Persistent Sea Surface Temperature Research Articles

Unprecedented cross-equatorial southerly wind anomalies during the 2020–2023 triple-dip La Niña: Impacts and mechanisms

The 2020–2023 La Niña event, the third triple-dip occurrence since 1950, stands out for its unique characteristics when compared to the preceding events in 1973–1976 and 1998–2001. Unlike its predecessors, the latest event emerged without a preceding strong El Niño episode, starting from a state of weak discharge. Moreover, the resurgence of cold sea surface temperature (SST) anomalies during 2020–2023 showed a loose link to ocean-atmosphere variability in the northern subtropical Pacific. The spatial pattern of anomalous conditions in this recent event featured persistent SST cooling in the southeast tropical Pacific, accompanied by pronounced meridional southerly wind anomalies. Our comprehensive investigation indicates a minimal role of subsurface ocean dynamics, underscoring the exceptional strength of cross-equatorial meridional southerly wind anomalies as a primary driver of sustained negative sea surface temperature anomalies. By considering the influences of intra-basin and inter-basin factors, we propose that the observed patterns may be the combined outcome of interannual variability within the tropical Pacific and decadal variations beyond the tropics. Noteworthy contributions from the low-frequency components of the South Pacific Meridional Mode and the Atlantic Warm Pool further shape the dynamics of this event. Our findings underline the increasing importance of meridional winds in shaping the evolution of ENSO in recent decades.

Diurnal warming rectification in the tropical Pacific linked to sea surface temperature front

Sharp and rapid changes in the sea surface temperature (SST) associated with fronts and the diurnal cycle can drive changes in the atmospheric boundary-layer stability and circulation. Here we show how a one-dimensional surface ocean model forced with either high-resolution or daily averaged surface fluxes can be used to distinguish diurnal versus frontal SST anomalies observed from an uncrewed surface vehicle. The model, forced with daily satellite fluxes, shows that the diurnal warming is largest within the equatorial Pacific cold tongue of SST. The strong persistent SST front north of the cold tongue is evident in both the oceanic and atmospheric boundary-layer stability scales and, as a consequence, in the magnitude of the diurnal ocean warming. Using SST, barometric pressure and surface wind measurements from moorings at 0°, 95° W and 2° N, 95° W, we show that the front in the SST diurnal warming results in a weakened SST front in the afternoon and a corresponding reduced meridional gradient in the barometric pressure that appears to contribute to a diurnal pulsing of the surface meridional winds. To the extent that these modulate the surface branch of the Hadley cell, these diurnal variations may have remote impacts.

An attempt using equatorial waves to predict tropical sea surface temperature anomalies associated with the Atlantic zonal mode

IntroductionThe forecast for anomalous sea surface temperature (SST) events associated with Atlantic zonal mode, also known as Atlantic Niño/Niña, is full of challenge for both statistical and dynamical prediction models. MethodsThis study combines SST, wind and equatorial wave signal to construct a linear model, aiming to evaluate the potential of equatorial waves in extending the lead time of a skilful prediction for Atlantic Niño/Niña events. Wave-induced geopotential simulated by linear ocean models and potential energy flux calculated using a group-velocity-based wave energy flux scheme are involved to capture the signal of equatorial waves in the model establishment. ResultsThe constructed linear prediction model has demonstrated comparable prediction skill for the SST anomaly to the dynamical models of the North American Multimodel Ensemble (NMME) during the test period (1992-2016). Compared with the statistical forecast using SST persistence, the model notably improves the six-month-lead prediction (Anomaly correlation coefficient increases from 0.07 to 0.28), which owes to the conservation of wave energy in the narrow Atlantic basin that the Rossby waves reflected in the eastern boundary will transfer the energy back to the central equatorial basin and again affect the SST there. ConclusionThis study offers a streamlined model and a straightforward demonstration of leveraging wave energy transfer route for the prediction of Atlantic Niño/Niñas.

