Relative roles of dynamic and thermodynamic processes in causing positive and negative global mean SST trends during the past 100 years
Introduction
Because of anthropogenic activities, there is steady increase of greenhouse gases such as CO2 in the atmosphere since the Industrial Revolution. The increase of the greenhouse gases (GHGs) traps more longwave radiation within the atmosphere due to the so called greenhouse effect. While the GHGs kept increasing during the past one hundred years, GMST didn’t increase all the time. For instance, during the past one hundred years (say, 1910–2012), there are four relatively warming and cooling phases. The periods from 1910 to 1940s and from mid-1970s to 1997 are so called global rapid warming phases, whereas the period from 1940s to mid-1970s and from 1998 to 2012 are known as warming hiatus periods.
Previous studies suggested that the bigger hiatus during the period from 1940s to mid-1970s was attributed to the increasing of artificial aerosols, which resulted in a decrease of cloud particle size and an increase of cloud albedo (Lohmann and Feichter, 2005; Hateren, 2012; Kosaka and Xie, 2016). Thus the cooling during that period was primarily attributed to the reduction of incoming solar radiation. Another possible factor is natural mode variability such as the IPO, which may have a far-reaching effect on GMST (Meehl et al., 2013; Kosaka and Xie, 2016). Various hypotheses have been proposed to interpret the recent hiatus phase. One is that the hiatus might be caused by data bias (Karl et al., 2015; Jones, 2016). The magnitude and statistical significance of a trend depend on time intervals considered (Fyfe et al., 2016). The second hypothesis emphasizes the effect of the IPO. Anomalous heat source in the tropics associated with the IPO may force a quasi-stationary atmospheric Rossby wave response to affect higher-latitude wind and temperature (Trenberth et al., 2014). It has been argued that GMST warming stagnates during the IPO negative phases (e.g., Kosaka and Xie, 2013; England et al., 2014; Meehl et al., 2014; Trenberth, 2015; Kosaka and Xie, 2016). The third hypothesis suggests that a positive Atlantic Multidecadal Oscillation (AMO) may promote a global cooling (Steinman et al., 2015). The fourth hypothesis suggests the shift of heat from the surface into the deep ocean over Indian Ocean, North Atlantic and/or Southern Ocean (Meehl et al., 2011; Chen and Tung, 2014; Kintisch, 2014; Roemmich et al., 2015; Lee et al., 2015; Liu et al., 2016; Li et al., 2017). Liang et al. (2017) suggested that ocean mixing processes, including isopycnal and diapycnal as well as convective mixing, are important for the decadal change of the heat exchange between upper and deeper ocean. Other hypotheses include the effect of that volcanic eruptions (Fyfe et al., 2013; Santer et al., 2014) and anthropogenic aerosols. For example, Smith et al. (2016) reckoned that the phase of the IPO during the hiatus period was modulated by external forcing of anthropogenic aerosols.
Because the ocean surface covers 71% of the earth’s surface, the trend of GMST is, to a large extent, determined by that of the global mean SST. Fig. 1 shows the time series of the global mean SST (blue curve) during the last one hundred years. Consistent with the GMST, there are two rapid SST warming periods and two SST cooling periods. The warming phases occurred from 1910 to 1942 (hereafter Phase 1) and from 1976 to 1997 (hereafter Phase 3), and the cooling phases happened from 1943 to 1975 (hereafter Phase 2) and from 1998 to 2012 (hereafter Phase 4). It is further noted that ocean mixed layer temperature experienced similar warming and cooling phases (see the red solid line in Fig. 1). Thus, to the first order of approximation, one may examine the ocean mixed layer temperature evolution, omitting the deeper ocean warming process.
Motivated by the results above, in this study we intend to reveal fundamental physical processes responsible for the distinctive warming and cooling trends during the four periods, based on an ocean mixed layer heat budget analysis. The objective of the current study is to quantitatively measure the relative roles of ocean dynamics (including three dimensional oceanic temperature advection) and thermodynamic processes (including shortwave radiation, longwave radiation, latent and sensible heat fluxes) in causing the warming and cooling tendencies during the different phases (Phase 1 to Phase 4) through a detailed mixed-layer heat budget analysis. Physically, the SST warming or cooling tendency is caused by many factors, include external GHG forcing, natural modes such as interdecadal variability, change of solar radiation due to artificial or volcano aerosol, and so on. Previous studies seldom paid attention to the detailed diagnosis of dynamic and thermodynamic terms that affect SST. In this study, we attempt to quantitatively assess dominant physical processes that caused the distinctive SST tendencies during the warming and cooling phases, using two complementary diagnostic methods. Through the quantitative assessment of individual SST controlling factors, one may better understand global mean SST change in the past century and tackle the global warming problem.
The remaining part of this paper is organized as following. In Section 2, we describe data and analysis methods to be used. In Section 3, we present the result from mixed layer heat budget diagnosis. Section 4 shows the result from a simple equilibrium state model. Finally, a conclusion is given in Section 5.
Section snippets
Data and method
The primary datasets used in this study include 1) monthly SST dataset from Met Office Hadley Center Version 1.1 (Rayner et al., 2003), 2) monthly ocean reanalysis data Version 2.2.4 and Version si.3 from SODA (Simple Ocean Data Assimilation) including zonal velocity, meridional velocity, vertical velocity, temperature and salinity (Giese et al., 2016), 3) monthly ocean mixed layer depth from GODAS (Global Ocean Data Assimilation System), and 4) 20th century atmospheric reanalysis data Version
Diagnosis of ocean mixed layer heat budget for Phase 1-4
The cause of the distinctive mixed-layer temperature tendencies during the four warming and cooling phases is addressed through an oceanic mixed layer heat budget analysis. According to Eq. (1), the time change rates of ocean mixed layer temperature in the four phases and throughout the entire period are diagnosed respectively. The results are shown in Table 1. Note that the sign and magnitude of the diagnosed temperature tendencies at the right side of Eq. (1) during each of these phases and
Results from a simple equilibrium state model
In the previous section, we compare the mixed-layer temperature tendencies averaged at each phase. Taking a different approach, we now focus on examining fundamental processes that cause SST difference between two adjacent equilibrium warmer and cooler states. As illustrated by black lines in Fig. 1, five such equilibrium states are selected. They are named as E1 (1903–1912), E2 (1937–1946), E3 (1965–1976), E4 (1997–1999), and E5 (2011–2013). For example, by diagnosing Eq. (3) between E2 and
Conclusion
In the past 100 years, the global mean SST exhibits four distinctive warming and cooling phases. The specific dynamic and thermodynamic processes that determine the relative warming and cooling tendencies are investigated through the diagnosis of the ocean mixed layer heat budget. It is found that the ocean thermodynamic process dominates during the warming phases, whereas the ocean dynamic process dominates during the cooling phases. Removing the long term tendency, the relative warming or
Acknowledgments
We thank the anonymous reviewers for their constructive comments that greatly improve the original manuscript. This work is jointly supported by China National Key R&D Program2017YFA0603802 and 2015CB453201, NSFC grants 41630423 and 41875069, NSF grant AGS-1565653, and NOAA grant NA18OAR4310298. This is SOEST contribution number 10657, IPRC contribution number 1366 and ESMC number 251.
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