118 E that were obtained from the National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP NCAR) reanalysis datasets (Kalnay et al. 1996) show an intensified horizontal gra- FIG. 3. (a) Accumulated precipitation (mm) in summer (Jun Jul) averaged from 1971 to 1999 at 160 stations in China, and (b) observed se (K) and velocities ( : ms 1, : 0.05 Pa s 1 ) in summer averaged from 1971 to 1999 along a vertical cross-section at 118 E.

4 1SEPTEMBER 2004 ZHAO ET AL FIG. 4. (a) Composite accumulated precipitation (mm) in summer (Jun Jul) in the light sea ice years at 160 stations in China; (b) the same as in (a), but in the heavy sea ice years; (c) their differences (the light and heavy shaded areas show the 90% and 95% confidence levels, respectively); and (d) the std dev of the summer accumulated precipitation from 1971 to dient of se and an apparent convergence belt between 28 and 35 N (Fig. 3b). This is a typical characteristic of the mei-yu front. The maximum se south of the meiyu front reflects the warm and moisture air mass. In order to explore a relationship between summer rainfall in China and the spring sea ice extent anomaly over the BOS, we made a composite analysis of rainfall based on the years with light and heavy sea ice extents defined above. Principally, the rainfall patterns in both the light and heavy ice cases are similar to the climatology. In the light ice case, there is precipitation over 500 mm within the area of N, E, with a maximum value of about 700 mm (Fig. 4a), while in the heavy ice case, precipitation in this area decreases remarkably, with a maximum value below 500 mm (Fig. 4b). The rainfall difference between the two cases shows an increase east of 110 E between 27 and 31 N (Fig. 4c). The maximum difference can reach 200 mm (at the 95% confidence level), which is comparable to the maximum standard deviation of about 200 mm in rainfall from 1971 to 1999 (Fig. 4d). This relationship between sea ice extent anomalies and rainfall variability weakens to some extent when the sea ice extent and rainfall anomalies are detrended (figures not shown). In summary, the extreme anomalies of summer rainfall in southeastern China could be accounted for by the changes in the spring sea ice extent over the BOS. A pertinent question to ask would be, what background atmospheric circulations support this relationship. c. The relationship of atmospheric circulation to the sea ice extent anomaly Similar to the rainfall composite described in the previous section, we made a composite analysis of the summer 500-hPa geopotential height in the light and heavy sea ice extent cases (Fig. 5a). One positive anomalous center occurs to the south of Lake Baikal and another occurs in the midlatitudes of Europe (at the 95% confidence level), which shows that the atmospheric cir-

5 3438 JOURNAL OF CLIMATE VOLUME 17 culation in the Eurasian summer is closely related to the spring sea ice extent over the BOS. In climatology, geopotential ridges are located over the Sea of Okhotsk (correspondingly, the Okhotsk high at surface) and the European continent, between which there is a trough in Asia. The anomalous high south of Lake Baikal implies a westward shift of the Okhotsk high and a weakened low pressure in the Asian continent. In the lower troposphere, anomalous northerlies or northeasterlies prevail in the East Asian continent (Fig. 5b), reflecting a strong cold air activity, which prevents the East Asian summer monsoon from advancing northward and makes the East Asian summer monsoon activities occur in southern China. These features are also reflected in the mei-yu front. The vertical section along 118 E shows an increased horizontal gradient of se and intensified upward motion in the light ice case (compared to the heavy ice case) in the lower troposphere between 28 and 35 N (Fig. 5c). All of this shows that the mei-yu front strengthens and stays in southern China. The intensified updraft is an important contributing factor to the increased rainfall. Motivated by Honda et al. s (1996) analysis of wave dynamics, we computed the anomalous wave activity flux between the light and heavy sea ice cases by using the NCEP NCAR reanalysis data, aiming to figure out the mechanism responsible for the generation of the anomalous high in East Asia. We used the conventional definition of the stationary wave activity flux F s by Plumb (1985): F x Fs p cos( ) F y, F z where 1 ( ) 2 Fx, 2 a sin2 1 (u ) Fy u, and 2 a sin2 ] [ 2 sin 1 (T ) Fz T. S 2 a sin2 Here, F x, F y, and F z are zonal, meridional, and vertical components of F s, respectively. All parameters in the above equations are the same as in Plumb (1985). In line with his view, thermal forcing and orographic effects in boundaries are two important sources of the generation of stationary wave activities. For convenience in the following sections, the horizontal vector of F s is defined as F h (F x, F y ). FIG. 5. Differences in composites for (a) geopotential height at 500 hpa ( 10 m; the shaded areas show the 95% confidence level); (b) winds at 850 hpa (m s 1 ; the thick dashed line denotes the 1500-m topographic contour); and (c) se ( 0.1 K) and velocities ( :ms 1, : 0.05 Pa s 1 ) along a vertical cross-section at 118 E between the light and heavy sea ice cases during summer in reanalysis data.