California margin temperatures modulate regional circulation and extreme summer precipitation in the desert Southwest

In August 2022, Death Valley, the driest place in North America, experienced record flooding from summertime rainfall associated with the North American monsoon (NAM). Given the socioeconomic cost of these type of events, there is a dire need to understand their drivers and future statistics. Existing theory predicts that increases in the intensity of precipitation is a robust response to anthropogenic warming. Paleoclimatic evidence suggests that northeast Pacific (NEP) sea surface temperature (SST) variability could further intensify summertime NAM rainfall over the desert southwest. Drawing on this paleoclimatic evidence, we use historical observations and reanalyzes to test the hypothesis that warm SSTs on the southern California margin are linked to more frequent extreme precipitation events in the NAM domain. We find that summers with above-average coastal SSTs are more favorable to moist convection in the northern edge of the NAM domain (southern California, Arizona, New Mexico, and the southern Great Basin). This is because warmer SSTs drive circulation changes that increase moisture flux into the desert southwest, driving more frequent precipitation extremes and increases in seasonal rainfall totals. These results, which are robust across observational products, establish a linkage between marine and terrestrial extremes, since summers with anomalously warm SSTs on the California margin have been linked to seasonal or multi-year NEP marine heatwaves. However, current generation earth system models (ESMs) struggle to reproduce the observed relationship between coastal SSTs and NAM precipitation. Across models, there is a strong negative relationship between the magnitude of an ESM’s warm SST bias on the California margin and its skill at reproducing the correlation with desert southwest rainfall. Given persistent NEP SST biases in ESMs, our results suggest that efforts to improve representation of climatological SSTs are crucial for accurately predicting future changes in hydroclimate extremes in the desert southwest.

Prevalent atmospheric and oceanic signals of the unprecedented heatwaves over the Yangtze River Valley in July–August 2022

Unprecedented heatwaves attacked the Yangtze River valley (YRV) in July–August of 2022. The heatwave-favorable circulation pattern for the YRV is identified by using a self-organizing map. Result shows that the dominant daily circulation pattern driving heatwaves in the YRV is the East Asian anticyclonic pattern (EAAP), characterized by an anomalous deep anticyclone over the East Asian mid-latitudes. Remarkably, the unprecedented frequent occurrence of EAAP in 2022 contributes about 84.6% of the temperature anomalies in the 35 YRV hot days, which is the key dynamical factor for the heatwave in 2022. Physical mechanisms for the occurrence of EAAP are revealed on interannual and interdecadal time scales. On interannual time scale, the dipole sea surface temperature (SST) anomalies over the North Atlantic, characterized by warm and cold SST anomalies in the midlatitudes and subtropics, respectively, exert an important influence on the EAAP days. This dipole SST pattern is unprecedentedly strong in 2022. It facilitates the EAAP through the maintenance of the coupled British–Okhotsk Corridor pattern and the Silk Road pattern in the mid-latitudes. Additionally, the persistent cold SST anomalies in tropical Indian ocean and the central-eastern Pacific from the preceding spring to summer contribute to cooling of the tropical troposphere and induce easterly anomalies in the East Asian subtropics, thereby further promoting the occurrence of EAAP. On interdecadal time scale, the negative Pacific Decadal Oscillation may establish favorable circulation conditions for the occurrence of the EAAP through modulating the interdecadal Silk Road pattern. These findings improve our understanding of the physical mechanisms for driving extreme heatwaves in the YRV and have an implication for enhancing regional heatwave prediction.

