This study aims to determine the role of the tropical ocean dynamics in the response of the climate to extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a reduced gravity ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics oppose the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates strong warming in the central-eastern basin from April to August balanced by cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño–Southern Oscillation, weakening its amplitude and low-frequency behaviour.
Paleoclimatic data (Wang et al., 2004), 20th century observations (Folland et al., 1986) and numerical simulations (Chiang and Bitz, 2005; Broccoli et al., 2006; Kang et al., 2008, 2009; Cvijanovic and Chiang, 2013; Talento and Barreiro, 2016, 2017) have all suggested the capability of extratropical thermal forcing to affect different features of the tropical climate. While Chiang and Bitz (2005) and Broccoli et al. (2006) were among the first to propose an atmospheric bridge mechanism connecting extratropical forcing with a tropical response, by performing experiments with atmospheric general circulation models (AGCMs) thermodynamically coupled to a motionless ocean, the other studies deepened the analysis and examined the physical mechanisms involved in the remote linkage.
The general picture emerging from these studies is that the Intertropical Convergence Zone (ITCZ) tends to shift toward the warmer hemisphere at the same time that the atmospheric energy transport is modified to favour the transmission of energy to the colder hemisphere. For example, if the net energy input into the Northern Hemisphere (NH) is higher than into the Southern Hemisphere (SH) an interhemispheric thermal contrast is generated. This interhemispheric thermal gradient (ITG) triggers an atmospheric response through changes in the Hadley circulation, leading to a partially compensating cross-equatorial southward energy flux and a southward shift of the ITCZ. Schneider et al. (2014) analyse the ITCZ displacements from an energy flux perspective, and find an anti-correlation between the latitude of the ITCZ and the cross-equatorial atmospheric energy transport.
First SVD pattern of SST and near-surface winds in the tropical Pacific Ocean (30
Forcing pattern. The sign convention is positive out of sea. Contour interval 20
Annual mean anomalies with respect to the control of NSAT for
Annual mean anomalies with respect to the control of precipitation for
Annual mean anomalies with respect to the control of near-surface (950
Northward atmospheric energy transport for the experiments:
Seasonal SST and near-surface wind anomalies with respect to the control for
Equatorial Pacific Ocean (2
Equatorial Pacific Ocean (2
Equatorial Pacific Ocean (2
First SVD pattern of SST and near-surface winds in the tropical Pacific Ocean (30
Talento and Barreiro (2016) use an AGCM coupled to a slab ocean model to quantify the relative roles of the atmosphere, tropical sea surface temperatures (SSTs) and continental surface temperatures in the ITCZ response to extratropical thermal forcing. They find that if the tropical SSTs are not allowed to change, then the ITCZ response strongly weakens (although not negligible), particularly over the Atlantic Ocean and Africa. If, in addition, the land surface temperature over Africa is maintained the ITCZ response completely vanishes, indicating that the ITCZ response to the extratropical forcing is not possible just through purely atmospheric processes, but rather it needs the involvement of either the tropical SST or the continental surface temperatures. With the same model configuration, Talento and Barreiro (2017) focus on the South Atlantic convergence zone (SACZ) and show that, during its peak in austral summer, its response to warming in the NH extratropics and cooling in the SH extratropics consists of weakening, mostly due to the NH component of the forcing. Both studies showed strong changes in the tropical band where SST, surface winds and precipitation are strongly coupled. Nevertheless, in these studies important ocean dynamics are missing as the slab ocean can only simulate the thermodynamic exchange between the atmosphere and the ocean.
Chiang et al. (2008) explore the impact of an ITG on the tropical Pacific climate through simulations performed with an AGCM coupled to a medium-complexity ocean model: a reduced gravity ocean (RGO) model. They find that when the NH is warmer than the SH, the annual mean equatorial zonal SST gradient strengthens, associated with an earlier onset and a later retreat of the seasonal cold tongue together with an intensification during the peak cold season. They also find that El Niño–Southern Oscillation (ENSO) activity is sensitive to the ITG, with small ITG optimal for the development of ENSO activity.
