On the relationship between low cloud variability and lower tropospheric stability in the Southeast Pacific

F Sun,X Qu,A Hall

doi:10.5194/acp-11-9053-2011

F Sun, X Qu + Show 1 more

Open Access

https://doi.org/10.5194/acp-11-9053-2011

Copy DOI

Abstract

Abstract. In this study, we examine marine low cloud cover variability in the Southeast Pacific and its association with lower-tropospheric stability (LTS) across a spectrum of timescales. On both daily and interannual timescales, LTS and low cloud amount are very well correlated in austral summer (DJF). Meanwhile in winter (JJA), when ambient LTS increases, the LTS–low cloud relationship substantially weakens. The DJF LTS–low cloud relationship also weakens in years with unusually large ambient LTS values. These are generally strong El Niño years, in which DJF LTS values are comparable to those typically found in JJA. Thus the LTS–low cloud relationship is strongly modulated by the seasonal cycle and the ENSO phenomenon. We also investigate the origin of LTS anomalies closely associated with low cloud variability during austral summer. We find that the ocean and atmosphere are independently involved in generating anomalies in LTS and hence variability in the Southeast Pacific low cloud deck. This highlights the importance of the physical (as opposed to chemical) component of the climate system in generating internal variability in low cloud cover. It also illustrates the coupled nature of the climate system in this region, and raises the possibility of cloud feedbacks related to LTS. We conclude by addressing the implications of the LTS–low cloud relationship in the Southeast Pacific for low cloud feedbacks in anthropogenic climate change.

Highlights

Marine low-level clouds are prevalent over eastern subtropical oceans just west of the continents, and are maintained through interactions with the lower portion of the atmosphere and the cool ocean surface (Schubert et al, 1979a; Randall et al, 1984; Albrecht et al, 1988)
We examine International Satellite Cloud Climatology Project (ISCCP) observed low cloud cover variability in the Southeast Pacific and its association with ERA-40 calculated lower-tropospheric stability (LTS) across a spectrum of timescales
The seasonal dependence of the LTS–low cloud relationship hinted in the seasonal cycle analysis is brought into sharp relief through an examination of interannual and daily variability

Summary

Introduction

Marine low-level clouds are prevalent over eastern subtropical oceans just west of the continents, and are maintained through interactions with the lower portion of the atmosphere and the cool ocean surface It turns out that the purely internally-generated component of the variability in Fig. 4 is characterized by a very similar seasonality in the LTS–low cloud relationship We demonstrate this by calculating the seasonal means of area-averaged LTS and low cloud amount for each year and stratifying the data by season, creating a timeseries of interannual variability for each season. To explore the spatial relationships underpinning the LTS– low cloud statistics of internally-generated interannual variability, we calculate for each season correlations between seasonal-mean, area-averaged marine low cloud amount and local LTS for the entire Southeast Pacific. For both DJF and MAM (solid lines), the values are larger near the surface than around 700 hPa. low cloud amount appears to be more sensitive to the surface temperature component of LTS.

D J FMAMJ J ASON

Findings

Discussion and implications