Poleward Shift in Ventilation of the North Atlantic Subtropical Underwater

• Published in: Geophysical research letters 2018-01, Vol.45 (1), p.258-266
• Main Authors: Yu, Lisan, Jin, Xiangze, Liu, Hao
• Format: Article
• Language: English
• Subjects: decadal variability; Deep water; Equatorial regions; Evaporation; Hadley circulation; High pressure; Hydrologic cycle; Hydrological cycle; Isohalines; Latitude; Mixed layer; North Atlantic salinity maximum; poleward expansion; Precipitation; Saline water; Salinity; Salinity effects; Sea surface; subtropical underwater; Surface salinity; Temperature (air-sea); Thermocline; Tropical climate; Tropical environments; Underwater; Ventilation; Water vapor; Water vapor transport; Water vapour; Widening
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL075772

We report the findings that the sea surface salinity maximum (SSS‐max) in the North Atlantic has poleward expanded in recent decades and that the expansion is a main driver of the decadal changes in subtropical underwater (STUW). We present observational evidence that the STUW ventilation zone (marked by the location of the 36.7 isohaline) has been displaced northward by1.2 ± 0.36° latitude for the 34 year (1979–2012) period. As a result of the redistribution of the SSS‐max water, the ventilation zone has shifted northward and expanded westward into the Sargasso Sea. The ventilation rate of STUW has increased, which is attributed to the increased lateral induction of the sloping mixed layer. STUW has become broader, deeper, and saltier, and the changes are most pronounced on the northern and western edges of the high‐saline core. Plain Language Summary The subtropical high pressure of the descending branch of the Hadley circulation is located between 20° and 40° of both north and south latitudes. Within the zone, a pool of sea surface salinity maximum (SSS‐max) exists in responding to the excess of evaporation over precipitation. Among all SSS‐max centers, the North Atlantic SSS‐max is highest, in excess of 37. In the past 50 years, the salinities in the subtropical North Atlantic have increased, and the trend is seen as an ocean supporting evidence of the “dry‐gets‐drier and wet‐gets‐wetter” paradigm , indicating that the global water cycle amplifies as a consequence of the increase in water vapor transport. However, we hypothesize in this study that the salinity increase is due to the poleward expansion of the SSS‐max center in association with the widening of the tropics in the recent decades and that the expansion has shifted the ventilation zone of subtropical underwater—a high‐saline water mass in the upper 50–300 m. The subtropical underwater has become broader, deeper, and saltier. Because of its connection to the tropical and subpolar regions via interior pathways, the change of subtropical underwater could have profound impacts on both the equatorial thermocline and the North Atlantic Deep Water. Key Points The subtropical surface salinity maximum water in the North Atlantic has expanded toward North since 1979 The expansion shifted and expanded the ventilation zone, leading to an increased production of the subtropical underwater The underwater has become broader, deeper, and saltier, and the changes are particularly noted on the northern/wester