Lee et al. (2015) also use an AGCM coupled to an RGO model and analyse the impact of the glacial continental ice sheet topography on the tropical Pacific climate. They suggest that the thickness of the ice sheets, separate from the ice albedo effect, has a considerable impact on the tropical climate. They identify two types of responses: a quasi-linear response directly associated with the topographic changes and a nonlinear response mediated through the tropical thermocline adjustment. They find that increasing the thickness of the continental ice sheets produces a southward displacement of the ITCZ and a weakening of the equatorial zonal SST gradient, caused by cooling (warming) in the western (eastern) equatorial Pacific, together with the thermocline deepening to the east. They note that the energy flux approach proposed in Kang et al. (2008, 2009) and Cvijanovic and Chiang (2013) does not appear to explain the ITCZ shifts in these experiments because even though the northern cross-equatorial energy transport increases with the ice thickness, the mid-latitude transport decreases.
Although most of the simulation studies on extratropical to tropical teleconnections focus on just one ocean model at a time, there is recent literature analysing the subject in a hierarchy of ocean model configurations.
Kay et al. (2016) study the effect of Southern Ocean cooling on the tropical precipitation, coupling an AGCM either to a slab or to a full oceanic model. They find that with dynamic ocean heat transport the tropical precipitation response is weaker with, in this case, most of the cross-equatorial heat transport carried out by the ocean and not by the atmosphere. Similar conclusions are obtained by Hawcroft et al. (2017) and Tomas et al. (2016) with different fully coupled models, suggesting that the results are not model specific. In the same direction, Green and Marshall (2017) perform a series of idealized simulations in aqua-planet mode with an AGCM coupled either to a dynamic or to a slab ocean model while an ITG is applied. They find that the oceanic circulation dampens the ITCZ shift in response to the ITG by a factor of 4 compared to the case when the ocean circulation is not allowed to respond to the forcing. They find that with a dynamic ocean the mechanical coupling of the tropical atmospheric and oceanic energy transport (through Ekman balance) ensures that the ocean circulation always transports energy across the Equator in the same direction as the atmosphere does, therefore helping offset the imposed thermal contrast and not requiring for the atmosphere to transport as much energy as when the ocean circulation is fixed.
From a theoretical perspective, Schneider (2017) confirms the simulation results and derives a quantitative framework that shows that the Ekman coupling of atmospheric and oceanic energy fluxes dampens the response of the ITCZ and calculates that, in the current climate in the zonal and annual mean, the factor of damping by Ekman coupling is of the order of 3.
To complement the results of the previously mentioned studies here we propose to analyse the tropical response to extratropical thermal forcing in a hierarchy of ocean model configurations, but by using an intermediate-complexity ocean model coupled only in the tropical oceans: an RGO model. These simulations, therefore, represent an additional and intermediate step into understanding the tropical ocean dynamics' role in the extratropical to tropical communication process.
The paper is organized as follows. In Sect. 2 we describe the models used, with special emphasis on the description of the RGO model and its validation against observational data. The experiments performed are explained in Sect. 3. The results can be found in Sect. 4, differentiated regarding changes in annual mean, seasonal cycle or ENSO. The summary and conclusions are presented in Sect. 5.
The atmospheric model used in this study is the Abdus Salam International Centre for Theoretical Physics (ICTP) AGCM (Molteni, 2003;
Kucharski et al., 2006), which is a full atmospheric model with simplified physics. We use the model version 40 in its eight-layer
configuration and T30 (
We analyse the outcomes of coupling the AGCM with two ocean models of different complexity. In the first configuration the AGCM is
coupled with a slab ocean model; a monthly varying ocean heat flux correction (derived from a previous 30-year model integration with
identical settings but with prescribed observed SSTs) is imposed in order to keep the simulated SST close to present-day conditions. In
the second configuration, and in order to better reproduce the tropical ocean dynamics, an RGO model is coupled in the tropical region (30
We proceed to describe the RGO model and to validate its results by comparison with observational analogous.
