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Kunpeng Yang
,
Haijun Yang
,
Yang Li
, and
Qiong Zhang

Abstract

Using a CESM1 control simulation, we conduct a follow-up study to advance our earlier theoretical research on the multicentennial oscillation (MCO) of the Atlantic meridional overturning circulation (AMOC). The modeled AMOC MCO primarily arises from internal oceanic processes in the North Atlantic, potentially representing a North Atlantic Ocean–originated mode of AMOC multicentennial variability (MCV) in reality. Specifically, this AMOC MCO is mainly driven by salinity variation in the subpolar upper North Atlantic, which dominates local density variation. Salinity anomaly in the subpolar upper ocean is enhanced by the well-known positive salinity advection feedback that is realized through anomalous advection in the subtropical to subpolar upper ocean. Meanwhile, mean advection moves salinity anomaly in the subtropical intermediate ocean northward, weakening the subpolar upper salinity anomaly and leading to its phase change. The salinity anomalies have a clear three-dimensional life cycle around the North Atlantic. The mechanism and time scale of the modeled AMOC MCO are consistent with our earlier theoretical studies. In the theoretical model, artificially deactivating either the anomalous or mean advection in the AMOC upper branch prevents it from exhibiting AMOC MCO, underscoring the indispensability of both the anomalous and mean advections in this North Atlantic Ocean–originated AMOC MCO. In our coupled model simulation, the South Atlantic and Southern Oceans do not exhibit variabilities synchronous with the AMOC MCO; the Arctic Ocean’s contribution to the subpolar upper salinity anomaly is much weaker than the North Atlantic. Hence, this North Atlantic Ocean–originated AMOC MCO is distinct from the previously proposed Southern Ocean–originated and Arctic Ocean–originated AMOC MCOs.

Open access
Mengyu Wei
,
Jun Yang
,
Yongyun Hu
,
Yonggang Liu
,
Shineng Hu
,
Xiang Li
,
Jiawenjing Lan
,
Jiaqi Guo
,
Shuai Yuan
, and
Ji Nie

Abstract

Both observations and simulations show that under global warming there exists warming deficit in the North Atlantic, known as the North Atlantic warming hole (NAWH). Here we show that similar warming hole occurs in the sub-polar Pacific ocean of paleo-climate simulations. As solar constant is increased, local surface becomes substantially cooler rather than warmer in the sub-polar paleo-Pacific ocean under the land-sea configurations of 70, 90, and 150 million years ago (Ma). The warming hole has a magnitude of ≈3 °C and locates in the Northern Hemisphere in 70Ma and 90Ma. The warming hole in 150Ma has a magnitude of ≈1 °C and locates in the Southern Hemisphere. Both atmospheric and oceanic processes contribute to trigger the warming hole. For 70Ma and 90Ma experiments, atmospheric teleconnection along a great circle from tropics to extratropics intensifies surface winds over sub-polar ocean and thereby increases relatively cool seawater transport from high to low latitudes. Meanwhile, global meridional overturning circulation (GMOC) becomes weaker, causing a divergence of the meridional ocean heat transport in the warming hole region. An increasing of regional cloud shortwave cooling effect acts to further enhance the warming hole. For 150Ma experiments, the warming hole is related to the meridional shift of mid-latitude jet stream and the weakening of GMOC in the Southern Hemisphere. The strength and phase of the atmospheric teleconnection and the response of GMOC strongly depend on land-sea configuration, resulting to the paleo-Pacific warming hole to occur in special periods only.

Restricted access
Kouya Nakamura
,
Shoichiro Kido
,
Takashi Ijichi
, and
Tomoki Tozuka

Abstract

The mean vertical advection of anomalous vertical temperature gradient is considered as the dominant generation mechanism of positive sea surface temperature (SST) anomalies associated with the canonical El Niño. However, most past studies had a residual term in their heat budget analysis and/or did not quantify the role of vertical mixing even though active vertical turbulent mixing in the upper ocean is observed in the eastern equatorial Pacific. To quantitatively assess the importance of vertical mixing, a mixed layer heat budget analysis is performed using a hindcast simulation forced by daily mean atmospheric reanalysis data. It is found that when the mixed layer depth is defined as the depth at which potential density increases by 0.125 kg m−3 from the sea surface, the development of positive SST anomalies is predominantly governed by reductions in the cooling by vertical mixing, and their magnitude is much larger than those by vertical advection. The anomalous warming by vertical mixing may be partly explained by an anomalous deepening of the thermocline that leads to a decrease in the vertical temperature gradient, giving rise to suppression of the climatological cooling by vertical mixing. Also, an anomalously thick mixed layer reduces sensitivity to cooling by the mean vertical mixing and contributes to the anomalous SST warming. On the other hand, the dominant negative feedbacks are attributed to both anomalous surface heat loss and anomalous deepening of the mixed layer that weakens warming by the mean surface heat flux.

