HTML
-
To find the interrelationship between the extratropical atmospheres of both hemispheres, we calculated the departures of spring areal mean surface pressure at 60°−90°N and 60°−90°S from 1979 to 2017 (Fig. 2a). The variations in high-latitude mean surface pressure are comparable, but with opposite sign in both of the hemispheres. The correlation coefficient reaches −0.304 at the significance level of 0.06 based on the t-test. This indicates that the air mass distributes in a seesaw pattern between the hemispheres during the spring season. Nevertheless, there is no such clear relationship between 0°−60°N and 60°−90°S, with a correlation coefficient of 0.02 (figure not shown). This fact shows that the interhemispheric surface pressure oscillation exhibits the inter-association of varying atmospheric mass mainly at bihemispheric extratropics. As documented in Table 1, the anticorrelation of regional mean SAP anomalies between the northern high latitudes (NHh) and the southern ones (SHh) is indeed most significant in boreal spring compared to other seasons. Furthermore, the correlation coefficient between IHO and both NHh and SHh in spring is 0.61 and −0.62 respectively, with the highest significance (> 0.01) among all the four seasons. Thus, IHO shows notable seasonality, which is consistent with Guan et al. (2010). In particular, the spring IHO links closely with the interhemispheric unbalance and connection of air mass, and is especially pronounced over high latitudes.
Figure 2. (a) Spring area-weighted mean surface pressure over the Arctic (solid line) and Antarctic (dashed line) regions. (b, c) Correlation coefficients between IHO and (b) surface pressure and (c) anomalous wind (units: m s−1) at 850 hPa regressed upon IIHO. The dotted areas in (b) denote values exceeding the 90% confidence level
NHh & SHh IHO & NHh IHO & SHh DJF 0.19 0.10 0.59*** MAM −0.30* 0.61*** −0.62*** JJA −0.09 0.35** −0.61*** SON −0.05 0.33** −0.46*** Note: Values with one, two and three superscript asterisks are statistically significant at the 0.1, 0.05 and 0.01 level, respectively, based on the t-test. Table 1. Correlation coefficients of IHO and regional mean SAP
IHO-associated surface pressure shows antiphase features between both the bihemispheric extratropics (Fig. 2b). In particular, the synchronous IHO−surface pressure correlation coefficients are positive (negative) in the NH (SH), with remarkable correlations dominantly at the bihemispheric extratropics. The perceptible positive correlation regions are largely in middle−western Eurasia, northern North America and the Arctic Ocean, in comparison to the latitudinal bands in 60°−90°S that have significant negative correlations generally. The significant positive correlation regions are also present in the middle and southern regions of Africa and South America and in the Northwest Pacific, while a large area of negative correlation is shown in the Southeast Pacific. As shown in the Appendix, the IHO indices and their correlation with SAP are highly consistent between NCEP1 and NCEP2 reanalysis data. Therefore, the following analysis will be based on NCEP1.
As shown in the regression coefficients of 850-hPa wind anomalies upon IHO (Fig. 2c), the IHO effect changes the surface pressure, which to a great extent determines the low-level winds. In particular, the positive pressure anomalies in the northern high latitudes decrease the meridional pressure gradient and thus decelerate the westerlies over the north of 60°N, while accelerated westerlies display over its SH counterpart. Furthermore, notable wave-train-like flow can be found in midlatitudes of both the SH and NH, indicating a potential interaction with stationary waves accompanied by IHO-associated large-scale airmass redistribution.
The above results show that the IHO indices are closely connected with the redistributed air mass at the extratropics in both hemispheres, and with the teleconnection between extratropical bihemispheric circulations. Therefore, the IHO may act as the bridge for the interaction of circulations in both hemispheres, which can be used to explain the seesaw feature of Fig. 2a.