We use an extension of the classical 1.5-layer RGO model, introduced by Cane (1979) to study the ENSO phenomenon. The extension of the model, as in Chang (1994), includes thermodynamics of the upper ocean and allows for the prediction of the SST.
The model consists of a 50
The resolution of the RGO model is 1
A 70-year control simulation in which the AGCM is coupled to the RGO in the tropics and to the slab ocean model elsewhere is produced. The last 50 years of the control run are used for averaging and comparison with observational analogues. We use the NOAA Extended Reconstructed SST V3b (Smith et al., 2008) and the near-surface winds from the NCEP/NCAR Reanalysis (Kalnay et al., 2006), for the period 1979–2013.
With the imposed heat flux correction, the annual mean SST in the control simulation strongly resembles the observed pattern (not
shown). In addition, the model reasonably captures the main characteristics of the seasonal cycle in the equatorial (2
The control simulation also reproduces the main mode of variability in the tropical Pacific Ocean quite realistically both in the spatial and temporal domains (Fig. 2; please note that in all the latitude–longitude maps in the manuscript the land and sea mask used by the model is the one depicted). The first coupled pattern arising from a singular value decomposition (SVD) of the monthly SST and surface wind characterizes ENSO and explains 81 % (62 %) of the variability in the observations (simulation). The simulated pattern is weaker than the observed, with the SST anomaly maximum located too far eastward. The phase-locking to the seasonal cycle of the simulated pattern peaks during the end of the calendar year as it does in the observations, but its distribution is more uniform throughout the year. Both simulated and observed spectra show statistically significant peaks relative to a red-noise null hypothesis from 16 to 60 months.
For each model configuration two runs are produced: a control run (in which no forcing is applied) and a forced run (in which
extratropical forcing is imposed). The applied forcing pattern consists of cooling in one hemisphere and warming in the other poleward
of 40
The forcing pattern is shown in Fig. 3, in which sign convention is positive out of sea and, therefore, positive values of the forcing
could be considered to represent a situation where the atmosphere is dry and colder than the ocean below it so that there is a strong
ocean-to-atmosphere net heat flux. This forcing generates a near-surface temperature (NSAT) anomaly response of up to 16
As mentioned before we use two ocean models. When the AGCM is coupled to
a slab ocean model, the experiments are named
Experiment summary.
First we analyse and compare the annual mean anomalies generated by the extratropical forcing with the two configurations implemented. Second, we will focus on the tropical Pacific climate and study the changes produced in the seasonal cycle for both setups. Finally, we will briefly investigate possible changes in ENSO activity when the RGO is coupled in the tropical oceans.
In this subsection we compare the results obtained with the two implemented configurations in terms of annual means of different fields. The results are presented in the form of anomalies with respect to the corresponding control case.
