Influence of meridional circulation on extreme high temperature and weakened rainfall over the Yangtze River Valley in August 2022

The Yangtze River Valley (YRV) experienced record-breaking extreme high temperature and weakened rainfall events in August 2022, which resulted in severe disasters and large socioeconomic losses. The importance of the intensification and westward movement of the western Pacific subtropical high (WPSH) with abnormal subsidence has been emphasized in previous studies. However, the source of the abnormal subsidence remains unknown. This study investigates the source of the abnormal subsidence over the YRV and discusses its possible causes by adopting the three-pattern decomposition of the atmospheric circulation (3P-DGAC). Meridional circulation (MC) was the main contributor to the abnormal vertical velocity (114%), while the contribution of zonal circulation (ZC) was negative (–14%). Additionally, the negative rainfall anomaly over the YRV can be explained mainly by the MC. The anomalous MC was characterized as a ‘negative-positive-negative-positive-negative’ quintuple distribution with sinking motion over the YRV. Anomalous MC is closely related to the sea surface temperature anomaly (SSTA) over the three oceans. The negative phase of the Indian Ocean dipole (IOD) and La Niña SSTA leads to an anomalous rising motion of the ZC over the Maritime Continent, favoring the existence of the rising motion of the MC by the coupling effect. The positive phase of the North Atlantic triple (NAT) SSTA results in an anomalous Rossby wave train, which further leads to a sinking motion over the YRV.


Introduction
The occurrence and intensity of extreme high temperature events during summer have significantly increased globally with climate warming, critically impacting human health, agricultural production, and social economics (Mora et al 2017, Pu et al 2017, Zheng andWang 2019, Perkins-Kirkpatrick andLewis 2020).For example, the mean temperature over eastern China in the summer of 2013 increased by 0.82 °C, resulting in economic losses of approximately 60 billion Chinese Yuan (Sun et al 2014, Xia et al 2016).Northeast China has experienced extreme high temperature events, with surface air temperature (SAT) exceeding double the standard deviation in the summer of 2018 (Ding et al 2019, Xu et al 2019).In South China, one of the economic and population centers, there was extensive weakened rainfall due to an extreme high temperature in the Meiyu period of 2020 (Ye and Qian 2021).Therefore, it is vital to explore the underlying causes of such extreme high temperature events.
As extreme high temperature events are largely determined by the abnormal sinking motion caused by a high-pressure system, the existence of extreme high temperature events over eastern China, especially in the middle to lower Yangtze River Valley (YRV), is closely connected to the variability in the intensity and location of the western Pacific subtropical high (WPSH) (Chen et al 2019).For instance, the westward expansion of the WPSH led to an anomalous high-pressure system over the YRV, which further led to a sinking motion and the occurrence of a record-breaking summer high temperature event in 2013 (Sun et al 2014, Li et al 2015, Wang et al 2016).A positive sea surface temperature (SST) anomaly (SSTA) over the Kuroshio and its extended area led to the strengthening and northwestward shift of the WPSH by encouraging the abnormal zonal circulation (ZC) in the summer of 2018, leading to an extreme high temperature event in Northeast China (Ding et al 2019, Xu et al 2019).Therefore, to enhance our understanding of extreme high temperature events, it is crucial to increase our understanding of the variability in the WPSH and source of the anomalous sinking motion associated with the anomalous WPSH.
Extreme high temperature events occurred over the YRV in the summer of 2022, particularly in August (figure 1(a)).Accompanied by extreme high temperature events, severely weakened rainfall occurred over the YRV (figure 1(b), S1a, and S1d).The SAT and rainfall anomalies averaged over the YRV (105°E-122°E, 25°N-33°N) reached 2.68 K and -4 mm d -1 , respectively, exceeding double the standard deviations and setting record highs since 1979 (figures 1(c) and (d)).Notably, both SAT and rainfall anomalies in the region showed significant linear trends, i.e., the warming trends of the SAT anomaly (0.31 K per decade) and a decreasing trend in rainfall anomalies (-0.44 mm d -1 per decade), which may be related to global warming.Linear trends are removed to eliminate the influence of global warming.After removing the influence of global warming, the SAT anomalies (2.05 K) and rainfall anomalies (-3.1 mm d -1 ) in August 2022 remain the hottest and driest records during 1979-2022 (figures 1(e) and (f)).The outcomes computed from the Global Precipitation Climatology Project (GPCP) and the near real-time Global Satellite Mapping of Precipitation (GSMaP_NRT) rainfall data are similar to those computed from the European Center for Medium-Range Weather Forecasts Reanalysis V5 (ERA5) data (figures 1 and S1).Therefore, the occurrence of the record-breaking high temperature and weakened rainfall events may be caused by interannual variability in addition to global warming.
Several  et al (2023) proposed that local soil moisture feedback was the main contributor.Notably, although previous studies suggested different mechanisms for this extreme high temperature event, they all agree that the anomalous WPSH and associated anomalous sinking motion played a significant role.Additionally, high temperature events generally coincided with weakened rainfall events over the YRV (figures 1(g) and (h)) during 1979-2022.This finding suggests that the weakened rainfall event is closely connected to the anomalous sinking motion in August 2022.However, the source of this anomalous sinking motion accompanied by the anomalous WPSH has not been explored in previous research.Therefore, investigating the source of this anomalous sinking motion, along with the anomalous WPSH, and the resulting extreme high temperatures and weakened rainfall events over the YRV in August 2022 is imperative.
The remainder of this paper is organized as follows.Section 2 outlines the datasets and methods adopted in this study.Section 3 explores the influence of the meridional circulation (MC) on the anomalous WPSH and related extreme high temperature and weakened rainfall events over the YRV in August 2022.Lastly, section 4 summarizes the conclusions drawn from this study.