6 1SEPTEMBER 2004 ZHAO ET AL FIG. 7. (a) Same as Fig. 3a for model simulations, and (b) the same as Fig. 3b, except for simulations at 115 E. FIG. 6. (a) Differences in F h at 500 hpa (arrows) and F z vertically averaged from 1000 to 850hPa (contours) for stationary waves between the light and heavy sea ice cases during summer (Jun Jul) in reanalysis data, and (b) the differences in F h at 300 hpa for stationary waves between the light and heavy sea ice cases during summer in reanalysis data. The unit of F h is m 2 s 2 and F z is 0.01 m 2 s 2. The thick dashed line denotes the 1500-m topographic contour. We calculated F z in the lower troposphere so as to examine the effect of surface diabatic forcing on anomalous wave activities. The differences in F h at 500 hpa and F z averaged arithmetically from 1000 to 850 hpa for stationary waves between the light and heavy sea ice cases in summer are shown in Fig. 6. It is evident that in Fig. 6a, there are generally positive values of F z in the midlatitudes between 30 and 60 E, with a maximum value over 0.30 m 2 s 2 in eastern Europe and western Asia, indicating an upward wave activity flux. This suggests that anomalous wave energy transfers upward in the lower troposphere, and the wave activity strengthens there. The force behind the upward wave activity flux in boundaries must be associated with surface diabatic heating. Relatively large F h differences in the midlatitudes of eastern Europe at 300 and 500 hpa can be seen to correspond to the upward wave activity flux (Figs. 6a b). The F h differences extend downstream to the neighborhood of Lake Baikal, which is more obvious at 300 hpa. This shows that the strengthened anomalous stationary wave energy in Europe reaches east Asia and may impact atmospheric circulations there. Moreover, F h differences at 500 hpa decreases in the west of the Tibetan Plateau and intensifies in the north of the Tibetan Plateau, which is probably associated with an interaction between the eastward-propagating wave energy and the topography. Through an analysis of the reanalysis and observed data, we have seen that extreme anomalies of spring sea ice extent over the BOS may have a close relationship to the monsoon circulation and associated rainfall in

7 3440 JOURNAL OF CLIMATE VOLUME 17 FIG. 8. Monthly mean sea ice extent (shaded area) in the heavy sea ice cover experiment in (a) Mar, (b) Apr, and (c) May. East Asia. Less spring sea ice extent over the BOS may lead to a strong anomalous high in the East Asian summer. Its generation may be accounted for by the eastward-propagating anomalous wave energy that is trigged in eastern Europe and western Asia. Under such circumstances, anomalous northerlies or northeasterlies prevail in the eastern Asian continent, which favors the invasion of cold air masses southward and intensifies the mei-yu front. The intensified updrafts on the front are directly responsible for the increase of rainfall. In order to ensure that the excitation of sea ice extent anomalies relates to changes of the atmospheric circulations and associated rainfall in East Asia and also to find reasons accounting for the persistence of the anomalous sea ice signal, we carried out numerical simulations with a climate model and made dynamically and thermodynamically consistent diagnoses. 3. Modeling experiments and interpretative diagnosis a. Model and experiment design We used the NCAR Community Climate Model version 3 (CCM3) with a triangular truncation at wavenumber-42 and -18 vertical levels (T42L18; Kiehl et al. 1996). The horizontal resolution approximately corresponds to 2.8 in both latitude and longitude. The distribution of sea ice extent in the model is prescribed on the basis of monthly climatic observations. In terms of sea ice treatment in the state-of-the-art model, the sea ice concentration is assumed to be either 100% or 0%, and the sea ice thickness is assumed to be 1 m at any model grid point where sea ice cover exists. The monthly climatic mean of sea surface temperature is prescribed at ocean grid points when sea ice is absent. Figure 7 shows the distribution of summer rainfall as well as the vertical cross-section of se and velocities along 118 E that are simulated in the control experiment. The figure shows that the major rainfall occurs in eastern China between 30 and 37 N, with a maximum value of about 500 mm. In the lower troposphere, a se center larger than 308 K exists between 25 and 34 N and a center smaller than 288 K exists to the north of 40 N. The large horizontal gradients of se between them correspond to the mei-yu front. Generally speaking, the control experiment captures major characteristics of the mei-yu front and associated rainfall although their positions are slightly northward.

