Cross-equatorial Gradient Research Articles

Tropical Atlantic multidecadal variability is dominated by external forcing.

The tropical Atlantic climate is characterized by prominent and correlated multidecadal variability in Atlantic sea surface temperatures (SSTs), Sahel rainfall and hurricane activity1-4. Owing to uncertainties in both the models and the observations, the origin of the physical relationships among these systems has remained controversial3-7. Here we show that the cross-equatorial gradient in tropical Atlantic SSTs-largely driven by radiative perturbations associated with anthropogenic emissions and volcanic aerosols since 19503,7-is a key determinant of Atlantic hurricane formation and Sahel rainfall. The relationship is obscured in a large ensemble of CMIP6 Earth system models, because the models overestimate long-term trends for warming in the Northern Hemisphere relative to the Southern Hemisphere from around 1950 as well as associated changes in atmospheric circulation and rainfall. When the overestimated trends are removed, correlations between SSTs and Atlantic hurricane formation and Sahel rainfall emerge as a response to radiative forcing, especially since 1950 when anthropogenic aerosol forcing has been high. Our findings establish that the tropical Atlantic SST gradient is a stronger determinant of tropical impacts than SSTs across the entire North Atlantic, because the gradient is more physically connected to tropical impacts via local atmospheric circulations8. Our findings highlight that Atlantic hurricane activity and Sahel rainfall variations can be predicted from radiative forcing driven by anthropogenic emissions and volcanism, but firmer predictions are limited by the signal-to-noise paradox9-11 and uncertainty in future climate forcings.

A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

Abstract. Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4–18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37∘ S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3–4 ∘C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas.

Eastern Pacific ITCZ Dipole and ENSO Diversity

Abstract The eastern tropical Pacific features strong climatic asymmetry across the equator, with the intertropical convergence zone (ITCZ) displaced north of the equator most of time. In February–April (FMA), the seasonal warming in the Southern Hemisphere and cooling in the Northern Hemisphere weaken the climatic asymmetry, and a double ITCZ appears with a zonal rainband on either side of the equator. Results from an analysis of precipitation variability reveal that the relative strength between the northern and southern ITCZ varies from one year to another and this meridional seesaw results from ocean–atmosphere coupling. Surprisingly this meridional seesaw is triggered by an El Niño–Southern Oscillation (ENSO) of moderate amplitudes. Although ENSO is originally symmetric about the equator, the asymmetry in the mean climate in the preceding season introduces asymmetric perturbations, which are then preferentially amplified by coupled ocean–atmosphere feedback in FMA when deep convection is sensitive to small changes in cross-equatorial gradient of sea surface temperature. This study shows that moderate ENSO follows a distinct decay trajectory in FMA and southeasterly cross-equatorial wind anomalies cause moderate El Niño to dissipate rapidly as southeasterly cross-equatorial wind anomalies intensify ocean upwelling south of the equator. In contrast, extreme El Niño remains strong through FMA as enhanced deep convection causes westerly wind anomalies to intrude and suppress ocean upwelling in the eastern equatorial Pacific.

A Hybrid Coupled Model Study of Tropical Atlantic Variability

A hybrid coupled model (HCM) is used to explore the underlying dynamics governing tropical Atlantic variability (TAV) and the dynamic regime that may be most relevant to TAV. By coupling an empirical atmospheric feedback model to an ocean GCM, the authors have conducted a detailed investigation on the potential importance of an unstable ocean–atmosphere interaction between wind-induced heat flux and sea surface temperature (SST) in driving decadal climate variability in the tropical Atlantic basin. The investigation consists of a systematic parameter sensitivity study of the hybrid coupled model. It is shown that in a strong coupling regime the local air–sea feedbacks can support a self-sustained decadal oscillation that exhibits strong cross-equatorial SST gradient and meridional wind variability. An upper-ocean heat budget analysis suggests that the oscillation results from an imbalance between the positive and negative feedbacks in the model. The dominant negative feedback that counteracts the positive feedback between surface heat flux and SST appears to be the advection of heat by ocean currents. The major imbalance in the model occurs in the north tropical Atlantic between 5° and 15°N, caused by a phase delay between the surface heat flux forcing and horizontal heat advection. It is suggested that this may be one of the crucial regions of ocean–atmosphere interactions for TAV. Based on the HCM results, a simple 1D model is derived to further elucidate key coupled dynamics. The model assumes that air–sea coupling takes place in a limited area within the deep Tropics of the Atlantic sector and the change of upper-ocean heat transport is regulated by the advection of anomalous temperatures by the mean meridional current and equatorial upwelling. The analysis shows that the simple model captures many of the salient features of the decadal SST cycle in the HCM, suggesting that the decadal oscillations simulated by the HCM are primarily controlled by the coupled dynamics local to the deep Tropics. The parameter sensitivity study further suggests that in reality the local air–sea coupling in the tropical Atlantic is most likely to be too weak to maintain a self-sustained oscillation, and stochastic forcing may be necessary to excite the coupled variability. Using a realistic representation of external “noise” derived from a 145-yr simulation of the National Center for Atmospheric Research atmospheric GCM (CCM3) forced with the observed SST annual cycle, the effect of stochastic forcing on TAV when the coupled system resides in a stable dynamical regime is examined. It is found that the local air–sea feedback and the North Atlantic oscillation–(NAO) dominated “noise” forcing are both required to simulate a realistic TAV. In the absence of the local air–sea feedback, the “noise” forcing can produce substantial SST anomalies in the subtropical Atlantic up to about 15°N, particularly off the coast of North Africa. The local air–sea feedback appears to be particularly important for generating the covarying pattern of interhemipheric SST gradient and cross-equatorial atmospheric flow within the deep Tropics. However, too-strong local coupling can lead to an exaggerated tropical response. It is therefore conjectured that TAV may best fit into a weakly coupled scenario in which at minimum the air–sea feedback plays a role in enhancing the persistence of the cross-equatorial gradient of SST and the circulation anomalies, while the NAO provides an important source of external forcing to excite the coupled variability in the Tropics. Furthermore, it is argued that the“noise” forcing can significantly weaken the correlation between the SST variability on either side of the equator, thus hiding any underlying weak “dipole” structure in the SST.