Datasets
The monthly average surface air temperature at 2m, rainfall, geopotential height, specific humidity, horizontal winds, vertical wind, surface pressure, and SST datasets for August from 1979-2022 were derived from the ERA5 reanalysis data (Hersbach et al 2020) and used in this study.The datasets were interpolated into the horizontal resolution of  ´ 0.5 0.5 .In addition to the ERA5 reanalysis data, this study also adopts rainfall data from the GPCP data with a  ´ 2 2 horizontal resolution (Adler et al 2018).The GSMaP_NRT rainfall data with a  ´ 0.1 0.1 horizontal resolution was also used.This is important to note that the temporal period of the GSMaP_NRT data is 2000-2022.The anomalies were calculated by subtracting the climatological mean values of 1991-2020, while those from the GSMaP_NRT data were based on the climatological mean values of 2001-2020.
A rectangular area (105°E-122°E, 25°N-33°N) was selected to represent the YRV region (purple boxes in figures 1(a) and (b)).As discussed in section 1, the SAT and rainfall anomalies over the YRV were the hottest and driest in August 2022 after the linear trends were removed.Therefore, the influence of global warming was eliminated by removing the linear trends of the variables in the remainder of this study.

Three-pattern decomposition of the global atmospheric circulation
The vertical velocity (w) can be divided into two elements using the three-pattern decomposition of the global atmospheric circulation (3P-DGAC) (Liu et al 2008, Hu et al 2017, 2018a, 2018b, 2020, Cheng et al 2018), i.e., the vertical velocities of MC (w H ) and ZC (w W ). Namely, w w w = + .
H W Therefore, the contribution of the MC and ZC to the abnormal subsidence accompanied by the anomalous WPSH in August 2022 can be explored by adopting the 3P-DGAC method.( ) where dP is the abnormal precipitation, and the four terms from left to right on the right-hand side of equation ( 1) are the abnormal precipitation triggered by horizontal circulation (HC), MC, ZC, and the residual term.The difference between the value for 2022 and the climatological mean value for 1991-2020 is written as d.dP M _ can be further decomposed as follows: where dMCDA M _ and dMCDD M _ (dTHA M _ and dTHD M _ ) are the abnormal rainfall related to dynamic term (thermodynamic term), which are triggered by the anomalous advection and divergence of MC.The terms on the right of equation (2) can be gotten using the following formulas: where r , W g,  V , M and q are water density, gravitational acceleration, horizontal wind, and specific humidity, respectively.The climatological mean variables for 1991-2020 are represented by the subscript 0. Similar decomposition can also be obtained for dP H _ and dP Z _ .In the present study, the novel moisture budget equation was used to explore the quantitative influence of three large circulations (i.e., HC, MC, and ZC) on the weakened rainfall over the YRV in August 2022.In addition to the two main methods proposed above, linear fitting and regression analysis were performed.Additionally, the empirical orthogonal function (EOF) decomposition was used to obtain the dominant modes of the anomalous circulations.

Source of the anomalous sinking motion over the YRV in August 2022
Figure 2(a) illustrates the 500-hPa geopotential height anomaly in August 2022.It is evident that the WPSH has considerably strengthened and shifted westward comparing the 5880-gpm isopleth in 2022 and the climatological mean.Moreover, the strengthening and westward movements persisted throughout the August 2022 (Figure S2).The abnormal geopotential height at middle to high latitudes exhibited a 'positive-negativepositive' triple distribution, resulting in a low-pressure center in the northern part of the YRV.This further hindered the northward shift of WPSH and contributed to the persistent of the WPSH in the region.The persistent sinking motion over the YRV was triggered by the anomalous WPSH (figure 3(a)), leading to extreme high temperature and weakened rainfall events over the region.This finding was verified by the regressions of the regionally averaged SAT and rainfall over the YRV onto the geopotential height (vertical velocity).The regressions correspond to the spatial distribution of the abnormal geopotential height (vertical velocity) as shown in figures 2(a)-(c) (figures 3(a)-(c)).The scatter plots in figure 4 illustrates the association between the area mean SAT (rainfall) anomalies and the vertical velocity anomalies over the YRV.Figures 4(a) and (d) show that the vertical velocity anomaly is closely related to the SAT and rainfall anomalies, with explained variances of 0.46 and 0.87, respectively.
To investigate the origin of the abnormal subsidence induced by the anomalous WPSH over the YRV, the vertical velocity was decomposed into two components using the 3P-DGAC method.Figures 3(a), (d), and (e) depict the abnormal vertical velocity and those of the MC and ZC at 500 hPa in August 2022.Figures 3(a), (d), and (e) show that MC is the primary contributor to the abnormal vertical velocity over the YRV, while the contribution of ZC was negative.Specifically, the quantitative contribution of the MC is 114%, while that of the ZC is -14%.Comparing figures 4(b) and (c) (figures 4(e) and (f)), the abnormal SAT (rainfall) is significantly related to the abnormal vertical velocity of MC, while the relationship between the anomalous SAT (rainfall) and anomalous ZC is marginal and nonsignificant, suggesting a leading role of MC in affecting the extreme high temperature and weakened rainfall events over the YRV through the excitation of the anomalous sinking motion.effect of the three-pattern circulations are rather similar.Furthermore, figure 6 shows that the area mean precipitation anomaly over the YRV triggered by the three-pattern circulations was -2.65 mm d -1 , comprising approximately 85% of the actual anomalous rainfall (-3.1 mm d -1 ).This implies that the joint effect of the three large circulations can generally explain the weakened rainfall over the YRV in August 2022.
Further decomposition of the precipitation anomaly triggered by the three-pattern circulations is displayed in figures 5(b)-(d).The precipitation anomaly triggered by the MC resembles that triggered by the three large circulations (figures 1(b) and 5(a)).Moreover, the regionally averaged precipitation anomaly caused by MC over the YRV was -2.38 mm d -1 , approximately 77% of the factual precipitation anomaly (figure 6).However, the  regionally averaged rainfall anomalies caused by HC and ZC over the YRV were merely -0.05 and -0.22 mm d -1 , which can explain 1% and 7% of the factual precipitation anomaly (figure 6).Therefore, the analysis concludes that the MC played a leading role in the extreme weakened rainfall event in August 2022.
The contribution of the MC to the rainfall anomaly can be decomposed into four components using equation (2).dMCDD M _ was the main contributor to the rainfall anomaly triggered by the MC when comparing figures 5(c) and (f), while the contributions of the other three components were small, implying that the weakened rainfall over the YRV in August 2022 is mainly controlled by the divergence of the abnormal MC, and role of moisture change is negligible (see 'Physical explanation of dMCDD M _ ' in the Supplementary Material).Specifically, the quantitative rainfall anomaly over the YRV caused by dMCDD M _ was -2.58 mm d -1 , which can explain 83% of the factual precipitation anomaly.
As the moisture is generally located below 500 hPa (Cheng et al 2023), integrating equation (3) up to 500 hPa does not generally alter the outcome of the analysis (comparing figures 5, 6(a) and S3, 6(b)).Therefore, the result obtained from the novel moisture budget equation theoretically support the conclusion that the MC dominated the extreme high temperature and weakened rainfall events in August 2022.