-
AO and AAO are the leading modes of atmospheric low-frequency variation at the extratropics for both hemispheres. Guan and Yamagata (2001) performed an EOF decomposition of zonally averaged surface pressure anomalies on a monthly basis, reaching AAO as the first mode, AO as the second mode, and IHO as the third mode, with variance contributions of 36.4%, 21.1% and 14.5%, respectively. These results indicated that the modes have a noticeable effect on the redistribution of atmospheric mass at a large scale. To reveal their interrelationships in spring, the correlation coefficients are computed between AAO and IHO, as well as AO and IHO, at the different levels given in Table 2, where the AAO calculation starts from 700 hPa because of the terrain. It is seen that AAO and AO are not closely related at any level except 700 hPa, which is in rough agreement with Baldwin and Thompson (2009), who showed no clear interaction happening between AAO and AO on a synchronous basis. However, there is a strong correlation between AAO/AO and IHO at these levels. In particular, IHO is positively correlated with AAO below 50 hPa, with correlation coefficients decreasing as a function of increasing height. In contrast, IHO and AO are negatively correlated from the troposphere to lower stratosphere. The above analysis demonstrates that the bihemispheric extratropical circulations are tied to IHO despite less inter-association between AAO and AO in the spring. In addition, the well-defined correlations between IHO and AAO/AO from the surface to the lower stratosphere show evidence for the associated troposphere−stratosphere atmospheric interaction.
Level (hPa) IHO&AAO IHO&AO AAO&AO 1000 − −0.57 − 925 − −0.57 − 850 − −0.56 − 700 0.61 −0.53 −0.35 600 0.58 −0.52 −0.31 500 0.55 −0.51 −0.26 400 0.51 −0.49 −0.19 300 0.46 −0.48 −0.14 250 0.45 −0.48 −0.11 200 0.44 −0.45 −0.08 150 0.43 −0.39 −0.02 100 0.40 −0.33 0.03 70 0.37 −0.29 0.05 50 0.33 −0.25 0.07 30 0.23 −0.19 0.12 20 0.17 −0.13 0.18 10 0.10 −0.02 0.27 Note: Bold numbers denote statistically significant values at the 0.05 level based on the t-test. Table 2. Correlation coefficients of IHO with AAO and AO, as well as AAO with AO, at all pressure levels
-
The redistribution of air masses is accomplished via wind transport, while the related pressure gradient change in turn modifies the wind field. To further study bihemispherical atmospheric linkage associated with IHO, we conduct regression analysis on zonal wind and meridional mass transportation based on the IHO index. The regressed anomalous zonal winds are characterized by a longitudinal teleconnection (Fig. 3a): a wavenumber 5 pattern is well organized in the middle to upper troposphere and lower stratosphere, and it extends meridionally from the Arctic to Antarctic regions. The maximum (minimum) values are centered at approximately 60°S (75°N) in the lower troposphere (stratosphere).
Figure 3. Coefficients of (a) phase mean zonal wind anomalies (shaded; units: m s−1) and (b) averaged meridional streamfunction anomalies (shaded; units: 109 kg s−1) regressed upon normalized IHO, with the contours (spaced at 20 × 109 kg s−1) in (b) denoting the climatology of mean meridional streamfunction. The dotted areas delineate the F-test significance values at 0.10
The connection between IHO and the wind field can also be shown by the pattern of anomalies of zonal mean meridional mass streamfunction (
$\Psi $ ). The$\Psi $ is calculated by a method of iteration using zonally averaged meridional winds (Qin et al., 2006). As depicted in Fig. 3b, the$\Psi $ anomalies notably modulate the classic tri-cell circulations; regions of remarkable positive values are at the bihemispheric extratropics, centered at approximately 60°S and 65°N in the lower troposphere, in agreement with the large-value bands of zonal wind anomalies in Fig. 3a. This suggests that the atmospheric mass redistribution in association with IHO gives rise to the change in pressure gradients, thereby making the adjusted wind field associated with the meridional transport of air mass. In addition, the rising (sinking) branch of the low-latitude Hadley circulation cell corresponds to the positive (negative) anomalies, indicating that when interhemispheric exchange is strengthened (i.e., the amplification of IIHO), the related Hadley-cell transport is accelerated