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Combined effects of recent Pacific cooling and Indian Ocean warming on the Asian monsoon.

Ueda H, Kamae Y, Hayasaki M, Kitoh A, Watanabe S, Miki Y, Kumai A - Nat Commun (2015)

Bottom Line: During the hiatus, the tropical Pacific Ocean displays a La Niña-like cooling pattern while sea surface temperature (SST) in the Indian Ocean has continued to increase.This SST pattern differs from the well-known La Niña-induced basin-wide cooling across the Indian Ocean on the interannual timescale.Overall, the tropical Pacific SST effect opposes and is greater than the Indian Ocean SST effect.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan.

ABSTRACT
Recent research indicates that the cooling trend in the tropical Pacific Ocean over the past 15 years underlies the contemporaneous hiatus in global mean temperature increase. During the hiatus, the tropical Pacific Ocean displays a La Niña-like cooling pattern while sea surface temperature (SST) in the Indian Ocean has continued to increase. This SST pattern differs from the well-known La Niña-induced basin-wide cooling across the Indian Ocean on the interannual timescale. Here, based on model experiments, we show that the SST pattern during the hiatus explains pronounced regional anomalies of rainfall in the Asian monsoon region and thermodynamic effects due to specific humidity change are secondary. Specifically, Indo-Pacific SST anomalies cause convection to intensify over the tropical western Pacific, which in turn suppresses rainfall in mid-latitude East Asia through atmospheric teleconnection. Overall, the tropical Pacific SST effect opposes and is greater than the Indian Ocean SST effect.

No MeSH data available.


Related in: MedlinePlus

Global anomalies of sea water temperature in the boreal summer during the hiatus period compared with typical La Niña years.(a) Observed SST anomalies for 1999–2013 minus 1979−1998. Regions of the Indian Ocean, the tropical Pacific Ocean and the tropical Atlantic Ocean bounded by thick lines correspond to the IO, TPO and ATL simulations, respectively (see Methods). (b) Composite SST anomalies for La Niña years (1950, 1954, 1955, 1964, 1970, 1971, 1973, 1975, 1984, 1985, 1988, 1995 and 1998) relative to the base period (1950–1998). (c) Longitude-depth section of SWT anomalies along the equator (5°S–5°N) during the hiatus period. (d) As for c, but for La Niña years. (e) Time series of SST anomalies from the base period (1979–1998) in the tropical Indian Ocean (30°S–20°N, 40°–120°E). Red line shows a linear trend (0.145 K decade−1).
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f2: Global anomalies of sea water temperature in the boreal summer during the hiatus period compared with typical La Niña years.(a) Observed SST anomalies for 1999–2013 minus 1979−1998. Regions of the Indian Ocean, the tropical Pacific Ocean and the tropical Atlantic Ocean bounded by thick lines correspond to the IO, TPO and ATL simulations, respectively (see Methods). (b) Composite SST anomalies for La Niña years (1950, 1954, 1955, 1964, 1970, 1971, 1973, 1975, 1984, 1985, 1988, 1995 and 1998) relative to the base period (1950–1998). (c) Longitude-depth section of SWT anomalies along the equator (5°S–5°N) during the hiatus period. (d) As for c, but for La Niña years. (e) Time series of SST anomalies from the base period (1979–1998) in the tropical Indian Ocean (30°S–20°N, 40°–120°E). Red line shows a linear trend (0.145 K decade−1).

Mentions: Figure 2a,c shows anomalies of SST and the underlying sea water temperature (SWT) during the hiatus period on the basis of historical ocean temperature analysis32. The SST patterns in the tropical Pacific Ocean (TPO) resemble the LNSST-related negative phase of the Pacific Decadal Oscillation. This indicates that natural variability accounts considerably for the observed hiatus LNSST22 rather than anthropogenic forcing (Supplementary Fig. 2, Supplementary Table 1 and Supplementary Note 1). A close relationship between the TPO and the Indian Ocean is widely recognized on the interannual timescale. In the post El Niño summer, while positive SST anomalies in the eastern Pacific vanish, or become slightly negative, positive SST anomalies persist over the tropical Indian Ocean33. The reverse is true for the tropical Indian Ocean during summer following La Niña with occupied by cold water (Fig. 2b,d). It is interesting to note that the entire Indian Ocean has continued to warm3031 since the 1950s (Fig. 2e). The SST pattern is inconsistent with the interannual relationship with La Niña-like cold SST anomalies in the TPO. In other words, the combination of the warming Indian Ocean and the Pacific LNSST is unprecedented with potentially large impact on the Asian monsoon. A number of recent studies have drawn attention to interannual as well as interdecadal variability in the Pacific Ocean and its influence on the Asian summer monsoon. Statistically, the Asian summer monsoon tends to be intensified during the La Niña condition34 and/or negative phase of the PDO (La Niña-like anomaly)35. Having noted these processes, we expect that enhancement of the Asian summer monsoon during the hiatus period. However, the observed rainfall distribution exhibits non-homogeneous pattern in the Indo-WP (Fig. 1a), which requires other mechanisms than the Pacific forcing.


