Epidemics

The monthly time series for KD cases in Japan since 1970 showed two dramatic nationwide epidemics lasting several months and peaking in May 1982 (16,100 annual cases) and March 1986 (14,700 annual cases), respectively. A third epidemic, much lower in magnitude, peaked around April 1979 (6,700 cases) (Figure 1a). These three peaks represent the largest KD epidemics events ever recorded worldwide and provide an opportunity to investigate KD dynamics and possible climate relationships. To investigate a possible influence from large scale environmental factors, sea level pressure and surface winds were averaged for the June-July-August summer months before the onset of the epidemics (panels b1, c1 and d1 in Figure 1) and from September to the month prior to the peak in KD cases (panels b2, c2 and d2).

Figure 1 Major epidemics of monthly KD incidence in Japan. The three main historical KD epidemics are highlighted in red in panel a (cases). Time averaged sea level pressure (hPa) and surface winds (m/s) prior to the March/May 1979, May 1982 and March 1986 epidemics are shown in panels b, c and d, respectively. Monthly atmospheric variables were averaged for the preceding summer (JJA 1978 in b1, JJA 1981 in c1 and JJA 1985 in d1), when winds from the south typically blow across Japan and for the rising phase of the epidemics, from September to the last month before the peak (Sep 1978 to Mar 1979 in b2, Sep 1981 to Apr 1982 in c2 and Sep 1985 to Feb 1986 in d2) , when winds shifted and blew from the northwest. Colored dots depict the increase in KD incidence (per million inhabitants) by prefecture between the preceding September and the peak (Apr 1979 minus Sep 1978 in b2, May 1982 minus Sep 1981 in c2 and Mar 1986 minus Sep 1985 in d2). Full size image

Prior to the beginning of the epidemic peaks, low numbers of KD cases coincided with southerly winds (winds from the south) blowing over Japan for the entire summer (panels b1, c1 and d1 in Fig.1), a wind pattern which corresponds to the typical summer climatological configuration (e.g. see Supplementary Video 1). Immediately after the beginning of autumn, the number of cases rapidly mounted all over Japan when winds turned northwest in direction. Colored dots in panels b2, c2 and d2 illustrate the synchronization of the increase in KD cases throughout Japan coinciding with a shift to northwesterly winds in both peaks. Just after the peak in each epidemic, the winds again shifted and blew from the south and a marked decrease in the number of KD cases occurred (results not shown).

Interannual anomalies

To investigate further whether the influx of air from continental Asia is associated with fluctuations in numbers of KD cases, the Japanese dataset spanning the period from 1987–2006 was examined (Supplementary Figure 1). This segment of the time series excluded the major KD epidemics in Japan discussed above. Therefore, major interannual peaks and troughs of KD cases in Japan were selected for this post-epidemic period (red and blue anomalies in Figure 2a, respectively) and corresponding atmospheric anomalies were then composited. Years with increased numbers of KD cases in Japan were significantly associated with enhanced local northwesterly winds, as a result of an anomalous area of low pressure centered to the north of Japan (Figure 2b). Conversely, the composite of the major troughs in KD cases was linked to a quite different pattern in which an area of anomalous high pressure developed over the border between Russia and China, thus driving northeasterly winds from the Pacific Ocean across Japan (Figure 2c).

Figure 2 Surface configuration for the major interannual peaks of KD incidence in Japan. Panel a depicts the standardized (unitless) interannual reconstructed component of the original monthly time series of KD shown in Supplementary Figures 1a. The 7 (5) interannual peaks (troughs) reaching the +1 (−1) standard deviation criterion are dashed in red (blue). Standardized (unitless) interannual anomalies of sea level pressure and surface winds were composited in panel b (c) for the set of months when these peaks (troughs) were observed. Thick contours depict regions with significant interannual sea level pressure anomalies (p <0.05). Full size image

To assess whether year-to-year variations in wind patterns are associated with interannual fluctuations in KD numbers on the other side of the North Pacific, similar analyses were conducted for San Diego. The atmospheric connection from continental Asia to Japan and San Diego is complex and the atmospheric pathways connecting both shores of the Pacific can result from many different trajectories throughout the North Pacific. However, it was possible from this analysis to allocate all major interannual peaks of KD cases occurring in San Diego during the 1994–2008 period as belonging to two main atmospheric configurations. On the one hand (during green peaks in Figure 3a: 1994/95, 1997/98, 2002/03, 2004/05), zonal winds were intensified in the subtropics between 25N and 35N (Figure 3b and Supplementary Figure 2a), thus connecting the Asian continent and Japan to San Diego along a direct zonal path developing at all vertical levels (Figure 3c and Supplementary Figure 2c). On the other hand (during orange peaks in Figure 3a: 1999, 2006, 2007/08), the zonal trajectory appeared to be blocked in the subtropics (Figure 3d and Supplementary Figure 2b), but an alternative and shorter (in terms of distance) geodesic path was opened across the northern extratropics (Figure 3e and Supplementary Figure 2d), with enhanced westerly winds developing there. Therefore, interannual wind anomalies were associated with peaks in the numbers of annual KD cases in San Diego with the trajectory operating at different latitudes in two main patterns. Similar analyses were performed for other meteorological variables exhibiting seasonality, including minimum and maximum temperature, soil moisture and precipitation, but a similar association with KD did not emerge (results not shown).

