Global tree-ring response and inferred climate variation following the mid-thirteenth century Samalas eruption

The largest explosive volcanic eruption of the Common Era in terms of estimated sulphur yield to the stratosphere was identified in glaciochemical records 40 years ago, and dates to the mid-thirteenth century. Despite eventual attribution to the Samalas (Rinjani) volcano in Indonesia, the eruption date remains uncertain, and the climate response only partially understood. Seeking a more global perspective on summer surface temperature and hydroclimate change following the eruption, we present an analysis of 249 tree-ring chronologies spanning the thirteenth century and representing all continents except Antarctica. Of the 170 predominantly temperature sensitive high-frequency chronologies, the earliest hints of boreal summer cooling are the growth depressions found at sites in the western US and Canada in 1257 CE. If this response is a result of Samalas, it would be consistent with an eruption window of circa May–July 1257 CE. More widespread summer cooling across the mid-latitudes of North America and Eurasia is pronounced in 1258, while records from Scandinavia and Siberia reveal peak cooling in 1259. In contrast to the marked post-Samalas temperature response at high-elevation sites in the Northern Hemisphere, no strong hydroclimatic anomalies emerge from the 79 precipitation-sensitive chronologies. Although our findings remain spatially biased towards the western US and central Europe, and growth-climate response patterns are not always dominated by a single meteorological factor, this study offers a global proxy framework for the evaluation of paleoclimate model simulations.

A Statistical Intra-Seasonal Prediction Model of Extended Boreal Summer Western North Pacific Tropical Cyclone Genesis

A L2 regularized logistic regression model is developed in this study to predict weekly tropical cyclone (TC) genesis over the western North Pacific (WNP) and sub-regions of the WNP including the South China Sea (SCS), the western WNP (WWNP), and the eastern WNP (EWNP). The potential predictors for the TC genesis model include a time-varying TC genesis climatology, the Madden-Julian oscillation (MJO), the quasi-biweekly oscillation (QBWO), and ENSO. The relative importance of the predictors in a constructed L2 regression model is justified by a forward stepwise selection procedure for each region from a 0-week to a 7-week lead. Cross-validated hindcasts are then generated for the corresponding prediction schemes out to a 7-week lead. The TC genesis climatology generally improves the regional model skill, while the importance of intra-seasonal oscillations and ENSO are regionally dependent. Over the WNP, there is increased model skill over the time-varying climatology in predicting weekly TC genesis out to a 4-week lead by including the MJO and QBWO, while ENSO has a limited impact. On a regional scale, ENSO and then the MJO and QBWO respectively, are the two most important predictors over the EWNP and WWNP after the TC genesis climatology. The MJO is found to be the most important predictor over the SCS. The logistic regression model is shown to have comparable reliability and forecast skill scores to the ECMWF dynamical model on intra-seasonal time scales.

Seasonality in the relationship between equatorial-mean heat content and interannual eastern equatorial Atlantic sea surface temperature variability

Interannual sea surface temperature (SST) variations in the tropical Atlantic Ocean lead to anomalous atmospheric circulation and precipitation patterns with important ecological and socioeconomic consequences for the semiarid regions of sub-Saharan Africa and northeast Brazil. This interannual SST variability is characterized by three modes: an Atlantic meridional mode featuring an anomalous cross-equatorial SST gradient that peaks in boreal spring; an Atlantic zonal mode (Atlantic Niño mode) with SST anomalies in the eastern equatorial Atlantic cold tongue region that peaks in boreal summer; and a second zonal mode of variability with eastern equatorial SST anomalies peaking in boreal winter. Here we investigate the extent to which there is any seasonality in the relationship between equatorial warm water recharge and the development of eastern equatorial Atlantic SST anomalies. Seasonally stratified cross-correlation analysis between eastern equatorial Atlantic SST anomalies and equatorial heat content anomalies (evaluated using warm water volume and sea surface height) indicate that while equatorial heat content changes do occasionally play a role in the development of boreal summer Atlantic zonal mode events, they contribute more consistently to Atlantic Niño II, boreal winter events. Event and composite analysis of ocean adjustment with a shallow water model suggest that the warm water volume anomalies originate mainly from the off-equatorial northwestern Atlantic, in agreement with previous studies linking them to anomalous wind stress curl associated with the Atlantic meridional mode.