Figure 4 shows the near-surface air temperature (NSAT) changes with respect
to the corresponding control for the two configurations. In both experiments
there is generalized warming (cooling) in the NH (SH), while in the southern
tropics a strengthening of the zonal gradient is evident. The most pronounced
differences between the two configurations are seen in the tropical region,
in which the slab plus rgo configuration
anomalies tend to be up to 1
As a consequence, tropical changes in precipitation are weaker when using the RGO: while in both experiments the most pronounced
feature is a northward shift of the ITCZ, anomalies for the slab
As expected from the above results, both experiments present similar patterns of near-surface (950
To summarize, in Fig. 7 we present the northward atmospheric energy transport for the control and forced runs in the two configurations
implemented. As can be seen, while the control runs display almost identical transport, the forced runs significantly disagree in
magnitude in the tropical region, with the slab
As the previous subsection showed, the most pronounced differences between the two implemented configurations are found in the tropical
band. Therefore, for the analysis of variations in the seasonal cycle we will focus on the 30
Three-month means of SST and near-surface wind changes for the tropics are shown in Fig. 8. In the Pacific Ocean, for the
Equatorial (2
The thermocline depth shows consistent changes when the RGO is used (Fig. 11): a deepening in the east of the basin starting around March and finishing in July, consistent with the warmer SSTs seen in the region (with 1-month lag). Considering the wind anomalies of the slab setup (Fig. 9a) as the forcing pattern for the ocean dynamics derived from the extratropical signal, this thermocline-deepening pulse appears to be initiated during the SH summer in the west of the basin (due to a weakening of the trades) and is propagated eastward as a Kelvin wave, reaching the eastern boundary 2–3 months later. The deepening of the eastern Pacific thermocline is concurrent with a shallowing in the western Pacific particularly from May to July, and vice versa (but less obvious) in other seasons of the year. In the second half of the year the strengthening of the trades locally shallows the thermocline in the eastern Pacific and the western Pacific recovers its mean depth.
In summary, the equatorial near-surface zonal wind changes caused by the extratropical forcing seen in the slab configuration induce dynamical ocean–atmosphere coupling that generates seasonal changes in the SST field when the RGO is used. This results in late austral autumn warming and cooling in spring and summer in the equatorial eastern Pacific, leading to a strengthening of the SST seasonal cycle with consistent changes in the thermocline depth.
In this subsection we investigate how the interannual variability in the tropical Pacific is affected by the interhemispheric SST gradient induced by the imposed forcing.
The leading pattern of co-variability of SST and near-surface wind in the tropical Pacific basin when the extratropical forcing is applied is weaker than that obtained when no forcing is implemented (Figs. 12a and 2a), and it explains a smaller percentage of the total variability (46 % compared to 62 %). The phase-locking to the seasonal cycle (Figs. 12b and 2c) is also modified, being more uniformly distributed and with a peak season from July to the end of the calendar year. The frequency spectrum of the ENSO pattern under the effect of the extratropical forcing is characterized by shorter periods than in the absence of the forcing and has a peak at 24 months (Figs. 12c and 2e).
The weakening of the ENSO activity can be understood in relation to the changes produced by the extratropical forcing on the SST seasonal cycle in the eastern Pacific Ocean. According to the nonlinear frequency entrainment mechanism (Chang et al., 1994) ENSO amplitude is anticorrelated with the strength of the SST seasonal cycle. The frequency entrainment implies that a self-exciting oscillator (like ENSO) will give up its intrinsic mode of oscillation in the presence of strong external forcing (like a strong seasonal SST cycle) and acquire the frequency of the applied oscillating forcing. Therefore, in our case, as the extratropical forcing generates significant strengthening of the eastern Pacific SST seasonal cycle, a weakening of ENSO is expected according to this mechanism.
Assuming linear behaviour holds, our result of ENSO weakening is also in agreement with Timmermann et al. (2007). These authors analyse fully coupled GCMs in the context of an Atlantic meridional overturning circulation (AMOC) slowdown, producing generalized cooling of the NH and warming in the SH, and find that most of the models predict a ENSO intensification attributed to a seasonal cycle weakening. The weakening of ENSO activity in the presence of a northward ITG is also consistent with the work of Chiang et al. (2008), who use an AGCM coupled to an RGO model, a model configuration similar to ours. Although they do not attempt to explain the causes, they find that ENSO is sensitive to ITG with maximal activity when the ITG is close to zero and a weakened performance as the gradient increases in any direction.
We investigated and compared the response of the tropical climate to extratropical thermal forcing in a hierarchy of models in which an AGCM was coupled either to a simple slab ocean model (just thermodynamic coupling) globally or with a combination of an RGO model in the tropical oceans and a slab ocean model elsewhere.