8 1SEPTEMBER 2004 ZHAO ET AL FIG. 9. (a), (b), (c) Same as Figs. 4a,b,c, respectively, but from model simulations (the shaded areas show the 95% confidence level). For the light and heavy sea ice cases, we designed two groups of ensemble modeling experiments that represent sea ice rapidly or slowly melting in spring. Each ensemble experiment includes five members with different initial atmospheric conditions. In the light sea ice case, sea ice over the BOS is completely removed in the spring. In the heavy ice case, the sea ice extent is enlarged in spring, compared to the control experiment (Fig. 8), with the monthly sea ice extent increasing by km 2 from March through May. For each case, the model is integrated for 5 months from initial conditions on 1 March. We made a diagnosis based on an average of five ensemble experiments in the light and heavy sea ice cases, respectively. b. Anomalies of rainfall and atmospheric circulation triggered by the sea ice anomaly The pattern of the simulated summer rainfall in the light and heavy ice experiments generally resembles that in the control simulation (Fig. 9), indicating that the sea ice extent anomalies do not change the pattern of summer rainfall in East Asia. Compared to the observations, the simulated rainfalls in eastern China in the light and heavy sea ice cases are weaker and more northward. This inconsistency may relate to systematic errors in the model. In spite of the general resemblance in rainfall patterns between the light and heavy sea ice cases, differences are still obvious. In the light sea ice case, the large rainfall amount (over 400 mm) occurs in eastern China between 29 and 37 N, while the small rainfall amount (below 100 mm) occurs in the adjacent southern region (Fig. 9a). In the heavy sea ice case, the large rainfall amount (over 400 mm) noticeably moves northward to the north of 33 N, while the small rainfall amount (below 100 mm) stays almost in the same location as in the light sea ice case but decreases remarkably in value (Fig. 9b). Accordingly, a dramatic increase in rainfall occurs in southeastern China between 25 and 35 N in the light sea ice case, relative to that in the heavy sea ice case. This rainfall difference can exceed 150 mm within the area of N, E (at the 95% confidence level), with a maximum value of about 200 mm (Fig. 9c). It is evident that the simulated rainfall difference is quite close to the observed one, which suggests that the model successfully reproduces the extreme anomalies of summer rainfall in southeastern Chi-

9 3442 JOURNAL OF CLIMATE VOLUME 17 FIG. 10. (a), (b) Same as Figs. 5a,b, respectively, but from model simulations. A 9-point smoother was applied for the shading in the t test. na that are induced by changes of the spring sea ice extent over the BOS. The difference in the simulated summer 500-hPa geopotential height between the light and heavy sea ice cases shows two anomalous highs, respectively, in the midlatitudes of Europe and near Lake Baikal (at the 95% confidence level; Fig. 10a), which is similar to the analysis from the reanalysis data although the simulated anomalous highs are slightly more northward than those in the reanalysis data. At 850 hpa, there is an anomalous anticyclonic circulation to the northeast of the Tibetan Plateau, which is similar to but slightly more eastward than that from the reanalysis data. As a consequence, anomalous northerlies or northeasterlies prevail in southern China (Fig. 10b), favoring the southward intrusion of cold air. Calculations of F h differences at 500 and 300 hpa FIG. 11. Same as Fig. 6, but from model simulations. and F z averaged from 1000 to 850 hpa with the model outputs for stationary waves between the light and heavy sea ice cases show that a broad positive F z anomaly occurs in the midlatitudes of Europe, with a maximum value of over 0.8 m 2 s 2 near 20 and 50 E; they also show that relatively large F h differences at 500 and 300 hpa appear in Europe and extend downstream to east Asia (Fig. 11). The F h differences at 500 hpa slightly decrease in the west of the Tibetan Plateau and then intensify in the north. These features principally resemble the ones from the reanalysis data in Fig. 6 except that the simulated stationary wave activity flux is stronger in Europe. Studies from the reanalysis data and modeling above have shown a remarkable agreement on the impacts of

