Oscillation Sea Surface Temperature Research Articles

Terrestrial evaporation response to modes of climate variability

Large-scale modes of climate variability (or teleconnection patterns), such as the El Niño Southern Oscillation and the North Atlantic Oscillation, affect local weather worldwide. However, the response of terrestrial water and energy fluxes to these modes of variability is still poorly understood. Here, we analyse the response of evaporation to 16 teleconnection patterns, using a simple supervised learning framework and global observation-based datasets of evaporation and its key climatic drivers. Our results show that the month-to-month variability in terrestrial evaporation is strongly affected by (coupled) oscillations in sea-surface temperature and air pressure: in specific hotspot regions, up to 40% of the evaporation dynamics can be explained by climate indices describing the fundamental modes of climate variability. While the El Niño Southern Oscillation affects the dynamics in land evaporation worldwide, other phenomena such as the East Pacific–North Pacific teleconnection pattern are more dominant at regional scales. Most modes of climate variability affect terrestrial evaporation by inducing changes in the atmospheric demand for water. However, anomalies in precipitation associated to particular teleconnections are crucial for the evaporation in water-limited regimes, as well as in forested regions where interception loss forms a substantial fraction of total evaporation. Our results highlight the need to consider the concurrent impact of these teleconnections to accurately predict the fate of the terrestrial branch of the hydrological cycle, and provide observational evidence to help improve the representation of surface fluxes in Earth system models.

Potential Predictability of Multidecadal Oscillations of Sea Surface Temperature in the Arctic and Their Sensitivity to External Forcings

The low-frequency variability of sea surface temperature and salinity in the Arctic is analyzed using the data of the 1200-year preindustrial experiment with the INM-CM5 climate model developed in the Marchuk Institute of Numerical Mathematics of Russian Academy of Sciences. It is shown that the leading variability pattern is a regular coupled oscillation of temperature and salinity with the period of about 50 years. The empirical method based on the fluctuation-dissipation theorem was applied to evaluate influence functions which provide the optimum excitation of this oscillation phases. It is demonstrated that salinity anomalies are the main driver of this variability. The time of potential predictability of sea surface temperature and salinity was determined using the analog method, it equals about six years for 15-year means. The main source of long-term predictability is the spatial pattern associated with the leading mode of low-frequency variability of the analyzed parameters in the Arctic.

Land-falling typhoons are controlled by the meridional oscillation of the Kuroshio Extension

Low-frequency variations of typhoon paths are often attributed to changes in the North Pacific subtropical high and monsoon through influences on the steering wind. Evidence indicates however a strong imprint of the Kuroshio on the atmosphere. Here we show that the meridional oscillation of sea surface temperature (SST) front over the Kuroshio Extension east of Japan significantly correlates with the number of land-falling typhoons along East Asia from June to October, accounting for 70% of the low-frequency variance since 1980. We used observations and a simple model to show that when the SST front shifts poleward (equatorward), SST gradient south of the current and westerly tropospheric wind weaken (strengthen), steering more typhoons to veer toward (away from) the East Asian continent. Our analysis also indicates that long-term weakening of SST gradient and westerly wind appears to be concomitant with poleward shifting of the Kuroshio, attributed to global warming in some studies, and suggests the potential for more land-falling typhoons in East Asia in the coming decades.

Java Sea Surface Temperature Variability during ENSO 1997 – 1998 and 2014 – 2015

Sea Surface Temperature (SST) is one of the important parameter to describe seawater characteristic. There is a strong linkage between SST and El Nino Southern Oscillation (ENSO). The purpose of this research is to investigate SST of Java Sea during in period 1997—1998 and 2014– 2015. We use datasets from Hycom archieves, INDESO, and SOI. The result shows El Nino is started in March 1997 until April 1998 (peak in March 1998), then La Nina is started in June to December 1998 (peak in July 1998). Maximum Sea Surface Temperature Anomaly (SSTA) is occurred in August – September 1998 (0.8 °C – 0.9 °C). During 2014–2015, a propagation of El Nino is founded. El Nino is started in August until November 2014 (-7.6 < SOI < -11.4, peak in August), and is followed in May to October 2015 (-12 < SOI < -20.2, peak in October). During 2014–2015, a maximum Sea Surface Temperature Anomaly (SSTA) is founded in May 2014 (0.5 °C).