The influence of cross-equatorial pressure gradients on the location of near-equatorial convection

Tomas and Webster note that in regions where there is a substantial cross-equator surface pressure gradient, there is locally anticyclonic absolute vorticity on the low pressure side of the equator and thus, the flow in this region meets the parcel criterion for inertial instability. They hypothesize that the atmospheric response to this absolute-vorticity distribution is a convergence-divergence doublet in the boundary layer. The convergence centre results in strong convection and thus is very important in determining the strength and location of convection (i.e. the intertropical convergence zones) whereas the divergence centre results in suppressed convection. In this work, a zonally symmetric model of the boundary layer is developed to investigate this hypothesis further. An analysis of a linearized version of the model indicates that although the observed flow in these regions meets the parcel criterion for instability, it does not satisfy the linear stability criteria, because of the stabilizing influence of dissipation and the finite vertical scale. Much greater shear than that observed is required for linear instability. It is noted, however, that when the parcel criterion is met, one of the nonlinear terms which was neglected in the linear analysis may have an important influence on the flow. Several experiments are performed integrating the full nonlinear model, relaxing back to a pressure distribution having a cross-equatorial gradient, to test whether the observed vorticity, wind and convergence distributions can be well simulated. It is found that the simulations reach a steady state in which there is a region of locally anticyclonic absolute vorticity on the low pressure side of the equator. This absolute-vorticity distribution results in an anomalous acceleration of the meridional wind and a convergence-divergence doublet, similar to that observed by Tomas and Webster. This response is explained as resulting from accelerations that act to bring the zonal flow out of geostrophic balance as it passes through the region of locally anticyclonic absolute vorticity. The response of the meridional wind to the strength of the pressure gradient is quasi-linear for small and somewhat larger values of forcing but, between these values, there is a range where the response is highly nonlinear, i.e. large increases in the strength of the meridional wind result from small changes in the pressure gradient. It is suggested that this nonlinear response may play a role in the observed sudden onset of the monsoon circulations. The nonlinear model is also forced using pressure distributions taken from observations. The resulting steady-state simulated vorticity, wind and convergence distributions closely match the observations for the cases of pressure distributions taken from the east Pacific and east Atlantic during July. When forced using the pressure distribution from the Indian Ocean region during July, the model produces an unrealistically strong meridional wind response. Some improvement is obtained by including the latitudinally dependent zonal pressure gradient force, which is moderately strong in this region. Finally, the model is forced using a pressure distribution taken from observations in the central Pacific Ocean during July. In this case the cross-equator pressure gradient is near zero. The simulated convergence is a maximum on the equator whereas observations show two convergence maxima that flank the equator; the observed convergence centres are associated with the convection that occurs in the region. Thus, the boundary layer dynamics as simulated by this model are not responsible for determining the location of convection in this region. It is suggested that in this case, the convection determines the boundary layer convergence, in contrast with the other cases, in which it is the boundary layer convergence that determines the convection.

環大西洋における大気海洋変動

Ocean and atmosphere variability is investigated over the whole Atlantic basin in a 51-year record of sea surface temperature (SST). In the tropics, SSTs variations are separated into two time scales: decadal (8-16 years) and interannual (<5 years). A strong cross-equatorial gradient mode dominates decadal SST anomalies with centers of action having opposite polarities at 15°N and 15°S, while interannual variations are characterized by SST anomalies of the same polarity throughout the tropics. Sea level pressure (SLP) regressions onto the cross-equatorial gradient index reveal extratropical teleconnections associated with the North Atlantic Oscillation (NAO), and with the South Atlantic. At the same time, a Pan-Atlantic spatial pattern is also found in SST regressions. Lagged regressions of SLP on the cross-equatorial SST gradient index shows the extratropical decadal oscillation that is out of phase between hemispheres. These results suggest that an extratropical forcing could excite the tropical Atlantic variability.