10 1SEPTEMBER 2004 ZHAO ET AL FIG. 12. (a) Same as Fig. 11a, but for spring (Apr May), and (b) the same as Fig. 10a, but for spring. A 9-point smoother was applied for the shading in the t test. the spring sea ice extent over the BOS on the East Asian summer monsoon and associated rainfall, not only in the apparent parameters, such as rainfall and atmospheric circulation, but also in the underlying large-scale wave dynamics. Therefore, the simulations confirm the fact that the summer monsoon rainfall in southeastern China can be triggered by the previous sea ice extent anomalies over the BOS. However, a question that still needs to be addressed is what process or mechanism could be responsible for the spring anomalous sea ice signal remaining until summer. 4. Mechanism for persistence of the atmospheric signal triggered by the sea ice anomaly Although the above observational analysis shows a close relationship between the spring sea ice extent anomaly over the BOS and extreme events of summer monsoon rainfall in China, we could not exclude the effects of other contributing factors due to complicated interactions in the climate system. For example, sea surface temperature in the North Pacific is correlated with the sea ice extent over the Bering Sea. Nonetheless, the modeling experiments provide us with an opportunity to isolate the impact of the sea ice extent anomaly in which sea surface temperature is taken from climatology. Therefore, we use the output of modeling experiments to diagnose the mechanism in this section. Considering the spinup time of the model, we removed the first month of modeling results. a. Spring anomalous atmospheric circulation triggered by the sea ice extent anomaly The horizontal flux of a stationary wave activity at 500 hpa and its vertical flux averaged from 1000 to 850 hpa in spring, triggered by sea ice extent anomalies, are depicted in Fig. 12a. The large F h differences have two branches: one extends eastward from the eastern Bering Sea to the Atlantic and Europe via the high latitudes of North America and the other extends southeastward from the BOS to the Atlantic and Europe via the lower latitudes of North America, thus showing that the anomalous wave energy reaches Europe. This results in the formation of an anomalous high at 500 hpa in spring (at the 95% confidence level; Fig. 12b). Figure 13 shows the difference in the simulated mean vertical velocity from 700 to 300 hpa in p-coordinates in spring (April and May) between the light and heavy sea ice cases. In general, corresponding to the anomalous high are positive vertical velocity anomalies in the midlatitudes between 10 and 60 E, with their maximum value of over Pa s 1 (at the 95% confidence level; Fig. 13a). Thus the air ascent weakens in the light sea ice case, compared to the heavy sea ice case, which reduces rainfall (Fig. 13b). Negative rainfall anomalies exceed 50 mm in the midlatitudes of Europe (at the 95% confidence level). Owing to the limited reliability of rainfall and soil moisture in the NCEP NCAR reanalysis data, we use the p-coordinate vertical velocity as a proxy of rainfall. The vertical velocity in spring reveals that its positive anomalies emerge in most of the midlatitudes between 10 and 60 E, with a maximum value of about Pa s 1 (at the 95% confidence level; Fig. 13c), which shows the weakened upward motion. This is consistent with the simulated result. Therefore, it is reasonable that the air ascent in spring weakens in the midlatitudes of Europe when there is less sea ice extent, indirectly supporting the decrease of rainfall and soil water input in the European spring.