Combined effects of recent Pacific cooling and Indian Ocean warming on the Asian monsoon.

Ueda H, Kamae Y, Hayasaki M, Kitoh A, Watanabe S, Miki Y, Kumai A - Nat Commun (2015)

Global anomalies of sea water temperature in the boreal summer during the hiatus period compared with typical La Niña years.(a) Observed SST anomalies for 1999–2013 minus 1979−1998. Regions of the Indian Ocean, the tropical Pacific Ocean and the tropical Atlantic Ocean bounded by thick lines correspond to the IO, TPO and ATL simulations, respectively (see Methods). (b) Composite SST anomalies for La Niña years (1950, 1954, 1955, 1964, 1970, 1971, 1973, 1975, 1984, 1985, 1988, 1995 and 1998) relative to the base period (1950–1998). (c) Longitude-depth section of SWT anomalies along the equator (5°S–5°N) during the hiatus period. (d) As for c, but for La Niña years. (e) Time series of SST anomalies from the base period (1979–1998) in the tropical Indian Ocean (30°S–20°N, 40°–120°E). Red line shows a linear trend (0.145 K decade−1).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4660201&req=5

f2: Global anomalies of sea water temperature in the boreal summer during the hiatus period compared with typical La Niña years.(a) Observed SST anomalies for 1999–2013 minus 1979−1998. Regions of the Indian Ocean, the tropical Pacific Ocean and the tropical Atlantic Ocean bounded by thick lines correspond to the IO, TPO and ATL simulations, respectively (see Methods). (b) Composite SST anomalies for La Niña years (1950, 1954, 1955, 1964, 1970, 1971, 1973, 1975, 1984, 1985, 1988, 1995 and 1998) relative to the base period (1950–1998). (c) Longitude-depth section of SWT anomalies along the equator (5°S–5°N) during the hiatus period. (d) As for c, but for La Niña years. (e) Time series of SST anomalies from the base period (1979–1998) in the tropical Indian Ocean (30°S–20°N, 40°–120°E). Red line shows a linear trend (0.145 K decade−1).
Mentions: Figure 2a,c shows anomalies of SST and the underlying sea water temperature (SWT) during the hiatus period on the basis of historical ocean temperature analysis32. The SST patterns in the tropical Pacific Ocean (TPO) resemble the LNSST-related negative phase of the Pacific Decadal Oscillation. This indicates that natural variability accounts considerably for the observed hiatus LNSST22 rather than anthropogenic forcing (Supplementary Fig. 2, Supplementary Table 1 and Supplementary Note 1). A close relationship between the TPO and the Indian Ocean is widely recognized on the interannual timescale. In the post El Niño summer, while positive SST anomalies in the eastern Pacific vanish, or become slightly negative, positive SST anomalies persist over the tropical Indian Ocean33. The reverse is true for the tropical Indian Ocean during summer following La Niña with occupied by cold water (Fig. 2b,d). It is interesting to note that the entire Indian Ocean has continued to warm3031 since the 1950s (Fig. 2e). The SST pattern is inconsistent with the interannual relationship with La Niña-like cold SST anomalies in the TPO. In other words, the combination of the warming Indian Ocean and the Pacific LNSST is unprecedented with potentially large impact on the Asian monsoon. A number of recent studies have drawn attention to interannual as well as interdecadal variability in the Pacific Ocean and its influence on the Asian summer monsoon. Statistically, the Asian summer monsoon tends to be intensified during the La Niña condition34 and/or negative phase of the PDO (La Niña-like anomaly)35. Having noted these processes, we expect that enhancement of the Asian summer monsoon during the hiatus period. However, the observed rainfall distribution exhibits non-homogeneous pattern in the Indo-WP (Fig. 1a), which requires other mechanisms than the Pacific forcing.

Bottom Line: During the hiatus, the tropical Pacific Ocean displays a La Niña-like cooling pattern while sea surface temperature (SST) in the Indian Ocean has continued to increase.This SST pattern differs from the well-known La Niña-induced basin-wide cooling across the Indian Ocean on the interannual timescale.Overall, the tropical Pacific SST effect opposes and is greater than the Indian Ocean SST effect.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan.

ABSTRACT
Recent research indicates that the cooling trend in the tropical Pacific Ocean over the past 15 years underlies the contemporaneous hiatus in global mean temperature increase. During the hiatus, the tropical Pacific Ocean displays a La Niña-like cooling pattern while sea surface temperature (SST) in the Indian Ocean has continued to increase. This SST pattern differs from the well-known La Niña-induced basin-wide cooling across the Indian Ocean on the interannual timescale. Here, based on model experiments, we show that the SST pattern during the hiatus explains pronounced regional anomalies of rainfall in the Asian monsoon region and thermodynamic effects due to specific humidity change are secondary. Specifically, Indo-Pacific SST anomalies cause convection to intensify over the tropical western Pacific, which in turn suppresses rainfall in mid-latitude East Asia through atmospheric teleconnection. Overall, the tropical Pacific SST effect opposes and is greater than the Indian Ocean SST effect.

No MeSH data available.


Related in: MedlinePlus