Atmospheric Mechanisms for MJO Decay Over the Maritime Continent

AbstractEastward propagating Madden‐Julian oscillation (MJO) events that develop in the Indian Ocean from November to April are separated into events whose convective anomalies do and do not propagate across the Maritime Continent (MC). Propagating (P) events are divided into strong (sP) and weak (wP) subsets based on the initial amplitudes of their convective anomalies. Eastward decaying (ED) MJO events have initial amplitudes similar to those of wP events. Roughly half of all MJO events encounter westward propagating transient dry precursor (TDP) signals over the MC. These TDPs, which are external to the MJO circulation system, overwhelm the weak, eastward propagating moist anomaly in ED events, leading to MJO termination. In wP events, the MJO moist anomaly is stronger than the TDP dry anomaly, and the MJO propagates beyond the MC. ED MJO events that do not encounter TDPs decay because of insufficient moistening in the southern MC region. The background states of TDP‐ and non‐TDP‐affected MJO events resemble La Niña and El Niño conditions, respectively. The regional regulation of TDP activity and background moisture gradients by El Niño–Southern Oscillation can both be understood in terms of the zonal shift and the meridional expansion or contraction of Warm Pool moisture in response to El Niño–Southern Oscillation sea surface temperature anomalies. Our results demonstrate that modulation of TDPs by the background state moisture can affect the MJO in a manner that reinforces its propagation or decay via the interaction of MJO circulation anomalies and background moisture gradients.

Trends in seasonal warm anomalies across the contiguous United States: Contributions from natural climate variability

Many studies have shown the importance of anthropogenic greenhouse gas emissions in contributing to observed upward trends in the occurrences of temperature extremes over the U.S. However, few studies have investigated the contributions of internal variability in the climate system to these observed trends. Here we use daily maximum temperature time series from the North American Land Data Assimilation System Phase 2 (NLDAS-2) dataset to identify trends in seasonal warm anomalies over the contiguous U.S. in the three most recent decades and explore their relationships to low-frequency modes of internal climate variability. The results reveal substantial upward trends in the frequency of warm anomalies in all seasons and in all regions of the U.S., except for portions of the Intermountain West in winter where significant downward trends occur. The strengths and regional coverage of the trends, however, differ considerably by season. These trends can be explained, in part, by the large-scale anomalous atmospheric circulations associated with low-frequency sea-surface temperature oscillations characterized by the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). The association between the upward trends in the seasonal warm anomalies and PDO and AMO is further confirmed by the century-long (1871–2012) Twentieth Century Reanalysis dataset.

The 30–50-Day Intraseasonal Oscillation of SST and Precipitation in the South Tropical Indian Ocean

The Sea Surface Temperature (SST) in the South Tropical Indian Ocean (STIO) displays significant intraseasonal oscillation (ISO) in two regions. A striking 30–50-day ISO found over the east of thermocline ridge (Region A, 80–90° E, 6–12° S), as identified by the Empirical Mode Decomposition (EMD) method, is distinguished from the SST signature over the thermocline ridge (Region B, 52.5–65° E, 6–13° S). The 30–50-day ISO of SST in the Region A is active in March–May (MAM) and suppressed in September–November (SON). Meanwhile, a 30–50-day ISO of precipitation correlates with the SST over the Region A. SST leads precipitation by 10 days, implying a pronounced ocean–atmosphere interaction at the intraseasonal timescale, especially the oceanic feedback to the atmosphere during Madden–Julian Oscillation (MJO) events. Analysis on mechanism of the ISO manifests heat fluxes are critical to the development of the intraseasonal SST variability. The local thermocline in Region A, as the shallowest in MAM and the thickest in SON, is likely to modulate the strength of ISO and contribute to its sustainability. It suggests that thermocline plays a more important role in Region A than in Region B, leading to the difference between the two regions.