accordingly. Trenberth et al. (1998) showed that ENSO events play a driving role in extratropical circulation anomalies. Previous studies have also suggested that there is a close connection between tropical Pacific SST and extratropical atmospheric teleconnections (e.g., Alexander et al., 2002; Chen et al., 2014, 2015, 2017, 2018). As shown in Fig. 4, large areas of significant positive correlation coefficient between IHO and SST present in the middle and east of the tropical Pacific, potentially leading to an enhanced local Hadley circulation. SST anomalies and accompanying air−sea interaction over these areas may play an important role in triggering the atmospheric teleconnections between the northern high latitudes and its SH counterpart.Figure 4. Correlation coefficients between IHO and SST. Dotted areas denote values exceeding the 90% confidence level
Figure 3 presents a marked meridional teleconnection pattern of winds between the middle−upper troposphere and lower stratosphere. This inspired us to further examine the zonal wind distributions at 250- and 500-hPa in Figs. 5a and b. The IHO-related 250- and 500-hPa zonal winds are clearly dominated by a meridional teleconnection. In particular, alternate positive−negative zonal bands are notably obvious in middle−eastern Eurasia, and the South/North Pacific and Atlantic. The meridional teleconnection of wind anomalies establishes the link between extratropical air masses in both hemispheres. These zonal wind anomalies are likely to produce effects on the midlatitude eddy−flow interplay, probably associated with overturning circulation and eddy dynamics (Chang, 1998; Seager et al., 2003), which may lead to an inter-association of tropical and extratropical air (Thompson and Lorenz, 2004).
-
The pattern of large-scale airmass anomalies is likely to change atmospheric angular momenta. Here, we address the maintenance mechanism of the interrelation between bihemispheric air at the extratropics through the changes in global angular momenta on the basis of its conservation, for which we use the coefficients of IHO regressed upon zonal mean wind
$\bar u$ and surface pressure$\overline {{p_{\rm{s}}}} $ to calculate associated omega angular momentum (${m_\Omega }$ ) and relative angular momentum (${m_{\rm{r}}}$ ) with Eq. (2). In Fig. 6, we see that the pattern of IHO-associated${m_\Omega }$ anomalies exhibits a seesaw feature, with high values in the north and low values in the south, wherein the large values are mainly at the extratropics of both hemispheres, with the positive-value band(s) over 30°−90°N and negative-value center at approximately 60°S. According to Eq. (2),${m_\Omega }$ ($\varphi $ ) is proportional to$\overline {{p_{\rm{s}}}} \cos \varphi $ , thereby demonstrating that the distribution of${m_\Omega }$ anomalies is closely linked to the redistributed air mass from interhemispheric mass oscillation.Figure 6. IHO-regressed zonal mean mΩ (black), mr(blue) and (mΩ+mr) (red), in units of 1023 kg2 m2 s−1
The numerical change in IHO-regressed
${m_{\rm r}}$ is greater in comparison to${m_\Omega }$ , and the pattern of${m_{\rm{r}}}$ anomalies exhibits a distinct meridional waveform. This resembles the pattern of$\bar u$ anomalies that are remotely related longitudinally on a global basis, showing that relative angular momenta play a role in maintaining the distribution of zonal wind anomalies. In view of the fact that the numerical change in${m_{\rm{r}}}$ significantly exceeds that in${m_\Omega }$ , the distribution of anomalies of IHO-related total angular momenta (${m_\Omega }$ +${m_{\rm{r}}}$ ) approximates the pattern of${m_{\rm{r}}}$ . Deserving of attention is the fact that the difference between (${m_\Omega }$ +${m_{\rm{r}}}$ ) and${m_{\rm{r}}}$ is pronounced at the extratropics (from 45° to the polar region in both hemispheres). Conservation of global angular momentum suggests that the effect of extratropical airmass anomalies on angular momentum is strengthened such that the combination of IHO-associated${m_{\rm{r}}}$ and${m_\Omega }$ anomalies can maintain the teleconnection of extratropical air masses in both hemispheres. On the other hand, AO- and AAO-related${m_\Omega }$ and${m_{\rm{r}}}$ present opposite spatial distributions primarily at bihemispheric middle−high latitudes—a result that is in agreement with that reported in von Storch (2000).