Wintertime Rossby Wave Breaking Persistence in Extended-range Seasonal Forecasts of Atlantic Tropical Cyclone Activity

In this study, we examine the wintertime environmental precursors of summer anticyclonic wave breaking (AWB) over the North Atlantic region and assess the applicability of these precursors in predicting AWB impacts on seasonal tropical cyclone (TC) activity. We show that predictors representing the environmental impacts of subtropical AWB on seasonal TC activity improve the skill of extended-range seasonal forecasts of TC activity. There is a significant correlation between boreal winter and boreal summer AWB activity via AWB-forced phases of the quasi-stationary North Atlantic Oscillation (NAO). Years with above-normal boreal summer AWB activity over the North Atlantic region also show above-normal AWB activity in the preceding boreal winter that tends to force a positive phase of the NAO that persists through the spring. These conditions are sustained by continued AWB throughout the year, particularly when El Niño-Southern Oscillation plays less of a role at forcing the large-scale circulation. While individual AWB events are synoptic and nonlinear with little predictability beyond 8-10 days, the strong dynamical connection between winter and summer wave breaking lends enough persistence to AWB activity to enable predictability of its potential impacts on TC activity. We find that the winter-summer relationship improves the skill of extended-range seasonal forecasts from as early as an April lead time, particularly for years when wave breaking has played a crucial role in suppressing TC development.

Asymmetric Boreal Summer Intraseasonal Oscillation Events Over the Western North Pacific and Their Impacts on East Asian Precipitation

The boreal summer intraseasonal oscillation (BSISO) is the most prominent tropical subseasonal signature especially over the western North Pacific (WNP). Due to restrictions of methodology in extracting BSISO with band-pass filtering or EOF decomposition, most of the previous studies ignored the asymmetry of BSISO. This study reexamines the BSISO events over WNP and their impacts on the East Asian precipitation. With a hierarchical cluster analysis, the BSISO events over WNP during the summers of 1985-2010 are classified into two categories, the long-period (30-60 day) and short-period (10-20 day) events. The long-period BSISO events manifest as a northward propagating mode with a significant phase asymmetry characterized by a fast development, but a slow decay of the intraseasonal convection. The fast development tends to cause a rapid reversal of the atmospheric anomalies over WNP from an anomalous anticyclone induced by the preceding slow convection suppression to an anomalous cyclone, leading to a fast northeastward retreat of the preceding enhanced western North Pacific subtropical high. Accordingly, the middle and lower reaches of Yangtze River valley experience a rapid reversal from the increased precipitation to the decreased, while the precipitation in coastal South China keeps decreased. The short-period BSISO events which are symmetric in phase act as a northwestward propagating mode, mainly affecting East Asian precipitation in an oblique belt extending from southwest China to southern Japan and southern Korean Peninsula. Therefore, the two types of the BSISO events especially the asymmetric long-period BSISO events over WNP and their impacts on the East Asian precipitation revealed in this study would provide a new potential for subseasonal-to-seasonal forecast of the East Asian summer monsoon precipitation.

The Photophysiological Response of Nitrogen-Limited Phytoplankton to Episodic Nitrogen Supply Associated With Tropical Instability Waves in the Equatorial Atlantic

