Structure and Evolution of Decadal Spiciness Variability in the North Pacific during 2004–20, Revealed from Argo Observations

Abstract

Ocean spiciness is referred to as density-compensated temperature and salinity variations with warm (cool) and salty (fresh) waters having high (low) spiciness, respectively. The structure and evolution of density-compensated (warm/salty or cool/fresh) spiciness anomalies are investigated in the North Pacific thermocline using Argo observations during the period 2004–20. Two well-organized decadal spiciness events are identified through isopycnal surface analyses. One warm/salty spiciness anomaly of about 0.15°C and 0.05 g kg⁻¹ temperature and salinity perturbations on the 25 kg m⁻³ isopycnal surface appeared in the eastern subtropical North Pacific at (18°–30°N, 120°–150°W) in 2007, which then migrated southwestward along the mean circulation and arrived in the western tropics at (∼15°N, 145°E–175°W) in 2012–13, with the reduced salinity perturbation of about 0.043 g kg⁻¹. Another cool/fresh spiciness anomaly of about −0.2°C and −0.07 g kg⁻¹ temperature and salinity perturbations originated from the eastern subtropics at (120°–150°W, ∼30°N) in 2014 and followed a similar advective pathway during the period from 2014–15 to 2019–20. The subduction pathway can be adequately determined by the mean Montgomery stream function on the isopycnal surface; the propagation direction and speed are in good agreement with the expectation for the role played by advection due to the mean geostrophic current. Moreover, the subducted decadal spiciness anomalies can have negative feedback on sea surface temperature (SST) variability in the western tropical Pacific through the diapycnal processes. The identifications of these density-compensated spiciness anomalies and their propagation pathways provide a clear illustration of the oceanic extratropics-tropics interactions in the North Pacific Ocean.

摘 要

海洋涩度是指等密度面上对密度起补偿作用的温、盐变化,如暖而咸的海水涩度较高,冷而淡的海水涩度较低。本文利用2004-2020年Argo观测资料,研究了北太平洋温跃层中表现为密度补偿 (暖而咸或冷而淡) 作用的涩度异常的结构和演变。通过等密度面分析,确定了两个结构清晰的年代际涩度事件。在25 kg m-3等密度面上于2007年观测识别出一个暖而咸的涩度异常(其温、盐扰动分别约为0.15°C和0.05 g kg-1),产生于副热带东北太平洋(18°N-30°N, 120°W-150°W),然后沿着平均环流向西南方移动,于2012-2013年前后到达热带西太平洋(145°E-175°W, 15°N左右), 此时的盐度扰动约为0.043 g kg-1。于2014年发现另一个冷而淡的涩度异常(其温、盐异常分别为-0.2°C和-0.07 g kg-1左右),起源于副热带东太平洋(120°W-150°W,30°N左右),于2014-2015至2019-2020年间沿着相似的平流通道向西南方向移动。利用等密度面上的Montgomery流函数可以确定这一潜沉通道;其传播方向和速度与平均地转流引起的平流作用十分吻合。此外,潜沉的涩度年代际异常可通过跨等密度面过程对热带西太平洋海表温度(SST)的变化产生反馈作用。本文对这一密度补偿的涩度异常的识别及其传播途径的确定,更明确地阐释了北太平洋中热带-热带外海气过程相互作用及机制。