11 3444 JOURNAL OF CLIMATE VOLUME 17 FIG. 13. Differences of (a) mean vertical velocity ( 0.01 Pa s 1 ) between 700 and 300 hpa in p-coordinates and (b) accumulated precipitation ( 100 mm) in spring (Apr May) between the light and heavy sea ice experiments. (c) The same as in (a), but from reanalysis data. b. The role of the land surface process in maintaining the anomalous sea ice signal Changes in rainfall affect land surface conditions that may produce feedback to atmospheric circulation in subsequent seasons (Shukla and Mintz 1982; Yeh et al. 1984). The simulated root zone volumetric soil water from the surface up to a 1-m depth shows that there are generally negative soil water differences in the midlatitudes of Europe and west Asia in May between the light and heavy sea ice cases (at the 95% confidence level; Fig. 14a), indicating a drier soil in the light sea ice case than in the heavy sea ice case. The changed soil condition leads to an increase in land surface temperature in these areas in summer, with a maximum value of about 6 C (at the 95% confidence level; Fig. 14b). The warmed land surface increases the surface sensible heat flux. It can reach 40 W m 2 (at the 95% confidence level; Fig. 14c). This diabatic heating may locally generate upward and eastward fluxes of the anomalous wave activity shown in Fig. 11. Therefore, the land surface process in Europe and west Asia may help retain the atmospheric signal excited by the spring sea ice extent anomaly until summer to impact the east Asian summer monsoon. 5. Concluding remarks We have investigated the relationship between the spring sea ice extent anomalies over the BOS and extreme events of summer monsoon rainfall in East Asia. Considering the availability and reliability of sea ice data over the BOS, as well as the recent dominant de-

12 1SEPTEMBER 2004 ZHAO ET AL FIG. 14. Differences of (a) root zone volumetric soil water in May (fraction, 0.01); (b) surface soil temperature ( C) in summer (Jun Jul); and (c) surface sensible heat fluxes ( 10Wm 2 ) in summer between the light and heavy sea ice experiments of the model. creasing trend, we chose the period from 1971 to 1999 and treated these two seas together. Both observational analysis and modeling experiments show that the reduced spring sea ice extent over the BOS leads to a significant enhancement of summer rainfall in southeastern China. The underlying atmospheric circulation and the stationary wave dynamics support this relationship. There are strong eastward anomalous stationary wave activity fluxes between the light and heavy sea ice cases in eastern Europe and western Asia during summer. They reach East Asia and intensity in the north of the Tibetan Plateau, giving rise to an anomalous high in the midlatitudes of East Asia. Accordingly, anomalous northerlies or northeasterlies occur in the lower troposphere in southeastern China. Under such circumstances, cold air masses go farther southward and prevent the East Asian summer monsoon from moving northward, which results in a strengthened mei-yu front and makes the associated rainfall belt stay in southeastern China. Moreover, from the mesoscale point of view, the intensified updrafts are directly responsible for the increased rainfall. As for the mechanism for the persistence of the atmospheric signals triggered by the sea ice extent anomalies from March to May, we found that the land surface process plays an important role. The reduced spring sea ice extent over the BOS triggers an anomalous high in Europe in the same season, which results in decreases of rainfall and soil water content. Thereafter, the land surface in Europe is warmer than usual and excites anomalous stationary waves, and this is responsible for the formation of the anomalous high in the East Asian summer. Hence, the European land

13 3446 JOURNAL OF CLIMATE VOLUME 17 FIG. 15. Schematic summary of the impact of the spring sea ice extent anomaly over the BOS on the summer rainfall in East Asia. surface process is an important interseasonal bridge linking the spring sea ice extent anomalies with the East Asian summer monsoon. Figure 15 summarizes what we found in this study. We have noted that there is an apparent declining trend of sea ice extent over the BOS during the entire time period, but the trend is not obvious before the 1990s. Although the strong relationship between sea ice and rainfall weakens to some extent for the detrended data, in this study we aimed to examine effects of the absolute sea ice anomalies caused by both the trend and the superimposed variability. Therefore, we did not separate sea ice anomalies caused by these two components. Specifically, we employed the CCM3 to perform modeling experiments in addition to the observational analysis. The modeling experiments did not include any trends and isolated the effect of the sea ice anomaly on the rainfall, which shows that the cause effect relationship does exist between the sea ice and the rainfall and that such a relationship is supported by atmospheric circulation and wave dynamics. Acknowledgments. We thank Drs. D. E. Parker and J. Arnott of the Hadley Centre for Climate Prediction and Research for providing GISST2.3b, the Chinese Meteorological Administration for providing precipitation data at the Chinese meteorological stations, NOAA/Climate Diagnostic Center for providing NCEP

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