In the Equatorial Atlantic nitrogen availability is assumed to control phytoplankton dynamics. However, in situ measurements of phytoplankton physiology and productivity are surprisingly sparse in comparison with the North Atlantic. In addition to the formation of the Equatorial cold tongue in the boreal summer, tropical instability waves (TIWs) and related short-term processes may locally cause episodic events of enhanced nutrient supply to the euphotic layer. Here, we assess changes in phytoplankton photophysiology in response to such episodic events as well as short-term nutrient addition experiments using a pair of custom-built fluorometers that measure chlorophyll a (Chl a) variable fluorescence and fluorescence lifetimes. The fluorometers were deployed during a transatlantic cruise along the Equator in the fall of 2019. We hypothesized that the Equatorial Atlantic is nitrogen-limited, with an increasing degree of limitation to the west where the cold tongue is not prominent, and that infrequent nitrate injection by TIW related processes are the primary source alleviating this limitation. We further hypothesized phytoplankton are well acclimated to the low levels of nitrogen, and once nitrogen is supplied, they can rapidly utilize it to stimulate growth and productivity. Across three TIW events encountered, we observed increased productivity and chlorophyll a concentration concurrent with a decreased photochemical conversion efficiency and overall photophysiological competency. Moreover, the observed decrease in photosynthetic turnover rates toward the western section suggested a 70% decrease in growth rates compared to their maximum values under nutrient-replete conditions. This decrease aligned with the increased growth rates observed following 24 h incubation with added nitrate in the western section. These results support our hypotheses that nitrogen is the limiting factor in the region and that phytoplankton are in a state of balanced growth, waiting to “body surf” waves of nutrients which fuel growth and productivity.

A simple coupled model of the wind-evaporation-SST feedback with a role for stability

The wind-evaporation-SST (WES) feedback describes a coupled mechanism by which an anomalous meridional sea surface temperature (SST) gradient in the tropics evolves over time. As commonly posed, the (positive) WES feedback depends critically on the atmospheric response to SST anomalies being governed by a process akin to that argued by Lindzen and Nigam (1987), and omits an alternative process by which SST anomalies modulate surface wind speed through vertical momentum mixing as proposed by Wallace et al. (1989) and Hayes et al. (1989). A simple model is developed that captures the essential coupled dynamics of the WES feedback as commonly posed, while also allowing for momentum entrainment in response to evolving SST anomalies. The evolution of the coupled system depends strongly on which effects are enabled in the model. When both effects are accounted for in idealized cases near the equator, the initial anomalous meridional SST gradient grows over a time scale of a few months, but is damped within one year. The sign and magnitude of the WES feedback depend on latitude within the tropics and exhibit hemispheric asymmetry. When constrained by realistic profiles of prevailing zonal wind, the model predicts that the WES feedback near the equator is stronger during boreal winter, while the domain over which it is positive is broader during boreal summer, and that low-frequency climate variability can also modulate the strength and structure of the WES feedback. These insights may aid in the interpretation of coupled climate behavior in observations and more complex models.

WWLLN Hot and Cold-Spots of Lightning Activity and Their Relation to Climate in an Extended Central America Region 2012–2020

Lightning activity has been recognized to have, historically, social and environmental consequences around the globe. This work analyzes the space-time distribution of lightning-densities (D) in an extended Central America region (ECA). World Wide Lightning Location Network data was analyzed to link D with dominant climate patterns over the ECA for 2012–2020. D associated with cold surges entering the tropics dominate during boreal winter. The highest D (hot-spots) was found to agree well with previously known sites, such as the “Catatumbo” in Venezuela; however, D was lower here due to different detection efficiencies. Previously reported hot-spots showed strong continental signals in CA; however, in this work, they were over the oceans near to coastlines, especially in the eastern tropical Pacific (ETP). Most cold-spots, implying a minimum of vulnerability to human impacts and to some industries, were situated in the Caribbean Sea side of Central America. The Mid-Summer-Drought and the Caribbean-Low-Level-Jet (CLLJ) markedly reduced the D during July-August. The CLLJ in the central CS and across the Yucatan and the southern Gulf of Mexico acts as a lid inhibiting convection due to its strong vertical shear during the boreal summer. The CLLJ vertical wind-shear and its extension to the Gulf of Papagayo also diminished convection and considerably decreased the D over a region extending westward into the ETP for at least 400–450 km. A simple physical mechanism to account for the coupling between the CLLJ, the MSD, and lightning activity is proposed for the latter region.