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Free access
Mingmei Xie
,
Bo Wu
,
Jia-Zhen Wang
,
Chunzai Wang
, and
Xiubao Sun

Abstract

On decadal timescales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls.

Restricted access
Keno Riechers
,
Georg Gottwald
, and
Niklas Boers

Abstract

Paleoclimate proxies reveal abrupt transitions of the North Atlantic climate during past glacial intervals known as Dansgaard–Oeschger (DO) events. A central feature of DO events is a sudden warming of about 10°C in Greenland marking the beginning relatively mild phases termed interstadials. These exhibit gradual cooling over several hundred to a few thousand years until a final abrupt decline brings the temperatures back to cold stadial levels. As of now, the exact mechanism behind this millennial-scale variability remains inconclusive. Here, we propose an excitable model to explain Dansgaard–Oeschger cycles, where interstadials occur as noise-induced state-space excursions. Our model comprises the mutual multiscale interactions between four dynamical variables representing Arctic atmospheric temperatures, Nordic seas’ temperatures and sea ice cover, and the Atlantic meridional overturning circulation. The model’s atmosphere–ocean heat flux is moderated by the sea ice, which in turn is subject to large perturbations dynamically generated by fast-evolving intermittent noise. If supercritical, perturbations trigger interstadial-like state-space excursions during which all four model variables undergo qualitative changes that consistently resemble the signature of interstadials in corresponding proxy records. As a physical intermittent process generating the noise, we propose convective events in the ocean or atmospheric blocking events. Our model accurately reproduces the DO cycle shape, return times, and the dependence of the interstadial and stadial durations on the background conditions. In contrast with the prevailing understanding that DO variability is based on bistability in the underlying dynamics, we show that multiscale, monostable excitable dynamics provides a promising alternative to explain millennial-scale climate variability associated with DO events.

Significance Statement

Recent research has highlighted the risk that some Earth system components might undergo abrupt and qualitative change in response to global warming. Proxy records provide evidence for past abrupt climatic changes fundamentally proving the possibility for highly nonlinear state transitions in the climate system. Understanding the dynamics that drove past changes of this kind may help to assess the risk of future tipping events. Here, we propose a new mechanism for the repeated sudden warming events over Greenland that punctuated the last glacial’s climate and reproduce the warmer interstadial intervals drawing on a multiscale, excitable conceptual climate model. Therein, the warmer intervals appear as state-space excursions following stochastic supercritical excitations caused by non-Gaussian noise, which is dynamically generated via fast intermittent dynamics.

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YaoKun Li

Abstract

The ENSO phase locking to the annual cycle is investigated by applying a spatiotemporal oscillator (STO) model, in which the annual cycle of the climatological thermocline depth and its associated parameter are introduced. It is easy to derive its analytic solution, which demonstrates a harmonic oscillation of a combined variable. The ENSO phase locking can be theoretically proven by discussing the distribution of the calendar months of the peak time of the sea surface temperature anomaly (SSTA) time series. The calendar months of the peak time can be divided into two parts. The first part can evenly distribute in any a month of a year and hence has no phase locking feature whereas the second part, directly associated with the annual cycle, adds an increment onto the first part to make it move toward the phase of the annual cycle to realize the phase locking feature. This is the physical mechanism of the ENSO phase locking. With observed seasonal variation of the climatological thermocline depth, the Niño-3.4 index time series approach to extreme values in November was calculated with higher probability, reproducing the observed phase locking phenomenon quite well. The maximum probability of the calendar month that the ENSO peak time occurs is directly determined by the phase of the annual cycle and the stronger the annual cycle is, the larger the maximum probability is.