Simulations of the North Tropical Atlantic Mode–ENSO Connection in CMIP5 and CMIP6 Models

AbstractThe North Tropical Atlantic (NTA) mode is a dominant component of sea surface temperature (SST) variability over the northern tropical Atlantic, which is crucial to the occurrence of El Niño‐Southern Oscillation (ENSO). This study utilizes the historical simulation of the Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5 and CMIP6) to evaluate the simulations of the NTA mode and NTA–ENSO connection. We found that the CMIP5 and CMIP6 could reproduce the spatial pattern of the NTA mode similarly. Most of them could reasonably reproduce the spatial pattern of the NTA mode similar to the observation. Still, there are some differences in the central values and position of the NTA mode. In addition, the analysis further suggests that the CMIP5/CMIP6 models have a large diversity in the NTA–ENSO connection, which may be due to their differences in simulating the spring climatological mean Atlantic Intertropical Convergence Zone (ITCZ) precipitation and spring‐to‐summer NTA SST persistence. Models with stronger spring climatological mean Atlantic ITCZ precipitation in favor of the spring NTA‐related SST anomalies stimulate local precipitation anomalies, resulting in a more powerful atmospheric circulation reaction over the tropical Pacific. Additionally, the stronger spring‐to‐summer NTA SST persistence for sustaining the teleconnection, which constantly transmits impacts from the tropical Atlantic into the tropical Pacific. The CMIP6 models show improvements to the CMIP5 models regarding simulations of the spring climatological mean Atlantic ITCZ precipitation and spring‐to‐summer NTA SST persistence. Therefore, the CMIP6 models simulate the NTA–ENSO connection better than the CMIP5 models.

A survey of coastal conditions around the continental US using a high-resolution ocean reanalysis

Obtaining a long-term perspective on the evolution of the ocean is hindered by a lack of sub-surface observations. Ocean reanalyses, which merge fields from an ocean model with observations using complex data assimilation techniques, provide a dynamically consistent estimate of the ocean in time and space. Recently developed ocean reanalyses, several with resolutions finer than 10 km, provide a three-dimensional view of the ocean, including on the continental shelf. Here, we use monthly fields from the 1/12° Global Ocean Reanalysis and Simulations (GLORYS) over the years 1993–2019 to provide a broad survey of temperature, salinity, and mixed layer depth over the shelf (depth < 400 m) around the continental United States. The analyses reveal noteworthy aspects of US coastal oceanography, including: i) how bathymetry shapes the seasonal cycle of bottom water temperature (BWT) and the role of the mixed layer in linking the BWT and sea surface temperature (SST) anomalies; ii) the persistence, trends and distributions of SST and BWT anomalies, which strongly influence extremes such as marine heatwaves; iii) the influence of the outflow from the Mississippi and Columbia rivers on the adjacent ocean; and iv) how the mean and anomalous vertical structure of temperature and salinity vary between the northeast, southeast, Gulf of Mexico and California Current Large Marine Ecosystems. In addition to documenting coastal ocean conditions, the broader goal of the survey is to encourage more detailed studies using high resolution ocean reanalyses such as GLORYS.

A Trans-Season Out-of-Phase Relationship of Tropical Cyclogenesis between the Western North Pacific and South China Sea

Abstract The present study investigates the relationship of tropical cyclone (TC) genesis between the western North Pacific (WNP) and South China Sea (SCS) from 1979 to 2020. A significantly out-of-phase variation is found between spring [March–May (MAM)] TC genesis over the WNP and the following summer–fall [June–November (JJASON)] TC genesis over the SCS. More TCs over the WNP in MAM are followed by fewer TCs over the SCS in the succeeding JJASON. Composite analysis and numerical model experiments show that negative sea surface temperature (SST) anomalies during MAM in the tropical central-eastern Pacific (CEP) and southeastern Indian Ocean work together to induce a lower-level cyclonic circulation over the WNP, with the latter more important. The positive specific humidity and ascending motion favor the TC genesis over the WNP in MAM. In the following JJASON, the SST anomalies are reversed in the tropical CEP. The positive precipitation anomalies over the western-central Pacific induced by positive SST anomalies further stimulate an anomalous zonal overturning circulation with anomalous descending motion and boundary layer divergence over the SCS. In addition, the persistent negative SST anomalies around the Maritime Continent (MC) induce an anomalous anticyclone to the west. Both processes lead to negative genesis potential index (GPI) anomalies and thus inhibit the TC genesis over the SCS. This out-of-phase relationship of TC genesis between the WNP and SCS also exists when the El Niño–Southern Oscillation (ENSO) transition years are removed. This finding may be helpful to improve the seasonal prediction of the SCS TC activity over the peak TC season.