Characteristics and possible causes of the anomalous MC in August 2022
As the MC dominated the extreme high temperature and weakened rainfall events in August 2022, the characteristics and possible causes of the anomalous MC in August 2022 should be investigated.Figure 7(a) displays the zonal mean vertical velocity anomaly of MC between 105°E-122°E.The abnormal vertical velocity of MC was characterized as a 'negative-positive-negative-positive-negative' quintuple distribution with sinking motion over the YRV (figure 7(a)).The quintuple distribution of the MC can also be found in other high temperature and weakened rainfall events, and the MC in the years of high temperature and weakened rainfall  events and that in the years of low temperature and enhanced rainfall events are nearly symmetric (Figure S4).The spatial distribution of the regressions of the area mean SAT and rainfall over the YRV onto the zonally averaged vertical velocity of the MC resembles that of the actual vertical velocity anomaly of the MC (comparing figures 7(a)-(c)), implying that the quintuple distribution of the vertical velocity anomaly of the MC is the main contributor to extreme high temperature and weakened rainfall events in August 2022.
To explore the possible causes of the abnormal MC in August 2022, an EOF analysis was applied to the vertical velocity anomaly of the MC in August (figure 8).Generally, the variances of the three leading empirical orthogonal function modes (EOFs) are 32%, 21%, and 15%, respectively.The first leading mode (EOF1) is characterized as a 'negative-positive-negative' triple distribution with rising motion over the YRV (figure 8(a)).The corresponding time series of EOF1 (i.e., PC1) is nearly zero in 2022 (figure 8(d)), implying that the anomalous MC is unrelated to EOF1.However, it should be noted that PC1 reaches the third largest value in 2021 since 1979, favoring the sinking motion south of the YRV and the rising motion over the YRV, which further leads to the reoccurrence of Meiyu in August 2021.The second leading mode (EOF2) and third leading mode (EOF3) are both characterized as the 'negative-positive-negative-positive-negative' quintuple distribution (figures 8(b) and (c)).However, the range and intensity of the centers are different in EOF2 and EOF3.PC2 is almost zero, and PC3 reaches its largest value since 1979 (figures 8(e) and (f)), implying that the anomalous MC in August 2022 is closely connected to EOF3.
To verify the results proposed above, the regression of PC3 onto the 500-hPa geopotential height, SAT, and rainfall in August during 1979-2022 (figure 9) was carried out.Figure 9 shows that when PC3 is positive, there is an abnormally strong WPSH, resulting in anomalous high temperatures and negative rainfall over the YRV.Namely, the spatial distributions of the regressions and those of the anomalous geopotential height, SAT, and rainfall are rather similar (comparing figures 9(a)-(c) and 1(a), (b), 2(a)).Therefore, the causes of the highest PC3 values in 2022 should be investigated.
To investigate the possible causes of the largest value of PC3 in 2022, this study regressed PC3 onto the SST in August during 1979-2022 (figure 10).The regression field of SST resembles the actual SSTA (comparing figures 10(a) and (b)), implying that the SSTA played a crucial role in the abnormal MC.The SSTAs are mainly characterized by the following three features.First, the SSTA is negative (positive) over the tropical west (southeast) Indian Ocean, which is an Indian Ocean dipole (IOD) pattern.Second, the SSTA is characterized as positive (negative) in the tropical west (east) Pacific, which is a La Niña pattern.Third, the North Atlantic SSTA is characterized by a North Atlantic triple (NAT) pattern with a positive (negative) SSTA over the central (north and south) North Atlantic.
Three SSTA indices are adopted in this study to explore the impact of SSTA on MC.The area mean SSTA over the tropical central-eastern (5°S-5°N, 160°E-150°W) Pacific is used to define the Niño 4 index Figure 11 displays the three SSTA indices and the regressions between the three SSTA indices and the vertical velocity of the MC. Figure 11 shows that the Niño 4 index (multiplied by -1) is the second largest, and the IOD index (multiplied by -1) and NAT SSTA index reach the largest since 1979 (figures 11(a), (d), (g)).After removing the influences of PC1 and PC2 using multivariate linear fitting, the main conclusions remain unchanged (comparing the first and second columns of figure 11).The spatial patterns of the regressions are all characterized as the 'negative-positive-negative-positive-negative' quintuple distribution, which is similar to EOF3, confirming the significant influence of the SSTA.high-pressure anomalies, which can further result in a sinking motion over the YRV.Therefore, the joint effect of the SSTA over the three oceans leads to the anomalous MC, which ultimately results in extreme high temperature and weakened rainfall events in August 2022.