Understanding the Warm Water Volume Precursor of ENSO Events and its Interdecadal Variation

AbstractA wind forced ocean model is used to decompose the equatorial Pacific warm water volume (WWV) between 1980 and 2016 into two components: the (i) adjusted wind response, which is found by letting the model evolve unforced for three months, and (ii) instantaneous wind response, which are the instantaneous WWV changes due to Ekman transports. Our results suggest that roughly half of WWV variability is only as predictable as the winds that drive the instantaneous change. Separate examinations of pre‐2000 and post‐2000 periods reveal (i) nearly equal importance of instantaneous and adjusted responses for the pre‐2000 period and (ii) dominance of the instantaneous response during the post‐2000 period, which is most apparent during the recharged phase. This increasing instantaneous contribution prominence explains the post‐2000 reduction in WWV/El Niño‐Southern Oscillation sea surface temperature lead times (from six to nine months pre‐2000 down to three months post‐2000) and is consistent with the reduction in post‐2000 El Niño‐Southern Oscillation prediction skill.

Horizontal advection of temperature and salinity by Rossby waves in the North Pacific

ABSTRACTThe Aquarius satellite has been used for the first time to characterize Rossby waves in sea surface salinity (SSS) measurements for the North Pacific Ocean. Westward propagating wave signals are delineated by the SSS zonal salinity gradients. The phase velocities and spectral properties obtained from zonal salinity gradients are closely correlated with corresponding values obtained from the sea surface temperature (SST) zonal gradient and the altimetry-derived meridional velocity. The westward propagating SSS signals are consistent with Rossby wave advection across the strong meridional gradients of water characteristics. Following Killworth, we attempted to provide satellite-based estimates of the contribution of horizontal Rossby wave advection to the surface transfer of temperature and salinity in the North Pacific Ocean. Westward propagating signals in the SST and SSS zonal gradient fields show that the observed intensity of meridional advection by the ambient gradients of SST and SSS is less than the intensity predicted by an analytical solution of the transfer equation for Rossby waves. Our results extend the previous studies of physical mechanisms of Rossby wave manifestation at the sea surface and we demonstrate that Rossby waves are responsible for low-frequency oscillations in SST and SSS concentration in the North Pacific.

Climate models with delay differential equations.

A fundamental challenge in mathematical modelling is to find a model that embodies the essential underlying physics of a system, while at the same time being simple enough to allow for mathematical analysis. Delay differential equations (DDEs) can often assist in this goal because, in some cases, only the delayed effects of complex processes need to be described and not the processes themselves. This is true for some climate systems, whose dynamics are driven in part by delayed feedback loops associated with transport times of mass or energy from one location of the globe to another. The infinite-dimensional nature of DDEs allows them to be sufficiently complex to reproduce realistic dynamics accurately with a small number of variables and parameters. In this paper, we review how DDEs have been used to model climate systems at a conceptual level. Most studies of DDE climate models have focused on gaining insights into either the global energy balance or the fundamental workings of the El Niño Southern Oscillation (ENSO) system. For example, studies of DDEs have led to proposed mechanisms for the interannual oscillations in sea-surface temperature that is characteristic of ENSO, the irregular behaviour that makes ENSO difficult to forecast and the tendency of El Niño events to occur near Christmas. We also discuss the tools used to analyse such DDE models. In particular, the recent development of continuation software for DDEs makes it possible to explore large regions of parameter space in an efficient manner in order to provide a "global picture" of the possible dynamics. We also point out some directions for future research, including the incorporation of non-constant delays, which we believe could improve the descriptive power of DDE climate models.

Effects of ENSO on the intraseasonal oscillations of sea surface temperature and wind speed along Vietnam’s coastal areas

Our study applied the Ensemble Empirical Mode Decomposition (EEMD) method to analyze intraseasonal variability (ISV) of sea surface temperature (SST) and wind speed using a 22-year monitoring data set from 10 coastal stations. Results show that the El Niño and Southern Oscillation(ENSO) significantly affected the ISO Quasi-Biennial Oscillation (QBWO) 10-20 day periods and Madden-Julian Oscillation (MJO) 30-60 day periods of SST and wind speed at the coastal stations. As seen with MJO, the effects of ENSO on SST tend to increase from the north to south, whereas its impact on wind speed decreases from the north to south of Vietnam’s coastal areas. In contrast, with QBWO, the effect of ENSO on SST reduces moving from the north to south, whereas its impact on wind speed increases from the north to south of Vietnam’s coastal areas.

