Subtropical clouds key to Southern Ocean teleconnections to the tropical Pacific
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Abstract:
Excessive precipitation over the southeastern tropical Pacific is a major common bias that persists through generations of global climate models. While recent studies suggest an overly warm Southern Ocean as the cause, models disagree on the quantitative importance of this remote mechanism in light of ocean circulation feedback. Here, using a multimodel experiment in which the Southern Ocean is radiatively cooled, we show a teleconnection from the Southern Ocean to the tropical Pacific that is mediated by a shortwave subtropical cloud feedback. Cooling the Southern Ocean preferentially cools the southeastern tropical Pacific, thereby shifting the eastern tropical Pacific rainbelt northward with the reduced precipitation bias. Regional cloud locking experiments confirm that the teleconnection efficiency depends on subtropical stratocumulus cloud feedback. This subtropical cloud feedback is too weak in most climate models, suggesting that teleconnections from the Southern Ocean to the tropical Pacific are stronger than widely thought.Keywords:
Teleconnection
Tropical Eastern Pacific
Shortwave
Walker circulation
Tropical climate
ABSTRACT The climate shift in 1976–1977 sets a clear distinction in the teleconnection between Indian summer monsoon rainfall ( ISMR ) and sea surface temperatures ( SST ) in the tropical oceans, particularly in the tropical Indian Ocean ( TIO ). Before the climate shift (Pre‐77), the observed correlation between ISMR and SST was positive in the eastern equatorial Indian Ocean ( EIO ) and negative correlation prevailed in the western equatorial Indian Ocean ( WIO ), whereas after the climate shift (Post‐77), the correlation pattern has reversed (i.e. positive correlation in WIO and negative correlation in EIO ). The major reason for this shift is the consistent warming of SSTs throughout the tropics and frequent occurrence of IOD events. Further, it is noticed that during the earlier decades, the Indian Ocean was responding to the atmospheric forcing, particularly along the axis of strong cross equatorial flow, whereas in the recent decades, Indian Ocean dynamics are playing a crucial role in determining the SST in major parts of the Indian Ocean particularly in the EIO . Seasonal re‐forecasts from six state‐of‐the‐art global coupled atmosphere–ocean climate models which participated in ENSEMBLES faithfully capture the pre‐77 dipole structure of correlations. But the post‐77 correlation pattern is not simulated by any of the six models considered in this study. Two possible reasons are pointed out for the models' inability to capture the reversal of teleconnection: (1) in the coupled models, unchanged mean state of the winds inhibits ISMR to be governed by coupled dynamics in the TIO ; (2) the strong, local SST –rainfall relationship in models even in post‐77 period limits the influence of the teleconnections on the ISMR .
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Teleconnection
Hadley cell
Intertropical Convergence Zone
Walker circulation
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Atmospheric models
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Walker circulation
Extratropical cyclone
Atmospheric Circulation
Atmospheric models
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Atmospheric Circulation
Tropical Atlantic
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Abstract A greater understanding of equatorial teleconnections is a key aspect of research into seasonal prediction and future climate change for tropical regions. Here the impact of Pacific sea surface temperature (SST) anomalies on rainfall fluctuations over the semi‐arid Sahel is explored, using a combination of observed and model data. The first key topic of this study is the identification of those aspects of anomalous Pacific SST variability that are most strongly linked to the Sahel. One of these, also illuminated by earlier studies, is similar to the classic El Niño Southern Oscillation pattern in the central and east Pacific, which in its El Niño phase increases the likelihood of Sahel drought. It is shown here that, although a part of this link is indirect (operating via Atlantic SSTs), its main effect appears to be through a direct atmospheric teleconnection. The other critical pattern, of equal importance and revealed here by a novel analysis technique, is the large‐scale zonal gradient of SSTs from the west Pacific to the east Indian Ocean. If weakened, this too enhances the likelihood of Sahel drought. Atmospheric general circulation model experiments, forced either by observed or idealized SSTs, are used to confirm these two influences on the Sahel. Crucially, their Sahelian impact is substantially reinforced when both are present and, additionally, further empirical analysis shows them to be largely independent. The second key topic is an investigation of the mechanisms for this Pacific‐Sahel teleconnection. These appear to involve anomalous stationary equatorial waves, with communication occurring in both the eastward and westward directions. In El Niño years (for example), a Kelvin wave emanates across the Atlantic from east Pacific convective heating anomalies, and an equatorial Rossby wave appears over the Indian Ocean in response to the anomalous west Pacific‐Indian Ocean SST gradients via convective heating anomalies over the Indian Ocean. These interact over Africa to enhance large‐scale subsidence over the Sahel, thus reducing seasonal rainfall totals. Interannual changes in propagating equatorial waves or in the residence of subseasonal regimes appear not to play a substantial role.