Conclusions
In the summer of 2022, record-breaking extreme high temperature and weakened rainfall events occurred over the YRV, resulting in severe disasters and large socioeconomic losses.The causes of these climate extremes have been explored in previous research, emphasizing the vital role of the increase and westward shift of the WPSH with abnormal sinking motion.However, the origin of the anomalous subsidence remained unclear.This study investigated the origin of anomalous subsidence over the YRV and possible causes by adopting the 3P-DGAC method and the novel moisture budget equation.The conclusions are described as follows.
(1) The SAT anomaly was 2.68 K and reached its hottest over the YRV in August 2022 since 1979, while the rainfall anomaly was -4 mm d -1 reaching its driest.After removing the linear trends, the SAT (2.05 K) and rainfall (-3.1 mm d -1 ) anomalies still remain the hottest and the driest records.
(2) The persistent sinking motion over the YRV was responsible for the extreme high temperature and weakened rainfall events.The MC (114%) was the primary contributor to the anomalous vertical velocity, while the contribution of ZC was negative (-14%).
(3) The area mean precipitation anomaly in the YRV triggered by the three large circulations was -2.65 mm d -1 , which explained 85% of the actual anomalous rainfall (-3.1 mm d -1 ), implying that the joint effect of the three large circulations can generally explain the weakened rainfall over the YRV.Additionally, the area mean rainfall anomaly triggered by MC over the YRV was -2.38 mm d -1 , explaining 77% of the actual rainfall anomaly.
(4) The abnormal vertical velocity of MC was characterized as the 'negative-positive-negative-positivenegative' quintuple distribution with sinking motion over the YRV, which is similar to EOF3.Additionally, PC3 reaches its largest value since 1979, implying that the anomalous MC is closely connected to EOF3.
(5) The joint effect of the SSTAs over three oceans leads to the anomalous MC.The negative phase of the IOD and La Niña SSTA leads to anomalous rising motion of the ZC over the Maritime Continent, favoring the rising motion of the MC through the coupling effect.The positive phase of the NAT SSTA leads to the anomalous Rossby wave train, which further resulting in sinking motion over the YRV.
Notably, the conclusions proposed above are mainly based on data analysis.Although theoretical tools including the 3P-DGAC method and the novel moisture budget equation were adopted in this study, further analysis based on model simulations is needed in future investigations.Additionally, the mechanisms for the westward movement of the WPSH in August 2022 should also be investigated in the future.While the mechanism proposed here may not be the same to other instances of high temperature and weakened rainfall events, similar processes of this study can be repeated for the investigation of other similar events in the future.
studies have investigated the underlying causes of the extreme high temperature event in August 2022 (Chen and Li 2023, He et al 2023, Jiang et al 2023, Tang et al 2023, Wang et al 2023).Wang et al (2023) suggested that high temperature events were regulated by anomalous anticyclones over northeastern China.Tang et al (2023) emphasized the significance of abnormal Pakistan floods and triple-dip La Niña events.Jiang

