Short wind turns with strong cooling effect
Why the ocean in the tropics is often colder than expected

Date:
July 31, 2020
Source:
Helmholtz Centre for Ocean Research Kiel (GEOMAR)
Summary:
Why is the sea surface temperature of the northern tropics in the
summer months often lower than expected? New research shows that
a short-term, wind-driven wave phenomenon provides very efficient
vertical mixing and cooling of the upper water layer.



FULL STORY ==========================================================================
Sea surface temperatures in the tropics have a major influence on
the climate in the tropics and the adjacent continents. For example,
they determine the position of the Intertropical Convergence Zone and
the beginning and strength of the West African monsoon. Therefore, it
is important to understand the variability of sea surface temperatures
for climate predictions. Until now, the seasonal cycle of sea surface temperature in the tropical North Atlantic could not be sufficiently
explained. "More precisely, the sea surface is colder than predicted
by the combination of previous direct observations of solar radiation,
currents and mixing, especially in the summer months from July to
September," explains Dr. Rebecca Hummels from the GEOMAR Helmholtz Centre
for Ocean Research Kiel and first author of a study now published in
Nature Communications.


========================================================================== Ship-based observations with the German research vessel METEOR in
September 2015 provided first measurements of a strong turbulent mixing
event below the sea surface, where mixing was up to a factor of 100 higher
than previously observed at this location. "When we noticed the greatly enhanced turbulence in the water column during data processing, we at
first suspected a malfunction of our sensors," says Dr. Marcus Dengler, co-author of the study. "But when we also noticed strong currents at the
ocean surface, we became curious." Precisely such events can explain
the lower temperatures at the ocean surface.

"We were able to isolate the process behind this strong mixing event,
which lasted only for a few days," explains Dr. Hummels. "It is a
so-called inertial wave, which is a very short but intense flow event,"
Hummels continues.

Inertial waves are horizontal wave phenomena in which the current at the surface rotates clockwise with time, whereas the movement rapidly decays
with increasing depth. The different velocities at the surface and in the
layer below cause instabilities and ultimately mixing between the warm
water in the surface layer and the colder water below. Such inertial
waves can be caused by brief variations in the near-surface winds. Up
to now, generally only weak currents have been observed in this region
and the rather steady trade winds at this time of year did not suggest particularly strong mixing events. However, wind variations are crucial
to trigger these waves in the upper ocean. The winds do not have to be particularly strong, but ideally should rotate the same way the ocean
currents do. Since such wind fluctuations are relatively rare and only
last a few days, it has not yet been possible to measure such a strong
wave phenomenon with the associated strong mixing in this region.

After the discovery of this event during the METEOR cruise in September
2015, the Kiel scientists wanted to know more about the frequency and the actual impact of such events. "Through model-based data analysis, we were
able to give a context to the in-situ observations," explains co-author
Dr. Willi Rath from the Research Unit Ocean Dynamics at GEOMAR. "Together,
we have scanned 20 years of global wind observations looking for similar
events triggered by wind fluctuations and described their occurrence
in the region and during the course of the year," Dr. Rath adds. This
has supported the hypothesis that the temporal and spatial distribution
of such events can indeed explain the gap in the heat balance of the
upper ocean.

The strong turbulent mixing caused by the inertial waves at the base of
the surface layer is also crucial for biology: For example, the cold
water that is mixed into the surface layer during such an event also
brings nutrients from deeper layers into the upper ocean penetrated by sunlight. "This also explains the hitherto largely unexplained occurrence
of chlorophyll blooms in this region, which could now also be attributed
to the seasonally increased occurrence of these inertial waves," explains
Dr. Florian Schu"tte, also co- author of the study.

The ship measurements in the tropical Atlantic were carried out in
close cooperation with the international PIRATA program. For more than
20 years, the PIRATA surface buoys have been providing valuable data for studies of ocean- atmosphere interaction, which were also used for this
study. "Indeed, the intensive mixing measurements resulted from a failure
in the hydraulic system of the METEOR, which made other measurements
impossible at that time," says Prof. Dr. Peter Brandt, chief scientist
of the expedition. Despite buoys and series of ship expeditions to this
region, new phenomena are still being discovered -- sometimes by chance -- which decisively advance our understanding of the tropical climate.


========================================================================== Story Source: Materials provided by Helmholtz_Centre_for_Ocean_Research_Kiel_(GEOMAR). Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Rebecca Hummels, Marcus Dengler, Willi Rath, Gregory R. Foltz,
Florian
Schu"tte, Tim Fischer, Peter Brandt. Surface cooling caused by
rare but intense near-inertial wave induced mixing in the tropical
Atlantic.

Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-17601-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200731102638.htm

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