Indrani Roy

Solar Cycle Signals in the Pacific and the Issue of Timings

AbstractThe solar cycle signal in sea level pressure during 1856–2007 is analyzed. Using composites of data from January–February in solar cycle peak years the strong positive signal in the region of the Aleutian low, found by previous authors, is confirmed. It is found, however, that signals in other regions of the globe, particularly in the South Pacific, are very sensitive to the choice of reference climatology. Also investigated is the relationship between solar activity and sea surface temperatures in the tropical eastern Pacific. A marked overall association of higher solar activity with colder temperatures in the tropical Pacific that is not restricted to years of peak sunspot number is noted. The ENSO-like variation following peak years that has been suggested by other authors is not found as a consistent signal. Both the SLP and SST signals vary coherently with the solar cycle and neither evolves on an ENSO-like time scale. The solar signals are weaker during the period spanning approximately 195...

Solar cyclic variability can modulate winter Arctic climate

This study investigates the role of the eleven-year solar cycle on the Arctic climate during 1979–2016. It reveals that during those years, when the winter solar sunspot number (SSN) falls below 1.35 standard deviations (or mean value), the Arctic warming extends from the lower troposphere to high up in the upper stratosphere and vice versa when SSN is above. The warming in the atmospheric column reflects an easterly zonal wind anomaly consistent with warm air and positive geopotential height anomalies for years with minimum SSN and vice versa for the maximum. Despite the inherent limitations of statistical techniques, three different methods – Compositing, Multiple Linear Regression and Correlation – all point to a similar modulating influence of the sun on winter Arctic climate via the pathway of Arctic Oscillation. Presenting schematics, it discusses the mechanisms of how solar cycle variability influences the Arctic climate involving the stratospheric route. Compositing also detects an opposite solar signature on Eurasian snow-cover, which is a cooling during Minimum years, while warming in maximum. It is hypothesized that the reduction of ice in the Arctic and a growth in Eurasia, in recent winters, may in part, be a result of the current weaker solar cycle.

Evaluating ENSO teleconnections using observations and CMIP5 models

Bias correction of global and regional climate models is essential for credible climate change projections. This study examines the bias of the models of the Coupled Model Inter-comparison Project Phase 5 (CMIP5) in their simulation of the spatial pattern of sea surface temperature (SSTs) in different phases of the El Niño Southern Oscillation (ENSO) and their teleconnections—highlighting the strengths and weaknesses of the models in different oceanic sectors. The comparison between the model outputs and the observations focused on the following three features: (i) the typical horseshoe pattern seen in the Pacific Ocean during ENSO events with anomalies in SSTs opposite to the warm/cool tongue, (ii) different signature in the tropical Pacific Ocean from that of the North and tropical Atlantic Ocean, and (iii) spurious signature in the southern hemisphere beyond 45° S. Using these three cases, it was found that the model simulations poorly matched the observations, indicating that more attention is needed on the tropical/extratropical teleconnections associated with ENSO. More importantly, the observed SST coupling between the tropical Pacific Ocean and the Atlantic Ocean is missing in almost all models, and differentiating the models between high/low top did not improve the results. It also found that SSTs in the tropical Pacific Ocean are relatively well simulated when compared with observation. This work has improved our understanding of the simulation of ENSO and its teleconnections in the CMIP5 models and has raised awareness of the bias existing in the models, which requires further attention by climate modellers.

Climate Variability and Sunspot Activity

Part I initially focused on basic definitions of Climatology, General Circulation, Climate Variability and Stratosphere Troposphere Coupling. There was a discussion on Climatology of Sea Level Pressure (SLP) and Sea Surface Temperature (SST) which was followed by defining Hadley and Walker circulation. In the subsequent chapter, major modes of climate variability are included with their spatial characteristic and temporal behavior. It is then followed by an overview of Stratosphere-Troposphere coupling. Later on, there is a discussion based on teleconnection among various modes. Discussion on how robust solar influences around different places are detected is included afterwards. Finally, there is a discussion on Total Solar Irradiance (TSI) and how it is reconstructed.

The role of natural factors (part 1): addressing on mechanism of different types of ENSO, related teleconnections and solar influence

The role of natural factors, mainly the sun, is explored on major tropospheric modes of variability in a holistic way. It formulates a flow chart, depicting coupling in the ocean-atmosphere system, initiated by solar decadal variability that involves El Niño Southern Oscillation (ENSO). Possible mechanisms for Canonic ENSO, Modoki ENSO and Canonic-Modoki ENSO are proposed considering their relevance to the decadal variation of Hadley, Walker circulation and mid-latitude jets. The upper stratospheric feature of the polar vortex is included too. Teleconnections by the ENSO on Indian Summer Monsoon (ISM) with a special emphasis on the later two decades of the last century is discussed. The disruption of usual ENSO-ISM teleconnection during that period is also attended. Subsequent analyses presented some results of solar signature which could possibly trigger different types of ENSO, agreeing with proposed mechanisms of the flow chart. It addressed the changing pattern of ENSO behaviour since the 1970s. The overall study can benefit the modelling community by an improved representation of ENSO in models and a better representation of ISM teleconnection via regional Hadley cell.

Solar Influence: ‘Top Down’ vs. ‘Bottom Up’

Two fundamentally different routes have been proposed for a solar influence on the troposphere: the first is the ‘bottom-up’ and the other the ‘top-down’ mechanism. In the ‘bottom-up’ pathway, the Sun can directly influence sea surface temperature (SST) without stratospheric feedback, whereas the ‘top-down’ solar influence is generated through the stratosphere without any influence from oceans. Those two routes are discussed in details with relevant supporting studies.

A Debate: The Sun and the QBO

This chapter initially discussed results of data analysis on solar and QBO separately. It is followed by a discussion on two very popular research relating to the combined influence of the Sun and QBO on the upper polar stratosphere. It reconciled why these two studies differ with each other at a certain point. Later the combined effect of the Sun and QBO was investigated on surface climate. The knowledge can be used for improving prediction skill in the polar region.

Total Solar Irradiance (TSI): Measurements and Reconstructions

This chapter focuses on total solar irradiance (TSI). It gives a very brief idea about their measurements and reconstructions. It also discussed different TSI, e.g. Solanki and Krivova, Foster and Hoyt and Schatten.

The Sun and ENSO Connection–Contradictions and Reconciliations

This chapter focuses on solar signal and ENSO connection (if any) in recent centuries. The discussion begins with two contradictory findings: Contradiction (I), solar signal on tropical Pacific SST-active solar years and ENSO and Contradiction (II), solar signal on tropical Pacific SST – El Nino or La Nina. It reconciled those contradictions and additionally proposed two different mechanisms. The mechanisms differ in the earlier period to that from the later.

Solar Influence Around Various Places: Robust Solar Signal on Climate

This chapter focused on the detected robust solar signal on climate. It presented some observational results that identified solar signature on sea level pressure (SLP), sea surface temperature (SST) and annual mean air temperature. A technique of Multiple Linear Regression (MLR) methods was discussed, and a detected significant signal on SLP around Aleutian Low (AL) was analysed. A solar signal was observed around AL and Pacific High also using other techniques. In terms of SST, the region of tropical Pacific was addressed. One study noted an in-phase relationship between the Sun and tropical Pacific SST, and another study even observed a phase locking between those. A widely debated study that used the method of solar maximum compositing on tropical Pacific SST was presented discussing the methodology.