Subtropical Pacific SST Variability Related to the Local Hadley Circulation during the Premature Stage of ENSO

Abstract

Formation processes of negative (positive) sea surface temperature anomalies (SSTAs) in the subtropical North and South Pacific associated with the ENSO warm (cold) events are examined using reanalysis and in-situ observational datasets. During the premature stage of the ENSO warm events, negative SSTAs appear over the subtropical North Pacific in the February-March period and over the subtropical South Pacific after April, and vice versa in the ENSO cold events. One month prior to the formation of these subtropical negative SSTAs, the negative air humidity anomaly and anomalous downward motion appear at the same location in either the Northern or Southern hemisphere. Associated with these atmospheric anomalies, the strengthened descending branch of local Hadley circulation is observed during the January-February period in the Northern hemisphere and after March in the Southern hemisphere, which coincides with the seasonal transition of the climatological local Hadley circulation from the Northern to Southern hemisphere. Our linear decomposition analysis of surface heat flux anomalies indicates that the negative air humidity anomaly, as well as anomalies in wind speed, contributes to the formation of the subtropical negative SSTAs through the enhanced latent heat flux induced by the anomalous air-sea humidity di¤erence. These results suggest that the anomalous downward motion associated with the changes in local Hadley circulation can induce the subtropical negative SSTAs through the surface humidity variability. A possible mechanism for the subtropical air-sea interaction associated with the local Hadley circulation is discussed.

Increasing trends for the surface heat flux and fresh water flux in the North Pacific eastern subtropical region

“…This result shows the possibility that the increase in U and the decrease in Qa induced by the strengthened descending branch of local Hadley circulation. These results correspond to the results from Chikamoto et al [2010]. Moreover, the regions of decreasing Qa correspond to those of increasing FWF (Figure 1b).…”

“…The question arises as to which factor decides an increasing trend U and a decreasing trend in Qa over a region. Recently, Chikamoto et al [2010] pointed out the following mechanism. The anomalous downward motion associated with the changes in local Hadley circulation can induce an increase in LHF through not only surface wind speed variability but also surface humidity variability during an ENSO warm event.…”

The Anomalous Regional Hadley Circulation in the ENSO Cycle

“…Thus, such a significant positive correlation area (SPCA) over Eastern Extratropical Pacific (EEtP) lasting until August or Figure 1 Evolutions of SST3a (unit: K) for El Niño and La Niña composites from January (0) to December (+1). The years for El Niño were 2003, 1987, 1994, 1976, 2006, 2018, 2002, 1965, 1986, 2009, 1991, 1972, 2015, 1982, 1997, and the years for La Niña were 19731975197020071999196719882010198420171998196419711996, 1995, and the year order was based on higher and lower values of SST3DJF respectively. For example, the SST3DJF of the 0 year of 2003 means SST3 averaged from December 2003 to February 2004, and so on.…”

Relationships between the Hadley circulation and tropical sea surface temperature with different meridional structures simulated in CMIP6 models

“…Several researches have illustrated that the SST's warming trend impacts the HC's rising branch location, and thereby strengthens the HC (Hou and Lindzen, 1992). Conversely, the anomalous sinking branch of HC also leads to anomalous SST by changing the surface humidity over land (Chikamoto et al, 2010). Such anomalous variations in the SST are often more pronounced during the ENSO events, with positive and negative abnormal affecting the configuration and intensity of the HC, and thus affecting the global climate (Guilyardi et al, 2012).…”