Figure 1 .
Figure 1.Spatial distribution of the (a) SAT (K) and (b) rainfall (mm d -1 ) anomalies in August 2022 derived from the ERA5 reanalysis data.The purple rectangle (105°E-122°E, 25°N-33°N) indicates the YRV region.(c) and (d) Time series of the regionally averaged (c) SAT and (d) rainfall anomalies over the YRV in August during 1979-2022.The red solid lines indicate the linear trends, and the dashed lines denote anomalies exceeding double the standard deviations.(e) and (f) Same as (c) and (d) except for the detrended (e) SAT and (f) rainfall anomalies.(g) Scatter plot between the area mean SAT anomaly and the precipitation anomaly over the YRV in August during 1979-2022.The blue dot represents 2022, and the black dots represent the other years.The red solid line is the linear fitting line, and the blue dashed lines represent anomalies exceeding double the standard deviations.The correlation coefficient and explained variance are marked in the upper-right corner, and the asterisks indicate that the significance of the linear correlation exceeds the 95% confidence level.(h) Same as (g) except for the scatter plot between the detrended SAT anomaly and the detrended rainfall anomaly.
2.2.2.Novel moisture budget equation of the three-pattern circulations In recent studies, a novel moisture budget equation of the three-pattern circulations (Han et al 2021, Cheng et al 2022, 2023) was established by combining the 3P-DGAC method with the moisture budget equation (Seager et al 2010).
3.2.Quantitative contribution of three-pattern circulations to the weakened rainfall over the YRV in August 2022 Figure5(a) displays the rainfall anomaly caused by the joint effects of the HC, MC, and ZC in August 2022.Figures5(a) and 1(b) show that the spatial pattern of the precipitation anomaly and that triggered by the joint

Figure 2 .
Figure 2. (a) Geopotential height anomaly (gpm) at 500 hPa in August 2022.Green and black lines represent the 2022 and climatological 5880-gpm isopleths, respectively.(b) Regression of the regionally averaged SAT (K) over the YRV onto the geopotential height at 500 hPa in August during 1979-2022.Black (purple) stippling shows that the significance of the regression coefficient exceeds the 95% (90%) confidence level.(c) Same as (b) except for the regression of the area mean precipitation (mm d -1 , multiplied by -1) over the YRV onto the geopotential height.

Figure 3 .
Figure 3. (a) 500-hPa vertical velocity anomaly (Pa s -1 ) in August 2022.(b) Regression of the regionally averaged SAT (K) over the YRV onto the 500-hPa vertical velocity in August during 1979-2022.Black (purple) stippling shows that the significance of the regression coefficient exceeds the 95% (90%) confidence level.(c) Same as (b) except for the regression of the area mean precipitation (mm d -1 , multiplied by -1) over the YRV onto the 500-hPa vertical velocity.(d) and (e) Same as (a) except for the 500-hPa vertical velocity anomaly of (d) MC and (e) ZC.The quantitative contributions to the abnormal velocity anomaly are marked in the upper-right corner.

