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Rui Jiang and Haijun Yang

Abstract

The effect of the Rocky Mountains (RM) on meridional overturning circulations (MOCs) is investigated using a fully coupled climate model. Located between the Atlantic and Pacific Oceans, the RM is a major mountain chain in North America. The presence of RM plays an important role in atmospheric moisture transport between the two oceans. Adding the RM to a flat global continent (OnlyRocky) leads to a weakening of the atmospheric moisture transport from the North Pacific to the North Atlantic, which is consistent with previous findings. However, the simulation also shows more atmospheric moisture is transported from the tropical Pacific and Atlantic to the North Atlantic. The net effect of moisture transport leads to a slight freshening of the North Atlantic. The Atlantic MOC (AMOC) is hardly changed, but the Pacific MOC (PMOC) declines by 40% due to more moisture retained in the North Pacific. The sensitivity experiment of removing the RM from a realistic global topography (NoRocky) gives roughly opposite atmospheric changes to the OnlyRocky experiment. The AMOC in NoRocky declines slightly and then recovers, while the PMOC is nearly unchanged. The paired experiments conducted in this study demonstrate that the presence of the RM plays a trivial role in Northern Hemisphere deep-water formation.

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Qin Wen and Haijun Yang

Abstract

The effects of the Tibetan Plateau (TP) on the Pacific Ocean circulation are investigated using a fully coupled climate model. Sensitivity experiments are designed to demonstrate that the presence of the TP is the reason for the lack of strong deep water formation in the subpolar North Pacific, because removing the TP in the model would enable the establishment of the Pacific meridional overturning circulation (PMOC). The processes involved are described in detail as follows. Removing the TP in the model would excite an anomalous high pressure over the subpolar North Pacific, causing anomalous Ekman downwelling that enhances surface water subduction north of 40°N. Removing the TP would also lead to less freshwater flux into the western Pacific, increasing sea surface salinity over the region. The high-salinity surface water can then be advected northward and eastward by the Kuroshio and its extension, subducting along the 26–27σ θ isopycnal surfaces to the deeper ocean, which enables the formation of deep water in the North Pacific and the setup of the PMOC. Afterward, more high-salinity warm water would be transported from the tropics to the extratropics by the Kuroshio, leading to the establishment of the PMOC. The role of the Rocky Mountains is also examined in this study. We conclude that the Rocky Mountains may play a trivial role in modulating the meridional overturning circulations in both the Pacific and Atlantic Oceans.

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Haijun Yang and Qiong Zhang

Abstract

A revisit on observations shows that the tropical El Niño–Southern Oscillation (ENSO) variability, after removing both the long-term trend and decadal variation of the background climate, has been enhanced by as much as 50% during the past 50 yr. This is inconsistent with the changes in the equatorial atmosphere, which shows a slowdown of the zonal Walker circulation and tends to stabilize the tropical coupling system. The ocean role is highlighted in this paper. The enhanced ENSO variability is attributed to the strengthened equatorial thermocline that acts as a destabilizing factor of the tropical coupling system. To quantify the dynamic effect of the ocean on the ENSO variability under the global warming, ensemble experiments are performed using a coupled climate model [Fast Ocean Atmosphere Model (FOAM)], following the “1pctto2x” scenario defined in the Intergovernmental Panel on Climate Change (IPCC) reports. Term balance analyses on the temperature variability equation show that the anomalous upwelling of the mean vertical temperature gradient (referred as the “local term”) in the eastern equatorial Pacific is the most important destabilizing factor to the temperature variabilities. The magnitude of local term and its change are controlled by its two components: the mean vertical temperature gradient T z and the “virtual vertical heat flux” −wT ′. The former can be viewed as the background of the latter and these two components are positively correlated. A stronger T z is usually associated with a bigger upward heat flux −wT ′, which implies a bigger impact of thermocline depth variations on SST. The T z is first enhanced during the transient stage of the global warming with a 1% yr−1 increase of CO2, and then reduced during the equilibrium stage with a fixed doubled CO2. This turnaround in T z determines the turnaround of ENSO variability in the entire global warming period.

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Haijun Yang and Qin Wen

Abstract

The Tibetan Plateau (TP) over the Eurasian continent has significant effects on both regional and global climate. It can even affect the remote Atlantic meridional overturning circulation (AMOC), as shown in this study. Through coupled modeling experiments, we demonstrate that removing the TP immediately weakens the meridional wind over East Asia, resulting in stronger westerlies in the midlatitudes. The stronger westerlies enhance the southward Ekman flow and surface latent and sensible heat losses in the subpolar North Atlantic, cooling the surface ocean and leading to stronger North Atlantic deep-water formation and stronger AMOC during the first few decades after the TP removal. At the same time, accompanying the weakened trade winds in the tropical Pacific, more moisture is transported from the tropical Pacific to the North Atlantic, freshening the surface ocean and triggering a weakening of the AMOC. The AMOC weakening in turn results in southward expansion and melting of sea ice, providing more freshwater to the North Atlantic, which furthers the weakening of the AMOC. The positive feedback between the AMOC and sea ice eventually leads to AMOC shutdown. We illustrate that there would be no AMOC without the TP. These results call for a revisiting of how ocean circulation and global climate may have responded to the TP uplift and other tectonic changes on the geological time scale.