Significance Statement

El Niño–Southern Oscillation (ENSO) events tend to be strongest in the boreal wintertime. This phenomenon is called ENSO phase locking. This study investigates the dynamics of ENSO phase locking to the annual cycle by introducing the annual cycle to a spatiotemporal oscillator (STO) model that can deal with the complex spatial and temporal variations in SSTAs. The analytic solution can be obtained and then the phase locking feature can be theoretically proven and numerically testified. Therefore, the dynamics and the mechanism of ENSO phase locking can be comprehensively understood. It may be beneficial for the community to have a better understanding of this complex phenomenon.

Open access
Zili Shen
,
Anmin Duan
,
Wen Zhou
,
Yuzhuo Peng
, and
Jinxiao Li

Abstract

Two large ensemble simulations are adopted to investigate the relative contribution of external forcing and internal variability to Arctic sea ice variability on different timescales since 1960 by correcting the response error of models to external forcing using observational datasets. Our study suggests that previous approaches might overestimate the real impact of internal variability on Arctic sea ice change especially on long time scales. Our results indicate that in both March and September, internal variability plays a dominant role on all time scales over the 20th century, while the anthropogenic signal on sea ice change can be steadily and consistently detected on a time scale of more than 20 year after 2000s. We also reveal that the dominant mode of internal variability in March shows consistency across different time scales. On the contrary, the pattern of internal variability in September is highly nonuniform over the Arctic and varies across different timescales, indicating that sea ice internal variability in September at different time scales is driven by different factors.

Restricted access
Po-Chun Chung
and
Nicole Feldl

Abstract

The ice–albedo feedback associated with sea ice loss contributes to polar amplification, while the water vapor feedback contributes to tropical amplification of surface warming. However, these feedbacks are not independent of atmospheric energy transport, raising the possibility of complex interactions that may obscure the drivers of polar amplification, in particular its manifestation across the seasonal cycle. Here, we apply a radiative transfer hierarchy to an idealized aquaplanet global climate model coupled to a thermodynamic sea ice model. The climate responses and radiative feedbacks are decomposed into the contributions from sea ice loss, including both retreat and thinning, and the radiative effect of water vapor changes. We find that summer sea ice retreat causes winter polar amplification through ocean heat uptake and release, and the resulting decrease in dry energy transport weakens the magnitude of warming. Moreover, sea ice thinning is found to suppress summer warming and enhance winter warming, additionally contributing to winter amplification. The water vapor radiative effect produces seasonally symmetric polar warming via offsetting effects: enhanced moisture in the summer hemisphere induces the summer water vapor feedback and simultaneously strengthens the winter latent energy transport in the winter hemisphere by increasing the meridional moisture gradient. These results reveal the importance of changes in atmospheric energy transport induced by sea ice retreat and increased water vapor to seasonal polar amplification, elucidating the interactions among these physical processes.

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Qi Sun
,
Haikun Zhao
,
Philip J. Klotzbach
,
Xiang Han
,
Jun Gao
,
Jin Wu
, and
Zhanhong Ma

Abstract

There has been increased focus in recent years on the impact of the Pacific Meridional Mode (PMM) and the Atlantic Meridional Mode (AMM) on weather and climate events. This study shows an increased synergistic impact of both the PMM and AMM on eastern North Pacific (ENP) extended boreal summer (June-November) tropical cyclone frequency (TCF) since the 1990s. This increase in the combined impact of both the PMM and AMM on ENP TCF is mainly due to a stronger modulation of the AMM on TCF since the early 1990s and of a stronger modulation of the PMM on TCF since the late 1990s. A budget analysis of the genesis potential index highlights the important contribution of changes in vertical wind shear to the recent strengthened AMM-TCF relationship, while potential intensity and vertical wind shear are the two most important drivers of the recent increase in the PMM-TCF relationship. This intensified association is largely explained by changes in the mean state of sea surface temperatures in the tropical Atlantic associated with the Atlantic Muit-decadal Oscillation (AMO) and trade wind magnitude in the subtropical Pacific Ocean associated with the Pacific Decadal Oscillation (PDO). This study highlights an asymmetric effect of the AMO and PDO on these two meridional modes and ENP TC genesis frequency and provides a better understanding of ENP TC activity on interannual-to-decadal time scales.

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