Unraveling the spatio-temporal structure of the atmospheric and oceanic intra-seasonal oscillations during the contrasting monsoon seasons

Using remotely sensed data sets of rainfall and outgoing longwave radiation (OLR) over Indian land and adjacent oceanic regions and sea surface temperature (SST) over adjacent oceanic regions; we examine the major characteristics of the intra-seasonal oscillations of Indian summer monsoon (ISM) during the flood and drought years. Intra-seasonal oscillations of rain, OLR and SST corresponding to 30–60days transpires to contribute more to the intra-seasonal variability over the Arabian Sea, whereas 10–20days' mode is found to be more dominating over the Bay of Bengal during the drought years. Therefore, suggesting that both of the Seas surrounding the Indian land region respond in a different way to the below normal rainfall conditions of Indian land region. Another important finding of the present work is that during the drought years, 30–60days intra-seasonal oscillations of SST over both of the seas follow the intra-seasonal oscillations of rain at 30–60days' time scale over central India approximately after 26days. Conversely in the flood years, intra-seasonal oscillations of SST at 30–60days over the Arabian Sea and Bay of Bengal lead the intra-seasonal oscillations of rain over central India by 6days. Present analysis also reveals that the intra-seasonal variability of ISM at two different time-scales (10–20 and 30–60days) possess different spatio-temporal characteristics during the contrasting monsoon conditions over the oceanic regions; therefore it is advisable to study the two modes individually for understanding the underlying physical mechanism. Results presented in this paper may be useful for improved ISM prediction.

Impact of the Upper Tropospheric Cooling Trend over Central Asia on the Indian Summer Monsoon Rainfall and the Bay of Bengal Cyclone Tracks

The Indian summer monsoon rainfall had three decade long alternate dry and wet epochs during the 150 years from 1840 to 1989. The dry epochs had frequent drought monsoons affecting agriculture, power generation and the overall economy of the country. A high percentage of severe cyclones in the Bay of Bengal moved northwards during the dry epochs causing disasters in Bangladesh, Myanmar and the Indian states of Odisha and West Bengal. These dry epochs have been shown to be associated with the cold phase of the Atlantic multi decadal oscillation in sea-surface temperature. Using the available tropospheric temperature (re-analysis) data since 1948, the recent dry epoch during 1960-89 which had 10 monsoon drought years was found to have cold upper tropospheric temperature anomaly over Central Asia. This cold anomaly region has also experienced a long-term cooling trend. Extrapolating the naturally occurring epochal nature of the ocean-atmosphere system into the future, we fear that the epoch 2020-49 is likely to be another dry one, and the cooling trend over the Asian continent is likely to make it even more severe in its impact than 1960-89. This article presents details of an ocean-atmosphere instability that generates frequent drought monsoons during dry epochs which needs urgent research.

Influence of the South China Sea Biweekly Sea Surface Temperature on the South China Sea Summer Monsoon Especially during the Indian Ocean Dipole

ABSTRACTThe influence of the biweekly sea surface temperature (SST) in the South China Sea (SCS) on the SCS summer monsoon, especially during the Indian Ocean Dipole (IOD) is presented using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) SST and rainfall data for April to June from 1999 to 2013. During positive IOD (PIOD) years the biweekly SST anomalies over the SCS lead the rain anomalies by three days, with a significant correlation (r = 0.8, at the 99% confidence level), whereas during negative IOD (NIOD) years, the correlation is only 0.2. The biweekly SST is observed to influence the westward and northward propagating rainfall anomalies over the SCS and, hence, affect the SCS summer monsoon, especially during PIOD years. No such propagation was seen during NIOD years. The biweekly intraseasonal oscillation of SST in the SCS results in enhanced sea level pressure and surface shortwave radiation, especially during PIOD years. The potential findings here indicate that the biweekly SST in the SCS is strongly (weakly) influenced during PIOD (NIOD) years. Further, it is observed that SST in the SCS has a strong (weak) effect on the SCS summer monsoon by westward and northward propagation of rainfall, especially during PIOD (NIOD) years. When a PIOD or NIOD exists over the tropical Indian Ocean, the SCS SST will be strongly (r = 0.6, at the 99% confidence level) or weakly correlated with the residual index, respectively.