Understanding the Drivers of Atlantic Multidecadal Variability Using a Stochastic Model Hierarchy

Abstract The relative importance of ocean and atmospheric dynamics in generating Atlantic multidecadal variability (AMV) remains an open question. Comparisons between climate models with a slab ocean (SLAB) and fully dynamic ocean components (FULL) are often used to explore this question, but cannot reveal how individual ocean processes generate these differences. We build a hierarchy of physically interpretable stochastic models to investigate the contribution of two upper-ocean processes to AMV: the role of seasonal variation and mixed-layer entrainment. This interpretability arises from the stochastic model’s simplified representation of sea surface temperature (SST), considering only the local upper-ocean response to white-noise atmospheric forcing and its impact on surface heat exchange. We focus on understanding differences between SLAB and FULL non-eddy-resolving preindustrial control simulations of the Community Earth System Model 1 (CESM), and estimate the stochastic model parameters from each respective simulation. Despite its simplicity, the stochastic model reproduces temporal characteristics of SST variability in the SPG, including reemergence, seasonal-to-interannual persistence, and power spectra. Furthermore, the unrealistically persistent SST of the CESM-SLAB ocean simulation is reproduced in the equivalent stochastic model configuration where the mixed-layer depth (MLD) is constant. The stochastic model also reveals that vertical entrainment primarily damps SST variability, thus explaining why SLAB exhibits larger SST variance than FULL. The stochastic model driven by temporally stochastic, spatially coherent forcing patterns reproduces the canonical AMV pattern. However, the amplitude of low-frequency variability remains underestimated, suggesting a role for ocean dynamics beyond entrainment.

Monthly-scale extended predictions using the atmospheric model coupled with a slab ocean

Abstract. Given the good persistence of sea surface temperature (SST) due to the slow-varying nature of the ocean, an atmospheric model coupled with a slab ocean model (SOM) instead of a 3-D dynamical ocean model is designed as an efficient approach for extended-range predictions. The prediction experiments from July to December 2020 are performed based on the Weather Research and Forecasting (WRF) model coupled to the SOM (WRF-SOM) with the initial and boundary conditions same as the WRF coupled to the Regional Ocean Model System (WRF-ROMS). The WRF-SOM is verified to have better performance of SSTs in the extended-range predictions than WRF-ROMS since it avoids the complicated model biases from the ocean dynamics and seabed topography when extended-range predictions are made using a 3-D dynamical ocean model. The improvement of SSTs can lead to the remarkable impact on the response of the atmosphere from the surface to the upper layer. Taking typhoon as an example of extreme events, the WRF-SOM can obtain comparable intensity predictions and slightly improved track predictions due to the improved SSTs in the initialized WRF-SOM system. Overall, the WRF-SOM can ensure the stability of extended-range prediction and reduce the demand for computing resources by roughly 50 %.

Strength and variability of the Oligocene Southern Ocean surface temperature gradient

Large Oligocene Antarctic ice sheets co-existed with warm proximal waters offshore Wilkes Land. Here we provide a broader Southern Ocean perspective to such warmth by reconstructing the strength and variability of the Oligocene Australian-Antarctic latitudinal sea surface temperature gradient. Our Oligocene TEX86-based sea surface temperature record from offshore southern Australia shows temperate (20–29 °C) conditions throughout, despite northward tectonic drift. A persistent sea surface temperature gradient (~5–10 °C) exists between Australia and Antarctica, which increases during glacial intervals. The sea surface temperature gradient increases from ~26 Ma, due to Antarctic-proximal cooling. Meanwhile, benthic foraminiferal oxygen isotope decline indicates ice loss/deep-sea warming. These contrasting patterns are difficult to explain by greenhouse gas forcing alone. Timing of the sea surface temperature cooling coincides with deepening of Drake Passage and matches results of ocean model experiments that demonstrate that Drake Passage opening cools Antarctic proximal waters. We conclude that Drake Passage deepening cooled Antarctic coasts which enhanced thermal isolation of Antarctica.

