Abstract

With the twentieth century analysis data (1901–2002) for atmospheric circulation, precipitation, Palmer drought severity index, and sea surface temperature (SST), we show that the Asian-Pacific Oscillation (APO) during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies, especially the Northern Hemisphere (NH) summer land monsoon. Associated with a positive APO phase are the warm troposphere over the Eurasian land and the relatively cool troposphere over the North Pacific, the North Atlantic, and the Indian Ocean. Such an amplified land–ocean thermal contrast between the Eurasian land and its adjacent oceans signifies a stronger than normal NH summer monsoon, with the strengthened southerly or southwesterly monsoon prevailing over tropical Africa, South Asia, and East Asia. A positive APO implies an enhanced summer monsoon rainfall over all major NH land monsoon regions: West Africa, South Asia, East Asia, and Mexico. Thus, APO is a sensible measure of the NH land monsoon rainfall intensity. Meanwhile, reduced precipitation appears over the arid and semiarid regions of northern Africa, the Middle East, and West Asia, manifesting the monsoon-desert coupling. On the other hand, surrounded by the cool troposphere over the North Pacific and North Atlantic, the extratropical North America has weakened low-level continental low and upper-level ridge, hence a deficient summer rainfall. Corresponding to a high APO index, the African and South Asian monsoon regions are wet and cool, the East Asian monsoon region is wet and hot, and the extratropical North America is dry and hot. Wet and dry climates correspond to wet and dry soil conditions, respectively. The APO is also associated with significant variations of SST in the entire Pacific and the extratropical North Atlantic during boreal summer, which resembles the Interdecadal Pacific Oscillation in SST. Of note is that the Pacific SST anomalies are not present throughout the year, rather, mainly occur in late spring, peak at late summer, and are nearly absent during boreal winter. The season-dependent APO–SST relationship and the origin of the APO remain elusive.

Highlights

  • In the past five decades, the global land monsoon precipitation showed an overall weakening (Wang and Ding 2006), which is mainly associated with the decreases of the North African and South Asian precipitation (Zhou et al 2008a)

  • With the twentieth century analysis data (1901–2002) for atmospheric circulation, precipitation, Palmer drought severity index, and sea surface temperature (SST), we show that the Asian-Pacific Oscillation (APO) during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies, especially the Northern Hemisphere (NH) summer land monsoon

  • Over the winter Southern Hemisphere (SH) (Fig. 3), corresponding to a positive APO, low-level anomalous anticyclonic systems occur over the subtropics, indicating the strengthened subtropical high

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Summary

Introduction

In the past five decades, the global land monsoon precipitation showed an overall weakening (Wang and Ding 2006), which is mainly associated with the decreases of the North African and South Asian precipitation (Zhou et al 2008a). Since 1980, the global land monsoon precipitation did not show a significant trend These results suggest that the variation over the last 50 years may be a part of longer time scale variations. Zhao et al (2007, 2010a) found that the variability of a summertime large-scale teleconnection pattern over the extratropical Northern Hemisphere (NH), with anomalous centers over the Asian-North Pacific sector, referred to as the Asian–Pacific Oscillation (APO), is highly correlated to the major atmospheric circulation systems over the NH and to the tropical–extratropical North Pacific SST. How are the NH summer land monsoon (NHSM) and tropospheric atmospheric circulation and hydroclimate (including surface air temperature, precipitation, and drought) related to the APO during boreal summer? With this question in mind, we conduct this study to systematically examine the varying features of summer APO-related atmospheric circulation, temperature, rainfall, drought, and SST as well as the NHSM using the 1901–2002 datasets. Following Zhao et al (2011a, b), in the present study, a mean of May, June, July, August, and September (MJJAS) is used to represent boreal summer

Definition and variability of the APO index
Dynamic structure of the APO
Northern Hemisphere summer monsoons associated with the APO
Surface air temperature
Palmer drought severity index
APO and lower boundary forcing over oceans
Findings
Summary and discussion
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