Correlations between Interannual SST Oscillations and Modeled Swell Impacts on Turbulent Mixing*

Abstract The possibility of teleconnections between Southern Ocean swells and sea surface temperature (SST) anomalies on interannual time scales in the eastern Pacific Niño-3 region and southeastern Indian Ocean is investigated using numerical wave models. Two alternative parameterizations for swell dissipation are used. It is found that swell dissipation in the models is not directly correlated with large interannual variations such as El Niño–Southern Oscillation (ENSO) or the Indian Ocean dipole (IOD). However, using one of the two swell dissipation parameterizations, a correlation is found between observed SST anomalies and the modification of turbulent kinetic energy flux (TKEF) by Southern Ocean swells due to the damping of short wind waves: modeled reduction of TKEF is opposite in phase to the SST anomalies in the Niño-3 region, indicating a potential positive feedback. The modeled bimonthly averaged TKEF reduction in the southeastern Indian Ocean is also well correlated with the IOD mode.

Biweekly Sea Surface Temperature over the South China Sea and its association with the Western North Pacific Summer Monsoon

The association of the biweekly intraseasonal (BWI) oscillation in the Sea Surface Temperature (SST) over the South China Sea (SCS) and the Western North Pacific Summer Monsoon is authenticated using version 4 the Tropical Rainfall Measuring Mission Microwave Imager data (SST and rain) and heat fluxes from Ocean Atmosphere Flux project data during 1998–2012. The results suggest that the SCS involves ocean–atmosphere coupling on biweekly timescales. The positive biweekly SST anomalies lead the rain anomalies over the SCS by 3 days, with a significant correlation coefficient (r = 0.6, at 99 % significance levels) between the SST-rain anomalies. It is evident from lead/lag correlation between biweekly SST and zonal wind shear that warm ocean surface induced by wind shear may contribute to a favorable condition of the convective activity over the SCS. The present study suggests that ocean–to-atmospheric processes induced by the BWI oscillation in the SCS SST results in enhanced sea level pressure and surface shortwave radiation flux during the summer monsoon. Besides, it is observed that the SCS BWI oscillation in the changes of SST causes a feedback in the atmosphere by modifying the atmospheric instability. This suggests that the active/break biweekly cycle of the SST over the SCS is related by sea level pressure, surface heat fluxes and atmospheric instability. The potential findings here indicate that the biweekly SST over the SCS play an important role in the eastward and the southward propagation of the biweekly anomalies in the Western North Pacific.

Pteropod time series from the North Western Mediterranean (1967-2003): impacts of pH and climate variability

Environmental changes resulting from anthropogenic CO2 emissions occur at global and local levels and have potentially harmful effects, particularly for calcifying taxa in the marine environment. A time series of pteropod abundance covering the period 1967−2003 was isolated from the Point B (northwestern Ligurian Sea) zooplankton time series. Inter- and intra-annual changes in the abundance of 3 families (Limacinidae, Cavoliniidae and Creseidae) were com- pared with the copepod time series to identify any differential effects driven by ocean acidification and temperature. pH values were hind-cast from total alkalinity estimated from local temperature and salinity measurements, and atmospheric CO2 taken from the Mauna Loa time series. Although surface waters were supersaturated with respect to aragonite throughout the study period, it is estimated that pH declined by 0.05 units. All pteropod groups displayed a trend of increasing abundance, suggesting that any deleterious effect of declining pHT in the range of 0.05 units has not caused sufficient reductions in fitness as to decrease local abundances between 1967−2003. Pteropod populations are influenced by inter-annual changes in summer tempera- tures. Spectral analysis identified a ~14 yr periodic oscillation in sea surface temperature. Simi- larly timed oscillations in abundance are present for all pteropod families but not for copepods, indicating a possible influence of the North Atlantic quasi-decadal mode on pteropod populations. While laboratory studies have shown pteropods to be sensitive to changes in pH, this analysis sug- gests that local and regional scale drivers have had a greater effect on pteropod populations in the northwestern Mediterranean Sea in recent decades. It should be noted that pH changes in labo- ratory studies exceed 0.05 pH units and that the saturation state with respect to aragonite (Ωar) is usually much lower than that of the Mediterranean.

Zooplankton functional groups from the California current and climate variability during 1997-2013.

