The tropospheric biennial oscillation defined by a biennial mode of sea surface temperature and its impact on the atmospheric circulation and precipitation in the tropical eastern Indo-western Pacific region

Abstract Multidecadal variations in the global land monsoon were observed during the twentieth century, with an overall increasing trend from 1901 to 1955 that was followed by a decreasing trend up to 1990, but the mechanisms governing the above changes remain inconclusive. Based on the outputs of two atmospheric general circulation models (AGCMs) forced by historical sea surface temperature (SST) covering the twentieth century, supplemented with AGCM simulations forced by idealized SST anomalies representing different conditions of the North Atlantic and tropical Pacific, evidence shows that the observed changes can be partly reproduced, particularly over the Northern Hemisphere summer monsoon (NHSM) domain, demonstrating the modulation of decadal SST changes on the long-term variations in monsoon precipitation. Moisture budget analysis is performed to understand the interdecadal changes in monsoon precipitation, and the dynamic term associated with atmospheric circulation changes is found to be prominent, while the contribution of the thermodynamic term associated with humidity changes can lead to coincident wetting over the NHSM domain. The increase (decrease) in NHSM land precipitation during 1901–55 (1956–90) is associated with the strengthening (weakening) of NHSM circulation and Walker circulation. The multidecadal scale changes in atmospheric circulation are driven by SST anomalies over the North Atlantic and the Pacific. A warmer North Atlantic together with a colder eastern tropical Pacific and a warmer western subtropical Pacific can lead to a strengthened meridional gradient in mid-to-upper-tropospheric thickness and strengthened trade winds, which transport more water vapor into monsoon regions, leading to an increase in monsoon precipitation.

Download Full-text

Coupled Mg/Ca and clumped isotope measurements at IODP Site U1488 confirm absence of Plio-Pleistocene sea surface temperature cooling in the Western Pacific Warm Pool

10.5194/egusphere-egu2020-16364 ◽

2020 ◽

Author(s):

Niklas Meinicke ◽

Maria Reimi ◽

Christina Ravelo ◽

Nele Meckler

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Planktonic Foraminifera ◽

Warm Pool ◽

Western Pacific ◽

Sea Surface ◽

Western Pacific Warm Pool ◽

Pacific Warm Pool ◽

The Western Pacific

The Western Pacific Warm Pool (WPWP) as a major source of heat and water vapor has a crucial influence on climate dynamics both in the tropics and globally. Yet, there is conflicting proxy evidence regarding the evolution of WPWP temperatures since the Miocene. On the one hand TEX86 data suggest a gradual cooling by ~2&#8451; (O&#8217;Brian et al., 2014, Zhang et al., 2014) from the Pliocene to today, while faunal (planktonic foraminifera) sea surface temperature estimates (Dowsett, 2007) and Mg/Ca data measured in planktonic foraminifera (Wara et al., 2005) on the other hand indicate the absence of any long-term temperature trends. It has been suggested that Mg/Ca temperatures could on these time scales be biased by long-term changes of the Mg/Ca ratio of seawater (Evans et al., 2016). To test the influence of the proposed seawater changes on Mg/Ca we combined data from two independent temperature proxies, Mg/Ca and clumped isotopes, measured on two species of planktonic foraminifera from IODP Site U1488 in the central WPWP. Our study finds good agreement between both proxies thereby verifying the validity of Mg/Ca records from the WPWP and confirming the absence of a Plio-Pleistocene cooling trend for the WPWP. This finding suggests that the persistent disagreement between foraminifer-based proxies such as Mg/Ca and biomarker data might be caused by different environmental parameters being recorded in the two archives.&#160;References:O&#8217;Brien CL, Foster GL, Mart&#237;nez-Bot&#237; MA, Abell R, Rae JWB, Pancost RD. High sea surface temperatures in tropical warm pools during the Pliocene. Nature Geoscience. 2014;7(8):606-11.Zhang YG, Pagani M, Liu Z. A 12-million-year temperature history of the tropical Pacific Ocean. Science. 2014;344(6179):84-7.Dowsett H. Faunal re-evaluation of Mid-Pliocene conditions in the western equatorial Pacific. Micropaleontology. 2007;53(6):447-56.Wara MW, Ravelo AC, Delaney ML. Permanent El Nino-like conditions during the Pliocene warm period. Science. 2005;309(5735):758-61.Evans D, Brierley C, Raymo ME, Erez J, M&#252;ller W. Planktic foraminifera shell chemistry response to seawater chemistry: Pliocene&#8211;Pleistocene seawater Mg/Ca, temperature and sea level change. Earth and Planetary Science Letters. 2016;438:139-48.

Download Full-text