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Haijun Yang and Zhengyu Liu

Abstract

The full spectrum of basin modes in a tropical–extratropical basin is studied comprehensively in a linear shallow-water system. Two sets of least-damped basin modes are identified. At the low-frequency end is the planetary wave basin mode, whose period is determined by the cross-basin time of the planetary wave on the poleward boundary of the basin, consistent with recent theories. At the higher-frequency end is the Kelvin wave basin mode, whose period is determined by the around-basin traveling time of the Kelvin wave. Sensitivity experiments are also performed on the eigenvalue problem to study the dynamics of these basin modes. It is found that the period of the planetary wave basin mode is determined by an effective basin boundary that is always at a latitude no higher than the geometric basin boundary. The effective poleward boundary is located at the most poleward latitude where the planetary wave can cross the entire basin. It is also found that the Kelvin wave basin modes are vulnerable to boundary perturbations. If the coastal Kelvin wave propagation is suppressed along the basin boundary, the Kelvin wave basin mode would degenerate to the equatorial basin mode that has been obtained theoretically from the long-wave approximation.

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Haijun Yang and Fuyao Wang

Abstract

The thermocline depth is defined as the depth of the maximum vertical temperature gradient. In the equatorial Pacific, the depth of 20°C isotherm is widely used to represent the thermocline depth. This work proposes that under the circumstance of a significant mean climate shift, it is better to use the original definition of the thermocline depth in studying the long-term changes in mean climate and tropical coupled climate variabilities. For instance, during the transient period of global warming, the tropical thermocline is usually enhanced because the surface layer warms more and faster than the lower layers. The depth of maximum vertical temperature gradient shoals, which is consistent with the enhanced thermocline. However, the 20°C isotherm depth deepens, which suggests a weakened thermocline. This discrepancy exists in both the observations and the future climate simulations of coupled models.

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Haijun Yang and Lu Wang

Abstract

The tropical oceanic response to the extratropical thermal forcing is quantitatively estimated in a coupled climate model. This work focuses on comparison of the responses between the tropical Atlantic and Pacific. Under the same extratropical forcing, the tropical sea surface temperature responses are comparable. However, the responses in the tropical subsurface in the two oceans are distinct. The tropical subsurface response in the Atlantic can be twice of that in the Pacific. The maximum subsurface temperature change in the tropical Pacific occurs in the eastern lower thermocline, while that in the tropical Atlantic occurs in the west and well below the lower thermocline. The different responses in the tropical Atlantic and Pacific are closely related to the different changes in the meridional overturning circulations. The Pacific shallow overturning circulation, or the subtropical cell, tends to slow down (speed up) in response to the extratropical warming (cooling) forcing. The changes in the upwelling in the eastern equatorial Pacific as well as the shallow subduction from the extratropical southern Pacific along the eastern boundary are accountable for the eastern Pacific temperature change. The Atlantic overturning circulation consists of the shallow subtropical cell and the deep thermohaline circulation. A weakened thermohaline circulation will result in a strengthened northern subtropical cell, in which the change in the lower branch, or the low-latitude North Brazil Current, can cause strong response below the western tropical thermocline. Here the coastal Kelvin wave along the western boundary on the intermediate isopycnal level also plays an important role in the equatorward conveying of the climate anomalies in the mid-to-high-latitude Atlantic, particularly during the initial stage of the extratropical forcing.

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Haijun Yang, Zhengyu Liu, and Qiong Zhang

Abstract

Oceanic response to stochastic wind forcing is studied in a tropical–extratropical basin using two shallow water models: a periodically forced model and a time-forward model. Consistent with theory, extratropical stochastic wind forces a decadal spectral peak in the tropical and eastern boundary ocean as a resonant response of the planetary wave basin mode. This resonant response is characterized by a rather uniform amplitude and phase in the equatorial and eastern boundary region. In comparison, away from the eastern boundary, the extratropical ocean is affected significantly by the local Ekman pumping forcing, with spectral peaks varying with location. A complex EOF (CEOF) analysis of the time-forward model simulation further suggests that these resonant responses are robust, and can be extracted as the leading CEOF modes. Thus, the resonance of the planetary wave basin mode gives a physically based guideline for the interpretation of decadal oceanic variability in the tropical–extratropical ocean.

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M. Stephens, Zhengyu Liu, and Haijun Yang

Abstract

The evolution of decadal subduction temperature anomalies in the subtropical North Pacific is studied using a simple and a complex ocean model. It is found that the amplitude of the temperature anomaly decays faster than a passive tracer by about 30%–50%. The faster decay is caused by the divergence of group velocity of the subduction planetary wave, which is contributed to, significantly, by the divergent Sverdrup flow in the subtropical gyre. The temperature anomaly also seems to propagate southward slower than the passive tracer, or mean ventilation flow. This occurs because the mean potential vorticity gradient in the ventilated zone is directed eastward; the associated general beta effect produces a northward propagation for the temperature anomaly, partially canceling the southward advection by the ventilation flow.

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Yingying Zhao, Haijun Yang, and Zhengyu Liu

Abstract

The Bjerknes compensation (BJC) refers to the tendency for changes in the atmosphere heat transport (AHT) and ocean heat transport (OHT) to compensate each other. However, the nature of this compensation varies with the time scale of changes. In this study, a new approach was developed to diagnose BJC for climate variability by considering the correlation between AHT and OHT and their relative magnitudes. The correlation is equivalent to the cosine of phase difference between AHT and OHT. For high-frequency climate variability, AHT lags or leads OHT by π/2, the correlation is zero, and BJC does not occur concurrently. For low-frequency climate variability, AHT lags or leads OHT by π, the correlation is −1, and BJC is concurrent. With increasing time scale, the phase difference between AHT and OHT changes from π/2 to π, and the BJC reaches equilibrium. A coupled box model is used to justify the approach and to understand the temporal change of BJC from a theoretical perspective. The correlation and BJC rate derived from theory and from the box model exhibit similar transient behaviors, approaching equilibrium monotonically with increasing time scale. The equilibrium BJC is established at decadal time scale. Since the BJC is closely related to climate feedback, a proper identification of BJC processes in climate variability can reveal the nature of dominant climate feedback processes at different time scales.

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