Zooplankton plays an important role in recycling matter and energy trough the pelagic ecosystem.The California Current is one of the large marine ecosystems with high productivity and bio-physical variability at multiple time scales. An interannual scale or longer periods requires data series sufficiently long to ensure reliable averages of zooplankton abundance in order to estimate their low frequency variability. Here, tendencies inphysical and biological variables are presented for the period 1997-2013 with data obtained from IMECOCAL cruises in the Mexican sector of the California Current. The area was divided into four regions, two oceanic (off North and Central Baja California) and two neritic (Vizcaino bay and Gulf of Ulloa). Sea surface temperature (SST) and El Niño Oceanic Index (ONI) showed correlation in all areas, while extratropical indices (PDO andNPGO) exhibited different tendencies among the regions. The PDO had significant correlation with SST only in the central and Vizcaino bay regions. The NPGO was not correlated with temperature but presented significantly strong correlation with sea surface salinity in all regions, which has been attributed to changes in large-scale circulation of the north Pacific subtropical gyre. In spite of a significant influence of the El Niño Southern Oscillation (ENSO) in SST, the correlation between ONI and zooplankton abundance was limited to gelatinous herbivorous (tunicates) from the North region. Local influence was remarkable in Vizcaino bay where the tunicates showed a period of negative abundance anomalies (2000-2004) followed by increasing positive anomalies between 2005 and 2013 associated with positive upwelling index anomalies. Geometric mean abundance of salps (per oceanographic cruise) averaged in Vizcaino bay 33.3 ind m-3 during 2005-2013 compared to 1.4 ind m-3 in 2000-2004. Salps partially displaced crustacean herbivores since they compete for feeding particles; copepods decreased from 88.2 ind m-3 during 2000-2004 to 59.7 ind m-3 in 2005-2013; and euphausiids from 16.1 ind m-3 to 10.4 ind m-3. In the oceanic domain a period of saline stratification during 2002-2006 was associated with positive anomalies of all trophic groups (crustaceans, tunicates and carnivores). Alternation of particular taxa of tunicates and carnivores is discussed. The increase of gelatinous organisms associated to higher stratification in the oceanic region and enhanced upwellng in the coastal shelf appears to be in detriment of crustaceans, though the time-series are shortto outline a more defined trend. That tendency is particularly disturbing in Vizcaino bay affecting the availability of food for fishes and other predators.

Interannual variability of tropical cyclone activity and regional Hadley circulation over the Northeastern Pacific

AbstractThe interannual variability of the regional Hadley circulation (HC) and tropical cyclone (TC) activity over the Northeastern Pacific (NEP) was investigated. The interannual variability of the HC in the NEP hurricane season is found dominated by a mode (M1EP) regulating the strength and meridional extent of the Intertropical Convergence Zone. M1EP has a more robust correlation with NEP TC activity than the El Niño–Southern Oscillation sea surface temperature indices. The strong correlation is attributed to the impacts of M1EP on environmental conditions. In addition, the leading mode of the Atlantic HC, which has an anticorrelation with M1EP, also strongly impacts NEP TC activity. The findings, together with Zhang and Wang (2013), highlight the role of the HC in controlling the variability of TC activity as well as the interbasin connection between the NEP and the Atlantic. The regional HC is thus recommended as a useful metric to benchmark models in simulating TC activity.

Thermal variations in the South China Sea associated with the eastern and central Pacific El Niño events and their mechanisms

AbstractIn this study, we investigate the interannual variability of the sea surface temperature (SST) in the South China Sea (SCS) associated with two types of El Niño, namely, the eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño. First, double warm peaks can occur during both types of El Niño events in the SCS. However, the strong warm basin mode can only develop in the EP El Niño, while the warm semibasin mode exists during the CP El Niño. Associated with an anomalous positive (negative) net surface heat flux in the EP (CP) El Niño, along with a shallower thermocline with weaker (stronger) northeasterly wind anomalies, the SST anomalies become warmer (cooler) in the developing autumn. Over the background of cooling SST in autumn of CP El Niño, therefore, only a weak warming can occur in the subsequent years, which is limited in the western boundary area under the forcing of warm ocean advection. Second, the SST oscillation periods are different in these two types of El Niño. The SST evolution in the EP El Niño is negative‐positive with a quasi‐biennial oscillation, but that in the CP El Niño is positive‐negative‐positive‐negative with an annual oscillation. It seems that the double cooling in the CP El Niño is phase‐locked to the late autumn season.