Figure 4 .
Figure 4. (a) Scatter plot between the area mean SAT anomaly (K) and the vertical velocity anomaly (Pa s -1 ) at 500 hPa over the YRV in August during 1979-2022.The blue dot represents 2022, and the black dots represent the other years.The red line represents the linear fitting line, and the blue dashed lines represent anomalies exceeding double the standard deviations.The explained variance is marked in the upper-right corner, and the asterisk indicates that the significance of the linear correlation exceeds the 95% confidence level.(b) and (c) Same as (a) except for the scatter plots between the abnormal SAT and the abnormal vertical velocity of (b) MC and (c) ZC. (d)-(f) Same as (a)-(c) except for the scatter plots between the area mean precipitation anomaly (mm d -1 , multiplied by -1) and area mean 500-hPa vertical velocity anomalies.

Figure 6 .
Figure 6.(a) Area mean precipitation anomaly (dP) and the abnormal precipitation triggered by changes in the moisture budget components over the YRV in August 2022.The quantitative contributions to dP are marked on the bottom. (b) Same as (a) except for the outcome when integrating equation (3) up to 500 hPa.Units for the precipitation anomalies are mm d -1 .

Figure 7 .
Figure 7. (a) Zonal mean vertical velocity anomaly of MC (Pa s -1 ) between 105°E and 122°E.(b) Regression of the regionally averaged SAT (K) over the YRV onto the zonally averaged (105°E-122°E) vertical velocity of MC in August during 1979-2022.Black (purple) stippling shows that the significance of the regression coefficient exceeds the 95% (90%) confidence level.(c) Same as (b) except for the regression of the area mean precipitation (mm d -1 , multiplied by -1) over the YRV onto the zonally averaged vertical velocity.

Figure 9 .
Figure 9. Regression of PC3 onto the (a) 500-hPa geopotential height, (b) SAT, and (c) rainfall in August during 1979-2022.Black (purple) stippling shows that the significance of the regression coefficient exceeds the 95% (90%) confidence level.Units for the geopotential height, SAT, and rainfall anomalies are gpm, K, and mm d -1 .

Figure 10 .
Figure 10.(a) Spatial distribution of the SST anomaly in August 2022.(b) Regression of PC3 onto the SST in August during 1979-2022.Black (purple) stippling shows that the significance of the regression coefficient exceeds the 95% (90%) confidence level.Units for the SST anomalies are K.

Figure 11 .
Figure 11.(a) Time series of the Niño4 index (K, multiplied by -1) in August during 1979-2022.(b) Same as (a) except for the time series with PC1 and PC2 removed by linear regression.(c) Regression of the Niño4 index in (b) onto the zonally averaged (105°E-122°E) vertical velocity of MC (Pa s -1 ) in August during 1979-2022.Black (purple) stippling shows that the significance of the regression coefficient exceeds the 95% (90%) confidence level.(d)-(f) and (g)-(i) Same as (a)-(c) except for the results of the IOD index (K, multiplied by -1) and NAT SSTA index (K).

Figure 12 .
Figure 12.(a) Meridional mean ZC between 5°S-5°N (shading: vertical velocity of ZC, vectors: zonal and vertical velocities of ZC).The vertical velocities are multiplied by -50.(b) Regression of the Niño4 index in figure 11(b) onto the meridionally averaged (5°S-5°N) ZC in August during 1979-2022.Black (purple) stippling shows that the significance of the regression coefficient between the vertical velocity and Niño4 index exceeds the 95% (90%) confidence level.(c) Same as (b) except for the regression of the IOD index in figure 11(e) onto the meridionally averaged (5°S-5°N) ZC.Units for the Niño4 index and IOD index are K, while those for the vertical velocity and zonal velocity anomalies are Pa s -1 and m s -1 .