Diurnal cycle of the South Pacific Convergence Zone in 30 years of satellite images

Colene Haffke,Gudrun Magnusdottir

doi:10.1002/2015jd023436

Colene Haffke, Gudrun Magnusdottir

Open Access

https://doi.org/10.1002/2015jd023436

Copy DOI

Abstract

AbstractA data set of three‐hourly South Pacific Convergence Zone (SPCZ) location and extent, from which IR temperature within the SPCZ may be obtained, for 1980–2012, November–April is used to examine the diurnal cycle of the SPCZ. Maximum SPCZ area occurs at 15:00–18:00 local standard time (LST). Two minima in mean IR temperature are evident, one at 13:00–16:00 LST, nearly coinciding with the maximum in area, and the second in the early morning hours at 05:00–07:00 LST, when area is at a minimum. The morning minimum in mean IR temperature is associated with a peak in deep convection while the afternoon minimum is associated with a peak in midlevel clouds. On average, the morning minimum in IR temperature dominates in the tropical regions of the SPCZ, while the afternoon IR temperature minimum dominates in the subtropical regions of the SPCZ. The relative strength of the two IR temperature minima is affected by the seasonal cycle, intraseasonal variability associated with active Madden Julian Oscillation events, and interannual variability due to the El Niño Southern Oscillation. The morning IR temperature minimum becomes more dominant when the SPCZ is more frequently present or when the SPCZ is shifted toward the equator. In terms of cloud height, midlevel clouds dominate at all times in all regions of the SPCZ and peak in abundance between 15:00–18:00 LST. Low‐level clouds peak near midnight and then transition to high‐level clouds, which peak between 03:00–06:00 LST, just before sunrise.

Highlights

The South Pacific Convergence Zone (SPCZ) is an elongated convection zone that stretches from the equatorial region north of Australia poleward and eastward toward 30°S in the central Pacific [e.g., Vincent, 1994]
We find that the amplitude of the diurnal cycle in terms of mean IR temperature is larger in the subtopics than in the tropics when we examine the IR temperature anomalies or subtract the mean IR temperature for the region at each time (Figure 2d)
We use a unique data set to quantify the diurnal cycle of SPCZ area, mean IR temperature, and cloud height distribution

Summary

Introduction

The South Pacific Convergence Zone (SPCZ) is an elongated convection zone that stretches from the equatorial region north of Australia poleward and eastward toward 30°S in the central Pacific [e.g., Vincent, 1994]. In terms of deep clouds and rainfall, most studies find an early morning peak and some note an afternoon maximum in warmer clouds Many of these studies are limited by the spatial resolution of the data using only surface station data, or by the time span of the data set. Using GOES IR temperature satellite data from January to February 1979, Albright et al [1985] define the SPCZ as the region where the average fractional cover of deep clouds (IR < 237 K) is greater than 20% They find a pronounced diurnal cycle in the fractional coverage of very deep clouds (IR < 218 K) in the SPCZ region that deviated from the daily mean by as much as 60%, peaking in the early morning hours, between 03:00 and 06:00 local standard time (LST). This allows for the quantification of SPCZ area as well as quantities within the labels, such as IR temperature

Results

Seasonal Changes to the Diurnal Cycle in SPCZ Area and Mean IR

Modulation of the SPCZ Diurnal Cycle During MJO Events

Impact of ENSO on the Amplitude of the SPCZ Diurnal Cycle

Conclusions