Figure 3 Upper-troposphere wind configuration for the major interannual peaks of KD incidence in San Diego. Panel a depicts the standardized (unitless) interannual reconstructed component of the original monthly time series of KD shown in Supplementary Figures 1c. Standardized (unitless) interannual anomalies (b,c) and actual observations (d,e) of 300 hPa wind direction and intensity were composited for the 4 green (b,d) and 3 orange (c,e) interannual peaks in panel a. Thick contours in panels b and c depict regions with significant interannual wind intensity anomalies (p < 0.05). Full size image

Seasonal cycle

Seasonality represents a prominent contribution to the overall variance in the number of KD cases in Japan, especially when compared to interannual variability of the disease (44% vs. 11%, respectively). Examination of KD time series from Japan, San Diego and Hawaii show a nearly synchronized peak in KD activity from November through March (cf. Figures 4a–c), suggesting a shared mechanism explaining the seasonality of the disease in the three sites. A comparison between the number of KD cases and wind patterns was performed for the interval 1996–2006, the period for which there was data for all three study sites (Figure 4 and Supplementary Figure 3). In Japan, northwesterly winds (NW-WIND) were studied by projecting the observed winds onto a unit vector in the northwest/southeast direction in order to conform to the results observed in Figures 1b2,c2,d2 and 2b. Thus, positive values of NW-WIND correspond to winds blowing from the north, northwest or west, while negative values correspond to winds from the south, southeast or east. To characterize the pathway across the north Pacific from continental Asia and Japan to Hawaii and San Diego (e.g. Figures 3b–e), a Pacific Zonal Wind Index (P-WIND) was defined as the mean of the zonal component of winds along the subtropical north Pacific (see the horizontal green line in Figures 5a,b).

Figure 4 KD and surface winds in Japan (a), San Diego (b) and Hawaii (c). Time series correspond to KD incidence (red lines in a, b and c), the northwestern component of surface winds in Japan (NW-WIND, blue line in a; m/s) and the Pacific Zonal Wind Index (P-WIND, blue lines in b and c; m/s). NW-WIND is defined as the projection of the horizontal two-dimensional wind onto a unit vector in the northwest/southeast direction (i.e. cos(45°)·uwind −sin(45°)·vwind). P-WIND is defined as the mean zonal wind along 35°N between longitudes 140°E and 240°E (see the horizontal green line in Figures 5a,b). P-WIND is shown here for the surface level, but similar results were found for the middle and upper troposphere (e.g. Figure 5d). KD time series for San Diego and Hawaii are reconstructed components obtained after applying an eigendecomposition analysis to the KD data, to better isolate the annual component (see Methods). KD cases in Hawaii were accumulated prior to the eigendecomposition. Full size image

Figure 5 Tropospheric winds in the North Pacific. Climatological January values are depicted for sea level pressure (hPa, a), surface winds (m/s, a), geopotential height at 300 hPa (m, b) and winds at 300 hPa (m/s, b). Panels c and d depict the seasonal cycle of the northwestern component of tropospheric winds in Japan (NW-WIND, m/s) and of the Pacific Zonal Wind Index (P-WIND, m/s), respectively. NW-WIND is here defined as the projection of the horizontal two-dimensional wind onto a unit vector in the northwest/southeast direction (i.e. cos(45°)·u wind −sin(45°)·v wind ). P-WIND is defined as the mean zonal wind along the parallel 35°N between longitudes 140°E and 240°E (see the horizontal green line in panels a and b). Full size image

The coherence between NW-WIND, wind intensity (WI) and KD cases in Japan mirrored the relationship found for the main KD epidemics and the interannual anomalies (Figure 4a and Supplementary Figure 3a). The number of KD cases was indeed highest in winter, when strong northwesterly winds blow across Japan from central Asia. In contrast, there are fewer KD cases during the rest of the year, when winds become weaker or even change direction. Farther east, a similar coherence between the number of KD cases in San Diego and Hawaii and P-WIND was observed (Figures 4b,c and Supplementary Figure 3b,c). Minor peaks in other seasons were also evident. The Empirical Orthogonal Function (EOF) decomposition, applied to both KD and winds, recovered all main sub-annual variability portions in the two variables. Both contained a summer peak, wherein there is a return to northwesterly wind currents, consistent with the association to the major KD peak in cases in winter (Supplementary Figure 4).

The seasonal structure of NW-WIND is very similar to that of P-WIND, as both share the same driving atmospheric configuration emerging in winter in the North Pacific (Figure 5 and Supplementary Videos 1–3). In the lower troposphere, the high and low pressure areas near Siberia and the Aleutian Islands, respectively, reach the highest intensity in December and January, when they cover the whole eastern Asian continent as well as the extratropical North Pacific. During the entire winter, but especially in these months, the low level circulation sweeps from continental China, along the lower mid-latitudes and subtropical north Pacific to the west coast of the U.S. The atmospheric path is also open at higher levels of the troposphere in winter. The Pacific jet stream crosses the north Pacific and reaches its greatest strength and farthest equatorward coverage during the boreal winter. Since the free troposphere westerly winds extend more to the south, they would also blow over Hawaii at these atmospheric levels. The linking of these distant regions through wind currents might therefore explain the nearly simultaneous annual peak of KD in Japan, Hawaii and San Diego.

In spring, the Aleutian low becomes much weaker and a strong high pressure develops in the subtropical north Pacific. At the same time, northwesterly winds are interrupted in Japan and the path across the north Pacific is redirected to even higher latitudes in the North Pacific. Similarly, in the free troposphere, the atmospheric path is much weaker and shifts more to the north in spring and summer. This springtime interruption coincides with the seasonal decline in KD cases in both San Diego and Hawaii.