First, we found that the two model configurations lead to considerably different climate responses. In particular, in tropical regions the signal produced in the RGO coupling case is weaker in terms of annual means, indicating that regional dynamical air–sea interaction opposes the remote signal. This result is in agreement with the quantitative framework proposed by Schneider (2017), who calculates an expression for the damping of the ITCZ shift in the case of atmosphere–ocean mechanical coupling. In addition, our result also agrees with the simulation experiments performed by Kay et al. (2016) and Green and Marshall (2017), who also obtained a weaker tropical response when using a fully coupled model than when the AGCM is only coupled to a slab model, therefore indicating that the ITCZ shift damping is also seen when an intermediate-complexity ocean model is coupled only in the tropics. In our experiments the energy flux equator (which can be regarded as an approximation for the ITCZ latitude) shift dampens by a factor of 1.5 in the case when tropical ocean dynamics are included, while the southward atmospheric energy transport experiences a damping by a factor of 1.9. In the simulations by Green and Marshall (2017) and in the quantitative work of Schneider (2017) the damping factors for the ITCZ shift were 4 and 3 respectively.
However, although the annual mean anomalies produced by the RGO setup are weaker, we find that the changes in the SST seasonal cycle are larger. In particular, over the equatorial Pacific Ocean, while the slab configuration produces no changes to the SST seasonal cycle, the RGO addition generates profound warming in the central-eastern basin from April to August balanced by cooling in the rest of the year, yielding almost null integration in the annual mean but also implying a significant strengthening of the seasonal cycle in the eastern Pacific. The response of the seasonal cycle to the imposed extratropical forcing is qualitatively similar to the one obtained by Chiang et al. (2008) in similar experiments, although in our case positive SST anomalies reach the eastern boundary of the basin preventing earlier onset of the seasonal cold tongue as found by these authors in their simulations. We hypothesize that the changes in the SST seasonal cycle are possible via the effect that the zonal wind stress has on the thermocline depth: the remote forcing produces positive anomalies of zonal wind stress to be exerted in the first half of the calendar year; in particular, the significant weakening of the trades over the western portion of the basin around February and March induces a thermocline-deepening pulse that propagates eastward in the form of a Kelvin wave, reaching the eastern boundary 2 months later, and generating warming of the SST over that region as a result. In the second half of the year stronger trades in the central-eastern basin shallow the thermocline producing local cooling of the SST. Since these mechanisms are not available under the slab configuration, the wind stress seasonal cycle changes are not able to produce any SST changes.
Finally, within the RGO setup, we briefly analysed possible changes in ENSO activity and found that under the effect of the extratropical forcing, considerable changes are produced in both the spatial and temporal domains with a weaker SST pattern and a time series that lacks low-frequency variability. We hypothesized that the weakening of the ENSO activity concurrent with the intensification of the SST seasonal cycle in the eastern equatorial Pacific Ocean could be due to the frequency entrainment mechanism. As future climate projections tend to agree on the fact that global warming will have an important northward ITG component (NH warming faster than the SH; Friedman et al., 2013), the possible sensitivity of ENSO to ITG is of utmost relevance. However, current state-of-the-art fully coupled climate models do not seem to agree on the projected future changes in ENSO characteristics, and no clear evidence for a correlation with ITG has been detected in future climate projections (Stevenson, 2012; Taschetto et al., 2014).
Data sets, codes and analysis scripts
used in this study can be obtained from
The authors declare that they have no conflict of interest.
Part of this work was performed while the first author was supported by Universidad de la República, Agencia Nacional de Investigación e Innovación (ANII, Uruguay) and the Belmont Forum and JPI-Climate Collaborative Research Action “INTEGRATE, An integrated data-model study of interactions between tropical monsoons and extratropical climate variability and extremes”. Comments by two anonymous reviewers are gratefully acknowledged. Edited by: Ben Kravitz Reviewed by: two anonymous referees