An increase in marine heatwaves without significant changes in surface ocean temperature variability

Marine heatwaves (MHWs)—extremely warm, persistent sea surface temperature (SST) anomalies causing substantial ecological and economic consequences—have increased worldwide in recent decades. Concurrent increases in global temperatures suggest that climate change impacted MHW occurrences, beyond random changes arising from natural internal variability. Moreover, the long-term SST warming trend was not constant but instead had more rapid warming in recent decades. Here we show that this nonlinear trend can—on its own—appear to increase SST variance and hence MHW frequency. Using a Linear Inverse Model to separate climate change contributions to SST means and internal variability, both in observations and CMIP6 historical simulations, we find that most MHW increases resulted from regional mean climate trends that alone increased the probability of SSTs exceeding a MHW threshold. Our results suggest the need to carefully attribute global warming-induced changes in climate extremes, which may not always reflect underlying changes in variability.

Small‐Scale Spatial Variations of Air‐Sea Heat, Moisture, and Buoyancy Fluxes in the Tropical Trade Winds

AbstractObservations from two autonomous Wave Gliders and six Lagrangian Surface Wave Instrument Float with Tracking drifters in the northwestern tropical Atlantic during the January–February 2020 NOAA Atlantic Tradewind Ocean‐atmosphere Mesoscale Interaction Campaign (ATOMIC) are used to evaluate the spatial variability of bulk air‐sea heat, moisture, and buoyancy fluxes. Sea surface temperature (SST) gradients up to 0.7°C across 10–100 km frequently persisted for several days. SST gradients were a leading cause of systematic spatial air‐sea sensible heat flux gradients, as variations over 5 Wm−2 across under 20 km were observed. Wind speed gradients played no significant role and air temperature adjustments to SST gradients sometimes acted to reduce spatial flux gradients. Wind speed, air temperature, and air humidity caused high‐frequency spatial and temporal flux variations on both sides of SST gradients. A synthesis of observations demonstrated that fluxes were usually enhanced on the warm SST side of gradients compared to the cold SST side, with variations up to 10 Wm−2 in sensible heat and upward buoyancy fluxes and 50 Wm−2 in latent heat flux. Persistent SST gradients and high‐frequency air temperature variations each contributed up to 5 Wm−2 variability in sensible heat flux. Latent heat flux was instead mostly driven by air humidity variability. Atmospheric gradients may result from convective structures or high‐frequency turbulent fluctuations. Comparisons with 0.05°‐resolution daily satellite SST observations demonstrate that remote sensing observations or lower‐resolution models may not capture the small‐scale spatial ocean variability present in the Atlantic trade wind region.

The Different Relationships between the ENSO Spring Persistence Barrier and Predictability Barrier

Abstract In this paper, we investigate the relationship between the El Niño–Southern Oscillation (ENSO) spring persistence barrier (PB) and predictability barrier (PD) and apply it to explain the interdecadal modulation of ENSO prediction skill using the anomaly correlation coefficient (ACC). Previous studies showed that a longer persistence (i.e., autocorrelation) tends to produce a higher prediction skill. Using the recharge oscillator model of ENSO, both analytical and numerical solutions suggest that the predictability (i.e., ACC) is related to the persistence of sea surface temperature (SST) and cross correlation between SST and subsurface ocean heat content in the tropical Pacific. In particular, a larger damping rate in SST anomalies will lead to a lower persistence and ACC and a stronger PD. However, a shortened ENSO period, which controls the cross correlation, will lead to a lower persistence but a higher ACC associated with a weaker PD. Finally, we apply our solutions to observations and suggest that a higher ACC associated with a weaker PD after 1960 is caused by the shortened ENSO period.