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Tropical Eastern Pacific
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Abstract The Eastern Pacific (EP) and Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) types and their impacts on the tropical North Atlantic (TNA) SST variability and 15°N–15°S South American precipitation during the warm and cold phases of the Atlantic Multidecadal Oscillation (WAMO and CAMO) were evaluated during the 1901–2012 period. The results show more frequent ENSO events during the CAMO. The El Niño (EN) (La Niña [LN]) events, regardless of type (EP or CP), during the WAMO (CAMO) were accompanied by a warming (cooling) in the TNA after its mature phases. In these cases, extratropical teleconnection patterns are established through variations in the Pacific/North America (PNA) teleconnection pattern and are accompanied by variations in the Walker circulation. For the EN (LN) in the CAMO (WAMO), the tropical teleconnections occur predominant, through the Walker cell and the zonal inter‐basin gradient, which is intensified due to the SST gradient between the eastern equatorial Pacific (non‐neutral anomalies) and the equatorial Atlantic (neutral anomalies). These circulation pattern changes affect the precipitation patterns in the 15°N–15°S South American sector during December–January–February (D(0)JF(+1)) and March–April–May (MAM(+1)). The EP EN (EPEN) events are associated with the intensification of the negative precipitation anomalies in northeastern Brazil (NEB) during the WAMO and in the central part of the Amazon during the CAMO. In the case of CP EN (CPEN) events, the greatest differences between the AMO phases occur during MAM(+1), with reverse sign anomalies over northwestern South America. In the case of LN events, the largest differences occur in NEB, with reduced rainfall in the WAMO, regardless of type EP or CP. The results presented here highlight the role of low frequency oscillations in defining the teleconnection patterns between tropical Pacific and Atlantic Oceans, not discussed previously.
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Tropical Atlantic
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The teleconnections of the El Niño/Southern Oscillation (ENSO) in future climate projections are investigated using results of the coupled climate model ECHAM5/MPI-OM. For this, the IPCC SRES scenario A1B and a quadrupled CO2 simulation are considered. It is found that changes of the mean state in the tropical Pacific are likely to condition ENSO teleconnections in the Pacific North America (PNA) region and in the North Atlantic European (NAE) region. With increasing greenhouse gas emissions the changes of the mean states in the tropical and sub-tropical Pacific are El Niño-like in this particular model. Sea surface temperatures in the tropical Pacific are increased predominantly in its eastern part and redistribute the precipitation further eastward. The dynamical response of the atmosphere is such that the equatorial east–west (Walker) circulation and the eastern Pacific inverse Hadley circulation are decreased. Over the subtropical East Pacific and North Atlantic the 200 hPa westerly wind is substantially increased. Composite maps of different climate parameters for positive and negative ENSO events are used to reveal changes of the ENSO teleconnections. Mean sea level pressure and upper tropospheric zonal winds indicate an eastward shift of the well-known teleconnection patterns in the PNA region and an increasing North Atlantic oscillation (NAO) like response over the NAE region. Surface temperature and precipitation underline this effect, particularly over the North Pacific and the central North Atlantic. Moreover, in the NAE region the 200 hPa westerly wind is increasingly related to the stationary wave activity. Here the stationary waves appear NAO-like.
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Walker circulation
Atmospheric Circulation
Hadley cell
Atmospheric models
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