【Introduction】

     Up to now, based on various observations and numerical simulation results, the long-term changes in present and future tropical Pacific Walker circulation (PWC) are still under debate. To understand the PWC changes under different climate background states, we examined the intensity and position of the tropical PWC during the mid-Holocene (6000 years ago) using all available 29 numerical simulations from the Paleoclimate Modelling Intercomparison Project Phases 2 and 3. Based on the zonal mass streamfunction, the annual mean of the PWC intensity strengthened (with an average increase of 5%), and both the western edge and center of the PWC cell shifted westward (by an average of 4° and 3°, respectively) compared to the pre-industrial period in the majority of the 29 models during the mid-Holocene. Those changes were close related to an overall increase in the equatorial Indo-Pacific east–west sea level pressure difference and low-level trade winds over the equatorial Pacific. Annual mean PWC changes come mainly from boreal warm seasons. In response to the mid-Holocene orbital forcing, Asian and North African monsoon rainfall was strengthened due to large-scale surface warming in the northern hemisphere in boreal warm seasons, which led to an intensified large-scale thermally direct east–west circulation, resulting in the enhancement and westward shift of the tropical PWC. The opposite occurred during the mid-Holocene boreal cold seasons. Taken together, the change in the monsoon rainfall over the key tropical regions of Asia and North Africa and associated large-scale east–west circulation, rather than the equatorial Pacific SST change pattern, played a key role in affecting the mid-Holocene PWC strength.

【Citation】

Zhiping Tian, Tim Li, and Dabang Jiang, 2018: Strengthening and westward shift of the tropical Pacific Walker circulation during the mid-Holocene: PMIP simulation results. Journal of Climate, 31(6), 2283–2298, doi: 10.1175/JCLI-D-16-0744.1.

【Link】

https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-16-0744.1


【Key figure】
 

Fig. 1 Mid-Holocene minus pre-industrial changes in the (a) annual mean of zonal mass streamfunction (shading and contours; units: 10^10 kg/s) and (b) monthly mean of vertically integrated zonal mass streamfunction (units: 10^14 kg²/m²/s) along the equatorial Pacific (5°S–5°N) from the ensemble mean of 29 PMIP2/3 models. The dotted areas represent regions where at least 70% of the models agree on the sign of the change. Vectors in (a) are the composite of pressure velocity (ω × −50; units: Pa/s) and zonal divergent wind (units: m/s) from the ensemble mean of 22 PMIP2/3 models with data available.






Fig. 2 Mid-Holocene minus pre-industrial changes in (left column) DJFMA and (right column) JJASO (top row) land surface 2-m air temperatures and SSTs (units: °C), (middle row) precipitation (units: mm/day), and (bottom row) sea level pressures (shading; units: hPa) and winds at 850 hPa (vectors; units: m/s) from the ensemble mean of 29 PMIP2/3 models. The dotted areas represent regions where at least 70% of the models agree on the sign of the change.