Long-term drought intensification over Europe driven by the weakening trend of the Atlantic Meridional Overturning Circulation

Study regionEurope, with a particular focus on the Czech Republic, Poland, Spain, and Norway. Study focusLong-lasting droughts have become a semi-permanent feature of the European climate, especially over the last two decades. These prolonged droughts are usually driven by persistent sea surface temperature anomalies over the Pacific and Atlantic basins. By employing complex statistical methods (i.e., Canonical Correlation Analysis and Convergent Cross-Mapping) in this study we make a comprehensive assessment of the observed drying trend over the central and southern parts of Europe and its underlying drivers. New hydrological insightsBuilding upon the potential relationship between drought variability and large-scale oceanic and atmospheric circulation, we show that the observed drying trend in the central and southern parts of Europe has been driven by a long-term slowdown of the Atlantic Meridional Overturning Circulation (AMOC), via changes in the large-scale atmospheric circulation. A weakening of AMOC leads to an increase in the frequency of atmospheric-blocking like circulation over the central part of Europe, which in turn inhibits precipitation and favors long-term drying. Since climate projections indicate a slowdown of the AMOC in the future, we suggest that this will potentially lead to an increase in the frequency of dry years, especially over the central and southern parts of Europe (e.g., the eastern part of Germany, the Czech Republic, Poland, Spain and Portugal).

Large-Scale Sea Surface Temperature Forcing Contributed to the 2013–17 Record-Breaking Meteorological Drought in the Korean Peninsula

Abstract This study examined the contribution of the Pacific decadal oscillation (PDO) to the record-breaking 2013–17 drought in the Korean Peninsula. The meteorological drought signal, measured by the Standardized Precipitation Index (SPI), in 2013 and 2016 co-occurred with a heat wave. The positive phase of the PDO during the mid-2010s was responsible for the precipitation deficit, particularly in 2014, 2015, and 2017, resulting in 5 years of meteorological drought. The enhanced atmospheric heating anomalies over the subtropical central Pacific, induced by the in situ PDO-related sea surface temperature (SST) warming, led to a low-atmospheric cyclonic flow centered over the midlatitude Pacific. The northerly wind anomalies at the western edge of this low-level cyclonic flow were responsible for the horizontal negative advection of moist energy, which contributed to the decreased precipitation and the resultant negative SPI over the Korean Peninsula in 2014, 2015, and 2017. The large-ensemble simulation supported the observational findings that the composited SST anomalies during the 5 years of persistent drought exhibited prominent and persistent SST warming over the subtropical central Pacific, along with large-scale cyclonic flow over the North Pacific. The findings of this study imply that the SST anomalies over the North Pacific and subtropical central Pacific can be a predictable source to potentially increase the ability to forecast multiyear droughts over the Korean Peninsula.

The Cooling Over Northeast Asia in June Over the Most Recent Decade: A Possible Response to Declining Bering Sea Sea Ice in March

AbstractIn sharp contrast to rapid global warming, the surface air temperature (SAT) in Northeast Asia (NEA) has exhibited strong cooling in June since 2010. Our results show that this cooling is a possible response to declining Bering Sea sea ice concentrations (BSICs) in March. A physical process analysis shows that decreasing BSICs provoke a south‐eastward‐propagating Rossby wave train and induce a strong anomalous anticyclone over the midlatitude Northwest Atlantic, resulting in tripolar anomalous sea surface temperature (SST) pattern with warm anomalies in the midlatitude Northwest Atlantic. These persistent tripolar SST anomalies generate a new Rossby wave across Eurasia in June, which enhances the cyclonic atmosphere circulation anomaly over NEA and leads to the cold SAT anomalies observed in this area. Correspondingly, in response to the sharp decline in BSIC in March, the NEA has exhibited an unprecedented cooling trend in June over the most recent decade.

