Recently, Xiao Ma (the first author), a PhD candidate of the School of Oceanography, Shanghai Jiao Tong University, and his supervisor, Hailong Liu (corresponding author) published their latest research results online in the Journal of Physical Oceanography, entitled: “International Variability of Barrier Layer in the Tropical Atlantic and Its Relationship with the Tropical Atlantic Modes”. The research was completed in cooperation with Professor Xidong Wang of Hohai University.
Fig. 1. (a) Climatological annual mean and (b-e) climatological seasonal means of BLT (shadings) and surface current (arrows) from ORAS5. All the above figures are derived from the average of monthly data from January 1987 to December 2016. The estuaries of 7 major rivers are marked with green dots. BLT = Barrier Layer Thickness.
The upper ocean stratification is affected by salinity, so the isothermal layer and the isopycnal layer do not necessarily coincide. If the isothermal layer is obviously deeper than the mixed layer, the interlayer between them is defined as the barrier layer. The barrier layer hinders the vertical exchange of heat between upper ocean and thermocline, leading to the warming of the upper ocean, and further affecting the local air sea interaction and even global climate.
Influenced by the significant seasonal variation of heat and salt fluxes, there is a barrier layer with large thickness and wide range in the tropical Atlantic (Fig. 1). On the interannual scale, there are two major interannual climate modes in the tropical Atlantic (Fig. 2), i.e., the Atlantic Meridional Mode (AMM) and the Atlantic Equatorial Mode (AEM), which have significant effects on the local sea surface temperature, precipitation and surface wind fields in spring and summer respectively. According to previous studies, the distribution area of low sea surface salinity and thick barrier layer is obviously consistent in the tropical Atlantic, so the current research results are limited to the relationship between the above two climate modes and sea surface salinity. Unfortunately, the physical mechanism and causal relationship between interannual climate variability and barrier layer have not been discussed in depth. Therefore, this paper focuses on the impact of two interannual climate modes on the interannual variation of local barrier layer thickness. Two questions that “Does the interannual climate modes of the tropical Atlantic regulate the interannual variation of the barrier layer?” and “What physical mechanism influences the interannual variability of the barrier layer?” are raised in this paper. Based on several sets of observational and reanalysis data, this study aims to discuss and answer the above key scientific questions.
Fig. 2. First combined EOF analysis maps performed on SST-wind anomalies in (a, b) MAM and (d, e) JJA from ORAS5 (a, d; magenta lines in the time series), ERA-Interim reanalysis (b, e; dashed lines in the time series), and (c, f) their respective normalized time series for the 1980–2016 period.
This study describes the characteristics of barrier layer variation in spring and summer under the control of two interannual climate modes in the tropical Atlantic, and quantitatively describes its spatio-temporal variation mechanism according to the analysis of heat and salt budget. The study shows that the characteristics of barrier layer thickness and surface salinity are not completely consistent under the control of tropical Atlantic interannual climate mode. During the positive phase of the meridional mode, the barrier layer thickness anomaly is characterized by a "negative-positive-negative" three pole strip from north to south, and is consistent with its own spring dominant EOF mode (Fig. 3a,b, and the correlation value of the PC1 of barrier layer thickness and AMM index reaches 0.78, and passes the 95% confidence test). Under the control of equatorial mode, the characteristics of barrier layer thickness anomaly are relatively weak, and its consistency with the summer dominant EOF mode of barrier layer thickness is slightly poor, which indicates that the equatorial mode has limited instantaneous impact (Fig. 3c,d).
Fig. 3. BLT signature of the positive phase of the (a–b) meridional and (c–d) equatorial modes from ORAS5 reanalysis. Figs. 3a and 3c represent regression coefficients of BLT anomalies onto the meridional and equatorial modes’ time function, respectively (Fig. 2c and 2f, magenta lines), and Fig. 3b and 3d represent the dominant pattern mode of the EOF analysis performed on BLT anomalies in MAM and JJA, respectively. The front between positive/negative SST anomalies in Fig. 2a is superposed on Fig. 3a (magenta line). Hatched regions 1 to 5 delimited by heavy black lines are discussed in the main text.
The analysis results of heat and salt budget in typical regions show that the salinity stratification caused by the change of fresh water flux can’t explain all the variation of barrier layers, and the climate mode also affects the temperature stratification through the heat budget processes, thus having a significant impact on the barrier layers. The study reveals the relationship between the tropical Atlantic interannual climate mode and the local barrier layer variability, which provides a reference for the study of the occurrence of the Atlantic Niño and the dynamic thermodynamic process of the Atlantic Warm Pool as well as its climate effect.
Ma, X., H. Liu and X. Wang, 2022: Interannual Variability of Barrier Layer in the Tropical Atlantic and Its Relationship with the Tropical Atlantic Modes. Journal of Physical Oceanography, https://doi.org/10.1175/JPO-D-21-0235.1.