Objective delineation of persistent SST fronts based on global satellite observations

A novel method is introduced to objectively and automatically identify persistent thermal fronts across the ocean from 60° N to 60° S: FROnt Delineation in the Ocean (FRODO), an algorithm developed to delineate sea surface temperature (SST) fronts, was applied to global satellite observations. By relying on long-term averaged frontal probability fields of SST, persistent SST fronts were located globally in an automated manner. The algorithm was applied to AVHRR Pathfinder and MODIS observations, spanning the period from 1982 through 2011, and resulted in new insights regarding frontal zones: new fronts were detected, the location of some fronts identified in the existing literature has been updated, the accuracy of the algorithm was tested through comparison with visible color and SST data, and the variability in frontal location over climatological seasons was explored. While significant spatial variability prevents the algorithm from accurately tracking some fronts, the results were found to agree very well with observations in the case of fronts associated with ocean bathymetry. The output of the algorithm consists of five digital global sets of persistent fronts: one based on the 30 year time series and four based on the climatological seasons derived from the same time series, which will facilitate the study of the links between SST fronts and other oceanic processes.

Recent Intensified Influence of the Winter North Pacific Sea Surface Temperature on the Mei-Yu Withdrawal Date

AbstractThe mei-yu withdrawal date (MWD) is a crucial indicator of flood/drought conditions over East Asia. It is characterized by a strong interannual variability, but its underlying mechanism remains unknown. We investigated the possible effects of the winter sea surface temperature (SST) in the North Pacific Ocean on the MWD on interannual to interdecadal time scales. Both our observations and model results suggest that the winter SST anomalies associated with the MWD are mainly contributed to by a combination of the first two leading modes of the winter SST in the North Pacific, which have a horseshoe shape (the NPSST). The statistical results indicate that the intimate linkage between the NPSST and the MWD has intensified since the early 1990s. During the time period 1990–2016, the NPSST-related SST anomalies persisted from winter to the following seasons and affected the SST over the tropical Pacific in July. Subsequently, the SST anomalies throughout the North Pacific strengthened the southward migration of the East Asian jet stream (EAJS) and the southward and westward displacement of the western North Pacific subtropical high (WPSH), leading to an increase in mei-yu rainfall from 1 to 20 July. More convincingly, the anomalous EAJS and WPSH induced by the SST anomalies can be reproduced well by numerical simulations. By contrast, the influence of the NPSST on the EASJ and WPSH were not clear between 1961 and 1985. This study further illustrates that the enhanced interannual variability of the NPSST may be attributed to the more persistent SST anomalies during the time period 1990–2016.

Asymmetric relationship between ENSO and the tropical Indian Ocean summer SST anomalies

AbstractDuring the summer following El Niño, a basin-wide sea surface temperature (SST) warming takes place over the tropical Indian Ocean (TIO), exerting profound influences on the Asian summer monsoon. This is an important source of seasonal predictability for the Asian summer monsoon. Based on observations, however, the present study finds that the relationship between El Niño-Southern Oscillation (ENSO) and the TIO SST anomalies during the decaying summer is asymmetric with a much weaker relationship between La Niña and the TIO SST anomalies relative to El Niño. The analyses show that this asymmetric relationship can be explained by the asymmetries in initial TIO SST, oceanic Rossby wave in the southern Indian Ocean and ENSO decaying rate. In contrast to El Niño events, La Niña events tend to have a stronger initial TIO warming and a less peak intensity with a weaker oceanic Rossby wave response in the southern Indian Ocean. On the other hand, La Niña events tend to decay more slowly with the persistent SST cooling over the central equatorial Pacific in the following summer. The equatorial Pacific SST cooling would induce an anomalous anticyclone via a Gill-type Rossby wave response, weakening the positive feedback between the anomalous cyclone spanning the tropical Northwest Pacific and North Indian oceans and the TIO summer basin-wide SST cooling. These results have important implications for the climate predictability of the Indian Ocean and Asian summer monsoon.